West Nile Virus in Missouri: What to Expect in 2002

Dr. Howard L. Pue, Chief
Section of Communicable Disease Control and Veterinary Public Health
Division of Environmental Health and Communicable Disease Prevention
Missouri Department of Health and Senior Services

[Download as a MS Word Document] [Download as an Adobe Acrobat Document]

West Nile virus (WNV) was first isolated in the West Nile province of Uganda in 1937. The first recorded human epidemics occurred in Israel during 1951-1954, and in 1957. The virus is now known to infect humans, birds, and other animals in Africa, Eastern Europe, West Asia, and the Middle East. In late summer 1999, the first domestically acquired human cases of WNV encephalitis in the U.S. were documented in New York City and surrounding areas. Although it is still not known when or how WNV was introduced into North America, international travel of infected persons to New York, importation of infected birds or mosquitoes, or migration of infected birds are all possibilities.

The annual human case incidence of WNV now ranks it second only to LaCrosse encephalitis as the leading cause of reported human arboviral encephalitis in the U.S. In 1999, there were a total of 62 human cases reported with 7 deaths. In 2000, there were 21 human cases with 2 deaths, and in 2001 there were 66 human cases with 9 deaths (2001 data are provisional).

Mosquitoes transmit WNV to humans. Mosquitoes can acquire the virus when they feed on infected birds, which act as amplifying hosts for the virus. Once infected, mosquitoes can then transmit WNV to humans and animals while feeding. In most areas where mosquitoes carry the virus, fewer than one percent of mosquitoes are infected. Most people infected with WNV do not develop any symptoms. About one of every four infected persons develops a mild illness 1 to 2 weeks after exposure with symptoms such as fever, headache, body aches, skin rash, and swollen lymph nodes. Less than one percent of infected people may develop a serious illness that includes encephalitis (inflammation of the brain). These persons might experience headache, high fever, neck stiffness, disorientation, convulsions, and muscle weakness. Infection may prove fatal, especially among the elderly, in a small number of those who develop encephalitis. West Nile virus is not transmitted directly from birds to humans nor from person-to-person.

Medical care should be sought as soon as possible for persons who have symptoms suggesting severe illness. There is no specific treatment for WNV infection or vaccine to prevent it. Treatment of severe illness includes hospitalization, use of intravenous fluids and nutrition, respiratory support, prevention of secondary infections, and good nursing care.

WNV moved from New York to other states in the Northeast and along the Atlantic seaboard during 2000. During the 2001 season WNV invaded the Midwest, extending the border of activity 800 miles west and 750 miles south from its end-of-2000 location. This rate of spread was much more rapid than any public health experts predicted, which added impetus to efforts at establishing surveillance, prevention, and control programs. By the end of 2001, the virus had been isolated from eight crows found in the St. Louis area. So far, there have been no documented human WNV cases or infection in species other than crows in Missouri.

The Department of Health and Senior Services (DHSS), along with numerous partners, is continuing to expand its mosquitoborne disease surveillance program to include the following proposed components in 2002:

• Passive surveillance will be conducted for human cases of mosquitoborne illness, which are reportable under 19 CSR 20-20.020, Reporting Communicable, Environmental and Occupational Diseases. Local health departments should encourage arboviral testing of patients presenting with aseptic meningitis and viral encephalitis if mosquitoborne illness has not already been ruled out. The State Public Health Laboratory (573-751-0633) or district communicable disease representatives can be contacted for assistance in submitting specimens for arboviral testing.

• Active surveillance for human cases of mosquitoborne disease will be conducted by seven health departments under contracts with DHSS.

• Active surveillance for equine cases of western and eastern equine encephalitis (WEE and EEE) will be conducted by DHSS through a network of equine veterinary practitioners located across the state.

• Equine serum samples submitted from 400 horses to the University of Missouri-Columbia, Veterinary Medical Diagnostic Laboratory, will be tested for WNV and EEE under a contract with DHSS.

• Surveillance of mosquito populations will be conducted by 14 local public health jurisdictions, 13 of which are partially or completed funded under contracts with DHSS.

• Mosquito speciation and testing will be conducted by Southeast Missouri State University (SEMSU) under a contract with DHSS. SEMSU staff will differentiate nuisance mosquitoes from vector mosquitoes and test the latter for evidence of major arboviruses.
  

• Live bird surveillance will be conducted by the U.S. Department of Agriculture under a contract with DHSS. The USDA will trap wild birds, draw blood samples, and submit them to state university laboratories for arboviral testing.

• Dead bird surveillance will be conducted by sending dead wild birds to the National Wildlife Health Center Laboratory in Madison, Wisconsin, for WNV testing. Only crows, blue jays, and hawks will be tested, since these are highly susceptible, indicator species. Specimens must be freshly dead and intact. Local health agencies are encouraged to submit these species for testing when citizens and other sources notify them of dead birds. There is no charge for the test, but local health departments must pay for packaging and shipment of specimens. A laboratory submission number must be obtained from the Section of Communicable Disease Control and Veterinary Public Health prior to shipment of specimens.

Obviously, a great deal of effort and expense is going into national and state efforts to track the spread of WNV in the U.S. as well as morbidity and mortality rates of WNV-associated disease. Is WNV going to cause another "AIDS-like" epidemic in the U.S.? Certainly no one is predicting that! Will WNV become endemic and present a continued threat to people, such as that posed by St. Louis and eastern equine encephalitis viruses? Probably. But it is simply unknown at this time what the final impact of WNV will be in the U.S., since it is a virus "turned loose" in a new ecosystem. But with the data that are collected, we can:

• Raise public awareness of this new potential health risk, and educate citizens about protective measures they can employ.

• Inform health care providers concerning WNV and raise their level of suspicion concerning this class of pathogens, so they can better diagnose and treat patients.
  

• Educate decision makers at the state and local levels so they can more effectively promulgate rules, laws, and ordinances and prioritize funding.
  

• Initiate and assess mosquito control activities, including habitat elimination, larvicide application, and spraying for adults.
  

• Design public health programs directed at current and emerging risks that are integrated at local and state levels.

For more information on WNV, please call the Section of Communicable Disease Control and Veterinary Public Health at 573-751-6113/6495 or visit the DHSS Intranet site at http://dhssnet/SCDVPH/index.html.

[Download as a MS Word Document] [Download as an Adobe Acrobat Document]

Top of Page