Arthropodborne viruses (arboviruses) are transmitted to humans primarily through the bites of infected mosquitoes or ticks. In the United States, West Nile virus (WNV) is the leading cause of arboviral disease in the country, according to an MMWR report today.


In fact, of the 2,291 cases of domestic arboviral disease in the US in 2017, 2,097, or 92 percent of cases were due to WNV. Overall, the 2,097 WNV disease cases were reported from 641 counties in 47 states and DC. Among these cases, 1,425 (68%) were neuroinvasive, and 1,814 (87%) patients had illness onset during July–September.

California accounted for 401 WNV cases in 2017, the most in the country.

The report breaksdown the arbovirus cases reported in 2017 as follows: In addition to the WNV cases, there were 75 Jamestown Canyon virus cases followed by La Crosse (63), Powassan (34), St. Louis encephalitis (11), unspecified California serogroup (six), and Eastern Equine Encephalitis-EEE (five).

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More Jamestown Canyon and Powassan virus disease cases were reported in 2017 than in any previous year.

The 75 Jamestown Canyon virus disease cases were reported from eight states, primarily in the Northeast and upper Midwest including 36 in Wisconsin and 14 in Minnesota. The 34 Powassan virus disease cases were reported from 10 states, primarily in the Northeast and Midwest. Minnesota recorded the most cases in 2017 with seven.

Of the five cases of human infection with EEE last year,  three of the cases were acquired through organ transplantation.

From Lyme to Powassan: Tick-borne diseases in the US 2016