Babesia divergens-like infection, Washington State

Abstract

Most reported U.S. zoonotic cases of babesiosis have occurred in the Northeast and been caused by Babesia microti. In Washington State, three cases of babesiosis have been reported previously, which were caused by WA1 (for "Washington 1")-type parasites. We investigated a case of babesiosis in Washington in an 82-year-old man whose spleen had been removed and whose parasitemia level was 41.4%. The complete 18S ribosomal RNA gene of the parasite was amplified from specimens of his whole blood by polymerase chain reaction. Phylogenetic analysis showed the parasite is most closely related, but not identical, to B. divergens (similarity score, 99.5%), a bovine parasite in Europe. By indirect fluorescent-antibody testing, his serum reacted to B. divergens but not to B. microti or WA1 antigens. This case demonstrates that babesiosis can be caused by novel parasites detectable by manual examination of blood smears but not by serologic or molecular testing for B. microti or WA1-type parasites.

Figure 1

Unrooted phylogenetic tree for the complete 18S rRNA gene of selected Babesia spp. The tree was computed by using the quartet puzzling maximum likelihood method of the TREE-PUZZLE program. The scale bar indicates an evolutionary distance of 0.01 nucleotide substitutions per position in the sequence. The GenBank accession numbers for the sequences used in the analysis are as follows: B. divergens (6), AY046576; B. odocoilei, AY046577; Babesia sp. EU1 (6), AY046575; the Babesia sp. from the patient in Washington State, AY274114 (see arrow); B. microti, U09833; WA1, from the index case of infection with WA1-type parasites (7,8), AF158700; and Theileria annulata, M64243.

Figure 2

Panel of Babesia-infected erythrocytes photographed from pretreatment, Wrights-Giemsa–stained smears of fresh blood obtained from the patient on July 31, 2002. The mean corpuscular volume of the erythrocytes was 103 (normal range 80–100 μm³). Note the multiply infected erythrocytes; the pleomorphism of the parasite; and the obtuse (divergent) angle formed by some of the paired structures, which, like the form in (F), is characteristic of B. divergens and related parasites isolated from various wild ruminants. The forms of the parasite shown in the panel include: (A) ring-like trophozoite; (B) paired merozoites; (C) maltese-cross (tetrad); (D) various dividing forms; (E) multiple merozoites; (F) appliqué (accolé) form on right border of the erythrocyte; (G) and (H) degenerate (crisis) forms. A glass slide of a peripheral blood smear from July 31 has been deposited in the U.S. National Parasite Collection, Beltsville, Maryland; the accession number (USNPC #) for the slide is 093041.00.