The emergence of West Nile Virus (WNV) in late summer 1999 — a disease never before isolated in the Western Hemisphere — caused the death of hundreds of birds, seven people and nine horses in the New York City/Long Island area. The arrival of cooler weather stemmed further spread of this mosquito-transmitted disease, but there’s a strong possibility it may return. With no vaccine available and factors that favor outbreaks, it’s wise to understand this disease and where it came from in order to protect your horses.
The WNV was first isolated from a woman in Uganda in 1937 and was soon recognized as the most widespread of its type, with distribution including Africa, the Mediterranean and Eurasia. WNV antibodies have been detected in a variety of mammals, but significant disease occurs only in humans, birds and horses. Infection rates can be high in people, although mortality is about 10%, predominantly in the elderly or those with compromised immune systems. Equine mortality is 40 to 50%. Severity of the disease in birds varies according to the species of bird and strain of the virus.
Last year, horses in the involved areas of Long Island had about a 45% chance of exposure to WNV, a 55.5% chance of becoming ill if exposed and a 36% chance of dying if symptoms developed.
Where Did It Come From’
We may never know exactly how the WNV made its way into the United States. Rumors were flying in late 1999, from introduction being a deliberate terrorist act to it having come in with smuggled Israeli geese. Government and private groups are also trying to determine whether the virus was present in this country before the 1999 outbreak.
Whatever the mechanism, it is difficult to ignore the fact this outbreak occurred in close proximity to two international airports. One of the drawbacks to our modern age is that diseases can spread internationally with ease. WNV might have arrived through the importation of infected birds or other animals, hitchhiking mosquito eggs, even mosquitoes themselves. In December 1999 researchers announced the West Nile Virus strain from the New York outbreak matches a strain found in Israel, making that country the most likely source.
Like EEE (eastern equine encephalitis) and WEE (western equine encephalitis), WNV encephalitis is transmitted via biting insects — most notably mosquitoes, although ticks may play a smaller role. The virus causes high infection rates in birds, which are the major source of virus. Mosquitoes feed on infected birds and transmit it to horses and other mammals, including man.
While mosquitoes could pick the virus up from any animal that has a sufficiently high level of the virus circulating in its blood, the chance of a mosquito spreading the virus from horse to horse is low compared to the bird-mosquito-horse route. Horses and other mammals have much lower levels of circulating virus than birds as a rule, and the viremia (high level of virus in blood, usually accompanied by fever) is usually short-lived. Nevertheless, it is wise to isolate a horse with fever if there is any chance of WNV infection.
Because horse-mosquito-horse transmission is less likely than bird-mosquito-horse, the USDA hasn’t imposed restrictions on movement of horses in the United States, but there are international restrictions in force (see sidebar, above).
How Outbreaks Occur
Many factors determine the likelihood of an outbreak, but mosquito activity is among the most important for control measures. If infected mosquitoes don’t bite your horse, he isn’t likely to get the disease. Patterns of mosquito breeding and feeding strongly influence when and if encephalitis outbreaks occur. Mosquitoes breed during the warmer months, becoming dormant during cold weather. And, although there are a few exceptions, female mosquitoes must have a blood meal before they lay their eggs.
Egg-laying will be most active after a heavy rain, and females choose sites like deep furrows in fields or rain gutters. In periods of drought, females will hold onto their eggs, allowing large numbers of gravid females to build up.
When rain does arrive, there will be massive egg laying, leading to the emergence of large numbers of mosquitoes at one time. If these mosquitoes happen to be infected with an encephalitis virus, the rapid surge in the mosquito population increases the chances the horse will be bitten and increases the number of bites he is likely to receive. This pattern — drought followed by heavy rains and increased mosquito activity — may have been involved in the 1999 WNV outbreak.
Breeding and feeding patterns can also influence the time of year outbreaks are most likely. In dry spring months, mosquitoes are most active in wooded areas where moisture levels are higher. Feeding on birds is common in this location. When mosquito numbers begin to rise, they must forage farther for their blood meals. The irrigation of fields and summer rains provide additional suitable areas for egg laying and also draw mosquitoes out of wooded areas to breed and feed. This is when animals are most likely to be bitten.
Mosquito Control Measures
You can minimize your horse’s chances of being bitten if you avoid turnout during peak feeding times (dusk to dawn) and maintain fence lines a distance of at least 500 yards from marshy or wooded areas. Remove all physical objects from around the barn, house and field that might trap and hold water after a rainfall, such as tires, buckets, uncovered wood piles, depressions in the dirt at the base of bushes, puddles formed under decks and ditches or natural depressions that hold water.
Stock ponds with minnows and Gambusia (mosquitofish), which are heavy feeders and will eat mosquito eggs and immature growth phases. Keep all rain gutters free of blockages. Empty and wipe out birdbaths at least every other day. Fields and lawns should be kept well mowed to encourage drying. Keep in mind that every puddle, every area of boggy ground is a potential breeding site for mosquitoes, and mosquitoes will rarely travel more than a few hundred yards from their breeding sites. Use effective mosquito repellents religiously (see sidebar).
Avoid electronic insect “zappers.” These are ineffective at reducing populations. However, the Mosquito Magnet is reported to attract and trap mosquitoes by using the natural attractant, carbon dioxide. The device has caught up to 1,500 mosquitoes in a single night. Mosquito populations begin to drop drastically after six to eight weeks of use. It is nontoxic and portable.
Some government authorities may institute insecticide spraying programs to kill feeding adults. However, control measures that target the larvae are more effective and virtually nontoxic. Methoprene (brand name Altosid) is a chemical insect growth regulator that does not kill larvae but prevents them from becoming adults. Products incorporating this chemical are available as liquids, granules, pellets (up to 21-day activity) or briquets (30- to 150-day activity).
Methoprene is nontoxic to fish, mammals or beneficial insects feeding on the larvae and has a reported effectiveness of near 100%. The chemical is either dispersed widely (over wet lands, flood plains) or deposited in the form of slowly dissolving briquets. The Environmental Protection Agency has approved Altosid for application in orchards and crop fields. All potential mosquito breeding areas should be treated before the spring thaw and at regular intervals throughout season.
Contacts: Altosid (Adapco, Inc.) 800/367-0659 www.altosid.com; Gambusia (mosquitofish) www.rci.rutgers.edu/~insects/gamb.htm; Mosquito Magnet 877/545-5737, www.mosquitomagnet.com.
Also With This Article
Click here to view ”Game Plan For West Nile Prevention.”
Click here to view ”Import Restrictions.”
Click here to view ”Will It Strike Again’”
Click here to view ”Vaccine Prospects.”
Click here to view ”Mosquito Repellents.”