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Convincing link between leptospiral infection and eye disease in horses

Image by Christel Sagniez

The important role that leptospiral infection plays in the development of equine recurrent uveitis in horses has been shown in a fresh study in Germany.

Various types of uveitis occur in horses, Tobias Geiger and his fellow researchers noted in the journal Veterinary Sciences.

Regardless of the cause, each acute uveitis episode requires well-aimed and meticulous conservative therapy. Whenever possible, the underlying disease must be treated.

In chronic uveitis, various therapeutic options have been described to prevent progressive damage to the internal structures of the eye and further relapses.

Typical equine recurrent uveitis (ERU) is characterized by painful recurrent inflammatory episodes of uveitis at unpredictable intervals.

Only recently it was found that biofilm formation by leptospira in the vitreous cavity leads to uveitis recurrences and prevents effective elimination of the infection by antibiotics or by the immune system.

Most eyes affected by ERU will lose vision over time unless further episodes can be prevented.

In about one-third of horses, both eyes are affected. Its incidence in Europe is reported to be 7 to 10%, representing a great economic burden. Studies from the United States show an even greater incidence of up to 25%.

“This high incidence in the US can be partially explained by the inconsistent use of the term ERU in the literature,” the authors noted. Often, horses with glaucoma and those with the insidious uveitis typical of leopard coat pattern horses are referred to as ERU. In these diseases, however, the recurrent episodes of uveitis typical of ERU do not occur.

The researchers said they used the term exclusively for the type of uveitis in horses associated with the typical recurrent and painful episodes of uveitis, as well as the typical eye changes associated with the disease.

The most effective treatment option for ERU is vitrectomy (irrigation of the vitreous cavity), which mechanically removes the leptospiral biofilm from the eye.

“This operation not only leads to the permanent absence of recurrence in up to 97% of the operated eyes but can also serve to permanently preserve vision in the affected equine eyes if performed early enough in the course of the disease,” they said.

In addition, vitrectomy allows examination of the removed vitreous material.

“Of course, vitrectomy is also associated with risks that can lead to blindness. However, when surgery is properly performed by an experienced equine ophthalmic surgeon and when equipment (devices and instruments) suitable for equine eyes is used, serious complications are very rare.”

In any case, they said, the risk of blindness is incomparably higher without vitrectomy, and a properly conducted vitrectomy shortens the suffering from a very painful eye disease.

This surgery has been performed in horses for more than 30 years, and thousands of specimens from within the eye have been analyzed for antibodies directed against leptospira and by molecular-based methods for leptospiral DNA.

In their study, the researchers examined the complete medical and laboratory records relating to 1800 eye specimens from horses treated between 2002 to 2017.

In all, 1387 specimens were from eyes affected by ERU, 237 specimens were from eyes affected with another type of uveitis, and 216 specimens were from healthy eyes.

In 83% of samples from eyes affected by ERU, antibodies against leptospira were detectable, with immunoglobulin A appearing to play an important role, they noted. In 72% of the intraocular specimens affected by ERU, leptospiral DNA was detectable by molecular-based methods.

No antibodies against leptospira were detectable in the samples from eyes with another type of uveitis or in the samples from healthy eyes. A positive test occurred in only one sample from a healthy eye.

“These results with a very high number of intraocular specimens demonstrate the great importance of an intraocular leptospiral infection for ERU,” they said.

The study, they said, convincingly demonstrates the role of chronic leptospiral infection in the eye in the development of the disease. Evidence of the infection is detectable in all stages of the disease as long as the eyes do not yet show significant atrophy or wasting. All specimens from diseased eyes in the study were from eyes that were still suitable for vitrectomy.

In the end stages of the disease, structures within the eye are altered, which may explain why some other studies examining the disease in its advanced stages did not find evidence of leptospiral infection.

The study team noted that, over time, the good prognosis of vitrectomy for long-term preservation of vision has led to most horses being taken for surgery at a relatively early stage of disease, which in turn improves the prognosis.

The researchers cautioned that, because of the unique characteristics of the eye and the ability of leptospires to form biofilm in a very short time, there are few initial immune responses and antibody detections, which are not always reliable. “Depending on the area in the vitreous from which samples are taken and where leptospiral biofilms are located, PCR (testing) may also be false negative.”

When the cause of uveitis is not clear, ophthalmological examination and laboratory testing with an aqueous humor sample should be considered, they said.

“In order to obtain the most reliable information and to reduce false-negative results, different test procedures should be used.”

In particular, enzyme-linked immunosorbent assay (ELISA) studies with detection of specific immunoglobulin A antibodies can significantly increase sensitivity in the examination of intraocular specimens, provided this test is available.

The study team comprised Geiger, with the University of Veterinary Medicine Hanover; Hartmut Gerhards and Bettina Wollanke, with Ludwig-Maximilians-University; Bogdan Bjelica, with Hanover Medical School; and Elke Mackenthun, with the Equine Clinic at the Racetrack, An der Rennbahn.

Geiger, T.; Gerhards, H.; Bjelica, B.; Mackenthun, E.; Wollanke, B. Analysis of 1840 Equine Intraocular Fluid Samples for the Presence of Anti-Leptospira Antibodies and Leptospiral DNA and the Correlation to Ophthalmologic Findings in Terms of Equine Recurrent Uveitis (ERU)—A Retrospective Study. Vet. Sci. 2022, 9, 448. https://doi.org/10.3390/vetsci9080448

The study, published under a Creative Commons Licensecan be read here.

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