Last updated:
12/15/2025
Years published: 1987, 1989, 1996, 1997, 2005, 2009, 2025
NORD gratefully acknowledges Gioconda Alyea, MD (FMG), MS, National Organization for Rare Disorders, for the preparation of this report.
Summary
Leptospirosis is a bacterial infection that can affect both humans and animals. Although it is rare in United States, it is one of the most common zoonotic (passed from animals to humans) infections worldwide, especially in warm, tropical, or flood-prone regions.1-3
People usually become infected through contact with water or soil contaminated with urine from infected animals. Rodents, livestock, pets, and wildlife can all carry the bacteria without appearing sick.1-6
Symptoms range from very mild to severe and potentially life-threatening, but early treatment improves outcomes.1
Symptoms of leptospirosis vary broadly. Some people have no symptoms at all. Others develop a sudden flu-like illness.1-3 A small percentage of people develop life-threatening complications. The onset of the symptoms after getting infected (incubation period) ranges from 2 to 30 days, most commonly 5 to 14 days (the early septicemic phase, when Leptospira bacteria are actively multiplying in the blood).1
Leptospirosis appears in two main clinical forms:
Most people recover fully, sometimes after passing through two phases – an early or acute phase, followed by a delayed “immune” phase. However, many people only have the acute phase, and in some cases, the two phases blend together.
Symptoms often resemble the flu and may include:1-4
Less common symptoms which may include:
Leptospirosis during pregnancy can resemble other pregnancy-related conditions, which may delay diagnosis. Infection can pass to the fetus, but the exact risks are not well known. Reported cases show a high rate of pregnancy loss, and information about long-term outcomes in surviving infants is limited.
Current evidence suggests that people with weakened immune systems generally have a similar course and outcome as those with normal immune function. A small number of people develop symptoms again after initial recovery (relapsing illness).
Leptospirosis is an infectious disease caused by the bacteria called Leptospira, which are tiny, spiral-shaped organisms that move easily through water and tissues. They are hard to see with regular lab stains, so special microscopes are used to identify them (using techniques like dark field or fluorescent microscopy). They have a curved “question mark” shape at one end, which helps distinguish them from other similar bacteria. It survives in fresh water or moist soil for weeks to months.1, 2
These bacteria live in the kidneys of infected animals. They are shed into the environment through urine, especially into fresh water or wet soil, where they can survive for weeks to months. Humans become infected when bacteria enter through broken skin or mucous membranes (eyes, mouth, nose). After entering a host through a mucous membrane or damaged skin, the bacteria circulate and multiply in organs before being cleared from the blood. Humans can also shed the bacteria in urine but do not typically become long-term carriers. 1, 7
Animals such as cattle, pigs, horses, dogs, rodents, and even marine mammals can shed the bacteria in their urine without appearing sick.1-5
Serovars (subtypes)
Leptospira bacteria come in many subtypes called serovars. These serovars differ based on the proteins or sugars on their outer surface. These surface markers are what the immune system recognizes. Different serovars can behave differently in the body, may be found in different animals, and may be more common in certain regions. Many experts still use the serovar naming system, e.g. Leptospira interrogans serovar Autumnalis.
Leptospiras are part of the Spirochaetales order and the Leptospiraceae family, and for many years they were grouped by the surface proteins on their outer layer, which allowed labs to sort them into “serovars” within each species. Under this system, the genus was divided into two main species, Leptospira interrogans, which causes disease and includes around 218 serovars, and Leptospira biflexa, a harmless, free-living species with at least 60 serovars. Leptospira interrogans and L. borgpetersenii are the two most common species causing diseases in both humans and animals.
Newer DNA-based testing has revealed that at least seven different species can cause illness. However, these results do not always line up with older testing methods. Bacteria that look identical under traditional tests can turn out to be genetically different, and bacteria that are genetically similar can appear different when tested using older serology (the scientific study of blood serum) methods. Because of this mismatch, many experts still rely on the traditional serovar system when diagnosing infections and tracking how leptospirosis spreads in communities and, usually, the recommendation for Leptospira nomenclature is using species name followed by the term ‘serovar’ and serovar name with initial capital letter and non-italic, e.g. Leptospira interrogans serovar Autumnalis.1-3, 6
Leptospirosis occurs worldwide but is most common in tropical and subtropical regions.1–3 In the United States, it is rare with about 100 to 200 cases are reported each year, with higher rates in Hawaii and in cities with heavy rodent (rat or mice) infestations.1,2 Outbreaks often follow flooding, hurricanes, or heavy rainfall because people are more likely to come into contact with contaminated water.1,3
Global estimates suggest up to 1 million cases and nearly 60,000 deaths annually,1 while the World Health Organization (WHO) reports about 873,000 cases and 40,000 deaths each year.2 Despite its impact, leptospirosis is still not included on the World Health Organization’s official list of neglected tropical diseases.7
Leptospirosis infection is increasing in developing regions because of population growth, rapid urbanization (crowded cities), poor sanitation, and climate change create conditions where bacteria can spread more easily. Rodents (especially rats and mice) are the main carriers and shed the bacteria through their urine, contaminating water and soil. People become infected through contact with this contaminated water, soil, or infected animals. In tropical climates, cases occur year-round and rise during rainy seasons.
Groups at highest risk include farmers, veterinarians, meat processing workers, people in crowded urban areas, and anyone exposed to freshwater (rivers, lakes, floodwaters) through work or recreation.
People living in crowded cities with poor sanitation and many rodents have a higher risk of getting leptospirosis. Studies show that areas with fewer basic services tend to have more rodents that live close to humans and carry more diseases. 5
Diagnosing leptospirosis depends on when symptoms begin and what tests are available. Doctors often start by ordering blood tests, including:
Doctors may also order tests to look for complications affecting different organs. These include:
Because leptospirosis can worsen quickly and early treatment makes a difference, doctors are advised not to delay starting antibiotics when they strongly suspect the infection, even if test results are still pending.1, 3
Treatment
Treatment for leptospirosis depends on how severe the illness is. For people with a mild form of disease, doctors often prescribe antibiotics such as doxycycline, amoxicillin, ampicillin, or azithromycin.1, 3 Antibiotics are medicines that kill bacteria or stop them from multiplying. Even though mild illness may improve on its own, antibiotics can shorten how long symptoms last and can reduce how long the bacteria remain in the urine, which lowers the chances of spreading the infection.3
In more serious cases, people may need stronger treatment in a hospital. The main medicine used for severe leptospirosis is intravenous (IV) penicillin G.1, 3 “Intravenous” means the medication is given through a vein using a small tube called an IV line, so the medicine reaches the bloodstream quickly. If someone cannot receive penicillin, or if another medicine is more appropriate, doctors may use other intravenous antibiotics such as ceftriaxone or other third-generation cephalosporins. These are powerful antibiotics designed to treat serious bacterial infections and work well against Leptospira.
Severe leptospirosis can affect the kidneys, liver, lungs, and sometimes the brain. When the kidneys are failing, patients may need dialysis, which is a medical treatment that filters waste and excess fluid from the blood when the kidneys can no longer do this on their own.3
If the lungs are affected and breathing becomes difficult, mechanical ventilation may be needed. Mechanical ventilation is a form of life support in which a machine helps move air in and out of the lungs to ensure the body receives enough oxygen.1, 3
People who have severe complications generally need close monitoring in an intensive care unit (ICU). This is a specialized hospital area where nurses and doctors can closely watch vital signs, adjust treatments quickly, and respond to any sudden changes in the person’s condition.
Early treatment is important in all forms of leptospirosis because it reduces the risk of complications and speeds up recovery.1 If doctors suspect leptospirosis based on symptoms and exposure history, they usually begin treatment right away without waiting for test results, since delaying therapy can worsen the outcome.3
The leptospirosis vaccine is a canine vaccine that protects dogs from the Leptospira bacteria, which can cause severe organ damage and can be transmitted to humans. Only a few countries have approved a human leptospirosis vaccine, but it is not available in the United States. Also, the human leptospirosis vaccines that exist mainly protect against rodent-linked serogroups.
For now, prevention relies on exposure control through behavior changes and personal protective measures:
Prophylactic antibiotics (medications administered to prevent infections from occurring) have been used, but research studies trials have not confirmed that they work.9
Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. government funding, and some supported by private industry, are posted on this government web site.
For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:
Tollfree: (800) 411-1222
TTY: (866) 411-1010
Email: [email protected]
For information about clinical trials sponsored by private sources, contact:
www.centerwatch.com.
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