Understanding the histopathology of reovirus lesions
Emily Martin
Animal Health Laboratory, University of Guelph, Guelph, ON.
AHL Newsletter 2025;29(1):20.
It has been over a decade since reovirus emerged as a cause of lameness in Ontario chickens. Prior to 2012, bacterial and developmental causes of lameness were the primary diagnoses. Since then, various strains of reovirus have emerged in Ontario and have contributed to varying lameness severity. Reovirus has also emerged as a cause of lameness in turkeys.
When examining tissues from lameness cases, we prefer to have both heart and gastrocnemius tendon (Fig. 1A) to examine as some reovirus lesions can be more developed in one tissue versus the other (Figs. 1B,1C, 1D). We also prefer to have 3 to 5 sections of each tissue to examine since not every tissue may contain lesions.
When sampling lameness cases for ancillary testing, it is important that the PCR and histopathology samples be collected from the same birds so the results can be correlated. Reovirus PCR is routinely run in conjunction with histology of at least gastrocnemius tendon (NOTE: flexor tendon is also required in turkeys). The combination of a positive PCR test as well as lesions of lymphoplasmacytic tenosynovitis and epicarditis on histopathology confirms the presence of a reovirus infection. Occasionally, there are other combinations of results that can be confusing. Sometimes there will be no histologic lesions suggestive of reovirus infection, but the reovirus PCR will be positive. This may indicate the virus is just being introduced into these birds and lesions have not yet developed. In other situations, there may be obvious well-developed histologic lesions (Figs. 1C, 1D) that pathologists consider consistent with or highly suggestive of a reovirus infection, but the reovirus PCR is negative. In this case, the reovirus organism may no longer be present in the tissue, but there has been sufficient time for severe lesions to develop. Based on case experience and the current disease environment in Ontario, lymphoplasmacytic tenosynovitis in broiler chickens is primarily caused by reovirus; however, similar lesions could be caused by Mycoplasma synoviae. In other chicken commodity groups (i.e., broiler breeders, layers) and turkeys, other disease rule outs should still be considered.
Figure 1. A. Normal tendon. B. Predominantly lymphocyte and plasma cell populations in the synoviae. 40X C. Development of a lymphoid nodule within the gastrocnemius tendon. 20X D. Heart. Multiple lymphoid nodules over the epicardium. H&E stain.