The effect of environmental stressors on the immune response to avian infectious bronchitis virus

Abstract

The first aim of this research was to determine the prevalence of IBV in broilers within the Canterbury province, New Zealand, in late winter and to search for associations with management or environmental factors. The second aim was to study how ambient stressors affect the immune system in birds, their adaptive capacity to respond, and the price that they have to pay in order to return to homeostasis. In a case control study, binary logistic regression analyses were used to seek associations between the presence of IBV in broilers and various risk factors that had been linked in other studies to the presence of different avian pathogens: ambient ammonia, oxygen, carbon dioxide, humidity and litter humidity. Pairs of sheds were selected from ten large broiler farms in Canterbury. One shed (case) from each pair contained poultry that had a production or health alteration that suggested the presence of IBV and the other was a control shed. Overall, IBV was detected by RT-PCR in 50% of the farms. In 2 of the 5 positive farms (but none of the control sheds) where IBV was detected there were accompanying clinical signs that suggested infectious bronchitis (IB). Ambient humidity was the only risk factor that showed an association (inverse) with the prevalence of IBV (p = 0.05; OR = 0.92). It was concluded within the constraints of the totally enclosed management systems described, that humidity had an influence on the presence of IBV, but temperature, ammonia, carbon dioxide, oxygen or litter humidity had no effect. In another study environmental temperatures were changed in order to affect the biological function and adaptive capacity of chickens following infection with IBV. The 'affective states' of the animal were assessed by measuring levels of corticosterone (CORT) in plasma and tonic immobility (TI). It was found that low (10 +/- 2°C) and high (30 +/- 2°C) temperatures exacerbated the respiratory signs and lesions in birds infected with IBV as compared to those housed at moderate (20 +/- 2°C) temperatures. The chickens housed at high temperatures showed significantly decreased growth, a higher proportion of hepatic lesions (principally haemorrhages) and a longer tonic immobility period, but there was no significant alteration in the plasma levels of CORT. The birds housed at low temperatures developed a higher proportion of heart lesions (hydropericardium, ventricular hypertrophy) and had significantly higher levels of plasma CORT than birds housed under moderate and/or high temperatures. The specific antibody response to IBV decreased in birds housed under high temperatures. Interestingly the birds housed at high temperatures developed significantly higher levels of haemagglutinin antibodies to sheep red blood cells (SRBC) than those birds housed under low or moderated temperatures. Cell mediated immunity was not significantly affected by heat or cold stress in the first 13 days of treatment but at 20 days the levels of interferon gamma in the birds subjected to low temperatures were lower than in the high temperature group. In other trials, the exogenous administration of low physiological doses of oral CORT (as compared to high pharmacological doses typically used in such experiments) to birds resulted in suppression or enhancement of the immune response depending on duration of treatment and/or dose and nature of the antigen. To our knowledge, this is the first study to show that exogenous CORT can produce an enhancement in the immune response in chickens. iv In conclusion, environmental stressors such as high or low temperatures do affect the physiology of the fast-growing broiler. The adjustments the birds have to make to maintain homeostasis impacts on the course of common infectious diseases, such as IB, that normally is mild in the New Zealand poultry industry. The administration of exogenous CORT showed that this hormone may be part of the physiological stress response and acts as a messenger to prepare the immune system for potential challenges (e.g., infection).