Pseudorabies in a group of Texas hog-hunting dogs
Erin Edwards, DVM, MS, DACVP, Gabriel Gomez, DVM, PhD, DACVP, Terry Hensley, DVM, MS, and Will Sims, DVM, MS
Pseudorabies, also known as Aujezsky’s disease, mad itch, and infectious bulbar paralysis, is a highly contagious disease of swine that can also be fatal in domestic species. This past spring, pseudorabies was diagnosed in a group of hog-hunting dogs from southern Texas. Four dogs total were affected. Two were found dead with no clinical signs and were initially suspected to have died from heat stroke. Two other dogs then developed neurological signs and supportive care was initiated. These dogs were noted to have seizures, ataxia, profuse salivation, and severe facial pruritis with excessive pawing at the face. One died during treatment and the other was euthanized due to a poor prognosis. Based on the clinical signs and history of exposure to feral hogs, the submitting veterinarian strongly suspected a diagnosis of pseudorabies.
As the name implies, pseudorabies causes clinical neurological signs that mimic those of rabies. As such, samples from the affected dogs were first submitted to the Texas Department of State Health Services’ laboratory in Austin for rabies testing. Once rabies testing was deemed negative, brain samples were sent to the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) for histopathology and to the Iowa State University Veterinary Diagnostic Laboratory for pseudorabies PCR. Histopathology of one dog showed a mild, multifocal, chronic, lymphocytic encephalitis with lesions consistent with viral infection. The pseudorabies PCR results were positive.
Pseudorabies is caused by suid herpesvirus 1. This virus has been eradicated from the commercial swine population in the United States, but the virus still circulates in feral swine populations. The prevalence of pseudorabies in Texas feral swine populations averages between 8-13% based on opportunistic sampling of adults done by the USDA/APHIS/Wildlife Services. This prevalence is based on antibody detection in serum samples. The prevalence seems to cycle within a particular sounder and may be as high as 40-50 percent at a given time. Pseudorabies does not appear to have a detrimental effect on the feral swine population. Domestic species such as cattle, dogs, small ruminants, and cats can be infected when exposed to feral hogs, and infection in these species tends to be rapidly fatal. In dogs, the incubation time ranges from 2-10 days. In this case, the initial exposure was suspected to have occurred 4 1/2 days before the onset of clinical signs during a hog hunting event in Brooks County.
The most characteristic clinical sign of pseudorabies is intense pruritis, which is why the disease is also known colloquially as “mad itch.” In dogs, the pruritis tends to be directed primarily at the face. A wide variety of other clinical signs can be seen, such as salivation, muscle stiffness, convulsions, vocalization, aggression, restlessness, or depression. Many methods are available for diagnosis. Polymerase chain reaction (PCR) may now be the preferred method due to rapid turnaround time. Other laboratory tests that can be used include: histopathology, direct fluorescent antibody examination, and virus isolation. Histologically, pseudorabies lesions are characterized by non-suppurative inflammation with intranuclear herpesvirus-type viral inclusion bodies (Figure 1). Histologic changes may be localized and very subtle. Histopathology along with other modalities of diagnosis is recommended. While no specific abnormalities are seen in hematological or cytological tests, mononuclear pleocytosis and elevated proteins may be seen in the cerebral spinal fluid; however, these findings are not specific for pseudorabies and are only suggestive of a viral infection. Various serological tests (virus neutralization, immunodiffusion, ELISA) are available, primarily to determine disease incidence and distribution in the pig population. There is no treatment for pseudorabies. Pseudorabies is reportable to the Texas Animal Health Commission.
In summary, pseudorabies should be considered a differential diagnosis for neurologic disease of domestic animals exposed to feral swine populations. Due to public safety concerns, rabies infection should always be excluded prior to pursuit of additional diagnostic testing.
– Greene, C. E. (2013). Infectious diseases of the dog and cat (4th ed.). St. Louis, MO: Saunders.