Leptospirosis and Moon Blindness

By Heather Smith Thomas

“Moon blindness” (recurrent uveitis) has been linked with leptospira infections. Dr. Ann Dwyer, a private practitioner (Genesee Valley Equine Clinic, Scottsville, NY), has worked with many cases of leptospirosis and uveitis, and has been involved in research looking at these diseases. When horses are acutely exposed to pathogenic (disease-causing) strains of leptospirosis, the disease is generally mild and may not be noticed.

“The few times I have diagnosed the acute disease, the horse is just a little bit off, and may have had fever for a few days, but is not very ill. If the veterinarian happens to do a blood test, he/she might find that the horse is mildly anemic, with some alteration in the CBC, but nothing to be alarmed about, and nothing that would point toward leptospirosis,” says Dwyer. The few horses she has diagnosed with this disease have recovered rather quickly.

A person generally doesn’t have the opportunity to treat a horse when it is initially ill, because usually the disease is not recognized as leptospirosis and the horses tend to recover without treatment. By the time a horse develops recurrent uveitis, the initial illness is long past.

The ocular disease occurs many months later. Regarding the role leptospirosis plays, research over the past 2 decades points toward an immune reaction. “The pathogens may not persist in the eye, in most cases, but they trigger the immune problem. Once the horse starts showing signs of uveitis, none of us have much success in treating it with antibiotic therapy, but many veterinarians use antibiotics for a few weeks anyway. You try to do something for the horse—and hope that maybe this might be one that treatment could help,” she says.

“Doxycycline, an antibiotic related to tetracycline, is often given for a few weeks to horses experiencing their first or second bout of uveitis, and it may mediate signs of disease because it mediates inflammation. Recurrent uveitis is a life sentence for the horse. Once you’ve made the diagnosis you might as well try something,” she says.

There are some better treatments for the affected eye today than what we had 40 years ago. “There is more awareness among horse owners about this disease. They may start some anti-inflammatory therapy if the horse has a bout with uveitis. Early intervention may ward off the blinding effects for that particular bout. Still, if the horse keeps having bout after bout, with progressive damage to the eye, blindness may eventually occur,” says Dwyer.

“Statistics from the cases I looked at here in New York indicate that the blindness rate—if you followed those horses for many years—wasn’t much different whether the owners were very diligent and treated the horse in every episode or whether the owners were casual and treated very few episodes. These horses all had about the same eventual outcome, with about 60% losing vision,” she says.

“It is worth recommending to people, if they can afford it, to have the suprachoroidal cyclosporine implant surgery that Dr. Brian Gilger of North Carolina State University developed. It is a useful aid to try to minimize severity of the recurrent attacks, but I don’t know if anyone has ever done an assessment of the lepto-associated uveitus cases versus the horses that have uveitis for some other reason. We don’t know whether one group does better with the cyclosporine than the other,” she says. But as a whole, horses suffering from recurrent uveitis have a better visual outcome if they have the implant.

“I am glad the suprachoroidal implant was developed as a surgical option for treatment, but it’s still not the whole answer. A number of horses that receive the implant still have trouble later or they get the implant late in the course of their disease and it only helps a little,” she says.