Lyme Disease Update
- May 1, 2005
Lyme disease is a problem more commonly thought to occur in our canine and human friends than horses. However, occur it does, and it can have a wide range of signs and symptoms. This disease is a perplexing and confusing one to diagnose and treat in horses, as it has similar clinical signs to many other diseases.
Lyme, Slyme, What's in a Name?
Lyme disease is a relatively recent disease for horses and humans, as it did not achieve widespread recognition until 30 years ago, when multiple humans--mostly children--were diagnosed with an infectious arthritis in Old Lyme and Lyme, Conn., hence the name. The disease is caused by the bacteria Borrelia burgdorferi, so it is also sometimes called Lyme borreliosis.
The organism B. burgdorferi is not a bacterial infection you can just pick up at any run-of-the-mill, bacteria-laden mini mart. This particular bacteria prefers to be hosted by ticks. As a result, the infection is thought to be transmitted to humans, horses, and dogs solely via a tick bite.
The ticks most frequently implicated in carrying B. burgdorferi bacteria are of the genus Ixodes. This particular genus has approximately 250 species of ticks worldwide (at least 25 of which live in North America). So, yes, Lyme disease occurs worldwide, with cases in humans and other mammals having occurred in Europe, Asia, Africa, and Japan. In North America, Lyme disease occurs most often in the Northeastern and Midwestern United States, but it can occur throughout the country.
To understand how humans and other mammals acquire this infection, you must understand how ticks acquire the bacteria first. The Ixodes species of tick (also known as a deer tick or black-legged tick) has a two-year life cycle from egg to adult, requiring a blood meal at each life stage. The larval form of the tick begins as it emerges from the egg, which is usually laid in decaying leaves by its mother, often in the spring. The larvae then feed on small mammals such as mice or birds to have a blood meal. Once fed, the larvae can molt into nymphs, but remain dormant until the following spring. The next spring, the nymphs are hungry and will feed on mice or other mammals, such as humans, dogs, or horses. Then the ticks molt again and develop into adults. The adults will again feed on humans, horses, or other large mammals, but especially deer. Then they mate and lay eggs. The adult female ticks will die after laying eggs.
The ticks become infected with B. burgdorferi after the first or second blood meal as larvae or nymphs. In the Northeastern United States, the ticks most often acquire the Borrelia organism from white-footed mice. The tick is considered a reservoir for the Borrelia organism, as it lives within the tick while causing no apparent disease.
The Borrelia organism has also been cultured from mouse urine, but at this point, we do not think that transmission to horses, humans, or other creatures occurs in any way except via a tick bite. There has been no evidence that transmission of Lyme disease can occur directly from animal to human; there must be a tick involved for transmission.
You've Been Sucking How Long?
A tick can be attached and sucking blood for days before you find it, if you find it. It is believed that the tick must be attached for at least 24 hours in order to transmit the Lyme disease organism.
Because of that fact, in people at least, the nymph stage of a tick's life cycle is the most likely stage to be infected and transmit the organism responsible for causing Lyme disease. The adult ticks are much larger and are more likely to be seen and removed while feeding on you, your horse, or your dog. The nymph stage however, is quite small (only two to three millimeters long) and much more likely to be missed on your, or especially your hair-covered horse's, body. That means they are more likely to be able to feed for days undisturbed and transmit any disease they might be carrying.
Got the Creepy Crawlies?
Now that everyone is checking their body for suspicious blood suckers, let's talk about the signs of Lyme disease. In humans, many times a tick that has transmitted the Lyme disease organism to you will leave a very noticeable bull's-eye red rash at or near the actual bite, which is screaming, "Warning, Warning, Lyme Disease Here!" This rash is followed by sore and painful joints and a fever.
In animals, if the rash occurs at all, it would be very difficult to see due to the hair coat. So what do you see?
In horses with Lyme disease, clinical signs range from stiffness to a shifting-leg lameness, or multi-limb lameness. They might have a fever or swollen joints. Some horses appear lethargic and unwilling to work. There has been one report of Lyme disease causing a neurologic disorder and uveitis (moon-blindness) in a single horse and pony, but this is rare.
So, if you have a horse with suspicious signs, and other diseases such as osteoarthritis or rhabdomyolysis (muscle disorders) have been ruled out, now what? In our practice area of upstate New York, we test often for Lyme disease, but only rarely find it. The diagnosis of this disease in horses has been a puzzle and a problem for veterinarians worldwide. To diagnose Lyme disease, the first step is a blood test. The horse's blood will be checked for antibodies to the causative organism. If the horse has a high ELISA titer (greater than 300 Kela units) and a positive Western blot test for the organism, then your horse has absolutely been exposed to the organism, but might not have active disease. If your horse has a high titer and has clinical signs of the disease, then he probably has Lyme disease.
Also, to add to the confusion, if your horse has a low titer, there are several possibilities. The horse could:
- Not be infected;
- Has been infected in the past, but has cleared the infection; or
- Is currently infected, but for less than 40-60 days, thus he has not yet mounted a large immune response.
The other problem with diagnosing Lyme disease is that we do not yet know how long it takes for a horse to develop clinical signs of the disease once bitten by an infected tick. According to Thomas Divers, DVM, Dipl. ACVIM, of Cornell University, the incubation period can be "as quick as a few days to weeks." Unfortunately, every horse is different.
In some cases, the disease can absolutely be confirmed by the use of a test called PCR (polymerase chain reaction test). Unfortunately, it can't be used on blood samples. PCR can detect the DNA of the B. burgdorferi organism, which can hide within the synovial tissue inside a horse's joint, causing pain and inflammation. A sample of this synovial tissue tested using PCR might provide proof that the organism is there and causing disease. Hence, you would have a confirmed diagnosis of Lyme disease.
The disadvantage to this procedure (called a synovial biopsy) is that it is invasive, and many veterinarians will not want to perform this procedure on the farm. The procedure can be performed in a standing, sedated horse, but the environment must be clean and dry. In our practice, a synovial biopsy is most often performed in the fetlock or carpal (knee) joint, but any joint that is suspicious for infection is appropriate.
Make a Plan
All signs point to Lyme disease, and you have a diagnosis. Treatment of Lyme disease is also a problem because it can be prolonged. Many horses come to the Lyme diagnosis after many other treatments have been attempted. Treatment of Lyme disease most commonly involves one or two antibiotics. Tetracyline is a very effective treatment for Lyme disease and is relatively inexpensive. However, it must be given intravenously once a day, so most horses will only be treated with this antibiotic for a short time. Doxycycline is a related antibiotic, but it can be given orally. This drug must be given twice a day, but because it is poorly absorbed from the gastrointestinal tract, treatment duration is often prolonged, frequently one month or more. Sometimes, it is necessary to treat for months at a time until the antibody level has dropped to a low level. In some cases, the organism (and therefore disease) is never eradicated from the body.
An Ounce of Prevention
We all know the old adage, but what can we do for our horses? For ourselves, if we are in an area that is likely to have ticks, we can wear long-sleeve shirts and pants to prevent ticks from gaining access to our skin. But it can be very hard to keep ticks from crawling onto our horses and latching on. Several topical sprays used on dogs have been used on horses with mixed results, and the sprays are not approved for use on horses. There is also a canine vaccine against Lyme disease, and an equine vaccine is currently being researched, but it is not available at this time. Some veterinarians have used the canine vaccine in horses, but the efficacy of this practice is unknown. Our best hope is the development of an effective equine vaccine.
Divers, T.J. "Lyme Disease." Current Therapy in Equine Medicine 5. (Ed) N. Edward Robinson. Philadelphia, Saunders, 2003, pp. 54-55.
Divers, Thomas J., DVM, Dipl. ACVIM, personal communication.
Madigan, J.E. "Lyme Disease (Lyme Borreliosis) in Horses." Veterinary Clinics of North America. Vol. 9-2, Philadelphia, Saunders,1993: pp. 429-434.
See the Lyme Disease category under Infectious Diseases at www.TheHorse.com.
About the Author
Christina S. Cable, DVM, Dipl. ACVS, owns Early Winter Equine in Lansing, New York. The practice focuses on primary care of mares and foals and performance horse problems.
POLL: Mosquito-Borne Diseases