Dogs can get it. Cattle can get it. Cats can get it. Pigs can get it. And horses get it, too. Leptospirosis is a zoonotic bacterial infection that can attack many species. In fact, it is carried by many wild animals that pass it on to others. It has several variations (known as serovars) and each can affect animals differently. And, yes, it can be passed along to humans as well. In humans, it can cause a wide range of symptoms, some of which may be mistaken for other diseases, such as fever, headache and vomiting. Some infected persons, however, may have no symptoms at all. Without treatment, leptospirosis can lead to kidney damage, meningitis (inflammation of the membrane around the brain and spinal cord), liver failure, respiratory distress and even death.
How widespread is equine leptospirosis in Canada?
Some research has suggested that 75 per cent of horses in the United States have been exposed to the leptospirosis bacteria. Dr. Craig Carter of the University of Kentucky did a study of 1,495 horses in 29 U.S. states and Ontario; he found evidence of leptospirosis in 45 per cent of the horses examined.
Dr. John Prescott, professor emeritus in the Department of Pathobiology at the Ontario Veterinary College, said that it is hard to know exact numbers in Canada, in part because some of the serovars that cause leptospirosis rarely cause problems. “The more [relevant] serovars are Pomona and Gripptyphosa, acquired from wildlife,” Dr. Prescott said. These are the variations which can cause Equine Recurrent Uveitis (ERU), also called “moon blindness” because it waxes and wanes like the moon, as well as spontaneous abortion in pregnant mares. In yearlings, it can sometimes cause kidney problems.
According to Dr. Prescott, ERU tends to be more common in certain regions of Canada such as the Ottawa Valley and the southern Ontario Amish community. Certain breeds may also be more at risk. Dr. Jacquelin Boggs, senior veterinarian with Zoetis Equine Technical Services, identified Appaloosas, drafts and warmbloods as the highest-risk breeds for ERU. However, all breeds of horses can be infected. She also said that leptospirosis-linked abortion is very significant in central Kentucky. However, the disease is seen all across North America.
Dr. Boggs added “This is a devastating disease for which there are multiple triggers and factors but no cure.”
How is it transmitted?
Leptospirosis is largely transmitted to horses through the urine of wild animals including raccoons, skunks, rats, mice, opossum and deer. The bacteria can live for a long period of time in stagnant water or moist soil. The horse might get it from eating food that has been contaminated with rodent urine, or from contaminated water splashing on to his eyes, nose or mouth (also the most likely route for human infection).
While it is possible for the disease to be passed from one horse to another (since at certain stages the horse will shed bacteria in his urine) it is more likely to be picked up from rodents and other wildlife.
Therefore, noted Dr. Prescott, “owners can and should control wild animals in and around the stables.”
The Ontario Ministry for Agriculture, Food and Rural Affairs (OMAFRA) has issued a guide on Rodent Control in Horse Stables. It points out that “The adaptability and agility of these animals make getting rid of them particularly difficult. Mice are capable of running up a vertical surface, negotiating a wire like the finest circus performer and can easily jump to a height of 30 cm (12 in.) from a flat surface. Holes in walls as small as 0.6 cm (¼ in.) in diameter can allow a mouse entry. A rat only needs a hole 1 cm (½ in.) in diameter to gain entry. Their keen sense of smell, hearing, taste and touch help them in their constant struggle for survival.”
Despite the challenges, the guide has some useful advice:
• Remove the food source. Wild animals are attracted by the grains and other foods often stored in the barn, but if you can block their access they will go elsewhere. Store all feed in metal bins with secure lids, ideally in a closed room, and be sure to sweep up any spilled feed promptly (and put it in an equally-secure garbage can).
• Block any entry for rodents. That means caulking or sealing any cracks in the foundation or gaps around pipes. (Steel wool or copper mesh can be used to temporarily close up holes.) Windows and doors should fit snugly and have metal frames – otherwise the mice will just gnaw their way in.
• Eliminate any nesting sites by keeping areas uncluttered and not storing hay or straw in the barn.
• Keep the outside of the barn clear of long grass or weeds where animals might hide or nest.
• Don’t rely on cats and terriers to keep rodents under control. They can multiply faster than your barn cat can catch them.
• Traps are generally preferred to catch rodents over poisons, because there is always a risk of children, dogs or cats getting into the poison or the poisoned carcasses. The basic snap-type trap works well to kill mice. Live traps are also available, but the animals caught must be taken a significant distance before being released.
Because it can survive in the wet and muddy conditions, spring and fall are the peak times for transmitting the leptospirosis bacteria.
How do you know if your horse has lepto?
You might not. Some horses appear healthy and have few symptoms even though they have antibodies to the leptospirosis bacteria. In some cases, the horse is lethargic and has a fever for a few days, but then recovers. Other horses might experience a variety of clinical signs, including fever, loss of appetite, swelling of the eyes, light sensitivity, tearing, ocular discharge, eye cloudiness and redness around the eye, as well as lethargy and mid- to late-term abortion. Adult horses have been known to develop jaundice and even die from kidney and/or liver failure. The incubation period for leptospirosis in horses is one to three weeks.
If your horse develops ERU, he will have eye pain, aversion to light, and the cornea of the eye will look cloudy and may be blue. This may improve for a period of time and then get worse again – the “waxing and waning” part. However, Dr. Prescott noted that this will progressively get worse and lead to blindness. According to Dr. Boggs, research studies and clinical experience have found that more than 70 per cent of cases of ERU are associated with leptospirosis.
Pregnant mares that contract leptospirosis may lose their foals late in pregnancy. Occasionally, a foal is born alive, but infected with leptospirosis. In some parts of North America, as much as 10 to 13 per cent of foals are lost due to spontaneous abortion linked to this disease. It is rare for these broodmares to also have uveitis or similar eye problems, so the only indicator of the infection may be the lost foal.
The bacteria may also accumulate in the kidneys causing inflammation and, in some cases, kidney failure, a serious problem.
Dr. Prescott said that a definitive diagnosis is not easy. Blood tests (serology) can reveal if the horse has antibodies to leptospirosis, indicating that he or she has been exposed. If the horse has uveitis, though, the highest concentration of the bacteria will be in the eye and blood tests may only show low levels of antibodies. If a mare has lost a foal by abortion, the antibodies increase if the blood is tested at the time of abortion and again two weeks later.
Dr. Boggs added that a more definitive diagnosis can be obtained by submitting fetal tissues including the placenta to a laboratory and specifically requesting testing for leptospires.
In acute cases, Dr. Prescott said, a DNA test on the horse’s urine will detect leptospirosis. This will not work for horses with uveitis, though, because the infection is localized to the eye and not shed in the urine by that point.
The challenge with testing is that it takes time. Definitive diagnosis of leptospirosis is very difficult since culture of leptospira takes up to 13 weeks and is often hampered by other bacterial contamination. If one mare in a herd has lost a foal through spontaneous abortion, and leptospirosis is suspected, there may not be time to do blood tests on the other mares and get the results before those mares also lose their foals. In that case, it is a matter of urgency, said Dr. Prescott, to stop the spread of the infection, and he would treat the at-risk mares with antibiotics.
While antibiotics can be used to treat renal disease caused by leptospirosis, Dr. Boggs said there are no cures for leptospirosis and antibiotics tend to have variable results.
Results Looking Positive with New Vaccine
Vaccinations against leptospirosis have been available for dogs, cats, pigs and cattle for some time. In the fall of 2015, the US Food and Drug Administration approved one for horses. Dr. Boggs worked on the development of this vaccine for Zoetis. “The initial work began over nine years ago at Cornell University under the direction of Dr. Tom Divers and Dr. Yung-Fu Chang,” she said. “After preliminary work was completed, Zoetis opted to continue research on challenge model development in-house at their Research and Development site in Kalamazoo, Michigan.”
Dr. Boggs reported that veterinarians and horse owners had been urging the company to develop a vaccine for some time.
The recommended vaccine schedule is for healthy horses six months and older to be given an initial dose, followed by a booster three to four weeks later and then be revaccinated annually. Leptospirosis is a bacteria (not a virus) and bacterial vaccinations do not provide long-term immunity. You must booster yearly. The vaccine has also been shown through research to be safe for use in mares in their second trimester of pregnancy and foals as young as two months of age.
Dr. Boggs reported that efficacy studies for the vaccine had extremely positive results. “In our studies, we identified the organism in 100 per cent of our controls (non-vaccinated animals) in one or more of blood, urine or kidneys. In the vaccinated group, we identified none of the leptospires, meaning 100 per cent protection against disease. This meant no urinary shedding of the bacteria, which is important in terms of minimizing contamination of the environment and spread of disease.”
To confirm safety, they conducted a hyperimmunization study with nine Appaloosas (because this breed is at high risk for ERU due to leptospirosis). The horses were vaccinated seven times in five months. Dr. Boggs said the physical exams and eye exams conducted on these horses over a year found no acute uveitis flares. However, the vaccine is not recommended to be used in horses who already have equine recurrent uveitis, whether it is active or not, and any horse being vaccinated should be healthy.
“In our safety studies, we saw very minimal side effects with a very low reaction rate of 0.02 per cent. So 99.8 per cent of horses vaccinated were reaction-free,” said Boggs. “The minimal local site swelling resolved uneventfully without treatment. Since the product launch in October 2015, we have had minimal to no side effects reported in the field.”
While the vaccine is currently not distributed in Canada, Dr. Prescott said he anticipates that plans are underway to bring it here. “I have long been a strong advocate for the development and use of a leptospirosis vaccine for horses,” he added.
