acute systemic illness of animals, occasionally communicable to humans, that is characterized by extensive inflammation of the blood vessels (blood vessel). It is caused by a spirochete, or spiral-shaped bacterium, of the genus Leptospira.

Leptospires infect most mammals, particularly rodents and certain domestic animals. These animals excrete live, fully virulent organisms in their urine and contaminate the environment. Outside the animal body, leptospires can live for several weeks in fresh water. Thus, infection takes place by direct contact with urine of infected animals or by indirect contact with contaminated food or water. Leptospires can readily penetrate mucous membranes but probably cannot gain entrance to the body through intact skin. A scratch or abrasion, as well as the nasal mucosa and eye, are excellent portals of entry; thus, the origin of many infections can be traced to wading, swimming, or other contact with water containing virulent leptospires. The incidence in humans depends upon the opportunity for exposure in swimming, the harvesting of rice, and contact with animals.

Usually after an incubation period of about a week, the first symptoms to arise in humans are the abrupt onset of fever, chills, muscle aches, headache, abdominal pain, and vomiting. Another characteristic symptom is congestion of the conjunctival blood vessels around the corneas of the eyes. After a latent period of five to seven days, during which the infected person may improve, fever returns and the infection may involve the brain. In a less-common, but more-severe, form of the illness known as Weil disease or icteric (associated with jaundice) leptospirosis, the infected person turns yellow as a result of the destruction of red blood cells and liver disease. The death rate is approximately 30 percent of the severely ill and jaundiced patients.

Diagnosis is established by identification of the causative organism in urine or blood and by blood cultures on special media. antibiotic therapy is effective only if given within four days of the onset of the illness. Supportive care and the maintenance of fluid balance are needed because dehydration, cardiovascular collapse, and acute kidney failure can all occur.