Seasonal variations of phagocytic response, immunoglobulin G (IgG) and plasma cortisol levels in Dhofari goats

Unpublished observations in Oman revealed that both the morbidity and mortality figures for a wide variety of diseases in goats are higher during the winter than during the summer season. The present study therefore, attempted to ascertain whether there were any measurable seasonal differences in the phagocytic response, levels of circulating immunoglobulin G, blood cells or plasma cortisol in six adult male Dhofari goats investigated over a 1-year period. Interestingly, we observed that during the summer, goats had significantly higher levels of circulating lymphocytes, increased phagocytic activity against the bacterial target, Staphylococcus aureus and lower levels of plasma cortisol. It was postulated that the decreased phagocytic activity observed during the winter season might be related to reduced levels of opsonins, early neutrophil exhaustion and/or suppression of neutrophil activity as a result of higher cortisol levels and reduced periods of exposure to daylight. These results might also explain in part why the incidence of a variety of diseases in goats is commonly higher during the winter season.