Hawk diagnosed with avian pox
By Will Sims, DVM, MS
With over 800,000 tests run annually, TVMDL encounters many challenging cases. Our case study series will highlight these interesting cases to increase awareness among veterinary and diagnostic communities.
A wildlife rehabilitation center submitted a biopsy from a hawk. The veterinarian examining the hawk noted a proliferative lesion on the non-feathered skin of the left talon. There are several infectious disease differentials for lesions on the talons of birds ranging from pododermatitis (bumblefoot), often caused by an underlying bacterial infection to various viral, fungal, and protozoal infections. In this case, the veterinarian decided to biopsy the lesion and submit it for histologic examination. If a bacterial infection were also suspected, bacterial culture and sensitivity would also have been a good additional diagnostic tool. However, the veterinarian’s primary differential was avian pox (fowl pox or contagious epithelioma).
Diseases caused by poxviruses are very amenable to diagnosis with histopathology when coupled with the characteristic gross lesion. On histopathology, the epidermal surface of the hawk’s skin was hyperplastic. Closer examination revealed the individual keratinocytes exhibited cytoplasmic vacuolation (ballooning degeneration) and contained eosinophilic intracytoplasmic inclusion bodies (Bollinger bodies). While individual viruses are too small to be resolved with routine light microscopy, large DNA viruses such as the poxvirus produce characteristic intracytoplasmic inclusions that are aggregates of replicating virus. This key cellular change allows for diagnosis with histopathology. In addition to the characteristic inclusion bodies, there was a large amount of secondary inflammation associated with the viral infection. More advance diagnostic techniques to confirm the histologic diagnosis that can be used include virus isolation, electron microscopy, serology, and immunohistochemistry.
Avian pox affects a wide variety of both domestic and wild species of bird. There are two forms of the disease. The type observed in this case is called the cutaneous (dry) form. Starting as vesicles on the unfeathered skin of the feet, legs, beak, or conjunctiva, it progresses to a proliferative nodule that can become infected with secondary bacteria. This form is often self-limiting and will pass in 2-4 weeks. Another form of the disease called the diphtheritic from affects the mucosa of the mouth, throat, trachea and lungs. This form may be more serious as it can obstruct the airway by producing large necrotic nodules that, if severe enough, lead to suffocation. This disease is seen throughout the world in mainly temperate regions and is often associated with seasonal mosquito cycles, which is its primary vector. The disease is spread not only by mosquitos, but may also be transmitted by direct contact from bird to bird. Because the poxvirus is stable months to years in the infected scabs from the lesions, it may remain in the environment for extend periods of time. This becomes significant to poultry producers as it may remain on the premises for extended periods.
While mosquitoes are typically responsible for local transmission of the virus, wild birds may spread the virus over a greater range and allow for emergence in new areas. Domestic poultry producers may vaccinate for the disease and are also advised to follow biosecurity measures. These include preventing wildlife contact with domesticated flocks and eliminate standing water to prevent spread through mosquito vectors. If it does infect the flock, birds should be isolated or culled, and the premises should be decontaminated.
For more information on this case, contact Dr. Will Sims, Veterinary Pathologist at the Amarillo lab. To learn more about TVMDL’s test catalog, visit tvmdl.tamu.edu or call 1.888.646.5623 for the College Station lab or 1.888.646.5624 for the Amarillo lab.