Summary Of West Nile Virus In The United States

Courtesy of USDA, APHIS, Veterinary Services, Emergency Programs
Infectious Disease Table Topics, AAEP Convention
(Report of November 18, 1999)

West Nile Virus (WNV) has been identified in birds, mosquitoes, people, and horses in a limited area of the northeastern United States. Specifically, this area consists of parts of Connecticut, New York, New Jersey, and one county in Maryland. The available literature on this virus demonstrates that the transmission cycle is primarily a mosquito-bird cycle, with occasional incursions into other vertebrates as terminal hosts only. The literature supports the conclusion that horses are terminal hosts for WNV and do not maintain a sufficient viremia to infect either other mammals (including humans) or mosquitoes.

An outbreak of human encephalitis of then unknown etiology began in New York City in early August 1999. On September 14, 1999, a virus was isolated from tissues of exotic birds from the Bronx Zoo and of wild birds from the New York City area. This virus was later identified as WNV and confirmed as the cause of the human encephalitis outbreak. The virus was also confirmed in clinically ill horses located in the Riverhead area of Suffolk County on Long Island, New York, on October 18, 1999.

Surveillance has continued in the affected area and in additional States down the eastern coast of the U.S. to monitor possible spread of WNV. This surveillance consists of investigating suspect cases in horses, testing wild birds, mosquito collection and testing, and sentinel chicken testing. To date, no apparent spread has been detected in horses, no cases have been detected in commercial poultry, and the number of wild birds and mosquitoes testing positive continues to decrease.

Studies have been initiated to obtain additional information on the virus. Specifically, small inoculation studies in horses, chickens, and turkeys have begun. Information gathered from these studies will be shared when available.

WNV activity in this part of the United States appears to be decreasing at this time, due to various factors including climate and previous vector control activities. The following information provides more specifics on the outbreak, documents surveillance and vector control activities, and validates the fact that virus activity continues to decrease and is limited in geographic area.


60 laboratory-positive cases (all in NY)

First human onset: August 4, 1999

Last human onset: September 22, 1999

[Dates are given for clinical onset, an earlier point in the course of disease progression than the dates given for birds (see below), for which only a date of collection is known.]

Active surveillance for human encephalitis cases in Connecticut and New Jersey has not detected any WNV cases to date.


23 Clinical Cases (all in New York, 20 in Suffolk County, and 3 in Nassau County)

First Horse Onset: August 26, 1999

Last Horse Onset: October 18, 1999

[dates are given for clinical onset.]

An epidemiological investigation was conducted on the cluster of equine cases in Suffolk county on Long Island, New York. The 20 cases were on a total of 15 premises. During the investigation, samples were obtained from both convalescing horses and contact horses on these premises. An effort was made to obtain samples from contact horses that were either commingled with a case or on a premises that was co-owned or managed by an owner of a case animal at the time the animal was showing clinical signs. At the time of the investigation, 146 horses resided on the affected premises, 83 of which were sampled. All premises shared a similar ecological environment, with all cases clustered within a five-mile radius. Standing water, either in large pools or in barrel/watering areas, was evident on all affected premises.

Two of the three cases in Nassau County were horses stabled at the Belmont Park track. Both of these horses had clinical illness compatible with WNV infection and both were found to have high WNV antibody titers. Both horses have recovered and no other clinically ill horses have been reported from Belmont Park.

The third case in Nassau County was detected by a retrospective investigation. The horse exhibited neurological signs consistent with WNV on September 4, 1999, recovered within a few days, and remains healthy. A serum sample obtained from the horse on November 2, 1999, was reported serologically positive for WNV by the National Veterinary Services Laboratories on November 26, 1999. The horse was stabled in an area on the North shore of Nassau County, New York, that was near WNV positive human cases and mosquito pools in Queens County, New York. It is expected that similar WNV cases associated with the outbreak will be detected through this and other surveillance mechanisms where the WNV had been active. This is not an active case of WNV.


First positive bird: August 8, 1999 (NY)

Last positive bird: October 29, 1999 (NY)

[Dates given are for bird collection, i.e., date of death or later.]

Birds that have died are submitted to one of the laboratories with WNV testing capability, with confirmatory testing currently being done at CDC. This surveillance has focused on American crows and other wild birds. As of November 5, 1999, a total of 392 birds have been tested at CDC. Of these, 200 were negative for WNV and 192 were positive. Surveillance in wild birds will continue throughout the winter in at least 20 eastern and southern States.


Last positive mosquito pool: October 14, 1999 (NY)

Mosquitoes are collected from light and gravid traps in various locations. Pools of mosquitoes from each location are then tested for WNV. Since the onset of freezing weather in WNV-affected areas, mosquito collection has stopped due to the low numbers of insects being collected. However, some pools were collected after the positive October 14 collection; those pools were negative for WNV.

Vector Control

The State Departments of Public Health in New York, New Jersey, and Connecticut have been conducting intensive mosquito control activities since the latter part of August because of suspect cases of St. Louis encephalitis. These efforts intensified once WNV was confirmed as the actual etiologic agent. Mosquito control activities include area-wide spraying for control of adult mosquitoes and source reduction, i.e., eliminating mosquito breeding grounds. Spraying for adults continued until first frost or until such time that surveillance indicated that adults were no longer active.

Climate/Weather Factors

In locations that have had a hard freeze (temperatures less than -2ûC) mosquito activity has, for practical purposes, now ended. With the cold temperatures seen in WNV-affected areas, and the concomitant reduction of mosquito activity, pesticide spraying activities have now been suspended.

For more West Nile information from the USDA, APHIS, Veterinary Services Emergency Programs, visit

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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