Leptospirosis and Equine Recurrent Uveitis in Horses
From time-to-time, horse owners and veterinarians see equine recurrent uveitis, also known as periodic ophthalmia or moon blindness, in horses. Horse owners will see this as a painful eye. Painful eyes result from either damage or disease to the cornea or to the uvea (iris, ciliary body and choroid). The differential diagnosis includes traumatic injury, parasite infection by Onchocerca cervicalis, face fly irritation, and leptospira-induced uveitis. Shapiro and Stevenson recently reported equine abortions and recurrent uveitis associated with leptospirosis infections in horses in Eastern Ontario (1, 2).
Leptospires are motile bacteria called spirochetes. The leptospires are grouped into serovars and serogroups of which pomona, grippotyphosa, hardjo, bratislava, canicola, and icterohaemmorhagiae are the ones we are most interested in. Leptospires are very common in domestic and wild animals and can also infect humans. In particular regions, there will be one or more maintenance hosts which serve as the reservoir for the infection. Leptospires will be transmitted to incidental hosts by shedding into the environment from a maintenance host. They can invade the mucous membranes and/or damaged skin and migrate to various body organs of an incidental host (3). The maintenance hosts can be wildlife such as deer, raccoons, and rodents.
Researchers have believed that recurrent uveitis was an immune-mediated reaction and a sequella to a Leptospira pomona infection which occurred months or years previously (4). However, a recent German study isolated leptospires from the eyes of 27% of 130 affected horses. These isolates belong to the grippotyphosa serogroup (n = 31) and to the australis serogroup (n = 4). This is a remarkable achievement because of the difficulty of isolating this bacterium.(5)
Clinical signs will often disappear followed by recurrence. Severe attacks can lead to calcific band keratopathy (calcification of the cornea), permanent blindness, atrophy of the eye and/or glaucoma. The horse is presented with signs of a very painful eye - severe blepharospasm (muscle spasms closing the eyes), photophobia (light sensitivity) and lacrimation (tearing). The eye is often difficult to examine due to partial or complete closure. Practitioners will often use an auriculopalpebral nerve block to improve evaluation of the eye.
Pregnant mares can also have leptospires localize in the uterus/placenta and, subsequently, in the fetus, resulting in death of the fetus. Equine abortions are most frequently associated with L. pomona infection. The horse is probably an incidental host. L. bratislava is thought to be "host adapted" to the horse. In other words, L. bratislava can exist in the horse without causing disease and the horse is probably the maintenance host for the organism.
The diagnosis of leptospirosis will often be one of exclusion. Veterinarians will collect paired sera taken three weeks apart as an essential for diagnosis. However, the leptospira infection associated with the uveitis may have occurred many months previously. A fourfold change in titre to the organism in the paired sera would provide a presumptive diagnosis. The positive titres may be detectable for life (6). Titres to L. pomona and bratislava, in the recent cases of recurrent uveitis in Eastern Ontario, exceeded a 1:1,500 dilution. In one case, the titre for L. pomona was 1:20,480 (2).
Reported risk factors include flooding of pastures or drinking from contaminated water sources. In addition, the Appaloosa breed seems to have a greater predilection for uveitis and for severity of vision loss associated with the disease (6).
There are no leptospira vaccines approved for use in horses. Dr. Ann Dwyer (6) suggested that vaccines for cattle were not useful for horses and may be detrimental. Horses should be restricted from drinking from municipal drains and ponds as these are easily contaminated by wildlife.
Steroids, either topically or subconjunctivally, are often used to reduce the inflammation from this disease. Caution should be exercised, due to the possibility of corneal ulceration following their use. Atropine (1-4%) will dilate the iris and relieve the ciliary spasms, blepharospasm, lacrimation and photophobia. Cyclosporine may be used when corneal ulceration is present and steroids would be contra-indicated (7). Antibiotics have only been indicated when there was evidence of an acute infection or secondary bacterial infections (4). However, the demonstration, that living Leptospira are present in the eyes of affected horses, opens the way for studies documenting better ways to treat this stubborn disease (5).
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