West Nile Fever Outbreak Linked to Increased Urbanization and Warmer Winters

A study of the recent outbreak of West Nile Fever (WFN) in the US state of Texas linked resurgence in cases of the disease to increased urbanization combined with warmer winters.

WNF, which occurs in 20% of those bitten by infected mosquitoes, is a febrile syndrome that causes flu-like symptoms. Most characterizations of WNF generally describe it as a mild, acute syndrome lasting three to six days after symptom onset. In addition to a high fever, the disease presents with headache, chills, excessive sweating, weakness, fatigue, swollen lymph nodes, drowsiness, pain in the joints and flu-like symptoms. Gastrointestinal symptoms that may occur include nausea, vomiting, loss of appetite, and diarrhea. Less than one-third of patients develop a rash.

West Nile neuroinvasive disease (WNND), which is found in less than 1% of cases, occurs when the virus infects the central nervous system resulting in meningitis, encephalitis, meningoencephalitis, or a poliomyelitis-like syndrome. Many patients with WNND have normal neuroimaging studies, although abnormalities may be present in various cerebral areas including the basal ganglia, thalamus, cerebellum, and brainstem.

After declining over the previous five years, mosquito-borne West Nile virus infection, which had been introduced into the US in 1999, resurged in 2012 throughout the US, most substantially in Dallas County, Texas. Investigators at the University of Texas Southwestern Medical Center (Dallas, USA) analyzed the epidemiologic, meteorologic, and geospatial features of this outbreak to guide future prevention efforts.

The investigators identified 173 cases of WNND, 225 of West Nile fever, 17 West Nile virus–positive blood donors, and 19 deaths in 2012. Outbreaks of the virus in Dallas County began early after unusually warm winters, revisited similar geographical distributions, and were strongly predicted by the mosquito vector index. This index was calculated from the abundance of mosquitoes and the percentage of mosquitoes infected with West Nile virus.

“After the infecting mosquito bite, it takes a week for the first symptoms to develop, a week to see people turning up at hospitals, and a week for laboratory confirmation of the diagnosis and reporting to health officials,” said senior author Dr. Robert Haley, professor of internal medicine at the University of Texas Southwestern Medical Center. “That three-week time period is crucial. Acting early from the vector index rather than after human case reports and deaths mount up can nip an outbreak in the bud. However, if mosquito data are unavailable or a decision to intervene takes longer, later intervention may still be important to terminate the outbreak.”

“When the vector index goes above 0.5 early—in June or July—large numbers of people are silently infected, and this is the best time to intervene,” said Dr. Haley. “In years when the vector index did not rise until late July or August, impending outbreaks just sputtered – in late summer mosquito abundance declines, and mosquitoes become less active and stop biting as much.”

The study was published in the July 17, 2013, issue of the Journal of the American Medical Association.

Related Links:
University of Texas Southwestern Medical Center