Babesiosis

Babesiosis is a zoonotic disease which is triggered by infection with a protozoon of the genus Babesia. The disease is also known as piroplasmosis. The order of Piroplasmida belongs to the Apicomplexa (cf. malaria). There are more than 110 species in the genus Babesia. Some infect fish, birds, reptiles or mammals. The rodent parasite Babesia microti (USA) and the bovine parasites B. divergens and B. bovis (Europe) cause most infections in humans. Occasionally other species may be responsible for human infections (e.g. the WA1 strain). The species was first described in febrile cattle by Victor Babes in 1888. In 1957 the first case in humans was described in Yugoslavia by Skrabalo.

The protozoa are closely related to Theileria, a very important pathogenic genus in veterinary medicine. East Coast fever is triggered by Theileria parva, a disease transmitted by Rhipicephalus ticks. In 1893 Theobald Smith demonstrated that Texas cattle fever, a disease triggered by Babesia bigemina, could be transmitted by ticks, which represented a fundamental breakthrough in the knowledge of disease transmission. It was partly due to his work that the hypothesis of malaria transmission by mosquitoes was taken seriously. It was proposed that Babesia microti would be renamed Theileria microti, and that the disease would be named human theileriosis.

Babesiosis, transmission

Voles form the reservoir. Transmission is via the bite of hard ticks such as Ixodes scapularis and Ixodes ricinus. The transmission of B. bovis is via Boophilus microplus. In the USA larval nymphs of Ixodes scapularis feed chiefly on Peromyscus maniculatus (“the white-footed deer mouse”). The adult ticks suck blood from deer (cf. Lyme disease). Strangely enough the deer are not infected with B. microti. In ticks transstadial transmission occurs. The parasite passes from larva to nymph to adult tick. There is no transovarian transmission. Infections in humans are accidental occurrences. After injection of saliva of the tick, the micro-organisms penetrate red blood cells and mature. Babesia microti trophozoites undergo asexual reproduction in human blood and divide into two or four merozoites. To date no exo-erythrocytic cycle, as exists in Plasmodium species, has been described. Infected red blood cells undergo haemolysis. This releases the protozoa which can then penetrate new red blood cells. Infections via blood transfusions have been described. Transplacental infection may occur. When a tick sucks blood, the parasites are in the blood meal. This blood meal is enclosed in a peritrophic membrane in the tick’s gut. The parasite changes shape and it is assumed that it produces gametes. Once fertilisation is complete the zygote penetrates the peritrophic membrane and passes through the intestinal epithelium by endocytosis. Once in the haemolymph it forms an ookinete, which penetrates the salivary glands. There, the parasite forms sporoblasts, a stage which can overwinter. Thousands of sporozoites are formed. At the time of the tick’s next feeding, the parasite is mature and ready to be injected with the saliva. The part of the cycle between sporozoites and intra-erythrocytic merozoites is as yet unclear.

Babesiosis, geographical distribution

Endemic regions in the USA include Massachusetts and New York State with Nantucket Island, Long Island, the coast of Connecticut as well as foci in Georgia, California and Wisconsin. Cases have also been reported from various European countries such as Ireland, Scotland, Sweden, former Yugoslavia, France and Russia. There have been isolated case reports from Africa, Asia and Latin America.

Babesiosis, clinical aspects

Asymptomatic infection may persist for months or years. If symptomatic, the first symptoms occur after an incubation period of one to two weeks. Malaise, tiredness, fever, headache, nausea and abdominal pain, myalgia and joint pain are early but aspecific symptoms. The body temperature may rise to 40°C. Hepatosplenomegaly with haemolysis and jaundice, haemoglobinuria, mild neutropenia and thrombocytopenia follow. There is no lymphadenopathy. In severe cases ARDS [acute respiratory distress syndrome] with shock may develop. Infections may have a dramatic course in asplenic persons, chiefly in the European forms. Here the infection is similar to P. falciparum malaria. Splenectomy or immunosuppression during asymptomatic infection may lead to a full blown infection.

Babesiosis, diagnosis

Diagnosis is made from a blood smear stained with Giemsa. The parasitaemia is generally 1 to 10%. Haemoglobinuria and proteinuria occur. Sometimes the mature parasite is in the form of a clover leaf: a so-called tetrad or Maltese cross. The intra-erythrocytic dimension of the merozoite is 1 to 2.5 µm. It is pear-shaped, oval or round. The circular appearance means that Babesia is often confused with Plasmodium falciparum, but malaria pigment cannot be detected. There are also no gametocytes or malaria schizonts. In Babesia infections, large parasites may contain a central white vacuole, which is not present in malaria. Serological tests and DNA analysis may help in diagnosis. Hamsters and gerbils can be inoculated to cultivate the protozoa, but this can only be done in specialised centres. It is possible that some of the "malaria" in Africa is in fact babesiosis.

Babesiosis, treatment

Quinine is the drug of choice, 650 mg TDS plus clindamycin 600 mg TDS or 1.2g BD IV for 7 to 10 days. Children receive 25 mg quinine/kg/day. Atovaquone (750 mg BD) and azithromycin (500 mg on day 1, then 250 mg daily) are also used and this combination is better tolerated. Exchange transfusion may be considered if there is life-threatening parasitaemia. A blood transfusion may be life-saving. Remember that ticks can be infected with more than one pathogen. In endemic regions co-infection with Borrelia burgdorferi, a Ricketssia species or an Ehrlichia species must be considered.

Babesiosis, prevention

Category: Medicine Notes