Respiratory Disease Complex in Poultry – Recognising Mixed Infections in the Field

Respiratory disease is one of the most common presentations in poultry practice. In Indian field conditions, these cases are rarely due to a single pathogen and are better described as a respiratory disease complex involving multiple infectious agents¹. 

Understanding the Respiratory Disease Complex 

Mixed infections involving Newcastle Disease, infectious bronchitis virus, and bacterial pathogens such as Mycoplasma gallisepticum are frequently reported². 

These pathogens may interact synergistically, leading to more severe clinical signs, prolonged disease, and increased flock-level impact¹. 

Field Presentation 

Affected flocks typically show coughing, sneezing, nasal discharge, and reduced feed intake. However, the severity and progression may not always match classical disease descriptions due to overlapping infections¹. 

A key field observation is poor or inconsistent response to routine therapy, especially when only antibacterial treatment is used. 

Why Mixed Infections Worsen Outcomes 

Primary viral infections can damage the respiratory tract, allowing secondary bacterial pathogens to establish infection more easily. This may result in complicated respiratory disease (CRD) and increased mortality¹.  

Practical Field Approach 

Veterinarians should avoid assuming a single causative agent in respiratory outbreaks. Instead, a combined approach that considers both viral and bacterial components, along with correction of environmental factors, is likely to be more effective. 

Conclusion 

Respiratory disease in poultry should be approached as a multi-factorial condition. Early recognition of mixed infections can help improve treatment outcomes and reduce economic losses. 

References 

1. 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 

2. Samy A, Naguib MM. Avian respiratory coinfection and impact on avian influenza pathogenicity in domestic poultry: field and experimental findings. Veterinary sciences. 2018 Feb 24;5(1):23. https://www.mdpi.com/2306-7381/5/1/23