Redwater is a tick-borne disease of cattle caused by tiny parasites called Babesia that invade and destroy red blood cells. The name comes from its most striking symptom: dark red or brown urine, caused by the contents of burst red blood cells filtering through the kidneys. It’s also known as bovine babesiosis, and it remains a significant cause of illness and death in cattle across much of the world. A closely related infection, babesiosis, can also affect humans.
Why the Urine Turns Red
Babesia parasites enter the bloodstream through a tick bite and multiply inside red blood cells. As the parasites reproduce, they rupture the cells from the inside out. When large numbers of red blood cells break apart at once, hemoglobin (the protein that carries oxygen and gives blood its color) spills directly into the bloodstream. The kidneys filter this free-floating hemoglobin into the urine, turning it anywhere from clear red to deep brown depending on how severe the destruction is. This process is called intravascular hemolysis, and the resulting discolored urine is the hallmark sign that gives redwater its name.
The rapid loss of red blood cells also causes anemia. Affected cattle become weak, lose appetite, and may develop a fever above 40°C (104°F). In severe cases, organs can’t get enough oxygen to function properly, and animals may collapse or die within days if untreated.
The Parasites Behind It
Three Babesia species cause most clinical cases in cattle. Babesia bovis and B. bigemina are widespread through tropical and subtropical regions across Africa, Asia, and the Americas, where hundreds of millions of cattle live in areas where these parasites circulate. B. divergens is the main species in cooler climates, particularly across Europe including the UK and Ireland. A few other species like B. major can infect cattle but tend to cause much milder disease.
Each species produces slightly different patterns of illness. B. bovis, for example, can affect the brain and central nervous system, sometimes called “cerebral babesiosis,” making it especially dangerous. B. divergens tends to cause the classic redwater picture of fever, anemia, and hemoglobin in the urine.
How Ticks Spread the Disease
Redwater spreads exclusively through tick bites. Different Babesia species rely on different tick vectors. In Europe, the common pasture tick (Ixodes ricinus) transmits B. divergens. In tropical regions, Rhipicephalus ticks carry B. bovis and B. bigemina. A single tick species, Rhipicephalus annulatus, can complete its entire life cycle on one host animal, making it a particularly efficient transmitter.
Most ticks that carry Babesia follow a multi-year life cycle with three stages: larva, nymph, and adult. Each stage feeds on a different host. A tick picks up the parasite by feeding on an infected animal, then passes it along when it feeds on its next host. This means the parasite can quietly circulate in a region for years, with outbreaks triggered by conditions that favor tick activity. Warmer-than-usual winters, for instance, triggered an outbreak of redwater in southern England in February 2019, when unseasonably mild temperatures allowed ticks to become active earlier than expected.
Where Redwater Occurs
Redwater is found on every inhabited continent. B. bovis and B. bigemina have the widest reach, with established transmission across Africa, South America, Central America, southern Asia, and parts of southern Europe. Over 600 million people live in African regions where B. bovis circulates in cattle, and similar numbers are at risk in the Americas, giving a sense of how widespread the parasite’s livestock hosts are.
B. divergens is concentrated in Europe, where an estimated 430 million people live in areas where the parasite circulates between ticks and cattle. In the UK and Ireland, redwater is considered sporadic rather than constant, meaning outbreaks appear in clusters rather than as a steady background rate. These clusters tend to follow weather patterns that boost tick populations.
Redwater in Humans
While redwater is primarily a cattle disease, the broader group of Babesia parasites can infect people too. Human babesiosis is most common in the northeastern United States, where it’s caused by B. microti and transmitted by the same blacklegged ticks that carry Lyme disease. In Europe, the cattle parasite B. divergens occasionally infects humans, typically those without a functioning spleen.
Many people infected with Babesia never feel sick at all. When symptoms do appear, they typically develop one to four weeks after a tick bite and can take weeks or even months to fully emerge. The illness often resembles the flu: fever, chills, sweats, headache, body aches, fatigue, nausea, and loss of appetite. Because the parasites attack red blood cells just as they do in cattle, human babesiosis can also cause hemolytic anemia, leading to yellowing of the skin and dark urine.
For most healthy adults, babesiosis is mild and treatable. It becomes dangerous in people over 50, those without a spleen, and anyone with a weakened immune system from conditions like cancer, HIV, or chronic liver or kidney disease. Severe cases can lead to dangerously low blood pressure, organ failure, uncontrolled bleeding, and death. Treatment for mild to moderate cases typically involves a course of two oral medications taken for 7 to 10 days. People with severe disease may need hospitalization and intravenous treatment until symptoms improve.
Diagnosis
In cattle, a veterinarian can often suspect redwater based on the combination of fever, anemia, and red urine in a region where ticks are active. Confirming the diagnosis requires identifying the parasite, either by examining a stained blood sample under a microscope or through DNA-based testing (PCR). Microscopy is fast but can miss infections early on, before parasite numbers build up in the blood. PCR testing is equally accurate and can detect the parasite even at low levels, making it useful for catching cases before they become severe or for screening animals that look healthy but may be carrying the infection.
In humans, the same two methods apply. A trained lab technician can spot the pear-shaped Babesia parasites inside red blood cells on a stained blood smear. PCR testing has proven just as sensitive and specific, and it can identify infections in people who don’t yet have symptoms. Antibody testing can also confirm recent exposure but doesn’t distinguish between active and past infections.
Prevention on the Farm
Controlling redwater in cattle centers on managing tick exposure. This includes treating animals and pastures with tick-killing products, rotating grazing land to break the tick life cycle, and keeping cattle out of heavily wooded or brushy areas where ticks concentrate. In some tropical regions, live attenuated vaccines against B. bovis and B. bigemina are available, though access and efficacy vary by country. No vaccine exists for B. divergens, the main European species.
Calves under about nine months old have a natural resistance to severe babesiosis. Some farmers in endemic areas deliberately expose young calves to the parasite so they develop immunity before they’re old enough for the disease to be dangerous. This approach requires careful management, since the goal is mild infection rather than full-blown disease. Adult cattle introduced to an endemic area for the first time are at highest risk, because they have no prior immunity and face the full force of the parasite.