AAEP Convention 2005: Preventing Contagious Diseases

"Prevention of contagious diseases is dependent on an effective vaccination program and a carefully developed and strictly enforced management program," said Steve Conboy, DVM, a Central Kentucky equine practitioner, at the 2005 American Association of Equine Practitioners' Convention, held Dec. 3-7 in Seattle, Wash.

"Because of training in infectious diseases, bacteriology, virology, and  asepsis, the veterinarian is in a unique position to develop and to help facilitate a disease prevention program," he added, before outlining recommendations he makes to help keep infectious diseases off his clients' farms. "The basic principles are about the same for any type of horse operation; they must be realistic, and it's very important that everyone is involved--from the owner to the groom, all should be well aware of the principles of the program."

The basic principles of disease prevention include maintaining a carefully designed vaccination program, implementing a pre-arrival policy and a post-arrival program for visiting animals, isolating horses that return to the premises after attending an event or sale, isolating ill and "shedder" animals, and avoiding over-population and stress. "The solution to pollution is dilution," Conboy said. "Don't overcrowd your populations, and avoid contact between different age groups, i.e., weanlings or yearlings and older horses, especially pregnant mares."

In the face of an outbreak, practice preventive hygiene. Because of the broad range of vaccine efficacy for different diseases, and the fact there are some diseases for which there are no vaccines, management is an extremely important part of the program.


Conboy says every facility should have a policy for accepting new arrivals. "Send a form out before the horse is to be shipped to that farm and define health requirements," he recommended. "You can require a medical history, a health certificate that requires a veterinary examination within 48 hours of shipping, and it would be nice to have a veterinarian or manager's certification of the facility's health history that the horse is coming from (for example, is it a strangles-endemic farm?)."

Minimal requirements for these animals would be a negative test for equine infectious anemia (EIA, the Coggins test) and proof that the animals are up-to-date on their dewormers and vaccinations.

"Upon arrival, all horses in my opinion should be isolated for a minimum of two weeks, although four weeks is probably better," he added. "You should monitor temperatures and observe the horses carefully. This is an important time to conduct tests to  detect shedder animals ( the ability of horses to shed infectious organisms that cause disease without actually showing clinical signs of the illness). This can happen readily when horses are stressed. Veterinarians should take samples during this isolation period to test for contagious diseases.

Isolation Protocol

Veterinarians and farm managers should assess the risk of  incoming animals when deciding on an isolation protocol. Conboy gave the example of a horse that ships in on a commercial van and reminded veterinarians, "Realize the horse on that van is exposed to every horse that's been on that van and where those horsescome from. It might represent a variety of farms and health problems, which is another reason for an isolation period."

The isolation criteria depends on the farm's physical capabilities, the purpose of isolation, and how much the farm is willing to risk.
Conboy said  a pre-export protocol is an example of a ideal isolation program.This protocol requires at least 30 feet from other horses and the quarantine barn must be stripped, disinfected, and power-washed before horses enter, and there must be antiseptic footbaths in place at all entrances and stalls. Additionally, all feed, bedding, and equipment for that barn  must be kept separate from the rest of the farm's feed, bedding and equipment, and all feed tubs, water buckets, forks, grooming utensils, and other supplies must be disinfected prior to entry. "It's an all-in or all-out type of isolation," he said. "This is for an extreme form of isolation,. In some cases, a simpler program could be acceptable."

A veterinarian should examine horses on arrival, and it might be worthwhile to photograph the horse as it gets off the trailer. This will be particularly important if the horse arrives in poor physical condition.. " Broodmares, should be palpated  to confirm their  reproductive status , in case it is different from what it is thought to be by the owner.," he said. "And by observing  the performance horse in motion, you can document lameness, etc., at the time of arrival," so there's proof the horse didn't acquire injuries at the farm.

Once horses are in isolation, it's good to monitor their temperatures twice daily to detect any sign of impending clinical disease.During the isolation period horses should be tested for such contagious diseases as strangles, influenza, equine herpesvirus (EHV), salmonella, and rotavirus. In the case of imported horses that will be used for breeding, tests for equine viral arteritis (EVA), and contagious equine metritis (CEM) should be conducted. .He reviewed each disease and its diagnostics:

Equine Infectious Anemia

Even though we're all aware of the importance of spreading EIA, we still cannot let our guard down," Conboy said. "Use disposable syringes and needles, clean and sterilize instruments after use, and never intermingle healthy horses with infected ones. It only takes one-fifth of a teaspoon of blood to infect 10,000 horses, and in the case of an acutely infected horse, a similar amount of blood can infect one million horses."

Diagnostic tests include the Coggins and the enzyme-linked immunosorbent assay (ELISA) tests. In recent years, approximately 400 positive cases have been documented annually by these tests. Horse owners spend more than $50 million on Coggins testing each year in the United States.

"There has been a proposal to regionalize the United States and reduce testing requirements, but it remains to be seen whether this will come to fruition," he said. Either way you slice it--whether officials keep the current Coggins requirements or if the industry accepts the risks posed by reducing requirements--EIA is a very expensive disease for the horse industry.


Asymptomatic (not showing clinical signs), shedding horses are typically the sources of a strangles outbreak.

Shedders are identified by polymerase chain reaction (PCR) testing and culturing nasal swabs for Streptococcus equi (the bacteria that causes strangles). A negative polymerase chain reaction (PCR) test indicates that the horse isn't shedding bacteria. If a horse is positive on PCR and has a negative culture, the horse might have had recent vaccination with the modified-live strangles vaccine, recently recovered from infection, or it could be shedding bacteria that were missed by culture.

"The source of bacteria is usually the horses' guttural pouches," Conboy said. "Shedders can be treated with penicillin infused into the guttural pouches. He recommends treating one time, and re-testing  in a week to see if you've cleared the infection."

Affected horses and shedders should be isolated immediately, and human contact with infected animals should be limited (this helps prevent the spread of the disease to healthy horses via someone's hands, clothes, or on barn equipment). "Designate caretakers that wear protective clothing and gloves to care for these horses," he said.

"Several years ago, I started using a medical feed after a strangles outbreak. The antibiotic tetracycline  at 50 ppm (parts per million) is incorporated into feed pellets. It's cost-efficient  but requires a veterinary prescription." This feed has been used for two months after an outbreak to discourage the development of  Streptococcus equi shedders.


Conboy calls EHV "the immune system escape artist," because  when the horse is infected with herpesvirus,  the virus often hides in the lymph nodes or nervous tissue, and reappears when the horse is stressed. When the virus becomes active the horse will become infective (a virus shedder) with or sometimes without showing obvious symptoms.. Since the virus establishes a latent infection, it is difficult to detect shedders of the disease. He reviewed the common EHV types (EHV-1 and EHV-4) and said it's good to observe for signs of EHV during the first two to 10 days of isolation.  Virus can be detected during these periods of disease by PCR test on blood (white blood cells) or nasal secretions.

Herpesvirus can cause respiratory disease, neurological signs, and it frequently causes abortion in pregnant mares. During isolation, the signs of herpesvirus infection include:

  1. Biphasic fever (one that comes and goes over the course of several days)
  2. Nasal discharge
  3. Conjunctivitis and runny eyes
  4. Occasional cough
  5. Lethargy (the horse generally appears depressed)

Herpes infection can be diagnosed during the febrile stage or the shedding periods by virus isolation or by PCR test. Since asymptomatic horses can shed virus when stressed, testing new arrivals might prevent introducing a shedding horse to the population.

In the case of the neurological infection, clinical signs can include incoordination, paralysis of the bladder, tail, and rectum, and  frequently the inability to stand. In the case of neurologic outbreaks, it's important to isolate  recovering horses for 21 days after all clinical signs disappear.

Because herpesvirus causes abortions in pregnant mares, Conboy suggests isolating pregnant mares from all other horses on the farm and particularly new arrivals. "Pregnant mare should always be separated from younger horses (those most frequently infected by herpes virus). Visiting mares should be segregated from resident mares when possible," Conboy said. "Following a herpes abortion, the mare will shed large amounts of virus for at least 48 hours. Any mare that aborts should be isolated until the cause is determined. If it is diagnosed as herpes, her pasture mates should be isolated in their group and observed for abortion. Aborting mares should not be bred until 30 days following their abortion."


It's difficult to shake influenza--it always seems to reappear in young, stressed equine populations. "The (influenza) carrier state is not known to exist in horses," Conboy said. "If there's no carrier state, then where does it come from? We don't know."

The virus is highly contagious and is spread by coughing horses and contaminated tack, blankets, water buckets, and humans. The virus can survive outside the horse for several hours. Infected horses shed virus for up to eight days after infection.

Influenza can be diagnosis is by the Directigen Flu A test or by virus isolation during the febrile period "A sample for virus isolation is obtained by passing a Dacron swab up the nostril or by flushing the pharynx with saline. The sample is submitted to a diagnostic laboratory. If influenza is diagnosed, a duplicate sample should be sent to an OIE reference laboratory to monitor for virus drift. (Dr. Tom Chambers, Gluck Equine Research Center, Lexington, Ky. 859/257-4757.)

It's important to isolate infected horses and limit human contact, eliminate stress, and make the horses comfortable. It has been recommended that infected horses be completely rested one week for every day of fever.


Up to 40% of horses are intermittent salmonella shedders, and that number is on the rise. Carriers are difficult to identify, and generally shed when stressed. This bacteria is highly infective and causes severe diarrhea and joint infections.

Diagnosis is made by culturing a fecal sample, although Conboy does not have much confidence in a single test. He recommends a minimum of three negative cultures on the horse from a suspect horse before considering it negative.

Conboy stresses personal hygiene in handling salmonella cases, since humans can also get the disease. Isolation and disinfection are essential until the three negative cultures are obtained. He suggests using Tektrol (a liquid disinfectant) in salmonella outbreaks. He suggests washing udders at the time of foaling, to reduce infected material accessible to the foal. Disinfectant footbaths, protective clothing, and stall disinfection are essential in managing an outbreak.


This disease appears to becoming less significant as a source of illness for foals. Its effects are generally mild, but it can cause severe diarrhea in susceptible foals.

Diagnosis is made by using the Virogen Rotatest on a fecal sample. Since the infected foal can shed this virus for up to two weeks after the diarrhea has stopped, it is wise to test any new animals with a history of recent diarrhea. Isolation of carriers for two weeks should prevent the spread of rotavirus. Always use a Tektrol foot bath where foals have diarrhea.


Leptospirosis remains a sporadic cause of equine abortions. It is diagnosed by running the Microscopic Agglutination Titer (MAT) on serum samples from mares or aborted fetuses, or fluorescent antibody tests (FAT) on a mare's urine sample or on the fetal liver, kidney, and aborted placentas.

"When you have a leptospirosis abortion, there is a lot of leptospirosis in the uterine fluids, so handle the mare and aborted fetus very carefully," he said. "Submit the fetus and a serum sample for necropsy and MAT. Vaccinate in-contact animals  (non-equine)  with a multivalent (multi-strain) leptospiral vaccine," and test mares that have been in contact with the abortive mare. Isolate aborted mares from all other horses. Bacterial shedding might last for up to 14 weeks. Mares that test with a significant titer (greater than 1:64,000 should be isolated to reduce the chance of abortion.

Venereal Diseases

On breeding farms, there are two venereal diseases of concern:

Contagious equine metritis (CEM, caused by Taylorella equigenitalis) has been eradicated in the United States, but Conboy cautions that it could be reintroduced at any time by infected stallions and/or mares. "We continue to identify carrier stallions and mares that have been imported to the United States," he said. "The greatest risk is coming from sport horses that enter with temporary permits, and then they get lost in the system and remain here for breeding. We need to be vigilant that these horses get tested before they are bred."

Mares become persistently infected by CEM, while the stallion serves as a mechanical carrier. The disease is diagnosed by test breeding and culturing the stallion's external genitalia, and by culturing the clitoral sinuses and endometrium of the mare. There is also a blood test for recently infected mares.

EVA is transmitted through the respiratory and reproductive systems. Infected stallions are subject to persistent infection of the accessory sex glands and they frequently shed virus in their semen without showing signs of the disease. Mares that will be bred to infected stallions or with infective semen are protected from the disease if they have antibodies from prior exposure or, if they are vaccinated against EVA at least 30 days before breeding..Mares should be isolated 30 days after vaccination and for one week directly after being bred to a shedding stallion or with infected semen, because these mares can shed the virus through the respiratory route during those time periods.

Stallions should be tested for EVA before beginning their breeding careers, and they should be vaccinated if they are seronegative. Young colts should be vaccinated at nine months of age, before they are exposed to the respiratory form of EVA. This vaccination will prevent them from developing the EVA shedder syndrome later in life. A negative EVA titer should be documented in all colts prior to vaccination if export might be a possibility. Diagnosis is by isolating virus from the semen.

Managers of EVA shedder stallions are obligated to inform those breeding to their shedding stallions of this condition so that the mares can be protected by vaccination an isolation. Using shedding stallions or infected semen can be safely done if the proper protocol is followed. If ignored, this virus can cause abortions, sick foals, or respiratory infections.


Managing a disease outbreak is common sense. Conboy summarized his message:

  • Isolate sick animals, even if it means isolating an entire barn, field or farm. 
  • Diagnose the cause of disease as soon as possible and initiate therapy to sick horses without delay. 
  • Limit barn traffic and restrict isolation areas to as few people as possible. 
  • Use disposable coveralls and foot guards. 
  • Use foot baths of suitable disinfectants at stall doors and entrances. 
  • Wash hands frequently and always after handling sick horses (instruct farm personnel to do so as well).
  • Use spray tanks of disinfectant for truck, tractor, and manure spreader tires. 
  • Compost or dispose of bedding and manure where other animals cannot get to it. 
  • Restrict access of wildlife to feed, water, and bedding.

Notify others who might be affected by the outbreak, including the van company, farrier, breeding sheds, the insurance provider, the feed/hay/bedding provider, neighbors, and future recipients of horses from the facility.

Additionally, the veterinarian should make checking the infected facility his or her last visit of the day, whenever possible. The veterinarian should put on and remove disposable coveralls and boots at the facility and disinfect all equipment after each use, or dedicate a set of equipment for the farm. Contact with sick animals should be limited. "Make sure you wear gloves and wash your hands frequently," he said. "Tell the farm to have its own twitch. Spray your vehicles' tires with Tektrol, and disinfect your (vehicle) floormats and your shoes.

"A successful disease prevention program is dependent on an effective vaccination program and a rigid management protocol," he concluded. "Preventive hygiene and the use of effective disinfectants are essential to the management program."

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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