Immunosuppression in Poultry – The Hidden Driver of Mixed Infections

Immunosuppression is a major but often under-recognised factor in poultry disease outbreaks. It plays a crucial role in the development of mixed infections, poor treatment response, and vaccine failure, particularly in intensive farming systems ^1, 2. 

Major Immunosuppressive Diseases 

Diseases such as Infectious Bursal Disease and Marek's Disease are among the most important causes of immunosuppression in poultry. These infections directly affect immune organs, leading to reduced resistance against other pathogens ². 

Impact on Field Cases 

Immunosuppressed birds are more susceptible to a wide range of infections, including viral, bacterial, and parasitic agents. This often results in simultaneous infections involving multiple pathogens, making disease presentation more complex. 

In field conditions, such flocks may show persistent illness, higher mortality, and poor growth performance despite treatment. 

Vaccine Failure and Recurring Disease 

One of the most important consequences of immunosuppression is reduced vaccine efficacy. Even when vaccines are administered correctly, the immune response may be inadequate in immunocompromised birds². 

This explains why some farms experience recurrent disease outbreaks despite following vaccination schedules. 

Practical Field Implications 

Veterinarians should consider immunosuppression in cases where disease persists despite appropriate treatment or vaccination. Addressing only secondary infections without identifying the underlying cause often leads to repeated failures. 

Improving overall flock immunity through better management, nutrition, and disease control is essential. 

Conclusion 

Immunosuppression is a key driver of mixed infections and should be an integral part of disease investigation. Recognising its role can help veterinarians implement more effective and sustainable control strategies. 

References 

1. Jaton J, Gómez E, Lucero MS, Gravisaco MJ, Pinto S, Vagnozzi A, Craig MI, Di Giacomo S, Berinstein A, Zoth SC. Study of coinfection with local strains of infectious bursal disease virus and infectious bronchitis virus in specific pathogen-free chickens. Poultry Science. 2023 Dec 1;102(12):103129. https://www.sciencedirect.com/science/article/pii/S003257912300648X 
2. Liu H, Pan S, Wang C, Yang W, Wei X, He Y, Xu T, Shi K, Si H. Review of respiratory syndromes in poultry: pathogens, prevention, and control measures. Veterinary research. 2025 May 17;56(1):101. https://veterinaryresearch.biomedcentral.com/articles/10.1186/s13567-025-01506-y