WNV Vaccine Challenge Study to be Presented Sept. 15

Fort Dodge Animal Health has just received approval from the United States Department of Agriculture to release the follow abstract regarding the West Nile virus vaccine. This paper will be presented at the OIE meeting "Vaccines for OIE list A and Emerging Diseases" in Ames, Iowa, on Sept. 15. This study was done on normal healthy horses, isolated from natural exposure.

The title of the abstract is "Equine Vaccine for West Nile Virus," by T. Ng, D. Hathaway, N. Jennings, Y.W. Chiang, H.J. Chu. The four men work for Fort Dodge Animal Health in Biological Research and Development.

They report the following:

The West Nile virus (WNV) was first isolated and identified from birds, mosquitoes, and mammals including horses in three states of the northeastern United States in 1999. Since then, WNV infection has been spread to southeastern and Midwestern states. In order to meet the urgent need of controlling the WNV infection in equine population, we have developed a killed WNV vaccine. A dose titration study in horses was first conducted to evaluate serum neutralization antibody responses against WNV in horses.

Horses were randomized into three vaccinated groups and one control group. Horses were vaccinated with the test vaccine at low, median, and high doses, respectively. All vaccinated horse were administered the test vaccine intramuscularly twice, three weeks apart. Serum samples were collected periodically and were measured for serum neutralization titers using plaque reduction neutralization test.

Twelve months after the second vaccination, horses vaccinated with the median dose of WNV vaccine and non-vaccinated control horses were experimentally challenged with WNV. After the challenge, horses were monitored for rectal temperature and any clinical signs twice daily for two weeks and once daily thereafter until 21 days post challenge (DPC). Serum samples were collected twice daily for two weeks and once weekly thereafter for detection of viremia. Horses were euthanized and necropsied on 21 and 22 DPC. Cerebrospinal fluid (CSF), spinal cord (cervical, thoracic, and lumbar) and brain (frontal, occipital, medulla oblongata, and brain stem) tissue samples were examined for gross pathology and collected for virus isolation.

Nine out of 11 (81.8%) controls developed viremia after the challenge, while only one out of 19 (5.3%) vaccinates had transient viremia. No WNV associated clinical signs were observed in any of the challenged animals throughout the observation period. No febrile responses were observed in any of the challenged horses. No WNV was isolated from any of the tissue or CSF samples collected from any of the challenged horses. Results from this study demonstrate a significant protection (94% of preventable fraction) against viremia in horses vaccinated with the killed WNV vaccine and the long duration of the protective immunity.