West Nile Virus (WNV) is a mosquito-borne disease that was originally identified in the West Nile district of Uganda in 1937. In 1999, it was detected in the eastern United States; since then the virus has spread throughout the United States and is well established in most states, including California. The virus was detected in the Coachella Valley in August 2003. The strain of virus found in the United States most closely resembles that found in the Mediterranean and Middle East. 

How is WNV Spread?

WNV is spread by the bite of an infected mosquito. Mosquitoes are WNV carriers (“vectors”) that become infected when they feed on infected birds. The virus lives in the mosquito and is transmitted to a new host in the mosquito’s saliva when the insect bites a person or animal. However, humans and horses are dead end hosts and cannot spread infection to other humans or animals.

In the Coachella Valley, WNV can be transmitted by two commonly found mosquitoes: Culex tarsalis and Culex quinquefasciatus.

Symptoms of West Nile Virus

Most people who become infected with WNV virus have no symptoms whatsoever. Up to 20% of the population infected will get West Nile fever causing mild symptoms including fever, headache, body aches, nausea, a skin rash on the trunk of the body and/or swollen lymph nodes, generally lasting a few days.

It is estimated that 1 in every 150 people who are infected with WNV will require hospitalization. Severe symptoms may include high fever, headache, neck stiffness, even disorientation, coma, tremors and paralysis, and may last several weeks. Some severe cases may lead to encephalitis and other serious brain and spinal membrane inflammation in which neurological effects may be permanent. The elderly, small children, and those with compromised immune systems are most susceptible. The time between the infected mosquito bite and the onset of illness, known as the incubation period, ranges from 3 to14 days. There is no specific treatment for infection with WNV, although supportive care is important.

An infected mosquito can bite any animal, but not all animals will become infected. The disease most often affects birds, but occasionally causes disease in other animals as well. Dogs and cats only rarely show illness if infected with WNV.

Wild birds, particularly crows and jays, infected with WNV can become ill and die, however, most infected birds do survive. Dead birds are often the first indication of West Nile virus activity in an area. However, the hot temperatures of the Coachella Valley cause quick degradation of dead birds and are often not viable for testing. West Nile-infected mosquitoes caught in local traps are usually our first indication of disease. 

Horses are highly susceptible and about 30% of infected horses die.

Fortunately, a vaccine is available through equine veterinarians. Clinical signs of WNV in horses include listlessness, fever, stumbling, staggering gait, weakness and paralysis. Occasionally horses have high levels of virus circulating in their blood, but research indicates that this does not play a significant role in passing the infection to feeding mosquitoes

Several other mammals that have shown symptoms of WNV include sheep, hamsters, mice and some species of monkeys. Symptoms range from mild illness to death. However pigs, rabbits and dogs appear resistant to infection.