Human Babesiosis: An Emerging Zoonotic Disease


Babesiosis is an emerging tick-borne zoonotic disease caused by obligate intra-erythrocytic protozoa of genus Babesia, which is widespread over the world. There are more than 100 species which infect several wild and domesticated animals; but only few have been confirmed as causative agents for babesiosis in humans. Babesia microti, Babesia duncani, Babesia divergens, and Babesia venatorum are the species which have been identified to cause infection in humans worldwide. B. microti is the most common species to cause infection in humans. Even though it is observed that the infection by a species varies with the geographical area. Infection of B. microti cases is common in United States; B. divergens in Europe and B. venatorum found to cause more infection in China.


The first human case of babesiosis was identified in 1957 in a Croatian herdsman. Babesiosis considered as occupational hazard for farmers, landscapers, hunters, and professionals who work in direct contact with the soil and animals. Travelers and health professionals are of more concern about babesiosis while considering the destination, duration and the risk of exposure to infection of babesiosis. Currently more than half a billion people are involved in international travel annually, movement of people and their interaction with the various environments could be a risk factor. In India the first case of babesiosis reported in 2005.

What is this parasite?

Babesia is included in the phylum Apicomplexa of the suborder Piroplasmida and family Babesiidae. They are oval, round or pyriform in shape presenting annular conformation and peripheral location, similar to the protozoa Plasmodium falciparum which complicates the diagnosis. In Plasmodium, hemozoin deposits on the ring and banana-shaped gametocytes observed which absent in Babesia spp.

Rout of infection/transmission-

Bite of Ixodes tick is the most common route infection. The previously infected Ixodes ticks, blood transfusion and congenital pathways are routes of transmission. Bite of a tick injects the sporozoites, which then penetrates into the erythrocytes directly in case of B. bovis and B. divergens; whereas, in case of B. microti initially it lodge into lymphocytes and then invade into red blood cells.


The sporozoite transform into trophozoites which then into merozoites by binary fission (merogony). During this process erythrocytic membrane breaks and released merozoites invade new cells resulting in an intra-erythrocytic cycle.

The parasitic cycle in tick

When ticks feed on blood it ingests trophozoites, merozoites and gametocytes. Only gametocytes could survive the intestinal lumen of the tick where gamogony occurs; which fuse to form diploid zygotes. These zygotes invade the digestive and undergo successive round of multiplication before emerging as haploid kinetes. The kinetes migrate by means of the hemolymph to many other organs including the ovaries, where further division occurs. After hatching of eggs kinetes migrate to the salivary gland; where they transform into multi-nucleated stages by sporogony which later forms sporozoites that are inoculated from saliva during the vector’s feeding. Period of incubation depends on the route of transmission. Following a tick bite it is up it takes up to six weeks and nine weeks following transfusion of contaminated blood. Babesiosis can be asymptomatic to life-threatening in humans. The severity of the infection may vary with age and host immune status.

The clinical signs

The symptoms were highly nonspecific which could pertain to many other febrile infective conditions. Infected individuals develop fatigue, myalgia, and malaise followed by persistent or intermittent pattern of fever. Other commonly reported symptoms were sweating, headache, anorexia, vomiting, diarrhea, abdominal pain, joint pain, sore throat, cough, breathing difficulty, redness of eyes, dark-colored urine, depression, and emotional lability, hepatomegaly, splenomegaly, jaundice.

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Diagnosis

Diagnosis of babesiosis is made from patient history, especially travel history to endemic areas, blood smear examination stained with Giemsa, for detection of intra erythrocytic pathogen. In low parasitemia molecular methods like polymerase chain reaction (PCR), DNA sequency, DNA microarray systems and serologic methods like indirect immunoflourescence (IFA) Imunoblot, immunochromatography, and ELISA (enzymelinked immunosorbent assay) may be used.

The most commonly used antibiotics are atovaquone, azithromycin, clindamycin, quinine. Supportive therapy like antipyretics, vasopressors, blood transfusion and mechanical ventilation required.

Prevention is better than cure

The best preventive measure for babesiosis is to avoid exposure to tick habitats in endemic area. Efforts should be made to prevent tick bites and transmission of infections. There is no vaccine available on babesiosis.

Authors

1. Livestock Development Officer, Veterinary Dispensary Zilpa, Katol, Nagpur, Maharashtra

2. PhD Scholar, Veterinary Parasitology Division, ICAR-Indian Veterinary Research Institute, Izatanar, Bareilly, U.P.

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