Texas is the largest producer of horses in the United States. In an annual report published by the American Horse Council Foundation for 2005 the horse industry was estimated to produce goods and services of approximately $3.1 billion with an estimated 979,000 horses residing in Texas. Horses (Figure 1.) are companion animals that are used for many recreational and professional outlets including competitive rodeo, equestrian competitions, as well as a means of therapy for disabled individuals.
Figure 1. Horses in corral. Photo credit: James Partin – CDC Image Library.
In 1999, the West Nile Virus (WNV), which is a mosquito-born pathogen that is extremely infectious in birds, as well as horses, was first discovered in the United State in New York City. To date WNV has been found throughout the United States. It poses serious health risks to horses with the possibility of death in some cases (approximately 2% of horses will die from the infection).
A study conducted in Colorado and Nebraska showed that the costs associated with the treatment, lost-use, and replacement of 1,478 animals infected with WNV in 2002 totaled approximately $1.26 million while the estimated cost of vaccinating an equivalent number of animals totaled about $74,000. Thus, the prevention of WNV in horses is also economically justified.
Figure 2 illustrates the number of confirmed West Nile Virus cases in Texas from 2002 through 13 October 2005 and shows the significant reduction in WNV cases over the past 3 years. This can be attributed to both the widespread vaccination of horses and the increase in naturally induced immunity in the US horse population. West Nile Virus is here to stay and there is no room for complacency about mosquito control or vaccination.
West Nile Virus is primarily spread by Culex mosquitoes which are chiefly bird-feeding mosquitoes but will take blood from large mammals when available. As depicted in Figure 3, Culex mosquitoes follow a typical mosquito life cycle with eggs laid in rafts that float on top of the water. The eggs hatch and mosquito larvae appear. Larvae will grow as they move through four stages and finally develop into pupae in as little as a few days. The adult mosquito will then exit the pupal stage at the water surface.
Figure 4 illustrates the basic transmission cycle for WNV. Once infected, the virus will multiply in the bird. Not all birds are affected by the virus but those that are will either recover or succumb to the infection. Crows, blue jays, and hawks are some of the primary birds that are susceptible and often die from the infection. Some county health departments in Texas track these bird species to determine areas of high WNV transmission. The Texas Department of State Health Services – Zoonosis Control Division maintains a list of local contacts, submission instructions, and identification tips for dead birds on their website at: http://www.dshs.state.tx.us/idcu/disease/arboviral/westNile/samples/
Figure 4. Basic West Nile Virus transmission cycle as it relates to horses. Photo credit: James Partin – CDC Image Library.
Mosquitoes that feed on infected birds with a high virus level in their blood, also known as a viremia, become infected with the virus. The infected mosquitoes can then feed upon other birds, horses, or even humans. The virus does not replicate at a high enough level in incidental hosts (horses and humans) to be infectious to mosquitoes that might feed on them. Consequently, infected people or horses cannot infect mosquitoes with the virus.
Ticks, which are also blood-feeders, are not efficient vectors of WNV and are unlikely to be important vectors of WNV in the United States. Ticks are, however, serious vectors of other diseases and should be avoided.
SYMPTOMS, TESTING AND TREATMENT
West Nile Virus in horses can cause swelling and inflammation of the brain and surrounding tissues, also known as encephalitis, which causes symptoms similar to many other diseases, including rabies and other mosquito-borne diseases. Not all horses that become infected with the virus display symptoms of infection, as approximately 5% of infected horses become ill with WNV. Of horses that display symptoms, about one third will die or need to be euthanized.
Symptoms can include:
- Weakness, depression
- Reduced appetite
- Altered gait
- Muscle tremors
- Coma and death
If WNV is suspected, contact a veterinarian to examine the horse. Unfortunately, because WNV can be confused with many other disorders in horses, only samples of blood serum, cerebral spinal fluid (CSF), and post-mortem brain tissue can be used to identify the virus. In order to determine if your animal is infected with WNV, a person should contact their veterinarian about taking blood samples for analysis. It is possible that other tests will have to be conducted to exclude the possibility of the horse having one of the previously mentioned infections. Blood samples are sent to the Texas Veterinary Medical Diagnostic Laboratories (TVMDL) located in Amarillo and College Station, which handles testing in the state of Texas and also reports results to the Department of State Health Services.
There is no specific treatment for WNV in horses. Supportive care, such as providing fluids, rest, respiratory support, and physical support for horses that can no longer stand, is the only option currently available.
New treatment options and products are being developed, however their efficacy has not yet been confirmed through research. These products work by stimulating the horse’s immune system and introducing antibodies; they do not kill the virus themselves. Individual veterinarians have varying opinions on the use of antibody treatments and the decision to use them will depend on discussions with your veterinarian and the needs of a particular animal. This makes prevention the most important tool we have at this time to keep horses healthy.
The prevention of WNV infection in horses can be broken down into three main areas: 1. source reduction, 2. avoidance, and 3. vaccination.
1. Source Reduction
Source reduction involves the reduction or removal of sources typically used as sites for mosquitoes to develop. Because the offspring of mosquitoes develop in water, the removal of these sites can reduce mosquito populations and therefore the likelihood of WNV infection. Larval development sites include any receptacle or area that holds standing water for longer than a one week period. Specific examples of larval source reduction include:
- Turning over wheelbarrows, pots, or cans that accumulate water or adding drain holes so they do not retain water.
- Cleaning out livestock water troughs on a weekly basis or supplying them with the mosquitofish, Gambusia affinis, which feed on mosquito larvae in the water (Figure 5) .
- The management of ponds can include the use of mosquito-eating fish, such as the mosquitofish, if they do not have an outlet to a stream.
- The use of the mosquito specific microbial larvicide Bacillus thuringiensis israelensis (Bti, products such as Mosquito Dunks® and Mosquito Bits®) can be used to kill mosquito larvae in water. The Bti microbe works by producing a toxin when it is ingested by the larvae. The microbe specifically targets mosquito larvae, and is not harmful to fish, animals or humans.
- Maintaining swimming pools with proper chlorination levels.
Areas that encourage mosquitoes to rest or that encourage the birds that mosquitoes feed upon should be avoided. Specific examples of adult resting site reduction include:
- Reducing the use of large hedges and dense plant landscaping near areas where horses are housed.
- Discouraging birds from roosting in and around stables.
The avoidance of mosquitoes can involve three different methods: spatial, temporal, and chemical.
Spatial Avoidance of mosquitoes includes keeping horses inside stables at night and utilizing screens on windows and doors to keep mosquitoes out. It has been suggested that fans might help to keep mosquitoes off of horses when they are stabled. Mosquitoes are also attracted to lights. Therefore, lighting in and around stables should be kept off during periods of mosquito activity.
Temporal Avoidance involves keeping horses inside during dawn and dusk which are the peak hours of mosquito feeding.
Chemical Avoidance involves the use of chemical repellents specifically designed for use on horses. Repellents designed for human use should not be used on horses as they can cause harm. The use of repellents is often less than 100% effective as mosquitoes are very good at finding an area on the body not covered by the repellent.
There are currently two vaccines registered for use in horses in the United States; the killed-virus vaccine West Nile Innovator™ manufactured by Fort Dodge Animal Health and the live canary pox vaccine Recombitek™ Equine West Nile Virus Vaccine manufactured by Merial.
West Nile Innovator™ was the first WNV vaccine available. Laboratory research has confirmed that it can offer up to 95% efficacy and field research is ongoing. It confers immunity by exposing the horse’s immune system to the killed components of the whole virus and allowing the animal to produce antibodies to the virus. This vaccine has been proven to be safe for use in all ages and classes of horses. As with any vaccine, consult with your veterinarian in designing a vaccination program for maximum efficacy and protection.
Recombitek™ Equine West Nile Virus Vaccine is a recently introduced modified-live vaccine. It utilizes the canary pox virus (harmless to horses) to carry proteins that are specific to WNV and thus allow the horse’s immune system to produce antibodies specific to the virus proteins. This vaccine has also been shown to have a high rate of efficacy. The live canarypox vaccine has been used as a technological tool to create other vaccines as well.
Regardless of which vaccine is chosen the steps for effective vaccination include:
- Two doses of the vaccine are required approximately 3-6 weeks apart prior to the start of the mosquito season. Foals in high risk areas may require an additional dose 6-8 weeks after the second dose.
- A booster is required each year also prior to the start of the mosquito season.
- The two vaccines are not interchangeable and if a horse is vaccinated with one type of vaccine and then switched to the other type it is recommended that the whole series of shots be restarted.
- Consultation with a veterinarian to determine the individual needs of the animal.
It should also be noted that vaccination for West Nile Virus does not protect a horse from other mosquito-borne diseases such as Eastern and Western Equine Encephalitis. Consult your veterinarian to design a comprehensive vaccination program suitable for the needs of your individual horse or horse operation.
Research has shown that vaccination alone is not enough to protect horses 100% of the time thus a comprehensive approach must be used. The three method approach described here; source reduction, avoidance, and vaccination should be effective in preventing WNV infection in horses.
American Association of Equine Practitioners (AAEP). 2005. West Nile Virus Vaccination Guidelines. http://www.aaep.org/pdfs/AAEP_WNV_Guidelines_2005.pdf
APHIS Veterinary Services, Centers for Epidemiology and Animal Health. 2003. Info Sheet: Economic Impact of West Nile Virus on the Colorado and Nebraska Equine Industries: 2002.
APHIS-USDA. 2006. Animal Health Monitoring and Surveillance, West Nile Virushttp://www.aphis.usda.gov/vs/nahss/equine/wnv/
CDC. 2003. West Nile Virus: Vertebrate Ecology. http://www.cdc.gov/ncidod/dvbid/westnile/birds&mammals.htm
Texas Department of State Health Services, Zoonosis Control Group. 2006. West Nile Virus in Texas http://www.dshs.state.tx.us/idcu/disease/arboviral/westNile/
US EPA. 2002. Pesticides: Topical and Chemical Fact Sheets. Larvicides for Mosquito Control. http://www.epa.gov/pesticides/health/mosquitoes/larvicides4mosquitoes.htm