US hit by massive West Nile virus outbreak centred around Texas

Texas is at the epicentre of an outbreak of West Nile virus in the United States. At the latest count (17 August) there had been 551 reported cases and 21 deaths in the state this year, compared with only two deaths last year.

The first case of West Nile in Texas was reported on 25 May and the first death on 5 July, Jim Schuermann, a staff epidemiologist with the Texas Department of State Health, told the BMJ. Cases with neuroinvasive disease (305 patients) have outnumbered cases where only fever was reported (246 patients).

It can be difficult to determine the scope of a West Nile outbreak because most infected people develop few or relatively mild symptoms. Only around a quarter develop fever, joint pain, vomiting, and diarrhoea that require medical attention. Although symptoms can be treated, the disease cannot. Severe disease can cause death and survivors often experience substantial neurological damage and loss of muscle control.

The current outbreak centres around Dallas, where half of deaths have occurred. It has prompted the first aerial spraying of insecticides in the area to control the mosquito population in nearly 50 years, although spraying is common in other parts of the US.

Night time spraying flights in Texas dispersing a synthetic pyrethroid insecticide began on 16 August. A second round of spraying may also be required. Each round costs around $1m (£63 000; €800 000).

Figures from the US Centers for Disease Control and Prevention (CDC) on 14 August show that there have been 693 infections and 26 deaths so far during 2012, nearly half of which have come from Texas.1 Most (406 cases, 59%) have been classified as neuroinvasive disease, involving meningitis or encephalitis. However, the US CDC aggregates data for the whole country, so its numbers lag behind those from individual states.

This year cases of West Nile have been reported as far west as California and as far east as the suburbs of Washington, DC. The period of risk begins in the spring as mosquitoes emerge as the weather warms and continues until a hard frost kills them in the autumn.

The disease was first identified in Africa and its natural range extends into southern Europe. It was unknown in North America until 1999, when the first case was identified in New York, and it has spread rapidly across the entire continent.

The blood borne disease is spread to humans through mosquito bites. The virus is known to infect other mammals and even reptiles. But its greatest natural reservoir is in birds, particularly crows and blue jays, which contributed to its rapid dissemination across the continent.

The current outbreak is attributed in part to favourable meteorological conditions such as drought, which has reduced water flow and created stagnant water pools ideal for breeding mosquitoes, and heat, which has accelerates the insect’s lifecycle. The recent rebuilding of flocks of birds most susceptible to infection has also created a larger pool of infection.