When Poor Weight Gain Has No Obvious Cause: Could Chlamydiales Be Involved?

A flock does not always need to look sick to have a production problem. When broilers fail to achieve expected body-weight gains, the first clinical question is often whether an obvious disease process is present. But some infections may have a less visible impact. Chlamydia gallinacea is relevant in this context because infected broilers have shown reduced body-weight gain despite the potential for asymptomatic shedding. 

For the practicing veterinarian, this raises a different question: Could a clinically quiet infection still be contributing to poor flock performance? 

Production Loss Without Dramatic Clinical Disease^[1],[2] 

C. gallinacea primarily infects poultry and has been identified in chickens, ducks, guinea fowl, turkeys, and wild birds. Field observations have reported asymptomatic shedding in poultry, while experimental inoculation of broilers demonstrated reduced body-weight gains in infected birds. 

This finding is important from a flock-management perspective. 

A bird does not necessarily need to show obvious illness for infection to have a production consequence. Therefore, when body-weight performance is below expectation, veterinarians may need to consider infectious factors that do not produce a striking clinical picture. 

This does not mean that poor growth can be attributed to C. gallinacea based on performance data alone. Rather, the finding provides a reason to consider the organism when the production pattern and other flock observations support further investigation. 

Why Asymptomatic Shedding Matters 

The combination of apparently limited clinical expression and organism shedding creates a particular challenge. 

A flock can contain birds that do not appear overtly affected while still participating in transmission. Chlamydiales can spread through direct contact and through indirect routes involving respiratory exposure, infected material, and environmental conditions. Husbandry systems, flock density, and biosecurity can influence transmission^{[1],[3],[4],[5]}. 

For veterinarians, this means that visible disease and production impact should not be treated as interchangeable indicators. 

A flock may have limited obvious disease but still require attention when performance concerns coincide with an epidemiological risk of Chlamydiales exposure. 

Keep C. gallinacea Distinct from C. psittaci 

Another important consideration is species identification. 

C. psittaci has traditionally received considerable attention in avian chlamydiosis because of its zoonotic importance. However, Guo et al. (2016) identified C. gallinacea as the endemic chlamydial species in chickens, distinguishing it from C. psittaci^[6]. 

For poultry veterinarians, this distinction matters when interpreting findings associated with Chlamydiales. A suspected chlamydial infection in chickens should not automatically be considered C. psittaci. 

The broader avian epidemiology is also complex, with C. gallinacea reported in domestic and wild birds and other Chlamydia species identified across avian populations^[5]. 

What Should the Veterinarian Look For? 

When poor weight gain is the main concern, the clinical assessment should remain broader than a search for obvious chlamydial disease. 

Useful observations include: 

• Body-weight performance: Is the flock consistently showing reduced weight gain? 
• Clinical expression: Are obvious signs present, or is the flock largely clinically quiet? 
• Shedding potential: Could apparently healthy birds be contributing to transmission? 
• Flock circumstances: Are there factors that could facilitate Chlamydiales transmission? 

These observations cannot establish C. gallinacea infection on their own. They can, however, help determine whether Chlamydiales should remain part of the differential considerations surrounding an unexplained production problem. 

Clinical Bottom Line 

C. gallinacea is relevant to poultry practice not only because of its association with infection, but because production effects may occur even when clinical disease is limited or absent. Experimental findings of reduced body-weight gain in infected broilers provide a specific reason to consider Chlamydiales when flock performance is unexpectedly poor. 

The practical message is straightforward: When growth performance and clinical appearance do not tell the same story, do not overlook infections that may circulate with minimal visible disease.

References:

1. Szymańska-Czerwińska M, Zaręba-Marchewka K, Niemczuk K. New insight on chlamydiae. Journal of Veterinary Research. 2023 Dec 19;67(4):559-65. https://reference-global.com/download/article/10.2478/jvetres-2023-0067.pdf
2. Narayan J, Yadav V, Prakash D, Diwakar RP, Joshi RK. A Review on Chlamydiales in Indian Poultry and its Zoonotic Transmission. https://www.researchgate.net/profile/Dp-Shrivastava-2/publication/394943252
3. Ravichandran K, Anbazhagan S, Karthik K, Angappan M, Dhayananth B. A comprehensive review on avian chlamydiosis: a neglected zoonotic disease. Tropical animal health and production. 2021 Sep;53(4):414. https://pmc.ncbi.nlm.nih.gov/articles/PMC8313243/pdf/11250_2021_Article_2859.pdf
4. Sachse K, Borel N. Recent advances in epidemiology, pathology and immunology of veterinary chlamydiae. Chlamydia biology: from genome to disease. 2020:403-28. https://www.researchgate.net/profile/Nicole-Borel/publication/338310086
5. Stokes HS, Berg ML, Bennett AT. A review of chlamydial infections in wild birds. Pathogens. 2021 Jul 28;10(8):948. https://www.mdpi.com/2076-0817/10/8/948
6. Guo W, Li J, Kaltenboeck B, Gong J, Fan W, Wang C. Chlamydia gallinacea, not C. psittaci, is the endemic chlamydial species in chicken (Gallus gallus). Scientific reports. 2016 Jan 18;6(1):19638. https://www.nature.com/articles/srep19638.pdf