West Nile virus has been detected in Delaware for the first time this year in a blood sample taken from sentinel chickens that are monitored for mosquito-borne diseases.


West Nile virus has been detected in Delaware for the first time this year in a blood sample taken from sentinel chickens that are monitored for mosquito-borne diseases. The samples are collected as part of a statewide surveillance program conducted by the Mosquito Control Section of the Department of Natural Resources and Environmental Control.

No cases of West Nile virus have been found in humans or horses so far in Delaware this year.

The virus-positive results were reported to DNREC July 22 by the Delaware Division of Public Health Laboratory. The chicken was sampled at a west Wilmington monitoring station July 19, according to Mosquito Control Section Administrator William Meredith, Ph.D. Based upon these virus-positive findings, Mosquito Control will increase its mosquito population monitoring activities in the area and take appropriate mosquito control actions. This first indication of mosquito-borne viruses in Delaware for 2010 occurring during mid-July is fairly typical of first-time occurrences in past years, Meredith added.

Mosquito Control operates 24 caged chicken stations statewide. The sentinel chickens are humanely kept and tended in the field. If exposed to West Nile or eastern equine encephalitis (EEE) viruses — both of which can affect humans and horses — the chickens develop antibodies that enable them to survive. Their blood is tested every two weeks for the presence of these antibodies, which indicate the birds’ exposure to these viruses.

Starting last week, in addition to testing sentinel chickens, Mosquito Control began testing mosquito collections on a trial basis as part of its monitoring efforts. This week, recent collections will be submitted for testing to the DPH Lab in collaboration with University of Delaware researcher Dr. Jack Gingrich. The section no longer collects and tests wild birds for West Nile.

Nationwide as of July 20, 26 West Nile human cases have been reported across the country, with half of these in Arizona, but the 2010 mosquito-disease transmission season is really only beginning.

“Concerns for mosquito-borne disease transmissions now really won’t subside until much cooler autumn temperatures set in, lasting in most years until mid-October and sometimes even beyond,” said William Meredith of the Mosquito Control.

By the end of 2009, 720 West Nile human cases and 32 deaths were reported nationwide. The worst year for West Nile virus was in 2003, with 9,862 human cases and 264 deaths nationwide. That year, the worst West Nile outbreak in Delaware occurred, with 17 confirmed human cases and two fatalities, plus 63 horse cases.

Some indications point to the possibility of enhanced transmission for West Nile virus in 2010. As of July 20, New Jersey reports finding West Nile in 40 mosquito collections from 10 counties, with eight of these from nearby Gloucester County. Pennsylvania reports finding West Nile in 47 collections from seven counties, with 30 of these from nearby Delaware County.  However, Maryland has found only one West Nile-positive mosquito collection and Virginia none.

Indications also show the possibility of increased EEE occurrence and transmission. As of July 20, EEE has been found in Florida in 49 mosquito collections from 14 counties. EEE has stricken 50 horses in 20 Florida counties and an EEE-caused human death was reported. Regionally, EEE was found in mid-July in two New Jersey mosquito collections from Cape May County, and another EEE-positive mosquito collection was recently reported from upstate New York.

“If this year’s hotter than normal conditions continue, there is some concern that the growth and replication of West Nile and EEE viruses may increase in mosquito hosts, leading to greater probability for virus transmission,” Meredith said. “However, if dry conditions also continue, mosquito breeding may be reduced, thus also reducing the number of mosquito hosts to transmit these viruses.”

An effective equine vaccine now exists to protect horses from West Nile virus and EEE. “I am urging horse owners to assist with prevention efforts by making sure their horses are vaccinated against West Nile virus,” said Dr. Heather Hirst, Delaware’s state veterinarian. “Initially, two doses of the vaccine are necessary for immunization. After the initial two vaccinations, a yearly booster is needed. Horse owners should consult their local veterinarian for advice on vaccination protcols. If horses are unvaccinated or have only recently been vaccinated, owners should keep horses inside during peak mosquito times, i.e., dawn, dusk, and throughout the night.”

There are no approved West Nile or EEE vaccines for humans. The majority of humans infected with West Nile virus typically have only symptoms similar to a mild flu, if they show any signs at all; 20% of those infected develop a mild illness which includes fever, body and muscle aches, headache, nausea, vomiting and rash. A very small percentage of patients, usually the elderly, develop severe neurological disease resulting in meningitis or encephalitis (inflammation of the brain). Symptoms may include sudden onset of severe headache, high fever, stiff neck, confusion and muscle weakness. Individuals with these symptoms should see their physician immediately.

“While the finding of West Nile virus in Delaware is not cause for alarm, it serves as a good reminder for people to take common-sense precautions against mosquito bites,” Meredith said. These include wearing long-sleeved shirts and long pants when outdoors in mosquito-prone areas, applying insect repellent containing 10% to 30% DEET in accordance with all label instructions, and avoiding mosquito-infested areas or times of peak mosquito activity around dusk, dawn or throughout the evening. To reduce mosquito-breeding, people should drain or remove items that collect water, such as buckets, birdbaths, rain barrels, old tires, flower pot liners, depressions in tarps covering boats, clogged rain gutters and unused swimming pools.

The Mosquito Control phone numbers below should also be used for citizens to report intolerable numbers of biting mosquitoes. The section uses this information about severe mosquito nuisance situations to help determine when and where to provide control services. Staff will answer phones between 8 a.m. and 4 p.m. Monday through Friday. Callers leaving a message after business hours or during weekends or holidays should leave their name, phone number, address and a brief message about their need or problem.

n Glasgow Office, serving New Castle County and northern Kent County (including the greater Dover area): 302-836-2555

n Milford Office, serving Sussex County and southern Kent County: 302-422-1512

For more information about mosquito biology/ecology and mosquito control, call the Mosquito Control Section’s Dover office at 302-739-9917.  

For more information about West Nile virus in humans and related medical issues, contact the Division of Public Health at 888-295-5156.

For more information about West Nile virus in horses and equine vaccines, contact the Department of Agriculture’s Poultry and Animal Health Section at 302-698-4500 or 800-282-8685 (Delaware only).

For more information on West Nile virus, visit the CDC website, www.cdc.gov/ncidod/dvbid/westnile/index.htm