A wide variety of clinical signs have been attributed to B. burgdorferi infection in horses including uveitis and encephalitis. More commonly mild fever, stiffness, lameness, sensitivity around the joints and muscles, and behavioral changes have been attributed to infection. It is known that horses contract the Lyme organism which then reproduces in the horse and in some cases persist in the tissues. As a result horses in endemic areas are frequently seropositive for B. burgdorferi. Beyond these facts we start to have problems with attributing signs of disease to this infection.

Though horses become infected it has been hard to find clear evidence of symptoms of disease despite repeated attempts on dozens of horses, some given immunocompromising drugs prior to infection. The finding of lesions is troubling and suggests that there is a potential for disease however multiple large epidemiological studies where the history of disease from both sero-positive and sero-negative horses is compared there has not been any clear indication that the sero-positive horses have had an increase incidence of disease. The number of horses in these studies now is up into the thousands. This throws into question of whether B. burgdorferi causes disease in horses or not. We can also posit that though experimentally it has been difficult to produce disease in horses or demonstrate a epidemiological association or seropositivity and disease that some horses react different to the infection. Two possibilities is a immune system that does not react well to the organism allowing more serious disease or an overactive immune system that produces a more severe inflammatory response and clinical disease.

There have been some cases where disease attributable to Borrelia has actually been caused by Anaplasma phagocytophilia (Ehrlicia equi) https://horseadvice.com/horse/messages/4/5384.html. A. phagocytophilia is also a tick borne pathogen who's range is approximately the same as B. burgdorferi and found in a significant portion of Lyme positive horses and sensitive to the tetracyclines, ...more.

Until clear evidence surfaces that Lyme infection causes clinical disease in horses, diagnosis and treatment remains speculative. At best it appears to be a very rare cause of disease in horses, if it occurs at all. In spite of the speculative nature many horses are being diagnosed and treated for Lyme disease, some who get better, so let's cover what is known about this disease with an emphasis on the horse.