Leptospirosis - Horses

Description

Leptospirosis is a disease caused by the bacteria Leptospira interrogans. It affects domestic species, wildlife and has zoonotic potential. There are several different serovars which vary in pathogenesis and their host specificity. Most infections result in a self limiting pyrexia and anorexia, however occasionally the condition is more serious causing recurrent uveitis, abortion, or renal and hepatic disease. Recurrent uveitis is the most important clinical syndrome associated with Leptospirosis. The association has been well recognised for many years, originally thought to be an immune-mediated disease involving a cross rection between antibodies against leptospira and ocular tissues. There is now additional evidence that live leptospira persist in the ocular fluids, providing continuous antigentic stimulation resulting in high antibodies titres in aqueous humour in comparision to the the serum, suggesting that the eye is an immune privilegded site.

Signalment

Exposure to the disease is worldwide, with the predominent serovar varying with location. Serological studies show that exposure is high but clinical disease is low.

Diagnosis

Clinical signs

Infection with pathogenic Leptospira spp cause a bacteraemia with the following clinical signs

• Pyrexia
• Depression
• Lethargy
• Anorexia

The disease can then invade a specific organ system resulting in associated clinical signs:

Opthalmic: The initial signs are blephrospasm, increased lacrimation, photophobia and corneal oedema, with progression to retinal detachment, synechia formation, and cataracts. Chronic leptospirosis causes recurrent uvietis, typically signs develop 2-8 months after initial infection. Up to 67% of recurrent uveitis cases are due to leptospirosis.

Reproductive: Leptospirosis can cause placentitis, abortion, still birth or neonatal disease depending on the serovar and the stage of gestation when infected. Approximately 3-4% of equine abortions are caused by Leptospirosis, and are most frequently due to serovar Pomona, or occasionally Harjo. Abortion is usually late term, at 9 months of gestation.

Renal: leptospirosis rarely causes acute renal failure, signs include polyuria/polydipsia, azotaemia, pyuria and haematuria.

Hepatic: Horses with the hepatic form of the disease will bei icteric, pyrexic and lethargic.

Horses with subclinical infections or those in the carrier state are asymptomatic.

Laboratory Tests

Culture of Leptospira is challenging but can be attempted from urine, blood and aqueous humour in antemortem diagnosis, or from liver, kidney, fetus or placenta in post mortem diagnosis.

A MAT or ELISA on serum is a more sensitive diagnostic method. Paired titres 4 weeks apart are optimal.

Ultrasound

Abdominal ultrasound is useful in assessing the degree of damage in the hepatic and renal forms of the disease.

Treatment

Treatement should be directed towards the specific organ system affected.

Horses which are acutely affected and those aborting due to leptospirosis should be isolated for 14-16 weeks. After this time urine should be tested to determine whether the horse is shedding the organism.

There is no safe vaccine approved for protection against leptosporosis in horses.

References

• Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial