Diagnosis of Potomac horse fever (syn. equine neorickettsiosis) in 2 foals in southwestern Ontario

Potomac horse fever (PHF) is characterized by fever, depression, anorexia, ileus, diarrhea, and occasionally, laminitis. The disease is caused by infection with Neorickettsia risticii and/or N. findlayensis. Equids of all ages may be affected; however, the condition has not been well-characterized in foals. This report describes clinical signs, laboratory findings, and treatment of 2 foals diagnosed with PHF in southwestern Ontario. Feces submitted for an equine PCR panel tested positive for Neorickettsia spp. and were subsequently confirmed to be N. risticii (Case 1) and N. findlayensis (Case 2). Both foals recovered following hospitalization and intensive care. Key clinical message: The purpose of this report is to make veterinarians aware that foals may develop PHF. During summer (July to September), when encountering foals in endemic areas with clinical signs compatible with PHF, veterinarians should consider PHF as a diagnostic rule-out. For confirmation of the diagnosis, blood and feces should be submitted for PCR testing for Neorickettsia spp.

Improved molecular detection of Neorickettsia risticii with a duplex real-time PCR assay in the diagnosis of Potomac horse fever

Neorickettsia risticii, an obligate intracellular bacterium, is the causative agent of Potomac horse fever (PHF). Diagnosis of PHF is based on demonstration of serum antibodies, isolation of N. risticii, and/or detection of nucleic acid by a PCR assay. An existing real-time PCR assay targeting the N. risticii 16S rRNA has been validated using blood samples from horses with colitis, and snails; to our knowledge, the performance of the assay for other sample types has not been reported. We describe here a modification of the 16S rRNA gene assay by the addition of a set of primers and probe targeting the N. risticii p51 gene to form a duplex assay. We validated the new assay using diagnostic specimens from 56 horses with suspected PHF. The assay consistently detected down to 5 copies of synthetic targets, and did not show any cross-reaction with common equine enteric pathogens. Although we did not establish the diagnostic sensitivity and specificity of the duplex assay, results for both gene targets were in complete agreement, with the exception of 4 fecal samples that tested positive for the 16S rRNA gene only. Further analysis indicated that testing of fecal samples using our 16S rRNA gene assay alone can produce a false-positive result.

Detection of Neorickettsia risticii from various freshwater snail species collected from a district irrigation canal in Nevada County, California

This study investigated the role of a district irrigation canal in Nevada County, California, USA, as the point source of infection for Neorickettsia risticii, causative agent of equine neorickettsiosis (EN). A total of 568 freshwater snails comprising Juga spp., Planorbella subcrenata (Carpenter, 1857) (Rough Rams-horn), Physella virgata (Gould, 1855) (Protean Physa) and feces from three horses with EN were collected and tested for N. risticii by real-time PCR. A total of four freshwater snails tested PCR positive for N. risticii. Phylogenetic analysis showed 99.8-100% homology between the different snail and horse N. risticii isolates. This study represents the first report of infection with N. risticii in Planorbella subcrenata and suggests that the irrigation canal was the aquatic environment responsible for the spread of N. risticii.

Cloning and expression of 51-kDa antigenic protein of Neorickettsia risticii NR-JA1

Neorickettsia (Ehrlichia) risticii is a causative agent of acute diarrheal syndrome in horses, commonly known as Potomac horse fever. Korean isolate of N. risticii NR-JA1 was cultivated in mouse macrophage cell line P388D1. A complete ORF of p51 antigenic protein gene was amplified and cloned into pQE32 and pcDNA3.1 vectors and the resultant clones were named as pQE32/Nr-51 and pcDNA3.1/Nr-51, respectively. Recombinant p51 (rp51) protein antigen was expressed in E. coli (pQE32/Nr-51) and cos-7 cell line (pcDNA3.1/Nr-51). The rp51 protein showed immunoreactivity with anti- mouse p51 antibodies. BALB/c mice were inoculated with recombinant plasmid DNA (pcDNA3.1/Nr-51). The serum samples collected from these BALB/c mice showed IgG ELISA titers of 1:128. In a Western immunoblot assay, these serum samples showed a strong reactivity to rp51 expressed in cos-7 cell line transfected with pcDNA3.1/Nr-51. The results of this preliminary indicate that N. risticii p51 protein is an immmuno-dominant antigen and may be a good target for the development of serological or a molecular diagnostic test and possibly an improved recombinant DNA based vaccine against Potomac horse fever.

Two cases of Neorickettsia (Ehrlichia) risticii infection in horses from Nova Scotia

Two horses from Nova Scotia were diagnosed with Potomac horse fever (PHF). Polymerase chain reaction analysis was performed on formalin-fixed colon tissue or whole blood to show the presence of Neorickettsia risticii DNA, the causative agent of PHF. These are the first reported cases of PHF in the Maritime Provinces.

Prevalence and sequence analyses of Neorickettsia risticii

The presence of Neorickettsia (Ehrlichia) risticii DNA was confirmed by PCR amplification and sequence analysis from cercaria in snails collected from stream water in Chungcheong and Jeonra provinces. A total of 3,219 snails were tested for trematode cercariae and N. risticii infection. N. risticii 16S rRNA gene fragment was amplified in cercariae from Semisulcospira libertina and Radix auricularia coreana snails by nested PCR. More than four genus cercariae (Schistosomatidae, Microphallidae, Furcocercus, and Xiphidiocercaria) as well as unidentified cercariae were found from Semisulcospira libertina snails. Three species of cercariae (E. cinetorichis, E. hortense, and Fasciola sp.) were found from Radix auricularia coreana snails. The cercariae were present in 429 (13.3%) snails of 3,216 collected at the Chungcheong and Jeonra provinces. The prevalence of N. risticii in these 429 cercariae was 17.9% (77 of 429 cercariae-infected snails). The amplicons of N. risticii 16S rRNA gene fragment (527 bp) from cercariae DNA had two genotypes (NR-JA1 and NR-JA2) with an identity of 96.4% between their nucleotide sequences. However, when compared to the sequence of N. risticii Shasta strain these sequences showed a 94.3% and 96.4% homology, respectively. The comparison of N. risticii 51 kDa major antigen gene sequences (572 bp) from NR-JA1 and NR-JA2 were 100% identical to the sequence of the isolates from Juga sp., Caddisfly larvae, Shasta, Juga yrekaensis, and trematode of California. This study reports for the first time the detection of N. risticii from cercariae found in Radix auricularia coreana snail. These data also indicate that N. risticii could be widespread in Korea.