BABESIOSIS

General Goal: To know the major causes of malaria, the life cycle of these parasites, the most common modes of transmission and the major manifestations of this disease.

Specific Educational Objectives: The student should be able to:

1. identify the infectious form of this parasite.

2. identify the cause of this disease that results in the most pathology.

3. identify the most common means of transmission of this disease. Know the most common cause of babesiosis.

4. identify a patient with this disease based on clinical manifestations. There are other diseases with fever that come and go. Knowing the length, severity, and time period between fevers would be very helpful.

5. identify the current prevention strategies.

Reading: MEDICAL MICROBIOLOGY by P.R. Murray, K.S. Rosenthal, and M.A. Pfaller, 6th Edition. pp. 840-841.

Overview

Babesiosis is a protozoan infection of the erythrocytes that is transmitted by ticks, and is most commonly seen in northeastern parts of the United States.

Etiology

There are over 100 species of Babesia; however, only B microti and variants of the protozoan parasite are endemic in the U.S. B microti is endemic in the northeastern and midwestern parts of the U.S. Babesia WA-1 is a variant of B microti and has been reported in California and Washington. Another B microti variant, MO-1, has been isolated from patients living in Missouri.

Manifestations

Babesia inhabits the erythrocytes and causes a hemolytic anemia that results in a variety of symptoms associated with this infection. In regions endemic for malaria, babesiosis has been misidentified as malaria. Symptoms include fatigue, myalgia, arthralgia, nausea and vomiting, and dark urine. Physical signs include fever, shaking chills, hepatosplenomegaly, and jaundice. Most infections with this parasite are asymptomatic and improve without treatment. Persons who develop symptoms tend to be elderly, immunocompromised, or have been splenectomized.

Epidemiology

• Babesiosis is endemic in northeastern and midwestern United States and in California and Washington.
• About 25% of patients infected with B microti are also infected with the Lyme disease bacteria Borrelia burgdorferi, and they usually have more severe symptoms for longer periods of time than if they had been infected with either one of these agents alone.
• Babesia is a protozoan parasite that is transmitted to humans by the Ixodes tick and can be acquired from transfusion of blood infected with the parasite.

Pathogenesis

Unlike malaria, which infects hepatocytes initially and then infects erythrocytes, B microti trophozoites only infect erythrocytes. Damage to the red blood cells causes a hemolytic anemia and thrombocytopenia. Increases in atypical lymphocytes are seen in complete blood counts of peripheral blood samples. The changes in the membrane of the erythrocytes causes the cells to be less likely to change shape as they move through the capillaries, making the cells much more adherent. This can lead to the development of acute respiratory distress syndrome (ARDS) in severely affected patients.

Diagnosis

A complete blood count of a peripheral blood sample can aid in the diagnosis of babesiosis, revealing a hemolytic anemia, thrombocytopenia, atypical lymphocytes, and leukopenia. Thick and thin Giemsa-stained peripheral blood smears should be performed, similar for patients with malaria. Blood smears of patients with babesiosis will frequently contain ring-cell forms that contain no pigment, similar to that seen in patients with malaria. Some erythrocytes may contain the pathognomonic tetrad of trophozoites, frequently called the Maltese cross. Liver function tests usually reveal mildly elevated hepatic transaminases, lactic dehydrogenase, and serum bilirubin. The patient’s urine may be dark, and urinalysis may reveal hemoglobinuria and proteinuria. In severe cases, the patient may have hypoxia.

In patients who have negative peripheral blood-stained specimens, there are variety of serologic assays that may help, including an immunofluorescent antibody assay, ELISA, and immunoblot assays. A polymerase chain reaction (PCR) test can be used to confirm the diagnosis.

Treatment and Prevention

Treatment of babesiosis usually includes clindamycin and quinine. However, the treatment regimen of atovaquone and azithromycin results in fewer side effects. Prevention of babesiosis involves identification of persons most at risk of developing severe disease and encouraging them to avoid exposure to ticks and educating them concerning good methods of tick prevention. Disease transmission requires about 24 hours; therefore, prompt removal of any attached ticks is helpful in preventing infections.