Marinel M. Manaig1, Joseph F. Dela Cruz1*,
Himmatul Khasanah2, Desy Cahya Widianingrum2 and Listya
Purnamasari2
1Department of Basic Veterinary Sciences, College of
Veterinary Medicine, University of the Philippines Los Banos, College, 4031
Philippines
2Department of Animal Husbandry, Faculty of Agriculture, University of
Jember, Jember - 68121, Indonesia
*For correspondence: jfdelacruz@up.edu.ph
Heat stress
becomes a consistent concern in animal production as the global temperature
rises due to climate change. This circumstance has led to significant economic
losses, especially in poultry production and industry. Different methods have
been extensively studied to address issues associated with heat stress, such as
using various plant extracts. Although several environmental, genetic and
nutritional strategies have gained traction, many researchers remain interested
in using plant extracts as it offers a safe, accessible and low-cost solution
to the problem. This review aimed to explore the existing studies on plant extracts
that might be used to combat the adverse effects of heat stress, such as
metabolic alterations, oxidative stress, and immune suppression. Plant extracts
deemed potential to improve the production performance of affected birds were
also reviewed. © 2022 Friends Science Publishers
Keywords:
Chicken;
Feed additive; Immune suppression;
Metabolic alteration; Oxidative stress;
Phytochemical
The global temperature is estimated to
rise by 2°C to 6°C in 2100 (Singh and Singh 2012). This massive increase
affects every living organism in several ways, one of which is its negative
consequence on human and animal health, be it direct or indirect (Rabinowitz
and Conti 2013). The indirect impacts involve the biological alteration and
distribution of vector-borne diseases (Lacetera 2018). To contrast, the direct
effects may be associated with increased temperatures and heat waves (Ebi et al. 2009). Heat stress plays an
essential role in mediating these direct effects. However, heat stress may
cause oxidative stress, metabolic alterations, immune suppression, and even
death, depending on the severity of infection (Lacetera 2018).
Heat stress occurs when the amount of
heat created by animals reaches the capacity to disperse the heat to their
surrounding (Akbarian et al. 2016).
The inequality can be caused by multiple environmental factors such as thermal,
sunlight, irradiation, movement, temperature, and humidity, and internal
factors of animals, including species, sex and metabolism (Naga and Narendra
2018). Poultry meat and egg can provide considerable protein sources with a
high amount of nutrients and low levels of fat as well as fatty acid, making it
one of the most widely consumed animal-source food worldwide. As the global
population increases, the demand for poultry products is also on the rise. As a
corollary, genetic improvement of broiler and layer chickens is continuously undertaken.
These improved birds often have higher metabolic rates and production
performances. Higher metabolic rates produce more body heat, predisposing the
birds to heat stress (Wasti et al.
2020). Heat stress is defined as the imbalance between heat production and heat
loss in the animal body. The studies of heat stress in poultry have been
examined extensively, significantly influencing poultry production. When
animals experience heat stress, they will manage to decrease their heat
production by restricting feed consumption, which potentially declines growth
rate and products qualities (Rashamol et al. 2020). Hence, animal heat
stress has been a pivotal issue among researchers and producers for decades,
especially in tropical arid areas, and regions with harsh climates due to the
stark difference in temperatures between spring and summer seasons.
Heat stress is distinguished into acute
and chronic. Acute heat stress is related to a temporary and quick increase in
the ambient temperature. In comparison, chronic heat stress is associated with
a high ambient temperature after a prolonged period, leading to environment
acclimatization. Chronic heat stress, also called “cyclic chronic heat stress,”
is related to a restricted time of heat exposure triggered by a comfortable
temperature, or called “constant chronic heat stress,” during which poultry is
continuously exposed to increased ambient temperature. The heat stress affects
the physiological properties of poultry, such as increased body temperature
(Liu et al. 2019), lower body weight
(Wasti et al. 2020), reduced feed
intake (He et al. 2018), immune
depletion, altered electrolyte, pH balanced alteration (Ratriyanto and
Mosenthin 2018), impaired endocrine and reproduction (Boni 2019), increased
cortisol level, digestibility and metabolism (Yin et al. 2021) and changes in gut microbiota profile (Shi et al. 2019). Acute and chronic heat
stress has a serious effect on performance, including a lower rate of egg
production (Nawaz et al. 2021),
increased chicken mortality (Barrett et
al. 2019), reduced meat quality, altered immunological parameters and
caecal microflora (Awad et al. 2019).
Furthermore, the poultry industries
suffer a significant financial loss worldwide due to heat stress. In the U.S.,
heat stress caused an economic loss of $128 to $165 million to the poultry
industry (Pierre et al. 2003; Lara
and Rostagno 2013). These financial losses result from decreased production
performance and increased mortality rate. Several environmental, genetic, and
nutritional strategies are now being used to counter heat stress effects (Naga
and Narendra 2018). One promising heat stress management strategy is using
plant extracts (Akbarian et al. 2016)
due to their beneficial compound such as an antioxidant, polyphenol,
antimicrobial and antiparasite. Those compounds have been proven can mitigate
heat stress in chickens (Dong et al.
2015; Akbarian et al. 2016; Ghanima et al.
2019). This review aimed to present and evaluate several studies on
different plant extracts for mitigating heat stress on poultry.
The average body temperature in
chickens is between 41–42°C, and the ideal ambient temperature for growing
poultry is 18–21°C. Studies have found that poultry may experience heat stress
if the environmental temperature is higher than 35°C (Charles 2002). Heat
stress commonly affects animal health by causing metabolic disorders, oxidative
stress, immunosuppression, and death (Lacetera 2018).
The autonomic nervous system (ANS) is
essential in heat stress response. During heat stress, ANS increases the heart
rate and enhances blood flow to the skin. This action maximizes the heat loss
to balance body temperature (Nawaz et al.
2021). However, birds commonly release heat through panting for evaporative
cooling since they lack sweat glands (Wasti et
al. 2020). An increased respiratory rate when the bird is panting leads to
a higher rate of CO2 excretion than its cellular production. This
physiological occurrence changes blood’s bicarbonate buffer system and directly
impacts blood pH, gases, and others circulating metabolites. As the rate of CO2
falls, the concentration of carbonic acid (H₂CO₃)
and hydrogen ions (H⁺) is also reduced. At the same time,
bicarbonate ions (HCO₃-)
concentration is increased, raising blood pH. As a corollary, birds will
excrete a more significant quantity of HCO₃- and
maintain H⁺
from the kidney. This high H⁺ affects the
acid-base equilibrium and causes respiratory alkalosis and metabolic acidosis.
Studies have found that maximal growth rate is achieved when blood pH ranges
from 7.20 to 7.30. Changes in these elements significantly decrease growth rate
and feed efficiency. During panting, pH values commonly exceed 7.25 (Borges et al. 2007). Additionally, metabolic
acidosis triggers myoglobin and haemoglobin's unusual redox reaction (Tang et al. 2013). Haemoglobin plays a vital
role in transporting oxygen, nutrients and wastes in the blood.
Elevated environmental temperatures also activate the
hypothalamic-pituitary-adrenal (HPA) axis. Increased plasma corticosterone
levels are generally observed in birds exposed to heat stress (Lara and
Rostagno 2013). The secretion of corticosterone from the HPA and pituitary
gland is normally detected in chronic heat stress. Extensive secretion of
corticosterone can significantly affect the body and is often linked to muscle
breakdown, cardiac issues, compromised immunity, and depression in broiler
chickens (Nawaz et al.
2021). Additionally, a decrease in triiodothyronine (T3) concentration is
consistently reported, while alterations in thyroxine (T4) concentration are
inconsistent in affected birds. T3 and T4 are essential in regulating body
temperature and metabolic activity. Changes in the poultry’s neuroendocrine
system due to heat stress facilitate lipid accumulation via raised de novo lipogenesis, lowered lipolysis, and
improved amino acid catabolism (Lara and Rostagno 2013). Furthermore, heat
stress stimulates the release of catecholamine, subsequently decomposing
glycogen into glucose in muscles and reducing muscles' capacity to store
energy. Catecholamine is responsible for inhibiting glycogen phosphorylase and
activating muscle glycogenolysis. It works on skeletal muscles' beta androgenic
receptors and triggers a series of responses, unsettling the regular enzymatic
action in skeletal muscles. Moreover, heat stress leads to the discharge of
glucocorticoids and causes vasodilation, proteolysis, and lipolysis in the
muscle through the activation of HPA and the sympathetic-adrenal-medullar axis
(SAM). Glucocorticoids improve the synthesis of glucose to ensure survival
under acute conditions. In addition, heat stress also plays a crucial role in
lipolysis and proteolysis. Increased lipolysis occurs when glucocorticoids
trigger the hydrolysis of circulating triglycerides, intensifying lipoprotein
lipase activity. It can also stimulate major proteolytic mechanisms by damaging
myofibrils in skeletal muscles and negatively regulating anabolic factors such
as growth factor (IGF-1), leading to increased proteolysis. Overall, HPA is
regarded as a better heat stress indicator than corticosterone by sending more
signals of danger or stress to an animal (Nawaz et al. 2021).
Oxidative stress is an imbalance
between reactive oxygen species (ROS) and the antioxidant capacity of animal
cells (Mihaela et al. 2020).
ROS delivers molecules for normal biologic processes. However, this condition
also occurs when oxygen-based molecule collection contains a free radical
oxidizing cellular component, leading to oxidative injury and oxidative damage
of proteins and DNA, when left untreated (Auten and Davis 2009; Shokryazdan et al. 2017). During oxidative
injury, the liver is one of the most affected tissues in the body (Saracila et al. 2019). Increased ROS
overwhelms the buffering system of the liver and leads to oxidative
deterioration of enzymes, mitochondrial membrane and cellular lipids. To
maintain the redox balance, the liver needs to neutralize the excess ROS with
the aid of antioxidants and antioxidative enzymes. Additionally, the liver
plays an essential role in avian lipogenesis as it synthesizes up to 90% of
fatty acids. These fatty acids are packaged as very-low-density lipoprotein
(VLDL) molecules and serve as the primary energy source for other tissues.
Furthermore, hepatic lipids are essential during egg production to nourish the
embryo via yolk targeted VLDL (VLDLy)
(Emami et al. 2020).
It is also noteworthy that aside from its role in lipid metabolism, the liver
plays a critical role in processing carbohydrate, protein, vitamin, mineral
metabolism, and detoxification (Zaefarian et al. 2019).
Cells possess two effective defensive
mechanisms to maintain normal cellular processes under elevated temperatures.
This mechanism includes producing heat shock proteins (HSPs) and increasing the
antioxidant production inside the cell. The HSPs are produced by cells to
address stress-induced conditions by regulating the heat shock factors (HSFs)
gene. The HSP70 and HSP90 have cytoprotective action and perform as chaperons
that assure the correct folding of proteins. An increased expression of HSP70
can stimulate the production of superoxide dismutase (SOD), glutathione (GSH)
and total antioxidant capacity (TAOC). During acute heat stress endotoxins,
HSFs and HSPs tend to rise (Shehata et
al. 2020; Wasti et al.
2021). Moreover, to increase the production of antioxidants inside the avian
cell, a redox-sensitive nuclear transcriptional factor (Nrf2) is transferred to
the nucleus in response to oxidative stress. Once Nrf2 reaches the cell
nucleus, it stimulates the production of different antioxidants through binding
in the promoter region of genes responsible for producing antioxidants, such as
GPX1, GPX3, PRDX1, SOD1, SOD2, TXN and NRF2. The predominant free radical
produced in the cell is superoxide radical catalyzed by superoxide dismutases
(SOD), such as SOD1 and SOD2, into hydrogen peroxide (H2O2).
SOD1 is a copper-zinc enzyme mainly found in the cytoplasm, nucleus, lysosomes,
mitochondrial intermembrane spaces, and peroxisomes. Meanwhile, SOD2, also
called manganese-dependent superoxide dismutase (MnSOD), is a manganese enzyme
typically found in the mitochondria. Given their function, SODs play an
essential role in cells' first level of antioxidant defense. Peroxide produced
by SODs is moderately more stable than superoxide and the other ROS species;
however, their diffusion within the cell can oxidatively damage proteins and
lipids. GPX1 and GPX3 are antioxidant-related genes that maintain peroxide at a
healthy level in the avian cell. They are known to be glutathione structured
selenium dependent. GPX1 is abundant in the mitochondria and cytoplasm, whereas
GPX3 is abundant in the plasma. They both catalyze the glutathione to lower
hydroperoxides and H₂O₂. H₂O₂ and
hydroperoxides and their proximities are also reduced by PRDX1. PRDX1 is a
member of the Peroxiredoxins family using thioredoxin to counteract ROS inside
the cell. Thus, TXN belongs to the thioredoxin family, essential in the
cellular antioxidant system involved in DNA and protein repairs, immune
response and cell death endotoxins (Wasti et al. 2021).
Furthermore, the rate of ROS production and electron transport are
inversely related. Exposure to heat stress for 3 h in broilers has been
reported to repress mitochondrial respiratory complexes (I, III and IV).
Mitochondrial respiratory complexes I and III are known to be the major sites
of ROS production in the electron transport chain. In the mitochondria, the
antioxidant enzymes, like superoxide dismutase (SOD), catalase (CAT) and
glutathione peroxidase (GSH-Px), play a vital role. GSH-Px converts ROS to more
inert species. SOD, as previously discussed, converts superoxide to hydrogen
peroxide (H2O2) to buffer the superoxide levels, while
CAT converts H2O2 into water and molecular oxygen to
balance the H2O2 level. Nevertheless, despite the
increased response of these antioxidant enzymes, 3 h of heat stress is usually
adequate for oxidizing cellular lipids and proteins, overwhelming the buffering
liver capacity. A compromised mitochondrial function affecting metabolism and
energy balance is also commonly found under heat stress. Mitochondria serve as
the main generator of cellular ROS but, simultaneously, denote a highly
susceptible target of ROS-mediated damage. Due to the proximity of
phospholipids in the mitochondrial membrane to the site of ROS generation, they
are very vulnerable to oxidative damage. This leads to the production of
reactive by-products, including malondialdehyde (MDA) which is commonly used as
a biomarker for oxidative injury, and 4-hydroxy-trans2-nonenal (Ismail et al. 2013; Emami et al. 2020; Wasti et al. 2021).
Several
factors need to be taken into account when examining the immune system and its
function. One of the notable factors in the immune response is environmental
effects (Lacetera 2012). According to Monson et al. (2018), heat stress makes poultry highly vulnerable
to disease as it represses its immune response. Moreover, heat stress
potentially causes pathological atrophy of primary and secondary lymphoid
tissues, which reduces lymphocytes that play an essential function in immune
reaction (Hirakawa et al.
2020). A decrease in circulating antibodies, like IgM and IgG, is generally
observed in birds under heat stress. According to Mashaly et al. (2004), the synthesis
inhibition of T and B lymphocytes leads to suppressing the phagocytic activity
of blood leukocytes influenced by high temperatures. They have found that birds
under heat stress have a lower rate of white blood cell (WBC) and higher
heterophil/lymphocyte ratio (H/L ratio). Reduced WBC indicates a decrease in
the number and activities of leukocytes, while a high H/L signals stress in
poultry. Furthermore, Hirakawa et
al. (2020) found that heat stress affects the induction of
antigen-specific antibody production in broilers. This coheres with Mashaly et al. (2004), who document that
heat-stressed birds show significantly lower antibody titers to sheep red blood
cells, indicating declined antibody synthesis. They suggest that this can be
associated with the growth of inflammatory cytokines underneath stress
conditions which can stimulate corticotropin-releasing-factor production from
the hypothalamus. This leads to increased adrenocorticotropic hormone from the
pituitary, which eventually promotes corticosterone production from the adrenal
gland. Corticosteroid is responsible for inhibiting antibody production.
Heat stress can also negatively impact nutrient digestion and
absorption. Heat stress is also reported to stimulate the
hypothalamic-pituitary-adrenal (HPA) axis and therefore generate corticotropin-releasing
hormone (CRH) and adrenocorticotropic hormone (ACTH). This occurrence
eventually leads to increased corticosteroid levels causing fat accumulation,
fatty acid synthesis, and protein catabolism. This also damages
gastrointestinal functions by decreasing jejunum villi height and increasing
organ permeability to microorganisms. Aside from morphological changes in the
gut epithelium, the alteration in jejunal fatty acid-binding protein1 (FABP-1),
glucose transporter (GLUT2), and a cluster of differentiation 36 (CD36) have
also been observed in heat-stressed poultry. These factors affect nutrient
absorption. Reduced digestibility induced by heat stress results from reduced
digestive enzymes, like amylase, maltase, trypsin, lipase and chymotrypsin.
Moreover, heat stress causes mucosal lesions in the small intestine and leaky
gut syndrome (Emami et al.
2020), leading to intestinal inflammation, bacterial translocation, and
compromised bird health and performance (Gilani et al. 2021). Heat stress also affects the ileum, which
serves as the small intestine terminal part and is associated with the
absorption of most nutrients in poultry. The gastrointestinal tract plays an
essential role in animals' immune response in regulating paracellular
penetration of endotoxins, pathogenic bacteria and feed-associated antigens.
However, this tract can be easily affected by the intestinal epithelial tight
junctional barrier. This tight junction is formed mainly of transmembrane
proteins, such as occludin (OCLN) and claudin (CLDN). Occludin aids in cellular
structure and barrier part, while claudin constructs the backbone of tight
junctions and determines the tight junction's ability to tape the paracellular
space. During heat stress, the blood's peripheral circulation is grown and this
reduces blood flow in the intestinal epithelium, causing hypoxia. Hypoxia can
lead to tight junction disorder, intestinal integrity reduction, and intestinal
permeability increase. These circumstances contribute to the rise of
circulating endotoxins (Wasti et
al. 2021).
The
thermoneutral zone measured by the Thermal-Humidity Index (THI) is relatively
narrow in most poultries. The selection for high growth or egg production rates
in commercial poultry lines, which leads to excellent metabolic activity,
higher heat production and decreased thermotolerance, makes them particularly
vulnerable to heat stress and its detrimental effects. Modern broilers have a
high growth rate and increased ROS generation due to increased metabolic demand
for oxygen induced by higher metabolic rates. (Emami et al. 2020). Decreased feed intake is typical in animals under
heat stress as they spend more time drinking water and panting. It also aims to
lower the endogenous production of body heat resulting from digestion and feed
absorption (Zaboli et al. 2017).
Reduced feed intake during panting and increased energy expense for maintenance
alters the energy balance and is believed to lower body weight and mobilization
of adipose tissue in heat-stressed birds (Lacetera 2018). In a study by Souza et al. (2016), they observed a
36 and 21% decrease in body weight in broilers exposed to continuous and
cyclical heat stress, respectively. Poultry under heat stress experiences a sharp
increase in body temperature, and the effort to dissipate excess heat, blood
circulation, and peripheral blood flow is increased while visceral blood flow
is decreased. This occurrence restricts nutrient utilization and lower
production performance as well as feed conversion efficiency (Emami et al. 2020). Moreover, heat
stress can alter hypothalamic peptides involved in appetite
regulation. Studies suggest that poor feed efficiency can be associated
with the reduced distribution speed of feed residue and a lower rate of
chymotrypsin, trypsin, and amylase activities. Poor feed efficiency also can
affect nutrient absorption and intestinal morphology. These factors
significantly reduce protein digestion in broilers and decrease the
digestibility of various diet components, such as proteins, starch, and fats
(Souza et al. 2016). Meanwhile, a
significant decrease in body weight, egg weight, egg production, and eggshell
quality is observed in laying hens exposed to heat stress conditions caused by
the alteration in the status of Ca²+, acid-base balance and
diminished capability of duodenal cells in calcium distribution, egg
production, and skeletal integrity (Mashaly et
al. 2004).
Panting decreases blood bicarbonate
levels, affecting calcium availability in the blood for eggshell
mineralization. This condition is hypothesized to cause poor egg quality.
However, its physiological mechanism is not yet fully understood. Heat-stressed
birds often raise their wings, rest more and move less (Lara and Rostagno
2013). The neuroendocrine of birds is also affected during heat stress. The
disruption of thyroid activity affects poultry reproduction as the thyroid
gland is involved during the onset of puberty, as in the case of hens and birds
(Elnagar et al. 2010). Overall, heat
stress causes multiple serious negative effects on poultry performance, like
decreased feed efficiency, growth, intestinal integrity, egg production and
survival (Monson et al. 2018).
Poultry may even die when the body
temperature exceeds 45°C, which can significantly affect normal body function.
Vale et al. (2010) report increased
mortality in broilers older than 31 days at a maximum temperature-humidity
index (THI) of 30.6°C. Increased mortality in broilers aged 31-40 days has also
been reported when the environmental temperature reaches 34.4°C. Al-Fataftah
and Abu-Dieyeh (2006) noted a higher mortality rate at ambient temperatures
over 25°C. Increased mortality by 31.7% in heat-stressed layers was also
reported in a study by Mashaly et al.
(2004).
Recent discussions have delved into
several strategies to alleviate the negative impacts of heat stress. Different
approaches to environmental management, nutritional manipulation, as well as
genetic selection are being introduced. Adding feed additives and water
supplementation with electrolytes are also commonly suggested.
Increased susceptibility to heat stress
can be addressed by adjusting the development rate and feed efficiency.
Fast-growing broilers have higher heat production; hence, they have a higher
heat load than slower-growing ones (Yalçin et
al. 2001). However, Gonet et al.
(2000) have found that three different lines of hen breeders have comparable
growth performance despite the exposure to a hot environment. This offered the
opportunity to find genes associated with an increased growth rate with more
heat tolerance. Lin et al. (2006)
enumerated three significant genes that may be important in mitigating heat stress,
including the naked neck (Na) gene, Frizzle (F) gene and Dwarf (dw)
gene. Na gene is reported to reduce feather mass by 20% in heterozygous
(Na/na) birds and 40% in homozygous (Na/Na) birds. They are found
to have better body weight and feed efficiency during lower body temperature
(na/na). On the other hand, F gene causes curling and a decrease in
feather size, which is said to reduce heat insulation. Lastly, dw gene
is the suspected cause of heat tolerance in dwarf broiler breeders and causes a
30–40% reduction in body size.
Previous studies also highlight the
function of heat shock proteins (HSPs) in heat stress. Although birds are
exposed to improved ambient temperature, synthesis of most proteins at the
genetic level is decreased except for heat shock proteins (Kumar et al. 2021); hence, they are utilized
in heat stress studies. HSPs bind with other cellular proteins to assist
intracellular transport. It also facilitates protein structures and folds
formation by acting as chaperones. Lastly, it prevents protein aggregation
during stress (Kang and Shim 2020). A number of studies have examined HSP
families, including HSP27, HSP60, HSP70 and HSP90. HSP60 and HSP70 are
responsible for preventing the accumulation of synthesized polypeptides and
restoring their native form. HSP70 is also the most conservative and joint
constituent of the HSP family in chickens. It prevents lipid peroxidation,
improves antioxidant levels and increases digestive enzyme action, which aids
in the adaptive reaction to the thermal stress of birds. Meanwhile, HSP90 is
associated with developing and modifying proteins pattern in older phases.
HSP27 is reported to undergo accelerated phosphorylation during heat stress,
resulting in actin polymerization and stress fiber formation. Studies suggest
that HSP70 and HSP27 are critical in preserving protein solubility in specific
cell compartments. They also protected cells against apoptosis in endothelial
populations (Shehata et al. 2020). A
significant addition in the expression of HSP27, HSP60, HSP90a, HSP70 and
HSP90b has been discovered in chronic heat stress (Vinoth et al. 2015).
Identifying single nucleotide polymorphisms (SNP) corresponding to
thermotolerance also assists in selecting birds that are more resilient against
high environmental temperatures. Kumar et
al. (2021) report that some genes with a differential expression during
heat stress are HSPH1, HSP25, BAG3, RB1CC, PDK, and ID1 and suggest that those
genes play a significant role during acute stress in chickens (Kumar et al. 2021).
Several factors should be considered
when planning environmental strategies for counteracting heat stress (Lin et al. 2006). Most of the recent
studies focus on environmental temperature and relative humidity. These factors
are proven to affect the evaporative cooling mechanism in birds (Ranjan et al. 2019). According to Lin et al. (2005), evaporative heat
loss is better in high temperatures with wind speed. However, it reduces with
higher humidity. The ventilation system also aids in heat stress management.
Good ventilation can assist in the removal of ammonia, moisture, and carbon dioxide
while providing oxygen in poultry houses (Ranjan et al. 2019). Moreover, in a study about thermal manipulation
conducted by Al-Zghoul et al.
(2018), where broilers are exposed to thermal stress on days 10 to 18 of
embryonic development, they observe improved thermotolerance acquisition in
birds as reflected by lower mRNA expression of the genes of redox pathway such
as NOX4, GPx2, SOD2 and catalase. This is indicative of a decrease in
heat-induced oxidative stress. Lower cloacal temperature upon exposure to acute
heat stress is also reported to support better adaptation to changing
environmental temperatures. Meanwhile, Basilio et al. (2003) indicated that thermal conditioning at 40°C
for 24 h of 5-day-old chicks can decrease body temperature. Aside from
improving body temperature, thermal exercise improves plasma MDA and glucose
levels (Oke et al.
2020).
Dietary manipulations are suggested to
alleviate the effects of heat stress. One of the most straightforward
techniques is providing drinking water with electrolyte solutions. This
technique will control the acid-base equilibrium in the heat-stressed bird.
Studies also suggest that giving vitamin C, E and selenium can mitigate the
harmful consequences of heat stress. Vitamin E is known to have free radical
quenching activity that helps to combat oxidative injury. It contains
tocopherols and tocotrienols, making it fat-soluble. The alpha-tocopherol, an
active vitamin E form, is affected in the glutathione peroxidase pathway by protecting
organisms from oxidative harm by responding with lipid radicals produced in the
lipid peroxidation reaction. Oxidized alpha-tocopherol radicals are recycled
back to their functional shape by lowering other antioxidants, such as vitamin
C. An antioxidant compound like vitamin C is a water-soluble substance that can
improve an animal's immune system, defends cells against oxidative
deterioration, and serve as a crucial co-factor in enzyme reactions. Vitamin C
can act as a co-antioxidant with different antioxidants by synergistic effects.
Studies confirm that adult birds can normally synthesize vitamin C to complete
their necessities. However, their needs are found to rise during stress. Both
vitamins C and E are also reported to enhance feed intake, eggshell quality and
body weight. Meanwhile, selenium is a vital micronutrient that acts as a
co-factor for antioxidant enzymes, such as superoxide dismutase, glutathione
peroxidase and thioredoxin reductase and is thought to be a co-factor for
iodothyronine deiodinase.
The activation or inactivation of these
enzymes release hormone T4 to T3 or reverses triiodothyronine (rT3) with the
aforesaid indirect influence on the principle of T3 and T4 production of
selenium, and it can influence the protein synthesis and the metabolism of fat,
protein, carbohydrate, and vitamins, and animal basal metabolic rate. It is
expected that the thyroid hormone synthesis is defective under stress, and
providing the selenium helps restrain the thyroid hormone synthesis and restore
body homeostasis (Shakeri et al. 2020;
Goel 2021). Adding fat to the diet pattern is another technique to mitigate
heat stress in poultry. Fat also produces lower heat compared to carbohydrates
and protein during metabolism. A lower rate of food passage, which increases
nutrient utilization on the GI tract and increases the energy value of food
constituents, is observed in birds given fat supplementation. Moreover,
providing a 5% fat diet benefits both layers and broilers and increases feed
intake by 17% in heat-stressed laying hens. Another advantage of adding fat is
the improved performance of broilers. Improving oil supplementation in higher
protein levels is acknowledged to relieve the harmful impacts of chronic heat
stress on meat lipids, chicken production, and its physiological and immunological
characteristics (Wasti et al. 2020). Htin et al. (2007)
mentioned that the addition of fat improves the palatability of feed and may be
considered as the contributing factor to higher feed consumption and better
performance of birds.
Another common practice to decrease
heat rate in poultry is reducing metabolic rate through feed restriction during
hot periods. Unfortunately, this may harm the growth rate, hence introducing a
dual feeding regime. In this practice, a high amount of protein is supplied
during cooler periods, while a high amount of energy is supplied during the
warmer periods of the day. This method takes into consideration the fact that
protein produces higher metabolic heat than carbohydrates. Lastly, wet feeding
may also be considered. This practice aims to improve water consumption and
reduce the viscosity in the gut, resulting in a faster feeding passage. In
addition, wet feeding stimulates pre-digestion, enhances the absorption of
nutrients in the gut, and accelerates digestive enzyme activity. Improved
performances associated with wet feedings, such as better body weight, feed
consumption and GI tract weight, are reported in broilers, while increased dry
matter consumption, egg weight, and egg production are found in layers.
However, this practice remains underexplored, as it poses the threat of fungal
growth in feed, causing mycotoxicosis (Wasti et al. 2021).
Nutrition management is more
economically feasible than genetic and environmental strategies when addressing
heat stress (Saracila et al.
2021). Furthermore, several investigations have proven that certain plant
extracts can be used to alleviate the harmful results of heat stress. Plant
extracts are also cheap and widely accessible (Shokryazdan et al. 2017). The following
discussion will discuss the effect of plant extract in alleviating heat stress
problems.
In dealing with metabolic disruptions
due to heat stress, most studies focus on alleviating the apparent signs of
heat stress, such as raising wings and panting, which significantly affects the
maintenance of the acid-base balance in affected birds. The assessment of
hematologic parameters is also commonly done. In a study by Zmrhal et al.
(2018), panting and weightlifting were significantly decreased in
broiler chickens provided with Scutellaria baicakensis L. in comparison
with the control group, indicating the association with the anxiolytic result
of plant’s functional compounds, such as wogonin, baicalin, and baicalein. These
compounds pose diverse influences on the nervous system, affecting the behavior
of heat-stressed birds. Additionally, although Scutellaria baicakensis L.
does not significantly impact feed and water consumption, an improved feed
conversion ratio has been observed in experimental birds. El-Shoukary et al.
(2014) also observed a significant decrease in panting behavior in
heat-stressed chickens given black seed (Nigella sativa). The authors
suggest that it may be due to some biological value of the chemical composition
of black seed. Likewise, Iraqi et al. (2013) also observed that
broilers with ginkgo (Gingko Biloba) and peppermint (Mentha piperita)
showed a decrease in panting and weightlifting. This can be associated with the
ability of Gingko Biloba to promote vasodilation in the brain, which
improves oxygenation in brain tissue and acts on the hypothalamic level.
Furthermore, it helps to reduce the expression and secretion of corticotropic
releasing hormone. Meanwhile, peppermint is known to improve circulation,
dispel fevers, and cool the skin and mucosa through stimulating cold receptors.
These interventions help to reduce the negative effects of heat stress on the
behavior of affected birds.
Moringa oleifera is
another plant helpful to combat the negative metabolic effect of increased
environmental temperature. When given to broiler chicks under heat stress, a
significant increase in hemoglobin concentration was observed compared to the
control group. Given the function of hemoglobin to transport oxygen, nutrients
and waste, a higher value may indicate a more significant potential in
addressing the negative impacts and better health status (Hassan et al.
2015). Black grape (Vitis vinifera) is another highly
valuable plant for birds suffering from heat stress. It is reported to decrease
the concentration of serum glucose. This action is associated with its high
flavonoid content that inhibits renal glucose reabsorption by inhibiting
sodium-glucose symporters found in the proximal renal tubule. Stress hormones
alter energy, protein, lipid and mineral metabolisms, blood gases, acid-base
and electrolyte balances, and haemoglobin concentration. The increased blood
glucose level in the presence of a stressor is commonly observed during stress.
By extension, it helps to mobilize or produce glucose for energy to retain
homeostasis; thus, the hypoglycaemic activity of grape seed helps alleviate the
negative effects of heat stress (Hajati et
al. 2015).
Few strategies have been suggested when
combatting oxidative stress in exposed heat-stress conditions on poultry, such
as reducing membrane potential, increasing electron transport chain efficiency,
and improving ROS detoxifying capacity, which remains the principal target in
heat-stressed birds. Scavenging ROS can improve ROS detox capacity by
interfering with enzymatic processes that lead to ROS development, chelating
trace elements affected in ROS development, or upregulating and shielding
endogenous antioxidant defense (Akbarian et
al. 2016). In a study by Wasti et al.
(2021), the supplementation of dried plum (Prunus domestica)
significantly improves the expression of NRF2, GPX1, GPX3, SOD1, SOD2, PRDXN
and TXN genes. They claimed that Nerf2-mediated antioxidant could be activated
by dried plum, resulting in the upregulation of the antioxidant gene in
heat-stressed birds and lower ROS and lipid peroxidation within the cell. On
the other hand, Ghanima et al. (2019)
reports that supplementing boldo (Peumus boldus Molina) to heat-stressed
birds decreases MDA, GPx, and SOD levels. Boldo leaves are known to have a high
content of catechin and boldine, associated with the potent antioxidant
activity and free radical scavenging of this plant. Boldine and catechin are
reported to counteract scavenging and neutralizing the overabundance of free
radicals that subsequently increase the antioxidant system of heat-stressed
birds and reduce the oxidative stress biomarkers to the normal level.
Similarly, birds under heat stress that were given both ginger (Zingiber
officiale) and thyme (Thymus vulgaris) showed reduced liver MDA
concentration. Increased serum total antioxidant capacity (TAC) and total
superoxide dismutase (TSOD) were also observed in birds supplemented with
ginger compared to the control group. Ginger root contains several antioxidant
compounds, such as shogaol, gingerol, zingerone, and diarylheptanoids (Habibi et al. 2014). Meanwhile, the ability of
thyme to act as a natural antioxidant is associated with phenolic hydroxyl
groups functioning as hydrogen donors to the proxy radicals produced during the
initial stage of lipid oxidation. This method effectively inhibits the
formation of hydroxyl peroxide (Abdel-Ghaney et al. 2017). Plants rich in vitamin C, such as Moringa oleifera
and Glycyrrhiza glabra, also help to relieve the negative effects of
heat stress through their antioxidant activity (Al-Daraji et al. 2012; Hassan
et al. 2015).
Furthermore, the induction of HSPs is
also essential in combatting oxidative stress. These molecules enhance cell
stability and help develop thermotolerance during heat stress states. They also
promote cell survival and prevent apoptotic functions in different cell types
(Shehata et al. 2020). Tang et al. (2018) reported that rosemary (Salvia
rosmarinus) could preinduce HSP70 expression before stress occurs. They
also note that during acute stress, HSP70 level is decreased in birds given
rosemary, suggesting sufficient HSP70 in myocardial cells under heat stress. By
contrast, HSP70 levels are reported to increase in birds under heat stress
conditions without rosemary supplementation, followed by a decrease in HSP70
levels when exposed to heat for a longer period. This decrease can be
associated with the inability of the body to tolerate the severe damage from
heat.
Several factors affect the immune
response of heat-stressed birds, thus allowing different approaches to be
implemented. Many studies focus on the health of the gastrointestinal tract
when dealing with an immune response, as it is regarded as one of the main
targets of heat stress. Nigella sativa, or black cumin, has long been
used as traditional medicine for various diseases in the Middle East and has
proven to have a wide range of pharmacological benefits. The antimicrobial
activity of black cumin can be very helpful to help birds combat different
infectious diseases. Thymoquinone (TQ) is attributed to the antimicrobial
effect of Nigella sativa. TQ is effective to fight against Listeria
monocytogenes, methicillin-resistant Staphylococcus aureus (MRSA),
Streptococcus spp., Pseudomonas
aeruginosa, Proteus vulgaris, Klebsiella penumoniae, Bacillus subtillis and
Escherichia coli. It is also effective against viruses, parasites, and
fungi (Forouzanfar et al. 2014). Other plants that may
help mitigate the negative effects of heat stress through their antimicrobial
property include rosehip (Rosa canina L.) and willow bark (Salix alba). Rosehip inhibits
the development of Escherichia coli colony and is effective against Clostridium
perfringes (Criste et al. 2017). On the other
hand, the phenolic compounds of willow bark have bactericide and bacteriostatic
properties for depressing the adhesion of pathogens, such as E. coli and
Clostridium spp. What is more, these plants can improve nutrient
utilization and animal performance (Saracila et al. 2019). Al-Daraji (2012) have documented the potential
antiviral action of licorice (Glycyrrhiza glabra) against Newcastle
disease virus (NDV), and it was also noted for its immunomodulatory activity.
Given to poultry, licorice can improve a bird’s humoral immunity by inducing
antibody titers against non-specific and specific antigens. Its glycyrrhiza
polysaccharide is said to have a sturdy immune action and is recognized to be
involved in immune regulation. It increases WBC counts and ultimately boosts
interferon levels; hence it can improve immunity. Licorice is also reported to
improve the cellular immunity of layers through the increased phagocytic
capacity of mononuclear cells and granulocytes of chicken. This is seen as very
beneficial as heat stress is documented to inhibit B and T lymphocyte
production and ultimately suppresses phagocytic activities. Additionally,
decreased H/L ratio is observed in birds given licorice compared with the
control group. An increase in H/L ratio usually indicates that a chicken
suffers from acute stress. Another plant with a similar effect on the immune
system of chickens is peppermint (Mentha piperita). Peppermint helps to
protect lymphocytes against damages brought by free radicals produced during
heat stress. It can also stimulate the production of interferon and enhance
phagocytosis. Peppermint supplementation also stimulates increased TIg, IgM,
and IgG titer against SRBC. Increased albumin: globulin ratio is also reported,
indicating improved liver function (Arab-Ameri et al. 2016).
Aside from enhancing the immune system
of birds under heat stress, improving gut health can be another beneficial
option. Oregano (Origanum vulgare) and rosehip (Rosa canina L.) were both
documented to have a favorable influence on the colonization of beneficial
bacteria in the gastrointestinal tract as it suppresses the development of
pathogenic bacteria, like E. coli (Criste et al. 2017). Gut microbiome plays a vital
function in birds' intestinal health and development. In the same vein,
according to Wasti et al.
(2021), Prunus domestica can significantly increase the expression of
the CLDN1 and OCLN in the ileum of heat-stressed birds, suggesting improved
integrity of the digestive tract. This approach can be attributed to the
flavonoid component of dried plums used in the experiment. Flavonoids are
reported to indicate protective and promotive results on intestinal tight
junction barrier functions. Additionally, an increase in IL4 and MUC2
expression was also observed in affected birds supplemented with dried plum.
Interleukine 4 (IL4) is a cytokine that plays a critical function in
controlling the immune system and cellular homeostasis; thus, its significant
IL4 gene expression suggests an enhanced immune response. Another gene, MUC2,
also associated with mucin, plays an essential role in defending the gut from
acidic chyme, pathogens, and digestive enzymes and in affecting nutrient
absorption and digestion.
Combatting the negative effects of heat
stress on production performance, such as decreased feed efficiency, growth,
intestinal integrity, egg production and survival, is also crucial as it not
only affects the wellbeing of birds but also the farmers. Dried plum has an
auspicious effect on the production performance of broilers under heat stress.
Wasti et al. (2021)
acknowledges that the supplementation of dried plum leads to a significant
increase in average daily gain (ADG), total body weight, average daily feed
intake (ADFI), and feed conversion ratio (FCR) compared to birds without dried
plum supplementation. As an antioxidant, dried plums are proven effective to
combat the negative effects of heat stress due to their tight junction-related
genes and capacity to improve the relative abundance of beneficial bacteria to
support nutrient absorption and the overall health of heat-stressed birds. Similarly,
Moringa oleifera helps heat-stressed birds to attain better body
weight gain and FCR, presumably due to the improvement in crude protein
digestibility and nutrient utilization in the presence of flavonoids which
serve as antibacterial and antioxidants. It can also be associated with the
positive effect of Moringa oleifera on the gut's microbial
environment, leading to enhanced digestion, absorption, and utilization of
nutrients (Hassan et al. 2015).
Meanwhile, rosemary, dill, and chicory extracts are reported to improve egg index,
eggshell weight, Haugh unit and yolk index in layers suffering from heat stress
compared to the control group (Torki et
al. 2018). Despite these promising findings, further studies need
to check for replicability and explain the mechanism behind these results.
Increased ambient temperature is the
primary aspect that negatively impacts animal performance and production in
tropical, subtropical, and arid regions (Habeeb 2020). In the Philippines, heat
stress caused by high environmental temperature is a common issue in poultry
farms, especially during summer. The poultry industry in the country is
dominated by smallholders (Chang 2004). As such, offering safe, accessible and
low-cost alternatives such as plant extract to mitigate heat stress's negative
effect is a highly beneficial solution. One of the most abundant plants in the
Philippines for addressing heat stress is Moringa oleifera. It is
notable for its ability to boost the immune response of heat-stressed birds.
Turmeric (Curcuma longa) is also reported to improve the productive
performance and health of broilers reared under heat stress conditions. It
enhances affected birds' immune response and antioxidant systems (Sugiharto
2020). Furthermore, Psidium guajava is another common plant in the
country that can mitigate the adverse impacts of heat stress. Ngoula et al. (2017) found that using
guava can significantly counteract the negative impacts on cavies' reproductive
system, such as sperm quantity and quality. Lemongrass (Cymbopogon citratus)
is also found to alleviate the harmful effects of heat stress on rabbits due to
its antioxidant activity (Daader et
al. 2018).
Heat stress can result in massive
financial loss to farmers due to a decrease in growth, reproduction, meat and
egg production, health through metabolic disorders, oxidative stress and
immunosuppression. Genetic improvement, environmental modification, and
nutrient intervention can help mitigate the heat stress in poultry. Adding
plant extracts can decipher the effects of heat stress in poultry while
improving its overall performance. The effective method and dosage of plant
extract supplementation need further investigation. Comprehensive in vitro
or in vivo studies are required to
identify the best approach to heat stress and examine the potential of other
plant extracts in strengthening poultry defense against heat. stress.
Author Contribution
MMM and JSDC conducted this study, writing and literature review. HK, DC and LP as co-author performed the manuscript
writing.
Conflicts of Interest
The author declares no conflict of
interest of any sort.
Data Availability
Not Applicable in this paper.
Ethics Approval
This study does not involve human
subjects. Thus, ethics approval is not required.
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