Commentary: Take West Nile Risk Seriously

As of Sept. 4, 2012, West Nile virus (WNV) infections in people, animals, or mosquitoes have been reported from all states except Alaska and Hawaii, according to the Centers for Disease Control and Prevention (CDC). Of 1,993 human cases, 54% involved the nervous system. When are mosquitoes especially active? Dawn and dusk. When are most horse people also active? Dawn and dusk. If WNV is reported from 48 states, people and horses in 48 states are at-risk of being bitten by infected mosquitoes.

The only U.S.-approved WNV vaccines are for horses, and the American Association of Equine Practitioners lists WNV as a core vaccine, emphasizing its importance. Compare the cost for WNV vaccination versus costs for veterinary examination, diagnosis, and treatment of a neurological horse. And, let's not forget that any neurological horse needs to be handled as a rabies-suspect until proven otherwise.

While extensive WNV data is available at the CDC ArboNet website, equine specific data is not. According to the Kentucky State Veterinarian's website, as of Sept. 11, 2012, 10 WNV-positive horses were reported from nine counties. Seven horses were unvaccinated; two were partially vaccinated, and one was reportedly vaccinated. A summary of Kentucky WNV equine cases (2005- 2011) shows that 91% of positive horses were not vaccinated; others were partially vaccinated or had unknown vaccination history. How complete is your equine vaccination program?

Understanding WNV vectors is important to human and equine health. Mosquitoes in the genus Culex are primary vectors of WNV with multiple species being able to transmit the virus throughout the continental United States. The house mosquito (Cx. pipiens complex) is common in areas across Kentucky. Inseminated females spend the winter in protected places near a moisture source and mortality is usually low following mild winters. Surviving females become active, laying rafts of eggs on the surface of water with moderately to very high organic content including grassy ditches, waste lagoons, and polluted ground pools. The life cycle from egg to adult takes about eight to 12 days during the summer.

Female Cx. pipiens usually remain within a half-mile of their breeding habitat. These night-flying mosquitoes feed primarily on birds but will enter buildings and feed on a variety of mammals and humans. They are particularly important in amplification of the virus in susceptible birds and along with other mosquito species can be "bridge vectors" that enable the virus to move from avian to mammalian species.

The two major preventive strategies in reducing risk of arthropod-borne viruses are: 1) using measures to reduce contact with mosquitoes, including insecticides and repellents; and 2) reducing, eliminating or treating known breeding sites. The success of both strategies is variable for many practical reasons.

Insect transmitted diseases rarely disappear. People need to take the WNV risk to themselves and their horses seriously. What are you willing to do?

CONTACT: Roberta Dwyer, DVM, MS, Dipl. ACVPM; 859/218-1122;; Maxwell H. Gluck Equine Research Center; University of Kentucky; Lexington, Ky.
Lee Townsend, PhD; 859/257-7455;; Department of Entomology, University of Kentucky; Lexington, Ky.

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd's, London, brokers, and their Kentucky agents.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.