How can you tell if a horse starts to get EHV-1?

Most EHV-1 infections cause mild respiratory symptoms and fever, lasting 5-7 days. It is impossible to predict if a horse will develop the neurologic form of the infection, although proximity to other horses suffering this complication does indicate that the neurologic form of the virus is present. Hind end weakness and muscle fasciculations are the first signs of neurotropic EVH-1, so any horse with a fever that starts to develop weakness in the hind legs must be considered a possible EHV-1 case. The diagnosis can only be confirmed with blood tests, but supportive care and appropriate quarantine should be instituted while waiting for lab results.

Melissa McKee DVM

Why are Vaccinations Annual?

Why are horses vaccinated annually for things like rabies and tetanus while people are vaccinated far less often?

There are a couple of reasons why we tend to vaccinate horses more frequently than humans. Take tetanus, a horrible and fatal disease in horses, for example. Humans generally receive a tetanus booster shot every 5 years. This is because we have established, through research and vast numbers of vaccines administered over many years, that the vaccine induces a strong immune response that protects an individual for at least five years from developing the disease if they have a wound inoculated with Clostridium tetanii bacteria. You will still usually have a booster shot if you go to emergency with a dirty wound, just for safety’s sake.

The potential strength of an individual’s immune response can be determined with a blood test called a “titre”, which essentially detects the amount of antibodies in your circulation, and data has been analyzed to determine what the minimum protective titre is to prevent clinical illness in humans and to some degree in our small animal pets. When I was a veterinary student we had to receive rabies vaccinations in our pre-vet year. Every year after that we were required to have a blood test to check our rabies titre, and those that were considered low received a booster shot. To this day I still have mine checked regularly since I could easily come into contact with a rabid animal in my line of work. Southern Ontario still has many active rabies cases each year and horses are just as susceptible to it as our dogs and cats.

Another reason horses are vaccinated more frequently is due to greater sensitivity to a particular disease (we don’t vaccinate small pets for tetanus because they are fairly resistant, but horses are exquisitely sensitive to the bacteria), high likelihood of repeated exposure, or the immune response after vaccination is weak and/or short lived. Most of the vaccines for respiratory illness in horses have a short duration of effectiveness, because the pathogen is continually changing and it is difficult to stimulate a lasting response. This is the same reason that we are encouraged to get a flu shot for ourselves every year while our childhood polio vaccination should provide decades of protection.

Finally, although we can measure blood titres to determine antibody levels, there is still little equine data to determine the line between protective and ineffective levels. Vaccine companies are aware of this and are working towards vaccines that allow a longer treatment interval, which is already evident in small animals where several vaccines are now on an every three-year interval.

Melissa McKee DVM


My neighbours horse just died of EEE. I was just curious of your opinion of what precautions if any I should be taking. My horse is vaccinated and I try to ensure she has insect repellant on as often as possible. Is there anything else I should do or be aware of?

OMAFRA was out to our place to trap mosquitoes to find out the activeness of the disease in my area. Will I be able to find out the results from this do you know? Do they publish this info publicly for all to be aware of?

As long as your horse is vaccinated, you have little to worry about. The encephalitis vaccines (EEE, WEE, WNV) are amongst the most reliable in our preventative health arsenal and as long as your horse had a proper primary series and annual boosters then you should be well covered. “Vaccine failure” is always a possibility but it would be very remote. The increase in clinical EEE cases (and other infectious disease) has been linked in part to the economic downturn- people are skipping vaccinations because money is tight, leaving more horses susceptible. Increased mosquito control is still encouraged because people are susceptible to diseases such as West Nile Virus and there are no human vaccines available for protection. In general, OMAFRA makes their disease surveillance reports available on their website.

Melissa McKee DVM

Vaccinating After Strangles Infection

We had Strangles go through our barn 2 summers ago. Our younger mare developed symptoms yet our older gelding did not.

Both had been vaccinated in the early spring with the Intranasal. I am not concerned that the mare actually had symptoms because I do understand that the vaccine isn’t 100%–just the best tool we have.

My question is re-vaccination. We did not do the vaccine last year as our vet indicated that there is some immunity that accompanies having the infection. Even though the gelding did not show symptoms, he was turned out with multiple horses who did because by the time it was discovered, he had already been in contact so moving him would have been a moot point:) Our vet indicated that with the level of exposure he had, he must have a pretty large defense built up as well.

So, Is there any clinical studies that show if there is or isn’t a natural immunity that occurs after the infection has cleared? If there is an immunity, how long does it last? I have read articles that claim everything from lifetime to 2 years.

As you have seen with your own animals, every horse’s physiology is a bit different and some naturally have a stronger and more lasting immune response than others. Various studies have found different durations of immunity because of the inherent variability in populations of the horses that were studied. The studies themselves were also different also- some would measure circulating antibodies while other would expose the horse to strangles infection and watch for clinical signs. Ultimately, nobody knows for sure.

Although with other infectious diseases cases, such as West Nile Virus, we would simply go ahead and revaccinate just to be safe, horses with high titres or subclinical strangles infections can react badly to the vaccine. If past strangles exposure is suspected or confirmed, I recommend that these horses have a blood test to determine their current strangles titre and base my vaccination recommendation on the results. Horses with a high titre are assumed to have sufficient circulating antibody to cope with any new exposure, so we can skip the vaccination and potential risk of reaction. I would repeat the titre annually and initiate vaccination once the levels dip significantly.

Melissa McKee DVM


Vaccination for Botulism

A local dairy farmer in is vaccinating his cows for botulism because several farmers have lost significant numbers in their herds in this area, this year.

He said it is spread by birds who pick it up from the chicken farms? Would you recommend vaccination for my horses?

Equine botulism is caused by the anaerobic bacteria called clostridium botulinum (a cousin of the bacteria that causes tetanus). This common organism persists in a very environmentally resistant spore form and is typically found in the soil. It is not carried or spread by any type of bird, including chickens. While there are eight types of C. botulinum, horses in North America are typically affected by type B and occasionally type C. The problem occurs when the spores are incorporated into decaying vegetable matter, where they produce large amounts of a neurotoxin that is subsequently ingested by the horse. This toxin causes a dose-related flaccid paralysis that eventually kills the horse by paralyzing the respiratory muscles. It usually takes several hours to days for the signs to become apparent, making early definitive diagnosis extremely difficult. Treatment involves the administration of antitoxin, a very costly substance especially for the vast amounts needed to treat a full-sized horse, and intensive nursing care. Unfortunately, most cases are fatal due to loss of respiratory function. Affected foals are often placed on a ventilator as their body metabolizes the toxin.

There have been several outbreaks of botulism in Southern Ontario related to the feeding of round bale hay, which provides a moist oxygen-free internal environment conducive to the proliferation of the bacterial toxin. This can be a problem in the dairy industry as well due to the large amount of silage incorporated into the typical diet. Animal carcasses incorporated into hay during the baling process can also harbour the bacteria so if you find a dead rodent or rabbit in a bale, throw the whole thing out. Shaker foal syndrome is another manifestation of botulism that occurs when a foal ingests the spores, which then proliferate in the intestine and produce the toxin, although this does not occur in adult horses. There is a vaccine available in North America to prevent botulism type B, which requires a primary series of three shots followed by annual boosters. Several of the farms in our practice that feed round bale hay include this vaccination in their yearly preventative health plan, although it is a fairly rare condition. The likelihood of a horse contracting botulism from good quality square bale hay is extremely low.

Melissa McKee DVM


Lyme Disease

I brought my dog to a friend’s farm the other week and a few days later found a deer tick on her. My vet is testing it to see if it has lyme disease, just in case. But I was wondering, can horses get deer ticks? And what kind of affect does it have on them if the tick is carrying lyme disease?

Lyme disease is an infection caused by a spiral shaped bacteria called Borrelia bergdorferi. The first cases were identified in the town or Lyme, Connecticut- hence the imaginative name. I am not sure where you are located in Canada, but the disease has by far the greatest prevalence in the Northeastern and Atlantic seaboard regions of North America, as well as pockets in Wisconsin and Minnesota. Deer ticks carry the bacteria and transmit it between the regular hosts, deer and the white footed mouse, but a deer tick can attach to a passing horse and potentially pass the infection along if it feeds for a prolonged period of time (generally >24 hours). Typically, Lyme disease causes intermittent shifting lameness, muscle soreness, and low grade fevers. There is no evidence that Borrelia burgdorferi causes recurrent uveitis, unlike the similarly shaped spirochete Leptosporosis. Diagnosis can be challenging, since the majority of horses in an endemic area will be serologically positive for Lyme antibodies. This does not necessarily indicate active infection, it just confirms that the horse has been exposed to the bacteria at some point and mounted an immune response to it. Some conclusions can be drawn from the level of the titre and what actual test was used, as well as checking two samples several weeks apart to check for an elevation in levels. Treatment involves medication with long term tetracycline or doxycycline. Clinical Lyme disease is not that common in horses, but some prevention in the form of topical insect repellents and performing frequent tick checks is advisable if your horse lives out or you hack a lot. There is a vaccine for Lyme disease but it is used mostly for dogs, and is only in experimental stages for the horse. It is a good idea to save any ticks you find on yourself and your pets for identification because not all ticks are deer ticks (although all ticks are pretty disgusting and upsetting, particularly if you find them attached to you!!). Although a tick could theoretically transmit Lyme disease from an infected horse to a human, it is very unlikely that an adult tick would move from a horse to a person, particularly for sufficient time to pass the bacteria along.

Melissa McKee DVM

Donkeys and Horses

I have a question in regards to keeping horses and donkeys together.

I heard that donkeys carry something called lung worm, that isn’t found in horses and that you must have a different worming schedule for your barn if a donkey is on the property in order to protect the horses from lung worm. Is there anything else that would need to be adjusted in terms of health care to keep horses and donkeys together?

Would the same apply to horses and mules?

The parasite you are asking about is called “dictyocaulus arnfieldi”, a lungworm that is carried by donkeys and can be spread to the horses that share their environment. The eggs are coughed up and swallowed by the donkey, and complete their life cycle by passing with the manure onto pasture, where they hatch into larvae and are consumed by grazing horses. While donkeys can carry a substantial parasite load with no evident problems, lungworm infestation can cause significant clinical pneumonia in the horse. While it is commonly believed that most donkeys carry lungworm, in reality only a small percentage are infected and rarely shed enough onto pasture to cause problems for their companion horses. The good news is that this is a rather wimpy parasite, easily controlled with dewormers from the ivermectin, moxidectin, and benzimidazole drug classes. A regular deowrming program for both the horses and donkeys on the farm should be quite sufficient to eliminate the risks. Mules should be treated as horses.

Donkeys are not just stubborn horses with long ears, there are some important medical differences that we are aware of and take into account when treating this species:

  • Donkeys have different sensitivity to tranquilizers and often require a much larger dose to have an effect
  • They seem to get a really itchy muzzle when certain tranquilizers are used (not a problem, just weird the first few times you see it)
  • They are very stoic and show very little pain compared to horses, for example, when they have laminitis or colic
  • They are prone to metabolic disorders and can develop life-threatening fat accumulation in the liver if they become ill and anorexic
  • They tend to bleed a lot more at castration and require ligation of the blood vessels as well as the standard application of emasculators
  • Their feet are normally much more upright and tubular than those of a horse
  • They are very hardy and prone to obesity if overfed, especially the miniatures

In general, they can be on the same vaccination and deworming program as horses, and have the same basic anatomy.

Melissa McKee DVM


Duration of Infection

There is currently a horse in my barn diagnosed with “an unknown” virus. The horse is under a vets care, but for my own information – what is the typical incubation period for a viral infection in horses? And what length of time after symptoms are gone are they “likely” to be infectious to other horses?

A “generic” respiratory virus usually has an incubation time anywhere between 3 and 10 days. Some, such as the herpesvirus, are very good at flying under the radar until the infected horse is sufficiently stressed by an event such as shipping. Then they develop clinical illness and may shed the virus to others. In general, the infectious period has passed once the clinical signs have completely disappeared in the original case.

Melissa McKee DVM

Strangles Shedder vs. Carrier

What is the difference between a strangles shedder and a strangles carrier?

A strangles carrier is a horse who has a persistent population of strangles bacteria in the guttural pouches. This can happen because the blind ended guttural pouches easily trap bacteria and provide a friendly environment for continued growth, especially if there is a lot of nutritious fluid and debris in there. A carrier can become a shedder at any time if the amount of bacteria exceeds the capacity of the pouch to contain it. This often happens when the immune system is depressed, such as during times of stress or illness. Therefore, all shedders are also carriers, but a carrier is not necessarily a shedder at a given moment in time. The potential for a carrier to spread strangles is a good reason to try to clear up any latent guttural pouch infections when they are identified.

Melissa McKee DVM

Can people get strangles?

I have read some conflicting information on the internet. Can dogs, cats or people catch strangles from an infected horse?

The official name of the strangles bacterium is streptococcus subspecies equi. This means that it is very well adapted to invade the body and cause disease in horses but is relatively harmless in non-equid species. Although the bacteria may gain access and attempt to colonize the respiratory mucosa in dogs, cats, and humans, it is usually quickly and easily killed off by our immune systems before any real infection can occur. Individuals with compromised immune systems (HIV, certain chemotherapy) should exercise more caution around any kind of infectious disease regardless of which species is normally affected.

Melissa McKee DVM