Immunosupressive nature of heat stress and its remedies
“Sudden increases in temperature as the spring moves into summer are particularly stressful for laying birds and causes immunosupression which may become pre-disposing factor for many diseases”
This is generally acceptable fact that all stresses are immunosupressive in nature. Among many others, the environment heat is one of the very important stress factors, especially in tropical areas. The importance of animal responses to environmental heat applies to all species; however, poultry seems to be particularly sensitive to heat stress, due to the fact that modern poultry genotypes produce more body heat, due to their greater metabolic activity. The thermo neutral zone for poultry is considered as 60-75 ºF, Even the temperature of 85 ºF is manageable but at 100 ºF and above heat gain exceeds heat loss and core body temperature becomes lethal.
MECHANISM OF HEAT REGULATION IN POULTRY:
Unlike most of the warm blooded animals poultry do not have sweat glands in Their body to assist in thermo regulation and homeostasis, they perspire by the way of Panting, besides increasing radiant, convective and evaporative heat loss by vasodilatation. Panting promote heat exchange between their body and the environment, using air sacs, as they promote and increase gas exchanges with the air.
EFFECT OF HEAT STRESS ON LAYING BIRDS:
In laying hens, heat stress depresses body weight, egg production, egg weight, egg shell quality and is generally accompanied by suppression of feed intake, which could be the cause of the decline in production.
CHANGES IN BODY PHYSIOLOGY:
The feed digestibility of different components of the diet (proteins, fats, starch) decreases, alterations in acid-base balance, the status of Ca2+, and diminished ability of duodenal cells to transport calcium could cause suppression of circulating white blood cells (WBC), reduction in antibody production and an increase in the heterophil/lymphocyte ratio (H/L ratio), which is an indicator of stress. Panting under heat stress conditions leads to increased carbon dioxide levels and higher blood pH (alkalosis), which in turn hampers blood bicarbonate availability for egg shell mineralization and induces increased organic acid availability, also decreasing free calcium levels in the blood.
High environmental temperatures alter the activity of the neuroendocrine system of poultry, resulting in activation of the hypothalamic-pituitary-adrenal (HPA) axis (disrupt the normal status of reproductive hormones at the hypothalamus), and elevated plasma corticosterone concentrations. Body temperature and metabolic activity are regulated by the thyroid hormones, triiodothyronine (T3) and thyroxine (T4). The T3 concentrations consistently decrease in high temperature conditions, whereas T4 concentrations are inconsistent. Due to the involvement of the thyroid in the onset of puberty and reproductive function of birds, a disruption of thyroid activity by heat stress would cause negative effect on reproductive performance of the hens. In male semen volume, sperm concentration, number of live sperm cells and motility decreased due to heat stress
Reduced thymus, spleen, and liver weights (Primary lymphoid organs in laying birds) has been found in the laying hens subjected to heat stress besides reduced macrophages performing phagocytosis and lower levels of total circulating antibodies (specific IgM and IgG levels, during primary and secondary humoral responses). Viruses like NDV, IBV, AIV, Some bacteria, such as Salmonella, Campylobacter, Mycoplasmas and aflatoxins are capable of exploiting the Immune and neuroendocrine system during times of heat stress, proving that heat stress can potentially alter the host-pathogen interaction by enhancing pathogenecity.
- A. IMPROVING MANAGEMENT:
- Keep chickens in a well-ventilated area with adequate air flow. Move heat away from them by placing circulation fans to blow with prevailing winds. Remove the shutters from continuously operating fans to increase air flow. Increasing ventilation to remove heat from the birds should be the first priority.
- Install some type of evaporative cooling, such as a misting or fogging system. Water on the chickens’ bodies helps to cool them.
- Feed during the coolest part of the day because digestion naturally produces heat.
- feeding during night time hours when cooling is expected
- Inspect emergency generators and temperature alarm systems. Keep fans, evaporative systems and other cooling equipment in good working order to maximize efficiency.
- Avoid overcrowding the chickens. It reduces body heat, as well as the corresponding amount of heat the ventilation system must move out of the poultry house.
- Don’t disturb the birds during hot times of day and let them rest.
- Regularly remove any accumulated litter from the chicken house, as decomposition produces heat. Removal also keeps pests to a minimum.
- Reduce radiant heat in the poultry house with adequate ceiling insulation.
- Shiny roof can reflect twice as much solar radiation as a rusty or dark metal roof. Roofs should be kept free of dust and rust. Roof reflectivity can be increased by cleaning and painting the surface with metallic zinc paint or by installing an aluminum roof.
- Outside of the chicken house, tall grass and weeds restrict air flow, while bare ground can reflect heat into the house. Low cut grass is best as it helps to absorb the sun and heat.
- B. SUPPLEMENTATION:
- Water is instrumental to chickens surviving hot weather, as water intake will increase by 2 to 4 times that of normal during heat stress. Have cool, clean drinking water available at all times in accessible locations is extremely helpful.
- Supplement drinking water with electrolytes.
- Potassium chloride appears to increase water intake when supplemented in the drinking water at concentrations of 0.6% and has been generally more effective than other potassium and sodium salts.
- Sodium bicarbonate in the feed or use of carbonated water is especially useful for hens in egg production as panting and carbon dioxide release impacts the acid base balance and bicarbonate, available for egg shell formation.
- In breeding poultry, Vitamin C has been shown to be effective in moderating warm temperature declines in egg production and eggshell quality in laying hens and sperm production in breeder males.
1. Effect of Heat Stress on Production Parameters and Immunesystem
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By Dr. Akbar Ali, Dr. Naila Siddique and Dr. Muhammad Athar Abbass
National Reference lab for poultry diseases, national agricultural research centre, Islamabad.
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