Preventing infectious diseases and parasites from attacking our horses are primary concerns for every horse owner. James D. Smith, DVM, a practitioner at Hagyard-Davidson-McGee Associates, PLLC, in Lexington, Ky., and Craig Reinemeyer, DVM, PhD, president of East Tennessee Clinical Research in Knoxville, Tenn., provided invaluable insight into how to keep the risks of infectious diseases and parasites to a minimum at the Thoroughbred International Exposition and Conference (TIEC) held in Lexington, Ky., June 20-22.

Smith began the hour-long presentation with his discussion on disease prevention. The goals of any disease prevention program are to reduce exposure and enhance the horse’s resistance. Management methods can include quarantining new arrivals and sick horses, and separating horses into groups by age and use.

Vaccination can prevent or lessen the effects of many diseases. Smith recommended that a horse have all appropriate vaccinations at least three to four weeks before travel. The vaccination program should be tailored either to the individual horse or to the equine operation. Smith reminded attendees that no vaccine offers absolute protection. When planning a vaccination program, Smith considers the risk of exposure, recommendations from the vaccine manufacturers and the American Association of Equine Practitioners, and his clinical experience, then he develops a plan that is right for that client.

Foal Vaccinations
Foals are born immunocompetent, meaning they have the ability for a normal immune response. Therefore, if a mare is not vaccinated, then a foal can be vaccinated at any time. However, if a mare is vaccinated, then she can pass along her antibodies in the colostrum (first milk). Adequate colostrum intake is essential. Smith said that previously, vaccines were being administered to foals too early and were running into interference with colostral antibodies. Today, vaccine recommendations for young horses have been pushed back, with each vaccine having a different timing for the initial dosing series. Smith then went on to talk about various diseases and some basic facts on each. For more information on these diseases, see the Infectious Diseases category at

Caused by the bacterium Clostridium tetani, which is abundant in the environment, this disease is often fatal. Luckily, this disease is not contagious, noted Smith. Contamination is through wounds, especially in the case of lacerations and deep punctures. The vaccine is very effective, providing a long duration of immunity, although the exact length is not known. Mares should be vaccinated four to six weeks before foaling. Foals can be vaccinated anywhere between three and six months of age and should have an initial series of three shots four weeks apart. Adult horses can be vaccinated yearly. There is also a tetanus antitoxin that only offers protection for up to three weeks, and it has the potential to cause liver disease.
These include Western equine encephalomyelitis (WEE), Eastern equine encephalomyelitis (EEE), and Venezuelan equine encephalomyelitis (VEE). These diseases are spread by mosquitoes and can affect people, but horses cannot pass on the disease. Vaccination lasts six months and should begin a few weeks before the local mosquito season. Mares should be vaccinated in the ninth and tenth months of gestation. Foals can be vaccinated at six months of age. They might be vaccinated earlier if there is a threat of the disease in the area.

The risk of influenza is higher for young horses than older horses. Influenza is considered endemic in the United States, with outbreaks occurring periodically. Since the virus can mutate frequently, vaccinations should contain the most recent strains. There is a killed and a modified live vaccine. Smith recommends the modified live vaccine for weanlings and yearlings. The recommended time to vaccinate foals is nine to eleven months of age. A booster three to four weeks later will provide even better protection. A booster should be given every six months. A killed vaccine is fine for adults, according to Smith. They should receive boosters every six to 12 months. Pregnant mares should be vaccinated four to six weeks before foaling.

Equine Herpesvirus
This virus can cause respiratory problems, neurologic problems, and abortion. Different strains of the virus might cause these problems. Vaccination might prevent abortion storms. Immunity to the vaccine only lasts two to three months, so the vaccination program must be continued year-round. There is no vaccine for the neurological form of herpesvirus. Foals should be started at four months of age with a three-dose vaccination regime.

Strangles is a highly contagious disease caused by Streptococcus equi. Smith said horses could carry the organism in the guttural pouch for at least two years. There is an injectable and an intranasal vaccine. Smith said the strangles vaccine is not a routine part of the vaccination program unless there is a problem with strangles in the area. This is due to a high risk of local reaction and other side effects with the injectable vaccine. The vaccine does not provide long-term immunity, and it should not be given with other shots.

If strangles is included in the vaccination program, foals should be given the killed vaccine, according to Smith, starting at three months of age with an initial series of three doses followed by biannual boosters. Adults can receive two initial doses with yearly boosters. Smith does not recommend the intranasal form of the vaccine.

Rabies is always fatal and might be spread to humans, although there are no reported cases in humans from equine exposure.. Smith called the rabies vaccine a “must-do.” The rabies shot is given in two initial doses four weeks apart followed by yearly boosters.

Potomac Horse Fever
Smith does not recommend vaccination because it does not appear to be effective.

This disease is found mainly in the mid-Atlantic states, and vaccination might not be indicated in your area. This disease can be fatal and is very expensive to treat. Foals are most commonly affected, but horses of all ages are at risk. Mares should be vaccinated at eight, nine, and 10 months gestation. All other horses should receive an initial three-dose regime with an annual booster, and foals can be started at an early age.  Consult with your veterinarian for his or her recommendations in your area.

Equine Viral Arteritis
Equine viral arteritis (EVA) is a contagious, sexually transmitted disease that can cause abortion, edema (fluid swelling), and various other symptoms. Vaccination is very effective and is required annually by law for Thoroughbred stallions in Kentucky, including teasers. Mares sent to a positive stallion should be vaccinated. Because the vaccine is a modified live vaccine, it is possible for a vaccinated animal to pass the disease on by respiratory droplet  infection to other horses in close proximity for a period of three to four weeks. Consequently, all horses in the same barn should be vaccinated at the same time and quarantined for three to four weeks. A blood test should be done before the first vaccination to differentiate a positive result from exposure vs. vaccination. Vaccination once per year should be sufficient.

Rotavirus causes diarrhea in foals anywhere between 12 hours of age to five months of age. The vaccine has some efficacy, therefore mares should be vaccinated at eight, nine, and 10 months of gestation. Foals can be vaccinated at a young age.

West Nile Virus
Smith said that the West Nile virus (WNV) vaccine seems to be very effective. He recommended that if mosquitoes are a year-round problem, then vaccination should be every four months. Otherwise, vaccination should be started before the beginning of the mosquito season. The initial vaccination should consist of three doses three to four weeks apart followed by a booster at least once a year. In many areas, Smith suggests revaccinating in late summer.
Internal Parasites
Reinemeyer opened his discussion on internal parasites by saying that using a “recipe” for a parasite control program eventually will fail. Research in 1966 found that deworming at bimonthly intervals throughout the year would be adequate because the dewormer tested in that study, thiabendazole, kept egg counts low for eight weeks after treatment. However, today many parasites are resistant to some dewormers, said Reinemeyer. Parasites also reproduce faster and can adapt to their environment and to deworming drugs.

He reminded owners that parasites do not amplify within a host, and disease is determined by the number of parasites to which a horse is exposed. The parasites must pass through the environment to complete their cycle and infect a new host.

So, what damage can parasites do to your horse and what is the best way to control their numbers? Parasites can cause colic, diarrhea, poor hair coat, and several subclinical signs such as weight loss, and suboptimal performance. Reinemeyer believes that the ultimate goal of equine parasite control should be to minimize environmental contamination with the parasite “offspring,” versus treating the adult parasites already in the horse. This changes the focus of a control program from therapeutic to preventive. Since the larval stage causes the damage, preventing infection with the larval stage is key. Reinemeyer said that killing adult worms is important most because it prevents them from laying eggs.
Small Strongyles
Small strongyles, also called cyathostomes, infect virtually all grazing horses, said Reinemeyer. Populations can reach the hundreds of thousands, and infections are acquired only from pasture, not from stalls or dry lots. Female worms lay eggs, which pass into the environment with the horse’s manure. When temperatures are between 45°F (7°C) and 85°F (29°C), eggs can hatch and develop into infective larvae. Reinemeyer said the rate of hatch is directly related to temperature. More eggs hatch at warmer temperatures, and the interval required for larval development is shorter. Parasites will die when temperatures go above 85°F, so horses in certain areas of the country are fairly safe during hot summer months. Reinemeyer debunked the myth of a “killing frost,” saying small strongyles are resistant to freezing.

Upon ingestion by the horse, third stage larvae invade the lining of the large intestine. They form small cysts, or sacs, within the tissue. This encysted stage will eventually emerge and become reproductive adults. Most damage takes place as the larvae emerge from the cysts. Killing the larval small strongyles is the best way to prevent reproduction and re-infection. Therefore a larvicidal dewormer that can kill the encysted larvae has distinct advantages.

Reinemeyer recommends scheduling treatments based on the duration of the egg reappearance period (ERP) for each drug used in a year. This is called suppressive deworming. For instance, the ERP for benzimidazoles and pyrantel pamoate is four weeks, ivermectin is six to eight weeks, and moxidectin is 10 to 12 weeks. The first treatment of the year should be done when the risk of infection shifts from minimal to inevitable.

For instance, in the southern United States, a deworming program would typically begin in September and continue until about March, when it can be suspended until the following autumn. Reinemeyer recommends allowing some limited exposure to infection during spring order to build up his natural immunity. The summer months should be risk-free due to hotter temperatures. Deworming would begin again in the fall, with dewormers being rotated and overlapping ERP so that the horse is always protected. Reinemeyer said that it is impossible to eradicate all parasites, but suppression should be effective.

Researchers are trying to deal with parasite resistance to deworming medications. He reported that some strains of roundworms in Canada and Europe have developed resistance to ivermectin and moxidectin.  Reinemeyer said that once worms develop resistance to a class of drugs, it usually is permanent and that product might never again be effective on the farm. Fecal egg counts can be done to test for resistance and to determine dewormer efficacy. Egg counts should be evaluated before treatment and seven to 10 days after treatment.

Until now, dewormers were not effective against tapeworms; however new products have been approved recently that contain the drug praziquantel, which can kill tapeworms. Although time will give more information, Reinemeyer believed that deworming every four to six months for tapeworms should be effective.

Post-mortem studies of horses in Kentucky found that 50-60% of them were infected with tapeworms. Reinemeyer believes this would be indicative of the general equine population. He said that a fecal exam is a poor diagnostic test for tapeworms, because it often fails to demonstrate eggs in infected horses. A new ELISA test can measure antibodies in the blood, but might not be very useful for detecting current infections. In a survey of 3,353 samples from 19 diagnostic laboratories in the lower 48 states, the prevalence of antibodies to tapeworms ranged from less than 5% in California to greater than 95% in Minnesota. Reinemeyer said that all states east of the Mississippi River had at least a 60% prevalence of antibodies. So tapeworms apparently are more common than previously believed, and now that new products are out on the market, horse owners can begin to manage the problem effectively.

About the Author

Sarah Evers Conrad

Sarah Evers Conrad has a bachelor’s of arts in journalism and equine science from Western Kentucky University. As a lifelong horse lover and equestrian, Conrad started her career at The Horse: Your Guide to Equine Health Care magazine. She has also worked for the United States Equestrian Federation as the managing editor of Equestrian magazine and director of e-communications and served as content manager/travel writer for a Caribbean travel agency. When she isn’t freelancing, Conrad spends her free time enjoying her family, reading, practicing photography, traveling, crocheting, and being around animals in her Lexington, Kentucky, home.

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