Merck Manual

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Pet Owner Version

Infectious Diseases of the Urinary System in Horses


Peter D. Constable

, BVSc (Hons), MS, PhD, DACVIM, College of Veterinary Medicine, University of Illinois at Urbana-Champaign;

Thomas J. Divers

, DVM, DACVIM, DACVECC, Department of Clinical Sciences, College of Veterinary Medicine, Cornell University

Reviewed/Revised Aug 2019 | Modified Oct 2022

Infections of the urinary system are often caused by bacteria. The infection may develop on its own (for example, when bacteria enter the body through the urethra) or as the result of a problem in the urinary tract (for example, obstruction of the lower urinary tract by stones).

Several factors increase the risk of urinary system infection. These include problems with urine flow (especially not being able to empty the bladder completely during urination), overly dilute urine, sugar in the urine (often a sign of diabetes), older age, a weakened immune system, and the presence of other diseases. Healthy horses seem to be relatively resistant to urinary tract infections.

Bladder Infection in Horses

Infection and inflammation of the bladder caused by bacteria is called bacterial cystitis. In horses, cystitis is likely to be the result of an obstruction in the urinary tract or paralysis of the bladder (which may be the result of nerve damage). Cystitis can also occur in mares with chronic inflammation of the vagina.

Signs of cystitis include loss of control over urination, frequent urination, urine dribbling, urine scalding, and straining to urinate. There may also be blood in the urine. If nerve damage is the cause, other signs such as paralysis of the anus or tail may also occur.

A urine sample is needed to diagnose bacterial cystitis. Treatment includes antibiotics targeted to be effective against the bacteria causing infection, as well as identifying and treating any underlying causes for the cystitis.

Kidney Infection (Pyelonephritis) in Horses

Pyelonephritis is inflammation of the kidneys. This is usually caused by a bacterial infection in the urinary tract that climbed upwards into the bladder and then continued into the kidneys. The risk factors for pyelonephritis and those for bacterial cystitis are similar. Stones in the kidney or ureter, which prevent urine from flowing normally, are a common cause.

Signs of pyelonephritis include pain in the sides, especially in the area around the kidneys; fever; weight loss; and a general sense of not feeling well. Other signs include excessive thirst or excessive urination. Diagnosis of this condition requires blood and urine tests. Longterm cases may not show abnormalities on these tests. If this is the case, x-rays or ultrasonography may be needed. Treatment for pyelonephritis includes high dosages of antibiotics. In some cases, intravenous fluids or even surgery to remove the affected kidney may be required.

Interstitial Nephritis in Horses

Interstitial nephritis is another type of inflammation of the kidney and is usually the result of bacterial infection. Infectious diseases that affect the blood vessels can spread to become interstitial nephritis. As part of the immune response to these infections, collections of antibodies damage the kidneys and cause kidney failure. Antibiotics may be needed to treat the infection. Kidney failure, if it has occurred, may be treated with supportive treatment, including fluids.

Leptospirosis in Horses

Infection with Leptospira Leptospirosis in Horses Leptospirosis is a disease caused by a Leptospira bacteria; there are approximately 21 species divided into groups based on their propensity for causing disease. Because the organisms... read more bacteria is most commonly associated with eye infections (uveitis) and abortions in adult horses. However, in foals it can also lead to kidney failure. Infection can be effectively treated with appropriate antibiotics.

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