The Harry M. Zweig Memorial Fund for Equine Research

Vaccine Against Equine Leptospirosis

Dr. Yung-Fu Chang

ChangLeptospirosis is caused by a group of highly invasive spiral bacteria (spirochetes) that are capable of infecting people and animals, including horses. Transmission occurs either through direct contact with an infected animal or through indirect contact with soil or water contaminated with urine from infected animals. Infection of horses with Leptospira interrogans serovar Pomonatype Kennewicki causes uveitis (periodic ophthalmia), corneal opacity, abortion, and fever and icterus.

Currently, there is no vaccine against equine leptospirosis. Many veterinarians use commercially available vaccines for cattle. These vaccines contain heat or formalin-killed leptospires and produce only incomplete, short-term immunity.  The need for an equine vaccine against leptospirosis is highlighted by recent outbreaks of the disease in New York and other States (Missouri, Pennsylvania and Kentucky). Last year, an outbreak of equine abortion was reported in Kentucky where leptospirosis is the second most commonly identified infectious cause of abortion in mares (following EHV1). Leptospira induced abortions in Kentucky are important enough that the Equine Disease Quarterly has featured an article on the disease in 2004 and again in 2007. There were outbreaks of renal failure in yearlings (Kentucky) and weanlings (Texas) caused by leptospirosis last year. Publications from both University of California at Davis and University of Munich in Germany, suggest that in those areas, 50% of the cases of equine recurrent uveitis, are associated with persistent leptospira infections of the eye. This indicates that both equine abortion and uveitis caused by pathogenic Leptospira spp. are economically important to the equine industry.

Although there are several reports that horses can be naturally infected by Leptospira spp., there is no detailed information available about experimental induction of equine leptospiropsis.  This study will provide details of the primary symptoms associated with equine leptospiral infection. We hope that we will be able to induce clinical leptospirosis and therefore establish a challenge model in horses. This is very important especially for conducting equine leptospiral vaccine trials and antimicrobial treatments.

Three ponies will be subcutaneously and conjunctivally inoculated with 109 of L. interrogans serovar Pomona and one will be used as a control (inoculated with buffer solution only). The animals will be observed for clinical signs (temperature response, uveitis, and others).  Periodic blood, aqueous humor and urine tests will be used to monitor for infection of L. interrogans serovar Pomona.  Three months after infection, all animals will be euthanized and subjected to gross and histopathological examination.  Culture and DNA testing (PCR, Polymerase chain reaction) will be performed from various tissues obtained at the post mortem.

After this study, we will have a preliminary data for the resubmission of our proposal to establish an experimental induction of leptospiral disease model in ponies.  This challenge model will be used for an equine leptospiral vaccine trial.