Babesia microti

Yabsley, Michael J. & Shock, Barbara C., 2013, Natural history of Zoonotic Babesia: Role of wildlife reservoirs, International Journal for Parasitology: Parasites and Wildlife 2, pp. 18-31 : 24-25

2.2.4. Babesia microti and related species

2.2.4.1. Humans. Currently, only a single case of B. microti -associated babesiosis has been confirmed in Europe: a German patient with leukemia who likely became infected by a transfusion ( Hildebrandt et al., 2007). Retrospective screening of blood donors for the patient revealed a single donor with a titer to B. microti . Neither person had travel history to North America or Asia. Several surveys have detected anti- B. microti antibodies in individuals in Croatia and Poland suggesting that infections are underdiagnosed ( Topolovec et al., 2003; Chmielewska-Badora et al., 2012).

In Asia, human cases with B. microti -like sp. are rare and sporadic infections have been reported from Japan, Taiwan, China, and possibly India ( Wei et al., 2001; Arai et al., 2003; Marathe et al., 2005). The first human case ( B. microti Kobe type) in Japan was diagnosed in a patient in 1999. This patient likely acquired the infection from an asymptomatic donor ( Matsui et al., 2000; Wei et al., 2001). Previously diagnosed cases have been reported in Tawain (asymptomatic) ( Shih et al., 1997) and in China ( Li and Meng, 1984), but the causative agents were not well characterized. Serologic studies in Asia have indicated that unrecognized infections have occurred. In Taiwan, individuals with antibodies to B. microti have been reported ( Hsu and Cross, 1977; Shih et al., 1997) and a retrospective survey of sera collected in 1985 from Japan indicated that 1.3% of 1335 samples had antibodies to B. microti Kobe type (n = 3) and B. microti Hobetsu type (n = 14), with the latter type having only been previously detected in rodents ( Tsuji et al., 2001; Arai et al., 2003). Outside of Southeast Asia, antibodies to B. microti have been detected in 6% of 273 individuals from northern Turkey (Poyraz and Güneş, 2010).

2.2.4.2. Reservoirs. In Europe, natural infections of B. microti have been reported from numerous rodent and shrew species including species of yellow-necked mice ( Apodemus flavicollis ), wood mice ( Apodemus sylvaticus ), bank voles ( Myodes (Clethrionomys) glareolus ), field voles ( Microtus agrestis ), common shrews ( Sorex araneus ), and Mus spp. in Germany, Poland, Croatia, Slovenia, Austria, Hungary, Bulgaria, Czech Republic, Slovakia, and the United Kingdom ( Sebek et al., 1977, 1980; Turner, 1986; Randolph, 1995; Bajer et al., 2001; Duh et al., 2003; Siński et al., 2006; Beck et al., 2011; Bown et al., 2011). Genetic characterization of various samples of B. microti has indicated that both zoonotic and presumed non-zoonotic strains are co-circulating in the same species of rodents ( Beck et al., 2011).

In Asia, at least three named types of B. microti parasites ( US, Kobe, and Hobetsu) have been detected in naturally infected rodents and shrews ( Zamoto et al., 2004a,b; Kim et al., 2007; Qi et al., 2011). In Japan, two field mice species (Large Japanese field mice ( Apodemus speciosus ) and Small Japanese field mice ( A. argenteus )) are natural hosts for B. microti Kobe type ( Shiota et al., 1984; Tsuji et al., 2001; Wei et al., 2001; Saito-Ito et al., 2004, 2007). Numerous rodents and shrews are infected with B. microti Hobetsu type including Large Japanese field mice, grey red-backed voles ( Clethrionomys rufocanus ), northern red-backed voles ( C. rutilus ), Japanese field voles ( Microtus montebelli ), long-clawed shrews ( Sorex unguiculatus ), and Laxmann’s shrews ( Sorex caecutiens ) ( Tsuji et al., 2001). Large Japanese field mice, grey red-backed voles, and northern red-backed voles are also hosts for B. microti US-like ( Zamoto et al., 2004a,b). In Taiwan and China, B. microti Kobe type or related parasites have been reported in Horsfield’s shrews ( Crocidura horsfieldii ) and spinous country-rats ( Rattus coxinga ), Chinese white-bellied rats ( Niviventer confucianus ) and striped field mice ( Apodemus agrarius ) ( Saito-Ito et al., 2008). B. microti US type has been found in striped field mice and Korean field mice ( Apodemus peninsulae ) from South Korea, yellow steppe lemmings ( Eolagurus (= Lagurus ) luteus) from China, and Korean field mice and grey red-backed voles from Eastern Russia ( Zamoto et al., 2004b). A related B. microti -like sp. has been detected in Eurasian red squirrels ( Sciuris vulgaris ) ( Tsuji et al., 2006).

2.2.4.3. Vectors. In Europe, the primary vector of B. microti is I. ricinus , which also transmits B. divergens and several other human and veterinary pathogens (e.g., Borrelia and Babesia ). This tick is common on large mammals, including people, throughout Europe and isolated parts of western Asia and northern Africa. Infections with B. microti -like species have been reported in I. ricinus throughout the range of the tick including Switzerland, Poland, Italy, the Netherlands, Czech Republic, Estonia, Belgium, Hungary, Germany, Russia, and the United Kingdom ( Alekseev et al., 2003; Hartelt et al., 2004; Rudolf et al., 2005; Sréter et al., 2005; Casati et al., 2006; Siński et al., 2006; Nijhof et al., 2007; Bown et al., 2008; Wielinga et al., 2009; Cassini et al., 2010; Burri et al., 2011; Gigandet et al., 2011; Katargina et al., 2011; Lempereur et al., 2011).

In England, both I. ricinus and I. trianguliceps can transmit B. microti -like spp. among voles but I. trianguliceps is believed to be the primary vector because exclusion of deer (with subsequent drop in numbers of I. ricinus ) did not affect density of I. trianguliceps or prevalence of Babesia in voles ( Bown et al., 2008). Naturally infected I. trianguliceps have also been reported in Poland and Russia ( Telford et al., 2002; Karbowiak, 2004). Interestingly, B. microti was recently detected in 4.5% of 468 questing Dermacentor reticulatus from Poland, suggesting a need to investigate other potential vectors ( Wójcik-Fatla et al., 2012).

In Asia, B. microti Hobetsu have been detected in questing Ixodes ovatus from Japan while B. microti US type and a B. microti type related to B. microti Kobe type have been detected in I. persulcatus from Russia and China ( Saito-Ito et al., 2004; Yano et al., 2005; Sun et al., 2008; Rar et al., 2011; Zamoto-Niikura et al., 2012). In Taiwan, Ixodes granulatus transmitted a B. microti strain to laboratory rats ( van Peenen et al., 1977).