Sennetsu neorickettsiosis: a probable fish-borne cause of fever rediscovered in Laos

Abstract

Neorickettsia sennetsu has been described from Japan and Malaysia, causing a largely forgotten infectious mononucleosis-like disease. Because it is believed to be contracted from eating raw fish, frequently consumed in the Lao PDR, we looked for evidence of N. sennetsu among Lao patients and fish. A buffy coat from 1 of 91 patients with undifferentiated fever was positive by 16S rRNA amplification and sequencing and real-time polymerase chain reactions (PCR) targeting two N. sennetsu genes. Lao blood donors and patients with fever, hepatitis, or jaundice (N = 1,132) had a high prevalence (17%) of immunofluorescence assay IgG anti-N. sennetsu antibodies compared with 4% and 0% from febrile patients (N = 848) in Thailand and Malaysia, respectively. We found N. sennetsu DNA by PCR, for the first time, in a fish (Anabas testudineus). These data suggest that sennetsu may be an under-recognized cause of fever and are consistent with the hypothesis that it may be contracted from eating raw fish.

Figure 1. Dendrogram representing phylogenetic relationships between sequences derived from Fish 1 (Channa gachua), Fish 2 (Trichopsis vittata), Fish 3 (Anabas testudineus), and Patient 1 and other Anaplasmataceae (16S rRNA gene sequence comparison). The MEGA 4.1 software (http://www.megasoftware.net) was used to infer the tree by using the neighbor-joining method Kimura-2 parameter. The support of each branch, as determined from 1,000 bootstrap samples, is indicated by the value at the node.

Figure 2. Western immunoblotting of (A) a N. sennetsu-positive serum sample (a febrile Lao patient with an IFA IgG anti-sennetsu titer of 1/400), (B) a negative serum sample, and (C) an O. tsutsugamushi-positive serum sample. This figure appears in color at www.ajtmh.org.