Babesiosis: the tick-borne red blood cell parasite

The CDC describes babesiosis as a disease caused by a parasite that infects red blood cells. Babesia microti is the species most often reported in the United States, while CDC laboratory notes say Babesia divergens predominates in Europe. The life cycle involves small mammals such as mice and Ixodes ticks. Humans are accidental, dead-end hosts: they become infected when a feeding tick passes the parasite on, and they do not pass it on further except through contaminated blood.

Because the parasite lives inside red cells, it is destroyed along with them. The CDC explains that the body can lose red cells faster than it replaces them, which is what produces haemolytic anaemia in more severe illness.

Tick bites are the main route. The CDC notes that infection is linked to bites in wooded or grassy areas, and in the United States cases concentrate in the north-east and upper Midwest. The NHS Lyme disease page explains that ticks are found throughout the UK, with higher numbers in grassy and wooded areas in southern England and the Scottish Highlands, so the exposure that matters most is the same one that underlies Lyme disease.

Transfusion transmission and passage from mother to baby have also been described, according to the CDC clinical overview, which is why blood services pay attention to the infection in regions where it is common.

• Cover skin outdoors and tuck trousers into socks, as NHS advice on ticks suggests.

• Use an insect repellent containing DEET on exposed skin.

• Check yourself, children and pets for ticks at the end of a walk.

• Remove an attached tick promptly with fine-tipped tweezers, pulling slowly upwards without squeezing it.

Many infections cause no illness. When symptoms appear they are flu-like: fever, chills, sweats, headache, body aches, loss of appetite, nausea and fatigue, according to the CDC. Symptoms can begin within weeks or months of exposure and, in immunocompromised people, may first appear or recur many months later.

In more serious cases there may be an enlarged spleen or liver, jaundice and a rapid fall in haemoglobin. The CDC clinical overview lists complications of severe disease that include low platelets, abnormal clotting, breathing difficulty, kidney failure and altered mental state.

The spleen filters infected red cells from the blood, so people with no spleen or a poorly working one are at particular risk. The CDC lists absent or reduced spleen function, older age, HIV, B-cell lymphoma, immunosuppressive treatment, especially rituximab, and heart failure as risk factors for severe disease. Newborns can be infected by transfusion or across the placenta.

If you have had your spleen removed or have been told it does not work properly, it is worth knowing that a fever after a tick bite, or after travel to a region where Babesia is common, needs prompt assessment rather than watchful waiting.

The usual test is a Giemsa-stained blood film examined under the microscope. The CDC advises asking for manual review of the film and notes that several films may be needed and that Babesia must be distinguished from Plasmodium, the malaria parasite. A tetrad of dividing parasites sometimes called a Maltese cross is characteristic. Unlike malaria parasites, Babesia does not produce pigment.

Antibody tests and PCR are used to detect low-level infection and to identify the species. Because the picture can look like malaria, a clinician who learns that you have travelled or been bitten by ticks can arrange the right combination of tests together, and a reference laboratory may confirm the result.

The CDC says people without symptoms generally need no treatment. For symptomatic patients with healthy immune systems it recommends a 7 to 10 day course of atovaquone plus azithromycin, with clindamycin plus quinine as an alternative. Exact doses and monitoring are for the treating team. Most symptoms settle and blood films clear during the standard course, though tiredness may last for some weeks.

Severe disease can need hospital care. The CDC describes exchange transfusion, in which part of the patient blood is replaced with donor blood components, for high parasite counts or severe haemolytic anaemia, along with supportive measures such as dialysis or ventilation when required. People with weakened immunity may need treatment for at least six weeks or longer, with regular blood films, and relapse can occur.

See a GP or call NHS 111 if you develop fever, chills or marked tiredness in the weeks after a tick bite, and say that you have been bitten. Use 999 or A&E if you are breathless, confused, passing very dark urine, look yellow, or you have no spleen and feel suddenly and seriously unwell.

Care for confirmed cases usually involves infectious diseases or haematology specialists, and a tropical medicine centre may advise on diagnosis when the blood film is hard to interpret.