Leptospirosis is a serious, zoonotic disease of worldwide significance that affects many animal species. It is caused by infection with different species of Leptospira, at least 4 of these can infect the dog.

Leptospira are transmitted between hosts through direct and indirect contact. Direct transmission occurs through contact with infected urine, venereal and placental transfer, bite wounds, or ingestion of infected tissues. Crowded conditions, as may occur in a kennel, facilitate the direct spread of the infection. Recovered dogs excrete microorganisms in the urine even for months after infection. Once outside the host, leptospira do not replicate. Indirect transmission occurs by exposing sensitive animals to sources of contaminated water, soil, food or litter. The ideal habitat for leptospira is warm and stagnant water, while temperatures under 0 °C kill them. Disease incidence or outbreaks increase in late summer and autumn.

Leptospira penetrate the organism through the mucous membranes of the mouth, nose, eyes or abraded skin. Organisms multiply rapidly after entering the circulatory system and, subsequently, they spread and replicate in many tissues, including kidneys, liver, spleen, central nervous system, eyes and genital tract.

Young animals are generally more severely infected than adults and large breed animals that live outdoors are more affected.