Leptospirosis is a zoonotic disease, meaning it is an infection that spreads from animals to people. It is found all over the world but is most common in tropical and subtropical regions with a warm, humid climate and high annual rainfall. It is considered one of the most widespread zoonotic diseases globally.

The illness is known for having a biphasic (two-phase) presentation in many patients.

1. Phase 1 (The Septicemic Phase): This is the initial, acute phase of the illness. During this time, the bacteria are present in the bloodstream, and the patient experiences an abrupt onset of flu-like symptoms.
2. Phase 2 (The Immune Phase): After a brief improvement, some patients develop a more severe second phase as the immune system mounts a powerful response. This is when severe complications can occur.

The most severe form of leptospirosis is known as Weil’s disease. This is characterized by a classic triad of jaundice (yellow skin and eyes), acute kidney failure, and a tendency for bleeding. Not all leptospirosis patients will develop Weil’s disease, but for those who do, it is a medical emergency that requires intensive hospital care.

In my experience, leptospirosis is often overlooked because it mimics so many common illnesses. But in areas with flooding or poor sanitation, I always keep it in mind, especially when a patient has fever plus muscle pain or jaundice.

Leptospirosis is caused by infection with bacteria from the genus Leptospira. These are spirochetes, which are spiral-shaped, highly motile bacteria. Their unique corkscrew-like shape and motility allow them to burrow effectively into tissues and navigate through viscous environments like mud and soil.

These bacteria are survivors. They can persist for weeks or even months in freshwater and moist soil, provided conditions are favorable (warm and not too acidic). This environmental resilience is key to their transmission from animal reservoirs to human hosts. When a person is infected, the bacteria multiply rapidly in the bloodstream and then spread to various organs, particularly the kidneys and the liver, where they cause inflammation and damage.

Many patients are surprised to learn that a seemingly harmless walk through flood water or swimming in a river can lead to leptospirosis. It’s a classic example of how environment and hygiene intersect with infectious disease.

Leptospirosis is transmitted through a complex cycle involving animals, the environment, and humans. It is not spread from person to person, except in very rare circumstances.

The Animal Reservoir

The bacteria are maintained in nature in the kidneys of various wild and domestic animals. These animals act as the reservoir hosts. Common carriers include:

• Rodents (especially rats)
• Cattle, buffalo, and other livestock
• Dogs
• Pigs
• Sheep and goats

Many of these animals can carry and shed the bacteria in their urine for their entire lives without showing any signs of illness themselves.

Environmental Contamination

When an infected animal urinates, it releases a large number of Leptospira bacteria into the environment. This contaminated urine seeps into the soil and water sources, such as rivers, lakes, streams, and, most importantly, floodwaters.

Human Transmission

A person becomes infected through direct or indirect contact with the urine of an infected animal or with a contaminated environment. The bacteria typically enter the body through one of the following routes:

• Through breaks in the skin, such as cuts, scrapes, or sores.
• Through the mucous membranes of the eyes, nose, or mouth.
• By drinking or swallowing contaminated water.

High-Risk Activities and Exposures

The risk of exposure is highest for individuals whose work or recreational activities bring them into contact with potentially contaminated water or soil.

• Wading, swimming, or playing in floodwaters following heavy rains or monsoons is a major risk factor for outbreaks.
• Occupational risks are high for farmers (especially rice paddy workers), sanitation workers, sewer workers, veterinarians, and slaughterhouse workers.
• Recreational risks include kayaking, rafting, or swimming in contaminated lakes or rivers.

Clinically, I associate leptospirosis with occupations or lifestyles that involve water or animals. I always ask patients about recent travel, outdoor activity, or flood exposure when their symptoms don’t add up.

Leptospirosis is known as a “great imitator” because its symptoms are highly variable and can mimic many other common infectious diseases. The incubation period is typically 7 to 14 days after exposure.

Phase 1: The Acute or Septicemic Phase

This phase usually lasts about 5 to 7 days and is characterized by the sudden onset of symptoms.

• High fever and shaking chills.
• Severe headache.
• Intense muscle aches (myalgia): This is a very characteristic symptom. Pain is often severe in the calves and the lower back.
• Conjunctival Suffusion: This is another classic sign. It is a redness of the whites of the eyes caused by dilated blood vessels, but importantly, it occurs without any pus or discharge.
• Nausea, vomiting, and diarrhea.
• A skin rash can sometimes occur.

After this first phase, many patients will recover completely. However, some will experience a brief improvement for a few days before progressing to the more severe second phase.

Phase 2: The Immune Phase (Weil’s Disease)

This phase occurs in a minority of patients (about 5-15%) but is much more severe. Symptoms are caused by the body’s immune response to the lingering infection.

• Jaundice (yellowing of the skin and eyes) due to severe liver involvement.
• Acute Kidney Injury / Kidney Failure, leading to a decrease in urine output.
• Bleeding: A tendency for abnormal bleeding, such as nosebleeds, coughing up blood, or bruising easily.
• Pulmonary Hemorrhage: This is the most feared and often fatal complication, involving severe bleeding into the lungs, leading to respiratory failure.
• Meningitis: Inflammation of the brain lining, causing severe headache and stiff neck.
• Myocarditis: Inflammation of the heart muscle.

What often tips me off is the muscle pain pattern, severe calf or lower back pain isn’t common in most viral illnesses, but it’s classic for leptospirosis. Add conjunctival redness, and I know to test for it.

Diagnosing leptospirosis can be very difficult, especially in the early stages, because its flu-like symptoms are so similar to those of other common tropical infections like dengue fever, malaria, and typhoid.

• Clinical Suspicion: The most important first step to diagnosis is a high index of doctor suspicion. This is based on the patient’s symptoms (especially the combination of high fever, severe muscle aches, and red eyes) and, crucially, a history of potential exposure, such as recent contact with floodwater or animals.
• Laboratory Tests: The diagnosis is confirmed with specific laboratory tests.
  • Serology (Antibody Tests): This is the most common method. A blood test is performed to look for antibodies the body has made against the Leptospira bacteria. A diagnosis is often confirmed by showing a significant rise in the antibody level between an initial and a later sample.
  • Polymerase Chain Reaction (PCR): This is a molecular test that can detect the DNA of the Leptospira bacteria directly in the blood (in the first week of illness) or in the urine (after the first week).
• Other Supportive Tests: Doctors will also perform a complete blood count and tests to assess the function of the kidneys and liver to determine the severity of the illness.

When I see a patient with unexplained fever, muscle pain, and recent flood exposure or travel to endemic areas, I don’t wait, I test early. Delayed diagnosis can mean missed treatment opportunities.

Treatment

Leptospirosis is treatable with antibiotics, especially when started early.

• For Mild Disease: The treatment of choice is an oral antibiotic, most commonly doxycycline.
• For Severe Disease (Weil’s Disease): Patients require hospitalization for treatment with intravenous (IV) antibiotics, such as penicillin or ceftriaxone.
• Supportive Care: Patients with severe leptospirosis require intensive supportive care in a hospital to manage the complications of organ failure. This may include dialysis for kidney failure and placement on a mechanical ventilator (breathing machine) for respiratory failure due to lung hemorrhage.

I always emphasize timing. If treatment begins within the first few days of symptoms, patients typically recover fully. But if delayed, complications can escalate quickly.

Prevention

Since leptospirosis is an environmental disease, prevention is focused on minimizing exposure to contaminated sources.

• Avoid Contaminated Water: The single most important preventive measure is to avoid wading, swimming, or playing in floodwater or any freshwater source that could be contaminated with animal urine.
• Use Protective Clothing: Individuals who must work in high-risk environments, such as farmers or sanitation workers, should wear waterproof protective clothing, including boots and gloves.
• Cover Skin Lesions: Ensure any cuts or scrapes on the skin are covered with waterproof dressings.
• Rodent Control: Measures to control rat populations around the home can help reduce environmental contamination.
• Animal Vaccination: Vaccinating pets and livestock can help reduce bacteria shedding.
• Chemoprophylaxis: For individuals with a known, unavoidable, short-term high-risk exposure (like rescue workers), a doctor may prescribe a weekly dose of doxycycline to prevent infection.

Leptospirosis is a widespread and potentially severe bacterial disease that poses a significant public health risk, particularly in tropical and subtropical regions during periods of heavy rainfall and flooding. Its ability to mimic other common febrile illnesses often leads to delayed diagnosis, increasing the risk of progression to the life-threatening complications of Weil’s disease. The key to protecting yourself and your community lies in prevention, primarily by avoiding contact with floodwater and contaminated environments. If you develop a high fever and severe muscle aches after such an exposure, see a doctor immediately and tell them about your exposure, as early antibiotic treatment can prevent severe illness.

Centers for Disease Control and Prevention (CDC). (2022). Leptospirosis. Retrieved from https://www.cdc.gov/leptospirosis/index.html

World Health Organization (WHO). (2023). Leptospirosis. Retrieved from https://www.who.int/news-room/fact-sheets/detail/leptospirosis