Ans. a (Babesia microti). (Ref. Harrison's, Principles of Internal Medicine, 16th ed., pg1232)Babesiosis (B. microti)# Babesiosis is a worldwide protozoan disease of animals that is transmitted by ticks; humans are infected incidentally and initially develop a nonspecific febrile illness that can lead to hemolytic anemia.# Babesia organisms enter RBCs and resemble malarial parasites morphologically, posing a diagnostic problem.# Ixodid (hard-bodied) ticks, in particular Ixodes scapularis (/. dammini) and I. ricinus, are the vectors of the parasite. Transfusions are another source of babesiosis.# The incubation period for B. microti infection is 1 to 4 weeks.# Immunosuppressed patients, splenectomized individuals, and the elderly have the most severe illness.# The clinical presentation varies widely and resembles malaria or rickettsiosis; symptoms and signs include a gradual onset of irregular fever, chills, sweating, muscle pain, and fatigue. Mild hepatosplenomegaly and mild hemolytic anemia may develop, but a rash is not present.# Giemsa-stained thick and thin blood films examined for small intraerythrocytic parasites. b. microti appears as a small ring form resembling P. falciparum.# Unlike infection with Plasmodium, however, Babesia does not cause the production of pigment in parasites, nor are schizonts or gametocytes formed.# A species-specific polymerase chain reaction test using the RNA gene has been used to show parasite persistence when the blood smear is negative.# Treatment of Babesiosis:OrganismAdultsBabesia microtiAtovaquone 750 mg bid PO plus azithromycin 600 mg/ d PO or clindamycin 1200 mg bid IV.Babesia divergens and other Babesia species, including MO-1,WA-1, and CA-1Quinine 650 mg tid PO plus clindamycin 1200 mg bid IV (or 600 mg tid PO) plus atovaquone 750 mg bid POo