Merck Manual

Please confirm that you are not located inside the Russian Federation




Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Last full review/revision Jul 2019| Content last modified Jul 2019
Click here for the Professional Version
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Topic Resources

Leptospirosis is a potentially serious infection caused by Leptospira, which are spiral-shaped bacteria called spirochetes (see figure How Bacteria Shape Up).

  • Most people are infected through contact with contaminated soil or water during outdoor activities.

  • Fever, headache, and other symptoms occur in two phases, separated by a few days.

  • A severe, potentially fatal form damages many organs, including the liver and kidneys.

  • Detecting antibodies against the bacteria in blood or identifying the bacteria in a sample taken from infected tissue confirms the diagnosis.

  • Infections are treated with antibiotics and, if severe, sometimes with fluids given intravenously and dialysis.

(See also Overview of Bacteria.)

Leptospirosis occurs in many wild and domestic animals, including rats, dogs, and farm animals such as cattle, horses, sheep, goats and pigs. Some animals act as carriers and pass the bacteria in their urine. Others become ill and die. People acquire these infections directly through contact with infected animals or indirectly through soil or water contaminated by urine from an infected animal.

Leptospirosis is an occupational disease of farmers and sewer and slaughterhouse workers. However, in the United States, most people become infected during outdoor activities when they come in contact with contaminated soil or fresh water, particularly while swimming or wading. Outside the United States, outbreaks have occurred after heavy rainfall or freshwater flooding. Leptospira can survive for several weeks to months in sources of fresh water (such as lakes and ponds). However, they can survive for only a few hours in salt water.

The 40 to 100 infections reported every year in the United States occur mainly in the late summer and early fall. Because mild leptospirosis typically causes vague, flu-like symptoms that go away on their own, many infections are probably unreported.


In about 90% of infected people, leptospirosis symptoms are not serious. In the rest, the disorder involves many organs. This potentially fatal form of leptospirosis is called Weil syndrome.

Leptospirosis usually occurs in two phases:

  • First phase (septicemic phase): About 5 to 14 days after infection occurs, fever, headache, sore throat, severe muscle aches in the calves and back, and chills occur suddenly. The eyes usually become very red on the third or fourth day. Some people cough, occasionally bringing up blood, and have chest pain. Most people recover within about 1 week.

  • Second phase (immune phase): In some people, symptoms return a few days later. They result from inflammation caused by the immune system as it eliminates the bacteria from the body. The fever returns, and the tissues covering the brain and spinal cord (meninges) may become inflamed. This inflammation (meningitis) causes a stiff neck and headache.

If leptospirosis develops during pregnancy, the risk of miscarriage is increased.

Weil syndrome

Weil syndrome can occur during the second phase. It causes fever, jaundice (yellowish discoloration of the skin and whites of the eyes that is caused by liver damage), kidney failure, and a tendency to bleed. People may have nosebleeds or cough up blood, or bleeding may occur within tissues in the skin, lungs, and, less commonly, digestive tract. Anemia can develop. Although the liver and kidneys are the organs most commonly affected, the lungs and heart may also be severely affected.

People who do not develop jaundice recover. About 5 to 10% of people with jaundice die, and this percentage is higher in those over age 60. Risk of death is higher if changes in mental function, kidney failure, respiratory failure, and internal bleeding occur.


  • Culture of blood and urine samples or sometimes a sample of cerebrospinal fluid (obtained by spinal tap)

  • Blood tests for leptospirosis antibodies or genetic material

Doctors suspect leptospirosis when characteristic symptoms occur in people who have traveled to an area where an outbreak is occurring.

To confirm the diagnosis of leptospirosis, doctors take a sample of blood and urine. These samples are analyzed.

If people have symptoms of meningitis, doctors do a spinal tap (lumbar puncture) to obtain a sample of the fluid that surrounds the brain and spinal cord (cerebrospinal fluid).

Usually, several samples are taken over several weeks. These samples are sent to a laboratory to grow (culture) the bacteria.

Identifying the bacteria in cultures or, more commonly, detecting antibodies against the bacteria in blood confirms the diagnosis. The PCR technique, which produces many copies of a gene, may be used. It helps doctors diagnose leptospirosis quickly.


The antibiotic doxycycline can prevent leptospirosis. It is given by mouth once a week to people who are likely to be exposed to the bacteria—for example, people who live in or travel to an area where an outbreak of leptospirosis is occurring.


  • Antibiotics

  • For Weil syndrome, possibly blood transfusions and hemodialysis

Mild infections are treated with antibiotics, such as amoxicillin or doxycycline, given by mouth. For severe infections, antibiotics such as penicillin or ampicillin are given by vein (intravenously). Fluids containing salts may also be given.

People with the infection do not have to be isolated, but care must be taken when handling and disposing of their urine.

People with Weil syndrome may need blood transfusions, and if they have kidney failure, they may need hemodialysis.

More Information

Drugs Mentioned In This Article

Generic Name Select Brand Names
No US brand name
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Others also read

Also of Interest


View All
Overview of Human Immunodeficiency Virus (HIV)
Overview of Human Immunodeficiency Virus (HIV)
3D Models
View All
Cold Sore
3D Model
Cold Sore