New genetic lineages, host associations and circulation pathways of Neorickettsia endosymbionts of digeneans

Abstract

Neorickettsia is a genus of intracellular bacteria endosymbiotic in digeneans that may also invade cells of vertebrates and are known to cause diseases of wildlife and humans. Herein, we report results of screening for Neorickettsia of an extensive collection of DNA extracts from adult and larval digeneans obtained from various vertebrates and mollusks in the United States. Seven isolates of Neorickettsia were detected by PCR and sequenced targeting a 527 bp long region of 16S rRNA. Sequence comparison and phylogenetic analysis demonstrated that four isolates matched published sequences of Neorickettsia risticii. Three other isolates, provisionally named "catfish agents 1 and 2" (obtained from Megalogonia ictaluri and Phyllodistomum lacustri, both parasitic in catfishes) and Neorickettsia sp. (obtained from cercariae of Diplostomum sp.), differed from previously known genotypes of Neorickettsia and are likely candidates for new species. All 7 isolates of Neorickettsia were obtained from digenean species and genera that were not previously reported as hosts of these bacteria. Members of four digenean families (Dicrocoeliidae, Heronimidae, Macroderoididae and Gorgoderidae) are reported as hosts of Neorickettsia for the first time. Our study reveals several new pathways of Neorickettsia circulation in nature. We have found for the first time a Neorickettsia from a digenean (dicrocoeliid Conspicuum icteridorum) with an entirely terrestrial life cycle. We found N. risticii in digeneans (Alloglossidium corti and Heronimus mollis) with entirely aquatic life cycles. Previously, this Neorickettsia species was known only from digeneans with aquatic/terrestrial life cycles. Our results suggest that our current knowledge of the diversity, host associations and circulation of neorickettsiae is far from satisfactory.

Large Scale Screening of Digeneans for Neorickettsia Endosymbionts Using Real-Time PCR Reveals New Neorickettsia Genotypes, Host Associations and Geographic Records

“…As posited by Vaughan et al [4], future studies focusing on screening digenean extracts for Neorickettsia will reveal additional host associations of these bacteria and new pathways of their circulation in nature. This has already been demonstrated in a recent study by Tkach et al [3] who found 4 species level genetic lineages of Neorickettsia in 7 species of digeneans belonging to 7 different families.…”

“…Although relatively small, this genus has received significant attention in the recent years, with a rapid increase in the number of known species level lineages in this group [1], [2], [3], [4], [5]. Neorickettsia are vertically transmitted through all stages of complex digenean life cycles.…”

“…At the same time, screening of digeneans for Neorickettsia has been limited, which prevents understanding of the actual diversity of these bacteria and potential sources of infection of vertebrate animals including humans. Only 21–22 digenean species, in most cases identified only to genus or family level, have been previously confirmed as hosts of Neorickettsia [3], [4], [12], [13], [14], [15]. Records resulting from PCR detection of neorickettsial DNA are known only from North and South America, eastern Asia, and Antarctica.…”

“…Currently, there are 3 named species and 10 not formally named genotypes of Neorickettsia that are likely to represent additional species based on levels of 16S rRNA sequence divergence [2], [3], [4], [5], [16], [17], [18], [19]. Of these 13 known species/genotypes of Neorickettsia , 6 have been found in North America.…”

“…Circulation of digenean hosts of known Neorickettsia occurs in either freshwater (Rainbow trout agent, Catfish agent 1, Catfish agent 2), freshwater/terrestrial ( N . helminthoeca , N. risticii , Elokomin Fluke Fever agent, Diplostomum agent), or fully terrestrial ( N. risticii ) environments [3], [4], [12], [13], [20], [21], [22]. Presently, however, there are no records of Neorickettsia from digeneans having fully marine life cycles.…”