West Nile Incidence and Vaccination, 1999-2007

West Nile virus (WNV), an arbovirus endemic in North America, is the causative agent of West Nile equine encephalomyelitis (WNEE) and an important consideration in the differential diagnosis of horses with signs of neurologic disease. West Nile virus vaccination is important in the prevention of WNEE and is considered one of the core equine vaccinations by the American Association of Equine Practitioners. Core equine vaccinations are those indicated for use in all horses irrespective of use or geographic location in the USA. (www.aaep.org/vaccination_guidelines.htm).  

Since the initial detection of WNEE in 1999 in New York, the virus has spread throughout the 48 contiguous states and the District of Columbia. Data available from the United States Department of Agriculture (USDA) indicate that from 1999 through 2006 a total of 24,841 WNEE equine cases have been laboratory confirmed. The peak of the equine cases occurred in 2002 (Figure 1).

Chart showing WNV incidence
Current as well as historical data regarding the number and location of WNEE is available at www.aphis.usda.gov/vs/nahss/equine/wnv and in the "Equine 2005 Part II: Changes in the U.S. Equine Industry," 1998-2005 at http://nahms.aphis.usda.gov. While still an endemic disease in the USA, a decline in WNEE is probably due to a combination of naturally acquired immunity, management, and frequent use of WNV vaccination. Management and control measures in the USA include pesticide use against larvae and adult mosquitoes, repellents (topical pyrethroids), and vector-resistant housing such as screened stalls or stalls with fans to reduce mosquito activity. It is important to acknowledge that the number of cases reported could be impacted by factors in addition to actual cases, including disease recognition on the part of equine owners, pursuit of an etiologic diagnosis by the examining veterinarian through laboratory testing, and reporting criteria and surveillance at the state level.

The frequency of use of the WNV vaccines has been estimated in the National Animal Health Monitoring Systems (NAHMS) "Equine 2005 Part I: Baseline Reference of Equine Health and Management." For operations that gave any type of vaccine to equids in the previous year, eighty-five percent vaccinated some or all of their resident horses against WNV, making this the most commonly used vaccine in horses in the USA. These facts illustrate the rapid and widespread response of both pharmaceutical companies and the equine industry to the disease risk posed by WNV in the USA. Three different types of WNV vaccines are now licensed and commercially available for use in horses in the USA: WNV killed with adjuvant; WNV canarypox recombinant with adjuvant; and WNV chimera with yellow fever virus vaccine. With an estimated equine population in the USA of approximately six million, the vaccine manufacturers marketed approximately 4.1 million doses in 2006.


Dr. Tim Cordes, 301/734-3279, USDA-APHIS-VS, Riverdale, Maryland


Dr. Josie Traub-Dargatz, 970/221-4535, Colorado State University and Centers for Epidemiology and Animal Health, USDA-APHIS-VS, Fort Collins, Colorado

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

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