What Is West Nile Virus and How Does It Affect Humans and Horses?

West Nile virus (WNV) is carried by mosquitoes and if transmitted to humans, it can cause severe encephalitis. It is closely related to St. Louis Encephalitis virus (SLEV) which is sometimes a problem in Florida. WNV was first isolated in 1937, from a woman in the West Nile province of Uganda in Central Africa. Epidemics of West Nile have occurred in Israel, France, South Africa, and Romania. West Nile virus was first documented in the United States in New York City (NYC) during an epidemic in August 1999.

Human West Nile Fever and West Nile Virus Encephalitis

Most humans who are infected with the WNV do not develop clinical illness. Approximately 20% of the people who are infected will exhibit fever, headache, body aches, swollen lymph glands, and a skin rash – this is defined as West Nile Fever.

More severe infections include headache, high fever, neck stiffness, disorientation, coma, convulsions, muscle weakness and paralysis. This severe form of the infection is defined as West Nile Virus Encephalitis. About 1 out of every 150 infections will result in encephalitis. Symptoms of WN Fever typically last a few days while the severe disease may last several weeks to months with some permanent neurological effects.

West Nile Virus in Horses

Horses infected with WNV can exhibit signs of ataxia (the most common sign) which more often affects the rear limbs, causing stumbling, staggering, wobbly gait, and in coordination. Other signs include teeth grinding, muscle fasciculation, going down with difficulty and inability to rise, facial paralysis, or twitching and blindness. Treatment of infected horses is often based on clinical signs and reducing the severity of the disease. Fluid and nutrient supportive therapy may be required.

According to USDA-APHIS Veterinary Services, horses that are infected with WNV are not required to be euthanized. Horses are incidental hosts and it is unlikely that mosquitoes feeding on infected horses could ingest enough of the virus to transmit it to other animals. Horses are euthanized only when they are suffering from severe encephalitis from which they will not be able to recover. Also, because horses are dead-end hosts, quarantines are unnecessary.

How Does the Virus Get into Humans and Horses?

The most important mode of transmission of WNV to humans and horses is from the bite of an infected mosquito. Mosquitoes usually obtain the virus from feeding on infected birds. The virus is then passed to humans when an infected female mosquito takes a second blood meal from a human instead of a bird. As they are feeding on human blood, they release saliva that contains the virus. The saliva then enters the human bloodstream carrying the virus with it.

The mosquito species that are most likely to be important in WNV transmission are members of the genus Culex. Several species have been implicated in West Nile outbreaks elsewhere in the world, and there is evidence in Florida that Culex nigripalpus is an important vector. These particular mosquitoes lay their eggs in flooded citrus groves, catch basins, sewers, cisterns, and temporary flood waters. The peak time for blood-feeding of this mosquito species is between sunset and sunrise.

Rare Forms of WNV Transmission

A very small proportion of West Nile cases have come from other routes of infection than the mosquito. A small number of confirmed cases have occurred from organ transplants, blood transfusion, mother-to-child, and laboratory workers who were working on WNV infected animals. WNV is not transmitted from one human to another. Also, it is not transmitted from birds to humans or horses to humans. There is no evidence that WNV can be transmitted to humans by consumption of infected birds or animals or their eggs. If a human, horse, or wild bird is infected with the virus, it is assumed that immunity will be lifelong.

WNV in Florida

WNV was first detected in a single dead crow in Jefferson County, FL, collected on June 18, 2001, heralding the arrival of West Nile to Florida. Since 2001, confirmed cases in humans and horses have been reported in Florida.

WNV has been reported from sentinel chickens throughout Florida. Mosquito control districts, state and local health departments, and the Centers for Disease Control may test dead birds, sentinel birds, mosquitoes, and horses for WNV. Tests that are confirmed positive are reported by the Florida Department of Health. Authorities will then make decisions about what strategies to use to reduce risk of exposure to WNV based on scientific information about the virus and vectors involved, the risk to humans, and local or regional environmental conditions.

The best method of reducing risk of exposure is to avoid mosquito bites through personal protection by wearing mosquito repellents and protective clothing during times when mosquitoes are most active.