Despite advances in vaccination, biosecurity, and nutrition global poultry industry continues to suffer from unexplained vaccine failure, repeated disease outbreaks, poor flock uniformity, and hidden economic losses. In most cases, the underlying problem is immunosuppression-a silent yet devastating condition that compromises the bird’s ability to respond to vaccines and resist infections. It is one of the most underestimated problems in modern commercial farming.
Immunosuppression defined as a state of temporary or permanent dysfunction of the immune response resulting from damage to the immune system and leading to increased susceptibility to disease, includes sub optimal responses in antibody production, innate and cellular immunities.
Major Causes of Immunosuppression
1. Infectious Causes of Immunosuppression
Almost all poultry diseases can suppress the immune system to some extent; however, certain viral diseases cause profound immunosuppression because their viruses replicate directly within primary immune organs (e.g. the bursa of fabricius and thymus). Major infectious causes are Infectious bursal disease, Chicken infectious anaemia, Marek’s disease, Reovirus infection, Newcastle disease, and Avian influenza.
2. Non-Infectious Causes of Immunosuppression

Mycotoxins in Feed
Mycotoxins are among the most common yet underdiagnosed non-infectious causes of immunosuppression in poultry. Even low-level, chronic exposure can impair antibody production, reduce macrophage activity, and weaken vaccine-induced immunity. Common immunosuppressive mycotoxins are Aflatoxin, Ochratoxin, T-2 toxin, and Fumonisin.
Nutritional Deficiencies
Optimal immune function in poultry depends heavily on adequate nutrition. Nutrients critical for maintaining strong immunity are vitamin A, vitamin E and vitamin C, Se and Zn.
Environmental & Management Stress
Environmental and management related stressors significantly suppress immunity through the elevation of stress hormones, particularly corticosterone, which inhibits lymphocyte function and antibody production. Common stress factors in commercial poultry production are overcrowding, heat stress, poor ventilation, high ammonia levels, transport stress, and poor brooding management.
Immunosuppression is not a secondary issue—it is the foundation of poultry health failure. It causes poor production and economic losses. The key to prevention of immunosuppression is to reduce stress, maintain appropriate management practices and follow strict vaccination programs.

Written by:
Dr. Sumaiya Islam
MS (Parasitology), B. Sc. Veterinary Science & Animal Husbandry, CT (India)
Sher-e-Bangla Agricultural University, Bangladesh