Investigation of an outbreak of encephalomyelitis caused by West Nile virus in 136 horses.

The research study focuses on investigating an outbreak of encephalomyelitis caused by the West Nile virus in horses in northern Indiana and determining the potential protective benefits of vaccination.

Research Methodology

• A total of 170 horses showing clinical signs suggestive of encephalomyelitis induced by West Nile virus were selected for the study.
• Clinical details such as the age, sex, breed, date of presentation, and survival status of horses were recorded.
• Blood samples were analyzed for the presence of antibodies against the West Nile virus, and brain tissue samples were collected and tested to identify the presence of the virus.
• Data regarding the local climate conditions was collected, and an epidemic curve along with case fatality rate was derived.

Study Results

• The most common symptoms identified in the affected horses were ataxia (a condition affecting movement coordination), muscle tremors, and hind limb paresis (partial loss of voluntary movement or impairment).
• Eight of the horses had been vaccinated against the West Nile virus within a span of 2 to 21 days before the onset of symptoms.
• The virus was isolated from the brain tissue of two non-vaccinated horses, while antibodies against the virus were observed in 132 non-vaccinated horses. Furthermore, in two other non-vaccinated horses, both the virus and antibodies were detected.
• A total of 31 horses, about 22.8%, succumbed to the disease or had to be humanely euthanized.
• The outbreak peak occurred on September 6, 2002, and a noteworthy reduction in ambient temperatures was observed after this peak.

Conclusions and Recommendations

• The study states that the highest risk period for equine encephalomyelitis caused by the West Nile virus in northern Indiana spans from mid-August to mid-September.
• An important takeaway from the study is that the number of cases dwindled coinciding with the decrease in ambient temperatures.
• Consideration of the substantial fatality rate led the researchers to emphasize the importance of full protection of horses against the West Nile virus through vaccination before June in northern Indiana.
• For other regions of the United States with a particular mosquito breeding season, it is advised that vaccination proceedings for non-vaccinated horses should commence at least 4 months prior to the anticipated spike in mosquito numbers. Moreover, horses that have been previously vaccinated should receive a booster vaccine dose not later than 2 months before this predicted surge period.