Submitted by Covey Washington,
Madison/Jefferson County Extension Agent 1
It’s that time of year again; football season, nice comfortable weather, cooler evenings, and the peak season for the potentially serious illness, West Nile Virus (WNV). There have been 15 human cases reported in Florida this summer. All of them are in Duval County and two cases were fatal. It has been detected in three sentinel chicken test locations in Leon County. WNV is a seasonal epidemic in North America that flares up in the summer months and continues into the fall. It has been common in Africa, West Asia, and the Middle East for more than 50 years but scientists believe it made its presence in the U.S. around 1999. WNV was first detected in Florida in a dead crow in June, 2001 in Jefferson County.
Transmission
The most important mode of transmission of WNV to humans and horses is through the bite of a WNV-infected mosquito. Mosquitoes usually obtain the virus by feeding on infected wild birds. After the mosquito feeds on an infected bird the virus goes through a temperature-dependent incubation period within the mosquito. At the end of the incubation period the virus can be passed on each time the infected mosquito feeds on a human or other animal. When the infected mosquito blood-feeds, WNV is mixed with the mosquito’s saliva and released into the bloodstream of the second host, where it can multiply and possibly cause illness. If the second host is susceptible to the virus, a WNV infection may result.
n a very small number of cases, WNV has been spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby.
WNV cannot be transmitted from one human to another through casual contact. Also, It is not transmitted from birds to humans or from horses to humans. There is no evidence that it can be transmitted to humans by the consumption of cooked infected birds, eggs, or other animals. A human or animal that survives a WNV infection is assumed to have a lifelong immunity to the virus.
Even in areas where the virus is circulating, very few mosquitoes are infected with the virus. If the mosquito is infected, less than 1% of people who get bitten and become infected will get severely ill. The chances you will become severely ill from one mosquito bite are extremely small.
Symptoms
Humans typically develop symptoms between 2 and 15 days after they are bitten by an infected mosquito. Approximately 80 percent of people who are infected with WNV will not show any symptoms at all. Up to 20 percent of people who become infected have symptoms such as fever, headache, body aches, nausea, vomiting, skin rash on chest, stomach or back, and sometimes swollen lymph glands. These symptoms can persist for as short as a few days up to several weeks. About 1 in 150 people infected with WNV will develop severe illness. The severe symptoms include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These severe symptoms may last several weeks, and neurological effects may be permanent.
Though the virus can infect humans of all ages, people over age 50 and some immunocompromised people (for example, transplant patients, HIV) are at higher risk for getting severely ill when infected with WNV.
Horses
Horses that are infected with WNV most often exhibit signs of ataxia (loss of muscle coordination) affecting the rear limbs causing stumbling, staggering, and a wobbly gait. Other signs include teeth grinding, lying down with difficulty and inability to rise, facial paralysis, twitching and blindness. Horses that are infected with WNV are not required to be euthanized. They should be euthanized only when they are suffering from a severe case and are unlikely to recover. Since horses do not produce substantial WNV viremias in their blood (does not replicate), they are dead-end hosts. This means that it is unlikely that mosquitoes feeding on infected horses ingest enough WNV to become infective and transmit the virus to humans and other animals. Also, since horses are dead-end hosts, quarantines of infected animals are not required.
The Florida Department of Agriculture and Consumer Services urges all equine owners to have their horses vaccinated against WNV and to be vigilant about scheduling booster shots at regular intervals. Horses that have been vaccinated against eastern equine encephalitis virus (EEEV) are not protected from WNV infection.
How to Reduce Risk of Infection
Since the primary source are mosquitoes, the easiest and best way to avoid WNV is to prevent mosquito bites. When outside, use an EPA approved insect repellent (follow directions on the package). If you must be outside during peak biting times(dusk and dawn), wear long sleeves and long pants. Make sure door and window screens are in good condition. Flush out water in bird baths, toys, flower pot overflow dishes, and outdoor pet dishes every 3-4 days. Remove leaf litter, standing water, and debris from gutters and boat covers.
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