Leptospirosis is caused by spiral-shaped bacteria called spirochetes, specifically leptospires, that enter an animal’s body through mucous membranes in areas such as the nostrils, lips, eyes, trachea, stomach, genitals, or anus, or through broken skin. In addition to mammals (horses, humans, squirrels, voles, and scores more), amphibians (such as frogs) and reptiles (including snakes) can become infected with and pass on the disease-causing bacteria. In horses, foals can become infected in utero.Leptospires most commonly live and multiply in the renal tubules (where urine collects in the kidneys) of reservoir or carrier hosts such as rodents, wildlife, and domestic animals. In addition to spreading in urine, leptospires can be transmitted via infected blood or tissues or by infected urine splashing into eyes or the mouth.When an infected reservoir host urinates, the leptospires pass out of its body in the urine and contaminate the surrounding ground and/or water. In fact, exposure to standing water, such as ponds or floodwaters, is the biggest risk factor for leptospirosis infection. Occasionally, says Carter, animals (horses, livestock, etc.) inhale leptospires, ingest them with feed, or transmit them via wounds or bites.