1,021 Cases Of West Nile Virus In 2010 In USA, Says CDC Report

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Main Category: Infectious Diseases / Bacteria / Viruses
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Last year there were 1,021 cases of West Nile Virus (WNV) disease in 40 US states, and the District of Columbia (DC), according to a Morbidity and Mortality Weekly Report (MMWR) report issued by the CDC (Centers for Disease Control and Prevention) - 62% (629) of them were cases of WNV neuroinvasive disease, an incidence nationwide of 0.2 per 100,000 people.

Arizona had the highest incidence, at 1.6 per 100,000 population, followed by Nebraska at 0.55 and Colorado (0.51).

Arboviruses (arthropod-borne viruses) are transmitted to humans mainly through the bites of infected ticks and mosquitoes. WNV was first identified in the Western Hemisphere in 1999. Since then it has become the major cause of neuroinvasive arboviral disease in America. Nevertheless, other Arboviruses are present in occasional seasonal outbreaks of neuroinvasive disease, i.e. acute flaccid paralysis, meningitis, or encephalitis.

The latest report includes details on cases reported to CDC in 2010 of WNC and other arboviruses that have to be reported. It does not include dengue, which is an a separate report.

The second most commonly reported cause of neuroinvasive arboviral disease, after WNV in 2010 was CALV (California serogroup viruses) with 68 cases, the third EEEV (eastern equine encephalitis virus with 10 cases, followed by SLEV (St. Louis encephalitis virus) with 8 cases, and POWW (Powassan virus with 8 cases.

Arboviruses, including WNV still cause severe illness in a considerable number of individuals in America, the authors report. It is crucial that surveillance is kept up so that arboviral disease prevention activities can be maintained.

Arboviruses are kept in transmission cycles in the USA typically between birds and small mammals (officially arthropods and vertebrate hosts). Humans are usually the dead end hosts - the cycle cannot usually continue by infecting feeding mosquitoes because virus levels in human blood are too low.

In the majority of cases, when humans are infected with an arbovirus there are no symptoms (asymptomatic cases). When signs and symptoms do appear they generally include fever and neuroinvasive disease (less common). The majority of arboviral diseases in the USA are notifiable. Cases are reported to CDC via ArboNET, an internet-based surveillance system.

ArboNET collects data on human disease cases, and also viremic blood donors, veterinary cases, infections in sentinel chickens, mosquitoes and dead birds.

Human cases are classed as neuroinvasive or non-neuroinvasive disease. Because of the higher death rate linked to human neuroinvasive disease, data on such diseases tend to be more consistent and complete than for non-neuroinvasive disease cases.

In this report, incidence included only data from neuroinvasive disease cases.

Below are some highlighted details from the report, for 2010: 1,021 WNV cases626 (62%) of them were of neuroinvasive diseaseThe number of cases peaked in mid-August; with 911 (89%) cases. In those cases most of the illness onset occurred in July.The meridian age for WNV infection was 56 years, but cases ranged in people aged from 1 to 98 years.56% (574) cases were in males.70% (718) patients had to be admitted to hospital with WNV disease.6% (57) of the patients died.Of the 629 WNV neuroinvasive disease cases: - 342 (54%) had encephalitis.
- 239 (38%) had meningitis.
- 48 (8%) had acute flaccid paralysis. 33 of them also had meningitis or encephalitis.
- 54 (9%) patients died.
- 55% (345) of cases came from just four states, Arizona (107), New York (89), Texas (77), and California (72).0.2 per 100,000 people developed neuroinvasive disease nationally.The highest incidence of neuroinvasive disease occurred in individuals aged at least 70 years.Below are some details on other Arboviruses reported diseases: CALV disease - 75 cases, of which 91% (68) were neuroinvasive disease cases in 11 states. 99% (74) cases were caused by La Crosse virus. Illness onset occurred from March to November, with 85% of them occurring during July-September. The average age of patient was 8 years, and cases ranged in age from 1 month to 74 years.EEEV disease - 10 cases, all neuroinvasive reported from 5 states. Human onset occurred in July-August. Nine of the patients were adults. 5 patients died.SLEV - 10 cases, reported from Texas (3), Arkansas (2), Michigan (2), DC (2), and Missouri (1). DC had not seen a case before since 1975 - both were non-neuroinvasive diseases. All other eight cases were neuroinvasive diseases. Illness onset ranged from June to November. All patients were adults, median age 65 years, age range from 23 to 75. One patient died.POWW - 8 cases, all neuroinvasive disease. Four in Wisconsin, three in Minnesota, and one in New York. Illness onset ranged from May to December. There were no deaths reported. Median age of patients was 56 years, ranging from 6 to 77 years. 7 of the patients were male.Written by Christian Nordqvist
Copyright: Medical News Today
Not to be reproduced without permission of Medical News Today Article Reference:
"West Nile Virus Disease and Other Arboviral Diseases --- United States, 2010"
Nicole P. Lindsey, MS, Jennifer A. Lehman, Dustin Weaver, DVM, Grant L. Campbell, MD, J. Erin Staples, MD, Marc Fischer, MD
Morbidity and Mortality Weekly Report (MMWR). August 5, 2011 / 60(30);1009-1013

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