Babesiosis

Babesiosis, or piroplasmosis, is a malaria-like parasitic disease caused by infection with the eukaryotic parasites Babesia or Theileria. These parasites belong to the phylum Apicomplexa, which includes other protozoan parasites such as those causing malaria, toxoplasmosis, and cryptosporidiosis.

Human babesiosis is primarily transmitted via tick bites and most commonly occurs in the Northeastern and Midwestern United States and parts of Europe during warm weather. Other modes of transmission include blood transfusions from infected donors and congenital transmission from an infected mother to her baby.

The disease often presents alongside other tick-borne illnesses such as Lyme disease.

Signs and Symptoms

Approximately half of all children and a quarter of previously healthy adults with Babesia infection are asymptomatic. Symptomatic patients typically develop fever and hemolytic anaemia, with an onset of symptoms 1 to 4 weeks after a tick bite or 1 to 9 weeks following a contaminated blood transfusion. Common symptoms include malaise, fatigue, fever, chills, sweats, and thrombocytopenia. These symptoms can last from several days to several months.

Less common symptoms and physical exam findings in mild-to-moderate cases include headache, muscle pain, anorexia, nonproductive cough, arthralgias, nausea, vomiting, sore throat, abdominal pain, conjunctivitis, photophobia, weight loss, emotional lability, depression, hyperesthesia, splenomegaly, pharyngeal erythema, hepatomegaly, jaundice, retinopathy, retinal infarcts, and neutropenia.

In severe cases, symptoms can resemble malaria, with high fevers, shaking chills, and severe anaemia. Organ failure, including adult respiratory distress syndrome, may occur. Severe cases are more likely in very young, very old, and immunocompromised individuals, such as those with HIV/AIDS.

Cause

The main species of Babesia that infect humans include B. microti, B. duncani, B. divergens, and B. venatorum. These parasites reproduce in red blood cells, causing hemolytic anaemia, similar to malaria. However, unlike malaria, Babesia species lack an exoerythrocytic phase, so the liver is usually not affected.

Diagnosis

Diagnosing Babesia infection requires specialised laboratory tests. The definitive diagnostic test is the identification of parasites on a Giemsa-stained thin-film blood smear, where "Maltese cross formations" are pathognomonic. Serologic testing for antibodies against Babesia (IgG and IgM) can detect low-level infections and differentiate babesiosis from malaria. Polymerase chain reaction (PCR) testing is also used for detection but is more expensive. Other laboratory findings include decreased red blood cells and platelets on complete blood count.

Treatment

Treatment for asymptomatic carriers is considered if parasites are still detected after three months. The preferred treatment for mild-to-moderate babesiosis is a combination of atovaquone and azithromycin, due to fewer side effects compared to clindamycin and quinine. In severe cases, clindamycin and quinine are preferred, and life-threatening cases may require exchange transfusion. In veterinary medicine, imidocarb is used for treating dogs, and extracts of Boophone disticha are used in South Africa for equine babesiosis.

Epidemiology

Babesiosis is a vector-borne illness usually transmitted by Ixodes scapularis ticks, the same vector as Lyme disease. In the United States, B. microti is the primary cause and is prevalent in the Northeast and northern Midwest. In Europe, B. divergens is the main cause and is transmitted by I. ricinus. Isolated cases have been reported globally, including in Australia, Africa, Asia, and South America.

History

The disease is named after the Romanian bacteriologist Victor Babeș, who identified the microorganisms in red blood cells in 1888. Theobald Smith and Frederick Kilborne discovered that a tick was the vector for transmission in 1893. The first human case was documented in 1957 in a splenectomized Croatian herdsman, and the first case in an immunocompetent individual was reported in 1969 on Nantucket Island.

Other Animals

Veterinary treatment of babesiosis primarily involves the use of drugs such as diminazen, imidocarb, or trypan blue. A vaccine is available for B. canis canis, but not for B. c. rossi. In acute cases in cattle, blood transfusion may be carried out.

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Self-assessment MCQs (single best answer)

What is the primary mode of transmission for human babesiosis?

Which phylum do the parasites causing babesiosis belong to?

What are the common symptoms of babesiosis?

Which species of Babesia is the primary cause of babesiosis in the Northeastern United States?

What is the definitive diagnostic test for babesiosis?

What is the preferred treatment for mild-to-moderate babesiosis?

In which year was the first human case of babesiosis documented?

What laboratory finding is commonly associated with babesiosis?

Who is babesiosis named after?

Which tick species is primarily responsible for transmitting babesiosis in the United States?