February 2019 - Drug Testing in the Horse
Are you looking forward to your first horse show later this year? Or do you have questions regarding what medications can and cannot be used prior to competition? Join us this month as our expert, Dr. Thomas Tobin, answers your concerns regarding equine drug testing.
Click here to read this month's questions and answers.
Equine Infectious Disease
My horse is losing weight rapidly for no apparent reason. My horse tested negative for parasites and there has been no change in diet.(View Answer)
The primary causes of weight loss in horses are parasitism, dental disease and inadequate nutrition. You have already ruled out parasitism and it sounds like his diet was previously adequate; have you had a change in dietary quality (new hay supplier, etc.)? Has your horse had a thorough dental exam? Once those three potential causes are ruled out, a weight loss workup generally includes an abdominal ultrasound, a belly tap and a rectal examination to start. In these workups, we are often looking for masses, which can be abscesses or neoplasia(cancer). You do not indicate your horse's age or whether he has had recent illness, so it is impossible to tell which we might be more likely to find. Some diseases that can cause internal abscesses can be detected via a blood test; these include Corynebacterium pseudotuberculosis (the causative agent of Pigeon Fever) and Streptococcus equi (the causative agent of strangles). These blood tests may help narrow down the cause of weight loss. Finally, some causes of muscle wasting may appear to be rapid weight loss, such as immune mediated myositis also caused by Streptococcus. There are also infiltrative bowel diseases that can manifest as weight loss, but these are very difficult to diagnose without exploratory surgery and biopsy. You may need to visit a referral center to obtain some of the above mentioned diagnostics; good luck with your horse! Tamara Gull, DVM, PhD, DACVIM, DACVPM, DACVM, Lieutenant Colonel, U.S. Army Reserve (Veterinary Corps) Stillwater, OK.
What is an auto immune disease in horses that causes spider webbing in hair that has not shed?(View Answer)
I think you may be referring to the condition known as reticulated leukotrichia. We don't know exactly what causes this condition, although one theory suggests that it may be secondary to erythema multiforme, which can occur following herpesvirus infection or vaccination. There are breed predilections to the condition in Quarter Horses.
The reticulated leukotrichia may be an unusual form of erythema multiforme, as some cases are temporally associated with herpes virus infection or vaccination and will recur with subsequent vaccination.
Reticulated leukotrichia is most commonly seen in quarter horses, thoroughbreds, and standardbreds, but it is occasionally recognized in other breeds. There is no apparent sex predilection. Most horses develop clinical signs as yearlings. Linear crusts develop in a cross-hatched or net-like, more-or-less symmetrical pattern over the back between the withers and the base of the tail. Tamara Gull, DVM, PhD, DACVIM, DACVPM, DACVM, Lieutenant Colonel, U.S. Army Reserve (Veterinary Corps) Stillwater, OK.
About three-four weeks ago, my 31-year-old gray gelding had his teeth floated and was started on pergolide (long hair coat, etc.). About a week ago, he started having yellow-colored discharge out of both nostrils. He had no fever. The vet started him on 10 SMZ tablets 2X a day. He got better for a few days and then had more discharge, started coughing, and began breathing with a flutter and gurgling sound. Still no fever. My vet gave him a Lasix shot and an Equioxx shot, took a blood and mucus sample. I am to give him Equioxx paste for 5 days, continue the SMZ and pergolide. He grazes in his limited pasture with his so-far unaffected buddy and seems alert enough but has been leaving his softened alfalfa pellets and low-NSC feed. I am puzzled as to what his problem/disease might be. Any thoughts?(View Answer)
There are a number of things that could cause nasal discharge in horses. In an old horse I would be concerned that a communication may have occurred between his oral cavity and his sinus, usually associated with tooth roots. This communication could be very small and may not have been obvious on dental examination, if it even existed at the time of floating. Elderly horses have very little reserve tooth crown, so communications between mouth and sinus happen all too frequently. However, those usually only result in discharge from one nostril rather than both and the discharge is usually foul-smelling. Nonetheless, that would fit with your horse being reluctant to eat if he has dental pain.
Bilateral nasal discharge can arise from the lower respiratory tract, the nasopharynx or the guttural pouches. His recent history of coughing and gurgling would suggest a lower respiratory tract problem. There is a possibility that your horse aspirated some of the nasal discharge and that is what is causing the lower respiratory signs. In working up such a problem we often will do a rebreathing exam to make the horse breathe deeply, a chest x-ray and a trans-tracheal wash to sample the lower airway secretions. I would recommend pursuing both evaluation of the oral cavity and sinuses as well as the lower respiratory workup. Tamara Gull, DVM, PhD, DACVIM, DACVPM, DACVM, Lieutenant Colonel, U.S. Army Reserve (Veterinary Corps) Stillwater, OK.
Can a horse have strangles without the swollen lymph nodes?(View Answer)
While the swollen lymph nodes and nasal discharge are characteristic of clinical strangles, a horse can definitely be harboring the strangles bacteria without showing any external signs whatsoever. Some horses get strangles and develop enlargement mainly in the retropharyngeal lymph nodes inside the guttural pouches instead of the submandibular lymph node under the jaw. Swelling of the retropharyngeal lymph nodes is often difficult to appreciate externally and is best seen by placing an endoscope in the guttural pouches. Horses will often maintain the strangles organism in their guttural pouches for weeks to months after recovering from clinical illness; the only way to know a horse if a horse is carrying Streptococcus equi equi is by doing several guttural pouch washes several days apart and submitting the fluid for culture or PCR testing. Your veterinarian can assist you with this. Many horse owners opt to do this after their horse has had strangles so that they know when it is safe to expose their horse to other horses again without potentially transmitting strangles. Some farms also require this testing of new horses before they are allowed to mingle with the general population of the farm. Tamara Gull, DVM, PhD, DACVIM, DACVPM, DACVM, Lieutenant Colonel, U.S. Army Reserve (Veterinary Corps) Stillwater, OK.
Do any equine vaccines exist for Lyme Disease?(View Answer)
There are currently no Lyme vaccines licensed for use in horses. There are anecdotal reports of canine Lyme vaccines being used in horses, but no safety or efficacy data exists for this off-label use. An equine experiment with a Lyme vaccine using the OspA antigen did show evidence of protection, but there is no commercial vaccine available that is precisely analogous to the experimental vaccine used in that study. For prevention of Lyme disease in horses, tick control and regular grooming are recommended. Tamara Gull, DVM, PhD, DACVIM, DACVPM, DACVM, Lieutenant Colonel, U.S.Army Reserve (Veterinary Corps) Stillwater, OK.
I have dealt with gas colic in my horse for 20 years. He is sensitive to barometric and temperature change. I feed orchard grass, SmartPak Ultra digest for seniors, loose white and mineral salt, watered down hay fed in a slow-feed net (3x per day). I use GasX tablets ground to a powder, or Pepto liquid when he has an episode. I put him on UlcerGard when extreme weather changes are due to occur. My veterinarian says I am doing all that is possible to manage this problem, but I wondered if I could give chamomile daily in a tea or flower form in order to minimize the excess gas that is occurring? I am really at my wits end, and have been afraid to leave home for ANY length of time! (N0 trips in over 20 years.) My husband is really discouraged too (putting it mildly.)(View Answer)
There is a great deal of interest in "natural" remedies for common equine maladies, and chamomile is one of many herbs that have been anecdotally recommended for the treatment of colic. Unfortunately, there have been very few rigorous scientific studies investigating their use. I found no equine studies in the peer-reviewed literature; all reports were anecdotal and covering very small populations. One report looking at rabbit intestine in a lab model did find modest antispasmodic effect of chamomile extract on intestinal tissue, but this research is a long way from enabling the recommendation of chamomile as a treatment for colic. Rather than adding a new treatment with unknown effects, I would recommend investigating other dietary options. You might try transitioning your horse to a complete pelleted feed such as Equine Senior, as perhaps he has difficulty digesting roughage. Make sure he gets a complete oral exam as well, as he is definitely old enough that dental problems could now be contributing to his frequent colics. It might also be worthwhile to have a chronic colic workup done at a referral center with an equine internist. Tamara Gull, DVM, PhD, DACVIM, DACVPM, DACVM, Lieutenant Colonel, U.S.Army Reserve (Veterinary Corps) Stillwater, OK.
My horse has been troubled with eye problems for about 2 years and I have been told by veterinarians that it was allergies. However, lately his eye is getting worse with swelling and driping blood. I opened up his eye and saw what looks just like the gills of a fish and red in color. I have been treating him by washing the eye out twice a day and applying an antibiotic cream my vet gave me, but it is not working. Could this be something underlying?(View Answer)
First, the history that your horse has been suffering with eye problems for two years despite visiting several vets strongly suggests that you should visit a veterinary ophthalmologist for expert diagnosis. The American College of Veterinary Ophthalmology website can help you locate an eye specialist. I would be concerned that your horse is experiencing repeated bouts of anterior uveitis, or inflammation within the eye. This condition is very painful and affected horses often hold the eye closed and have excessive lacrimation (tearing) that may be misinterpreted as allergic conjunctivitis. I suspect the red gill-like tissue you are seeing is irritated conjunctiva; when it swells (known as chemosis) it can be very prominent. Unfortunately, excessive manipulation of the eye can contribute to chemosis, as can washing with anything other than a dedicated eye wash solution. Plain water can be very irritating to this tissue if used repeatedly. In summary, I recommend that you stop washing the eye and have your horse seen ASAP by a veterinary opthalmologist, as anterior uveitis is a difficult disease to get under control, requires lifelong surveillance and management, and can result in blindness if left untreated. Tamara Gull, DVM, PhD, DACVIM, DACVPM, DACVM, Lieutenant Colonel, U.S.Army Reserve (Veterinary Corps) Stillwater, OK.
Which canine lyme vaccine is being used on horses off-label ? What is the recommended vaccine protocol?(View Answer)
First, I’m sure all of the canine Lyme disease (caused by the tick-borne bacterium Borrelia burgdorferi) vaccines have been tried in horses. However, none are labeled for equine use and no published studies have been done on the safety or efficacy of commercial canine Lyme vaccines in horses. One small study published in 2000 using an experimental vaccine against the Borrelia protein OspA did demonstrate antibody production and apparent protection in ponies, but the vaccine formulation used is not commercially available. The experimental vaccine used a recombinant protein and an adjuvant, while most of the commercial vaccines use a killed whole-cell bacterin (2 strains of killed Borrelia organisms) as a source of protein. The one canine Lyme vaccine that uses recombinant protein is nonadjuvanted.
Because of the lack of studies investigating the effects of canine Lyme vaccines in horses, no specific protocols can be recommended. There are also legal concerns for any veterinarian administering a vaccine off-label, so veterinarians may be reluctant to administer or sell Lyme vaccine for use in horses. If you do choose to vaccinate it would be prudent to check whether the horse has already been exposed to Lyme disease. In humans with previous exposure to Borrelia, vaccination occasionally made the clinical signs worse; this contributed to the withdrawal of the human Lyme vaccine from the market. It is unknown if this effect could occur in horses. Serum samples should be submitted for diagnostic testing before vaccinating; I recommend the Lyme multiplex test offered by Cornell University.
Finally, in Lyme-endemic areas, diligent tick control and grooming is necessary. It takes 48 hours of attachment for the tick to transmit Lyme disease, so daily tick surveillance and removal and use of equine-appropriate tick repellent is highly encouraged. Tamara Gull, DVM, PhD, DACVIM, DACVPM, DACVM, Lieutenant Colonel, U.S.Army Reserve (Veterinary Corps) Stillwater, OK.
I have a new horse with no vaccination history and I recently vaccinated her with a 4-way vaccine and West Nile virus. Shortly thereafter, an acquaintance just told me she was already vaccinated. Will that hurt her or cause problems?(View Answer)
Re-vaccinating an already-vaccinated horse with the vaccines you describe is unlikely to cause any problems. The immune system relies on occasional reminders to stay ready to fight off infection; that’s why most vaccines require a 2-injection series in foals being vaccinated for the first time. It’s also why annual revaccination is recommended for most vaccines, and some (such as influenza) are boostered even more frequently in some high-risk populations. You could just think of your horse as having received a very early booster.
However, vaccination is not always benign. Anaphylaxis, a severe allergic reaction, is rare but can occur within minutes of injection. Many vaccines can cause injection site reactions (sore necks), and other vaccines have been associated with systemic inflammatory responses (fever, malaise and inappetence). The strangles vaccines have been reported to trigger cases of purpura hemorrhagica when administered to horses with high levels of circulating antibody, such as horses recently recovered from strangles. Before vaccinating a horse with an unknown vaccine history or one that has had strangles in the last two years, check a Streptococcus equi-specific SeM antibody titer. If it is 1600 or greater, do not vaccinate that horse. Tamara Gull, DVM, PhD, DACVIM, DACVPM, DACVM, Lieutenant Colonel, U.S.Army Reserve (Veterinary Corps) Stillwater, OK.