Powassan viral encephalitis: a review and experimental studies in the horse and rabbit.

The researchers studied the effects of the Powassan virus, known to infect humans and animals in Canada, on horses and rabbits by inoculating them with the virus. They found evidence of widespread encephalitis in rabbits and serious neurological symptoms in horses after eight days, although they were unable to isolate the virus from the brains of the animals post-inoculation.

Purpose of the Research

• The aim of this study was to investigate the effects of Powassan virus, a member of the Flavivirus genus, on different animals, specifically horses and rabbits.
• The researchers were particularly interested in understanding the mechanism of infection, the symptomatology it may cause, and any potential pathogenic features it may leave behind.

Methodology

• The scientists intracerebrally (directly into the brain) inoculated rabbits and horses with the Powassan virus strain M794.
• Following this, they closely monitored and recorded any observable clinical symptoms.
• They also evaluated the brains of the animals post-infection to isolate and study the virus, and to understand the pathologic features caused by the infection.

Findings

• In rabbits, even though there were no apparent clinical symptoms, signs of widespread encephalitis were found. This was characterized by lymphoid perivascular cuffing (a type of inflammation where immune cells crowd around blood vessels), lymphocytic meningitis (inflammation of the protective membranes of the brain and spinal cord), and lymphocytic choroiditis (inflammation of the vascular layer of the eye).
• The horses exhibited more clear-cut effects – eight days post-inoculation, they began to show prominent neurological signs. The brain lesions were suggestive of non-suppurative encephalomyelitis (an inflammatory brain disease), neuronal necrosis (death of neurons), and focal parenchymal necrosis (localized tissue death).
• Interestingly, the researchers could not re-isolate the Powassan virus from the brains of the infected animals.

Implications of the Study

• This study illuminates the pathologic consequences of Powassan virus infection in animals, which can help in earlier detection and treatment of the disease.
• The findings also highlight the differences in symptomatology and disease progression depending on the host species.
• It underscores the importance of further research, given that the virus could not be re-isolated from the brain tissue of either species, which raises questions about the antigenic process and immune response.