West Nile Virus Spreading Via Mosquitos

Houston could see its first mosquitoes infected with the West Nile virus this year, signaling a health threat to people and certain animals.

The virus has spread rapidly since its detection in New York in 1999, when 62 cases of severe disease occurred, including seven deaths. Louisiana health officials reported their first human case last summer. The mosquito season in Houston runs from May to October, and warming temperatures provide an opportunity for mosquitoes to re-emerge after lying dormant through the winter.

"Houston may soon have its first pools with mosquitoes positive for West Nile virus or perhaps infected birds," said Dr. Raouf Arafat, Bureau of Epidemiology chief at the Houston Department of Health and Human Services. "The virus might even be here already."

West Nile virus infection, in severe cases, can develop into encephalitis, an inflammation of the brain. Still, the probabilities of becoming severely ill or dying are extremely low. The Centers for Disease Control and Prevention estimates that less than 1 percent of people bitten by a mosquito with the virus will become infected and get severely ill.

Most people infected with the virus experience mild illness with symptoms that include fever, headache, body aches and occasionally skin rash and swollen lymph glands. Severe infections can cause high fever, headache, stiff neck, disorientation, coma, tremors, muscle weakness and paralysis.

Although most infections have been in birds, West Nile virus also infects horses, cats, bats, skunks, chipmunks, squirrels and domestic rabbits. Mosquitoes become vectors, or carriers of the virus after feeding on infected animals. The infected mosquitoes then transmit the virus when feeding on the blood of people and animals.

Houston and the Texas Gulf Coast also lie along both the north- and southward paths of some species of migrating birds that journey every year from the northeastern United States to their winter grounds in Mexico and Central America. The birds avoid flying across the large expanses of open water in the Gulf and instead follow the Texas coast.

Health experts believe birds are the principal introductory hosts of West Nile virus into new regions. Birds that fly along the Gulf route include the cattle egret, black-crown night heron, turkey vulture and different types of ducks and gulls.

This spring, the CDC doubled a grant to the Houston Department of Health and Human Services for local West Nile virus surveillance activities. The $182,000 grant will enable the department to expand surveillance activities started last year. The additional money will permit the department to collect up to 300 cerebral spinal fluid and blood samples from people with encephalitis of an unknown cause from eight area hospitals. In the event of an outbreak, the department would collect additional samples.

Last year, the department’s Texas Medical Center laboratory, with the help of the Texas Department of Health’s laboratory in Austin, tested cerebral spinal fluid from 230 people hospitalized with encephalitis. Those tests found no evidence of West Nile virus.

As part of its surveillance activities, the department will arrange for courier pickup of the cerebral spinal fluid and blood samples from the participating hospitals, test them for the presence of West Nile virus and provide results in two days. Testing will be free to the patients and hospitals. The department’s lab is the only area facility with the ability to test for evidence of West Nile virus infection in people. Still, all positive tests would need to be sent to the CDC for confirmation and definitive diagnosis because cross-reactivity occurs often during the testing of arboviruses such as West Nile.

The grant will also fund a part-time epidemiologist who will function as the department’s liaison to the Harris County Mosquito Control Division, the local agency primarily responsible for pest control and prevention of mosquito-borne diseases.

"The liaison will need to rotate regularly between the department and Mosquito Control so we can quickly find out about any positive pools," Dr. Arafat said. "We can help Mosquito Control from an epidemiological point of view figure out a better way of looking at pool sites and positive results."

The liaison will also gather and analyze data and assist with hospital surveillance.

Although the elderly and those with weakened immune systems are the highest risk for severe illness, the virus – once detected in a region – poses a risk to all residents. People usually develop symptoms three to 15 days after infection.

Currently, no vaccine exists against West Nile virus encephalitis in humans, and there is no specific treatment for West Nile encephalitis, but in severe cases people will receive intensive support therapy that may include hospitalization, intravenous fluids, airway management, ventilator support, prevention of secondary infections and nursing care.

---

©2006 Texas Medical Center

---

E-Mail: tmcinfo@texmedctr.tmc.edu
URL: http://www.tmc.edu/tmcnews/06_01_02/page_06.html