Leptospirosis is a blood infection caused by bacteria of the genus Leptospira that can infect humans, dogs, rodents, and many other wild and domesticated animals. Signs and symptoms can range from none to mild (headaches, muscle pains, and fevers) to severe (bleeding in the lungs or meningitis). Weil's disease ( ), the acute, severe form of leptospirosis, causes the infected individual to become jaundiced (skin and eyes become yellow), develop kidney failure, and bleed. Bleeding from the lungs associated with leptospirosis is known as severe pulmonary haemorrhage syndrome.
More than 10 genetic types of Leptospira cause disease in humans. Both wild and domestic animals can spread the disease, most commonly rodents. The bacteria are spread to humans through animal urine or feces, or water or soil contaminated with animal urine and feces, coming into contact with the eyes, mouth, or nose, or breaks in the skin. In developing countries, the disease occurs most commonly in pest control, farmers, and low-income people who live in areas with poor sanitation. In developed countries, it occurs during heavy downpours and is a risk to pest controllers, sewage workers, and those involved in outdoor activities in warm and wet areas. Diagnosis is typically by testing for antibodies against the bacteria or finding bacterial DNA in the blood.
Efforts to prevent the disease include protective equipment to block contact when working with potentially infected animals, washing after contact, and reducing rodents in areas where people live and work. The antibiotic doxycycline is effective in preventing leptospirosis infection. Human vaccines are of limited usefulness; vaccines for other animals are more widely available. Treatment when infected is with antibiotics such as doxycycline, penicillin, or ceftriaxone. Mortality from severe disease forms, Weil's disease and severe pulmonary haemorrhage syndrome, is high (>10% and >50%, respectively) even when optimal treatment is provided. When the lungs are involved, the risk of death increases to the range of 50–70%.
Environmental and One Health considerations Leptospirosis is a known One Health issue because its transmission involves complex interactions among humans, animals, and the environment. Transmission commonly occurs through water when it is contaminated with urine from infected animals like rodents and livestock.
Environmental factors such as heavy rainfall, flooding, and poor sanitation significantly increased the risk of leptospirosis transmission by allowing the spread of contaminated water. Additionally, urbanization and inadequate waste management promote the growth of rodent populations, which act as primary reservoirs for the bacteria.
Climate change is associated with a higher incidence of leptospirosis because extreme weather events create a suitable environment for outbreaks. The One Health approach highlights the significance of coordinated efforts among human health, veterinary health, and environmental health, and environmental management sectors to prevent and control the disease.
Leptospirosis disproportionately affects populations in environments where exposure is unavoidable, particularly in regions with inadequate drainage, poor sanitation, and frequent flooding. In these environments, individuals are routinely exposed to contaminated soil and water, which increases infection risk. These patterns underscore the role of environmental conditions and structural inequalities in shaping disease distribution.
Beyond direct transmission from animal reservoirs, growing evidence indicates that Leptospira can persist in environmental reservoirs such as soil and freshwater systems, thereby extending the window of exposure beyond intermediate contamination events. Studies show that the organism survives for prolonged periods in moist environments, with factors such as temperature, pH, and nutrient availability affecting its viability. This persistence complicates traditional transmission models by enabling indirect exposure pathways that do not require recent animal contact.
Public health interventions and One Health implementation
Effective control of leptospirosis requires coordinated interventions across human, animal, and environmental health sectors. Public health strategies include surveillance systems to monitor outbreaks, particularly in regions prone to flooding and poor sanitation. Early diagnosis and improved laboratory capacity are essential for timely case identification and response.
Animal health interventions include vaccination of livestock and domestic animals, as well as rodent control programs aimed at reducing reservoir populations. In agricultural and occupational settings, the use of protective equipment can reduce exposure risk among workers.
Environmental interventions focus on improving sanitation infrastructure, ensuring safe water management, and implementing waste control measures to limit contamination of soil and water. Urban planning and drainage improvements are also important in reducing standing water that facilitates bacterial survival.
The One Health framework emphasizes interdisciplinary collaboration among healthcare providers, veterinarians, environmental scientists, and policymakers to improve prevention and outbreak response.
An estimated one million severe cases of leptospirosis in humans occur every year, causing about 58,900 deaths. The disease is most common in tropical areas of the world, but may occur anywhere. Outbreaks may arise after heavy rainfall. The disease was first described by physician Adolf Weil in 1886 in Germany. Infected animals may have no, mild, or severe symptoms. These may vary by the type of animal. In some animals, Leptospira live in the reproductive tract, leading to transmission during mating.
Signs and symptoms
The symptoms of leptospirosis usually appear one to two weeks after infection, but the incubation period can be as long as a month. The illness is biphasic in a majority of symptomatic cases. Symptoms of the first phase (acute or leptospiremic phase) last five to seven days. In the second phase (immune phase), the symptoms resolve as antibodies against the bacteria are produced. Additional symptoms develop in the second phase. The phases of illness may not be distinct, especially in patients with severe illness. About 90% of those infected experience mild symptoms, while 10% experience severe leptospirosis.
Leptospiral infection in humans causes a range of symptoms, though some infected persons may have none. The disease begins suddenly with fever accompanied by chills, intense headache, severe muscle aches, and abdominal pain. A headache brought on by leptospirosis causes throbbing pain and is characteristically located at the head's bilateral temporal or frontal regions. The person could also have pain behind the eyes and a sensitivity to light. Muscle pain usually involves the calf muscle and the lower back. The most characteristic feature of leptospirosis is the conjunctival suffusion (conjunctivitis without exudate), which is rarely found in other febrile illnesses. Other characteristic findings on the eye include subconjunctival bleeding and jaundice. A rash is rarely found in leptospirosis. When one is found, alternative diagnoses such as dengue fever and chikungunya fever should be considered. Dry cough is observed in 20–57% of people with leptospirosis. Thus, this clinical feature can mislead a doctor to diagnose the disease as a respiratory illness. Additionally, gastrointestinal symptoms such as nausea, vomiting, abdominal pain, and diarrhoea frequently occur. Vomiting and diarrhoea may contribute to dehydration. The abdominal pain can be due to acalculous cholecystitis or inflammation of the pancreas. Rarely, the lymph nodes, liver, and spleen may be enlarged and palpable.
Resolution of symptoms occurs for one to three days. The immune phase starts after this and can last from four to 30 days, and can vary from brain to kidney complications. The hallmark of the second phase is inflammation of the membranes covering the brain. Signs and symptoms of meningitis include severe headache and neck stiffness. Kidney involvement is associated with reduced or absent urine output.
The classic form of severe leptospirosis, known as Weil's disease, is characterised by liver damage (causing jaundice), kidney failure, and bleeding, which happens in 5–10% of those infected. Lung and brain damage can also occur. For those with signs of inflammation of membranes covering the brain and the brain itself, altered level of consciousness can happen. A variety of neurological problems such as paralysis of half of the body, complete inflammation of a whole horizontal section of spinal cord, and Guillain-Barré syndrome are the complications. Signs of bleeding such as petechiae, ecchymoses, nose bleeding, blackish stools due to bleeding in the stomach, vomiting blood, and bleeding from the lungs can also be found. Prolongation of prothrombin time in coagulation testing is associated with severe bleeding manifestations. However, low platelet count is not associated with severe bleeding. Pulmonary haemorrhage is alveolar haemorrhage (bleeding into the alveoli of the lungs) leading to massive coughing up of blood, and causing acute respiratory distress syndrome, where the risk of death is more than 50%. Rarely, inflammation of the heart muscles, inflammation of membranes covering the heart, abnormalities in the heart's natural pacemaker and abnormal heart rhythms may occur.
Cause Bacteria
Leptospirosis is caused by spirochaete bacteria that belong to the genus Leptospira, which are aerobic, right-handed helical, and 6–20 micrometers long. Like Gram-negative bacteria, Leptospira species have an outer membrane studded with lipopolysaccharide (LPS) on the surface, an inner membrane, and a layer of peptidoglycan in the cell wall. However, unlike Gram-negative bacteria, the peptidoglycan layer in Leptospira lies closer to the inner than the outer membrane. This results in a fluid outer membrane loosely associated with the cell wall. In addition, Leptospira bacteria have a flagellum located in the periplasm, associated with corkscrew-style movement. Chemoreceptors at the poles of the bacteria sense various substrates and change the direction of their movement. The bacteria are traditionally seen using dark-field microscopy without staining.
In total, 66 species of Leptospira have been identified. Based on their genomic sequence, they are divided into two clades and four subclades: P1, P2, S1, and S2. The 19 members of the P1 subclade include the eight species that can cause severe disease in humans: L. alexanderi, L. borgpetersenii, L. interrogans, L. kirschneri, L. mayottensis, L. noguchii, L. santarosai, and L. weilii. The P2 clade comprises 21 species that may cause mild disease in humans. The remaining 26 species comprise the S1 and S2 subclades, which include "saprophytes" known to consume decaying matter (saprotrophic nutrition). Pathogenic Leptospira species do not multiply in the environment. They require high humidity for survival, but can remain alive in environments such as stagnant water or contaminated soil. The bacteria can be killed by temperatures of and can be inactivated by 70% ethanol, 1% sodium hypochlorite, formaldehyde, detergents, and acids.