Trichomonads, such as Pentatrichomonas hominis, are obligate protozoan symbionts found in vertebrates and are considered to be among the most primitive of eukaryotic organisms. They are characterized morphologically by multiple anterior flagella and a single recurrent flagellum that functions as an undulating membrane [1]. P. hominis inhabits the large intestine of a number of mammalian hosts, including cats, dogs, non-human primates, and pigs [2-6]. Opportunistic overgrowth of P. hominis can cause disease, typically involving diarrhea [7,8]. The potential zoonotic risk of this organism remains to be determined, in part due to the limited number of reports. Recently, sensitive and highly specific oligonucleotide probes enabling the identification of P. hominis were reported [9], as were the optimum reaction conditions and detection limits of polymerase chain reaction (PCR)-based identification of P. hominis in DNA extracted from canine feces [10]. The present report is the first identification of P. hominis infection in puppies with diarrhea in the Republic of Korea. As well, the taxonomic status of the P. hominis isolates are compared with those isolated elsewhere.

P. hominis is a trichomonad species found in humans and animals. Although P. hominis inhabits the large intestine of mammalian hosts and is considered to be a commensal [14], their molecular characterization and pathogenicity are ill-understood. The organism is characterized morphologically by five anterior flagella and a single recurrent flagellum that functions as an undulating membrane [15,16]. In this study, trichomonad-like parasites were observed in three of 14 fecal samples by light microscopy. A more detailed morphological examination, which is possible using a recently reported methylene blue staining regimen, was not undertaken because the fecal samples were frozen prior to refer to our laboratory.

We used the primer pair Th3 and Th5, which have 100% sequence identity with a canine isolate of P. hominis 18S rRNA genes [10]. P. hominis infection rate in the present study, based on the aforementioned light microscopy examination, was 21.4% (3/14; one female and two males). As previously reported, dogs presenting with trichomoniasis and diarrhea range widely in age from < 13 weeks to 6 months [10,17-20]. In this case, the affected puppies were 2-months-old (n = 1) and 9-months-old (n = 2), which is similar to previous studies in age. Interestingly, the parents of the affected puppies and other adult dogs in the same dog farm displayed no symptoms of diarrhea and were PCR-negative for P. hominis (data not shown). This is similar to a previous report [21]. It may be that puppies are more susceptible to P. hominis infection than adults, although this remains to be rigorously ascertained.

Trophozoites of P. hominis reproduce by binary fission and undergo direct host-to-host transmission without formation of environmentally stable cysts [1]. Because all of the 14 puppies we sampled experienced diarrhea at a similar time, the spread of P. hominis infection in the dog farm presumably occurred by contacting of the puppies during their regular communal exercise periods. The affected puppies were successfully treated empirically with metronidazole. In spite of the general vaccine program, whether P. hominis infection was a cause of diarrhea over one week in this case could be concluded from these observations alone. More detailed epidemiological studies of P. hominis infections should be required.

According to the phylogenetic tree, P. hominis (ac: AF124609), P. hominis (ac: DQ412643), P. hominis (ac: DQ412642), P. hominis (ac: DQ412641), P. hominis (ac: DQ412640) and P. hominis (ac: DQ899948) formed the 1st Clade. P. hominis (ac: AY349178), P. hominis (ac: AY245137), P. hominis (ac: AF342741), P. hominis (ac: AF156964), P. hominis (ac: AY758392) and P. hominis (ac: U86616) formed the 2nd Clade. P. hominis (ac: AF288745), P. hominis (ac: Z18250), P. hominis (ac: Z18259), P. hominis (ac: AY886880) and P. hominis (ac: AY886879) formed the 3rd Clade. P. hominis (ac: Z18258) was placed independently between the three Clades. P. hominis (AY245137) and P. hominis (AY886880 and AY886879), which are normal human commensals, were assigned to both the 2nd and 3rd Clades, but the P. hominis identified in this study was assigned to the 1st Clade, where the host of origin comprises animals such as Bos taurus and felines [10,13]. These natural hosts are consistent with the presence of P. hominis found in puppies with diarrhea in the present study; the dog farm is located in a mountainous and rural setting where contact with wild animals naturally harboring the bacterium could easily occur. Although the pathogenic potential of P. hominis in dogs remains unknown, the present study is the first to establish a molecular identification of P. hominis in puppies with diarrhea and confirm the taxonomic status of P. hominis in the Republic of Korea. Further studies concerning morphological identification, clinical isolation and confirmation of pathogenicity of P. hominis are required.