Image caption The virus is transmitted to humans through mosquitoes that have bitten infected birds

The death toll from US cases of West Nile virus, which has killed scores of people, has risen by nearly a third in a week, according to health officials.

Eighty-seven people have now died from the mosquito-borne illness, up from 66 the week before, said the Centers for Disease Control and Prevention (CDC).

The number of cases in 2012 has meanwhile risen by a quarter in the last week, to 1,993 from 1,590.

It is the largest outbreak since the virus first appeared in the US in 1999.

At least 40 of the deaths have been in Texas, the state's health department said on Wednesday.

Texas has also recorded at least 495 cases of the most serious, neuro-invasive form of the illness.

West Nile virus peaked in 2002 and 2003, when severe cases of the disease reached nearly 3,000.

Only about one in five people infected with West Nile gets sick, and one in 150 of those will develop severe symptoms, including neck stiffness, disorientation, coma and paralysis.

This year there have been reports of the virus in 47 states, although about 75% of the cases are from five states: Texas, Mississippi, Louisiana, South Dakota and Oklahoma.

Health officials think a mild winter and early spring fostered the breeding of mosquitoes that bite infected birds.