An 84-year-old woman with fever and dark urine.

Figure 1. Wright Giemsa-stained thin prep peripheral blood smears (original magnification, ϫ1000) demonstrating appliqué (B) and headphone (C) morphologies, which are characteristic of Plasmodium falciparum, and extracellular ring forms (D), which are characteristic of Babesia microti. Diagnosis: Babesiosis due to Babesia microti infection from a tick bite. Peripheral blood smear from the state laboratory showed intraerythrocytic ring forms as well as appliqué and headphone morphologies (figure 1B and 1C). Although some of the ring and appliqué forms were consistent with the morphological characteristics of Plasmodium falciparum, the patient had not traveled to an area where infection with P. falciparum was likely. Rare cases of Plasmodium vivax but not P. falciparum are reported in Quintana Roo, the state in Mexico where Cozumel is located. Although there have been recent outbreaks of P. falciparum infection in the Caribbean (i.e., Dominican Republic , the Bahamas, and Jamaica), Florida and the Cayman Islands are not areas in which malaria is endemic. In addition, B. microti infection was considered because of the travel history to an area where Babesia species are endemic (Minnesota) and because of a history of blood transfusions. Her blood smear showed many appliqué forms that are commonly seen in P. falciparum infection but are typically not characteristic of Babesia. Further examination of the blood smears at the US Centers for Disease Control and Prevention showed several extracellular ring forms that are more commonly observed with Babesia species infection (figure 1D). To determine conclusively the genus and species of the infecting organism, further diagnostic testing with polymerase chain reaction (PCR) was performed. PCR results were positive for B. microti infection and negative for all 4 Plasmodium species. There were 2 possible sources of the patient's babesiosis: tick-borne transmission or transfusion transmission. To evaluate the latter, all donors of blood products received by the patient were contacted and tested for B. microti antibodies. All donors were negative for such antibodies. Therefore, it is likely that the patient acquired babesiosis from a tick bite while visiting Min-nesota. The patient recalled no tick bite; however, recall of a tick bite does not always accompany tick-borne diseases [1, 2]. The typical incubation period for babesiosis after a tick bite is 1–6 weeks [3]. The number of cases of babesiosis reported annually in Min-nesota has increased in recent years. Eighteen cases were reported in 2006, compared with a median of 3 reports per year from 1992 through …