Lameness often comes and goes, shifts from one leg to another, and appears uninfluenced by the horses’ activity levels. Horses may show a rapid decline in performance, such as when racing or showjumping. Laminitis is another feature of Lyme disease in horses, along with anterior uveitis, an inflammatory eye condition sometimes referred to as moonblindness.
Symptoms of Lyme disease in horses can be quite variable however, making the illness difficult to spot, particularly where it is unexpected. Horses with Lyme disease may experience problems with multiple organ systems leading to both acute problems and possible permanent damage and chronic health issues, especially where the infection remains untreated. Lyme disease complications in horses can include liver damage and hepatitis, or severe neurological injury from encephalitis, resulting in ataxia, and/or behavioural changes.
Diagnosing Lyme Disease in Horses
There is a striking variance in the symptoms suffered by individual horses infected with Lyme disease, with some able to tolerate infection without showing signs of illness, and others becoming severely ill. Poor recognition of the infection in horses means that there is little available research material to guide veterinarians. Diagnosis of Lyme disease in horses is further complicated by problems detecting the spirochaetal bacteria, Borrelia burgdorferi, responsible for Lyme disease in samples taken from horses.
Veterinarians will usually apply a process of elimination when investigating symptoms common to Lyme disease and other illnesses in horses. Blood tests, such as ELISA and Western blot, are carried out to check for antibodies to Lyme disease bacteria, although the accuracy of these Lyme disease tests has been questioned, especially in cases of early Lyme disease where insufficient antibodies have built up to trigger detection.
Even if antibodies are detected in a horse this still does not mean that clinical signs are due to Lyme disease. Vaccinated horses, as well as those exposed to Lyme disease bacteria but not suffering from it, will show some level of antibodies to the spirochaetes. In Lyme-endemic areas there may be as many as 75% of horses who will test positive for antibodies to Borrelia burgdorferi, but the majority of these horses show no signs of illness. Where a veterinarian can find no other cause of the symptoms they will usually administer antibiotics to treat a suspected infection with Lyme disease in a horse. Further blood tests help determine the success of treatment at eradicating the infection.
Lyme Disease Vaccination for Horses
Vaccination of horses against Lyme disease is not currently standard procedure, nor is there an available vaccine approved by the USDA for use in horses. Some horse-owners use the Lyme disease vaccines for dogs, despite a paucity of evidence showing this to be safe or helpful. Indeed, there are some reports of horses becoming clinically ill after vaccination with a Lyme disease vaccine for dogs. The use of such vaccines also makes diagnosis more difficult, due to confusion over the presence of antibodies to Borrelia, should an owner or veterinarian suspect that a horse has been infected with Lyme disease. Most veterinarians recommend the use of insecticides to protect horses and minimize tick-exposure.
In delaying diagnosis of Lyme disease in horses whilst test results are returned the primary window for treating the infection may be missed. Early symptoms are also often put down to exhaustion from work, arthritis, or muscle strain from over-activity. Horses out to pasture have lower detection rates for early infection compared to those in active duty. However, active horses, such as show-jumpers, dressage, or race horses, are more likely to experience misdiagnosis rather than late diagnosis.
Treating Lyme Disease in Horses
Lyme disease treatment for horses is similar to treatments for Lyme disease in humans or dogs, namely antibiotics such as tetracycline or doxycycline (intravenously and orally, respectively). One study of Lyme disease using ponies deliberately infected with Borrelia bacteria showed that tetracycline treatment appeared most effective in eradicating infection. A four-week course of tetracycline, doxycycline, or Ceftiofur resulted in a drop in antibodies in all groups but then a subsequent rise in antibody levels after three months in 75% of the ponies treated with doxycycline and 50% of the Ceftiofur group. Where the antibodies did not begin to rise again after treatment there was no sign of infection with Lyme disease upon autopsies of the ponies.
Tetracycline appears then to be the best option for treating Lyme disease in horses but, unfortunately, the drug is highly damaging to tissues if it leaves the bloodstream. Oral doxycycline is easier to give, therefore, but a course longer than four weeks may be needed. In some cases, particularly where diagnosis is delayed, persistent symptoms of Lyme disease may arise. Even where the infection appears to have been cleared there may still be signs of Lyme disease in horses. Reports of such occurrences lend weight to the argument against Chronic Lyme disease symptoms simply being an ‘imaginary’ syndrome dreamt up by sufferers and disregarded by many. Autoimmune dysfunction may be at the root of some of these symptoms, perhaps connected to the ability of Borrelia bacteria to evade the immune system in some capacity. Such symptoms can also be the result of recurrent infection, permanent damage to tissues induced by the infection, or even persistent infection which is poorly detected by diagnostic tests such as ELISA and Western blot.
New Lyme Disease Test for Horses
A study published in July 2011 looked at antibody profiling for Borrelia in horses with a focus on those living in the New England and Mid-Atlantic regions of the US. Infection with Borrelia burgdorferi is common in such horse populations and the researchers used these animals to assess the efficacy of Luciferase Immunoprecipitation Systems (LIPS) for detecting antibody responses to three antigens used in diagnosing equine Lyme disease. LIPS testing verified that 75% of the horses were seropositive against the synthetic antigen used (VOVO), which was checked against an immunofluorescence assay (IFA) showing 51% to be infected. Just 15% showed seronegativity on both tests however, which strongly suggests that infection is endemic in horses in these regions.
In this study the use of more specific antigens led to higher sensitivity when testing horses for Lyme disease making it possible that commercially available tests for Lyme disease in horses might soon be available. Considering the difficulties in diagnosing and treating the infection and symptoms, as well as the expense of veterinary care for horses, more accurate testing of Lyme disease in horses, such as the new LIPS testing is likely to be warmly welcomed by horse-owners across the US.
Burbelo PD, Bren KE, Ching KH, Coleman A, Yang X, Kariu T, Iadarola MJ, Pal U., Antibody profiling Borrelia burgdorferi infection in horses. Clin Vaccine Immunol. 2011 Jul 20.