Leptospirosis - Horses

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Introduction

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 reaction between antibodies against leptospira and ocular tissues. There is now additional evidence that live leptospira persist in the ocular fluids, providing continuous antigetic stimulation resulting in high antibodies titres in aqueous humour in comparison to the serum, suggesting that the eye is an immune privileged site.

Signalment

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

Clinical signs

Infection with pathogenic Leptospira species 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:

Ophthalmic: The initial signs are blephrospasm, increased lacrimation, photophobia and corneal oedema, with progression to retinal detachment, synechia formation, and cataracts. Chronic leptospirosis causes recurrent uveitis, 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 be 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 monoclonal antibody test (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

Treatment should be directed towards the specific organ system affected.

Systemic antibiotics are indicated for pyrexic horses and those with acute renal failure. Appropriate antibiotics include pencillins, cephalosporins, enrofloxacin and tetracyclines. IV fluid therapy is also warranted as supportive therapy in cases of acute renal failure.

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.

Prognosis

Prognosis depends on the severity of the disease and the degree of organ involvement. In ophthalmic cases recovery may be incomplete, alternating between times of acute disease and symptom-free periods. In cases of abortion the mare will usually recover without complications. Prognosis is guarded for the renal, hepatic and neonatal syndromes in acute and severe cases.


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References

  • Brown, C.M, Bertone, J.J. (2002) The 5-Minute Veterinary Consult- Equine', Lippincott, Williams & Wilkins
  • Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial
  • Robinson, N.E., Sprayberry, K.A. (2009) Current Therapy in Equine Medicine (Sixth Edition) Saunders Elsevier




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