Forty cases have so far been confirmed in Central Kentucky this season, compared with only two at the same time last year.

The disease, normally spread through contact with the urine of infected wild animals, has caused most infected mares to lose their foals. Some domestic animals, such as cattle, can also transmitt the disease through their urine.

Leptosporosis can, in rare circumstances, be transmitted from animals to people.

Adult horses can suffer liver and kidney damage through the infection, for which there is no vaccine.

Authorities in Kentucky are urging horse owners to keep wildlife away from horse paddocks. They also suggest people keep their horses away from stagnant water which could be containing infected cattle urine.