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The disease is caused by the organisms collectively referred to as Leptospira hardjo (L. borgpetersenii serovar hardjo and L. interrogans serovar hardjo).

After infection these bacteria localise in the reproductive tract and in the kidneys. The agent is passed in urine and infection spreads when cattle are exposed to urine from infected animals.

As with other diseases included in the PCHS, disease is nearly always introduced into the herd by the purchase of infected cattle. Infection may also be introduced by contaminated water supplies.

L. hardjo can be maintained in sheep but does not appear to cause disease in sheep; however sheep may be a source of infection for cattle. Of more importance is the risk of infection in man where infection causes flu-like symptoms and severe headaches.

Dairymen working in the parlour are most at risk of exposure to infected urine. Herd owners must therefore be aware of their responsibilities under the COSHH regulations.

Confirmation of infection in premature or stillborn calves is difficult, but evidence of infection in the herd can be gained by blood sampling the cows. In the scheme the ELISA test is used to detect antibody in the blood or milk.

Some cows only test positive for a short time after infection. Another problem lies in the fact that other types of Leptospira organisms can infect cattle and result in a positive blood test while causing no disease.

These two features mean that a test and cull policy is not a reliable option for control. But where testing in a herd shows no evidence of disease, breeding stock from a herd can be considered to be free of infection and be purchased safely.

It is for this reason that some herds may wish to pursue accreditation of freedom from disease or monitor for the presence of disease. Where a herd test shows evidence of infection some owners may wish to enter a monitoring programme to help them manage the disease.

Alternative options for a herd with evidence of active infection include vaccination and medication.