While no animal or human cases have yet been reported this year in Delaware, the Division of Public Health (DPH) reminds healthcare providers that WNV is endemic in Delaware, and cases may occur any time there is mosquito activity.
West Nile Virus first appeared in Delaware birds and horses in 2000. Delaware’s first confirmed human case of WNV occurred in 2002, and was not fatal. There were no further human cases of WNV detected during 2002. In 2002, 214 wild birds tested positive for WNV in Delaware, as did six mosquito pool samples and 24 horses. During 2003, seventeen human cases, including two deaths, were reported. Additionally in 2003, 119 birds and 67 horses tested positive for WNV.
Most WNV infections (80 percent) are clinically inapparent. Approximately 20 percent of those infected develop a mild illness (West Nile fever), which includes sudden onset of fever and which may be accompanied by malaise, anorexia, headache, myalgia, nausea, vomiting, rash, lymphadenopathy, and eye pain. Symptoms generally last three to six days. Approximately 1 in 150 infections result in severe neurological disease, more commonly encephalitis than meningitis. The most important risk factor for developing severe neurological disease is advanced age. Neurologic presentations have included ataxia and extrapyramidal signs, optic neuritis, cranial nerve abnormalities, polyradiculitis, myelitis, and seizures. Several patients experienced severe muscle weakness and flaccid paralysis. Other associated symptoms include fever, weakness, and gastrointestinal symptoms. Myocarditis, pancreatitis, and fulminant hepatitis have also been described. The incubation period of WNV is thought to range from 3 to 14 days after the bite of an infected mosquito.
Diagnosis of WNV infection is based on a high index of clinical suspicion and obtaining specific laboratory tests. WNV should be strongly considered when unexplained encephalitis or meningitis occurs in summer or early fall. Local evidence of WNV enzootic activity or other human cases should further raise suspicion. Obtaining a recent travel history is also important.
The DPH Laboratory performs WNV testing for birds, horses and humans. Methods include antibody testing and confirmatory neutralization, as well as PCR and culture. Detection of IgM antibody in human serum or spinal fluid is an effective method for diagnosing WNV infection. Serum should be collected within eight days of illness onset. False positive antibody results for WNV may occur in patients recently vaccinated for or infected with related flaviviruses (e.g. yellow fever, Japanese encephalitis, dengue).
For submission of specimens call the DPH virology laboratory at 302-653-2870.
All WNV testing should be performed at or confirmed by the DPH Laboratory. Patient samples obtained at hospital labs in Delaware should automatically be sent to the DPH Laboratory for confirmatory testing. Healthcare providers may request WNV from commercial laboratories. However, these results should be confirmed by DPHL because some commercial laboratories have reported high false positive rates in the past. This may require the patient to have a second serum sample drawn. Alternatively, patients can have blood samples drawn, by appointment only, at the Delaware State Service Centers (see contact information below).
Treatment is supportive, often involving hospitalization, intravenous fluids, respiratory support, and prevention of secondary infections for patients with severe disease.
To avoid mosquito bites and reduce the risk of infection, patients should be encouraged to: