Dear Editor,
We have read with extreme interest the recent publication entitled “Possible zoonotic implications of the discovery of the advanced third stage larva of Gnathostoma turgidum (Spirurida: Gnathostomatidae) in a Mexican fish species” by Mosqueda-Cabrera et al., which was appeared in your journal, Helminthologia, 2023; 60(1): 112 – 116. They reported the discovery of one advanced 3rd stage larva (AL3) of G. turgidum from the muscles of one out of 17 wild fish, of which common name is a big mouth sleeper and the scientific name Gobiomorus dormitor, in Veracruz, Mexico. Since Cole et al. (2014) previously found 5 AL3 of G. turgidum in 3 (4.5 %) out of 67 wild-caught swamp eels Monopterus albus from Tampa, Florida, USA, discovery of G. turgidum AL3 in edible fish in Mexico by Mosqueda-Cabrera et al. (2023) strengthened the possibility of G. turgidum as the zoonotic pathogen in the Americas. By now, however, G. binucleatum is the only confirmed agent of indigenous human gnathostomiasis in Mexico (Diaz-Camacho et al., 2020) but actual infection in humans with G. turgidum has not been reported. In addition to G. binucleatum and G. turgidum, several other Gnathostoma species have been recorded in the wild life in Mexico (Pérez-Álvarez et al., 2008). Combined morphological and molecular identification of the causative species of human gnathostomiasis is, thus, critically necessary in the endemic areas. In the present study, Mosqueda-Cabrera et al. (2023) could not detect G. turgidum AL3 by the compression of the sliced fish meat between 2 backlit glass plates, but eventually found one AL3 after artificial digestion in of the whole meat examined by compression, and put an emphasis on the advantage of digestion method. Nevertheless, the authors group previously found encysted G. turgidum AL3 in the musculatures of amphibians (4 larvae in a turtle Kinosternon integrum, and 8, 2 and 1 AL3 in 3 out of 5 frogs, Lithobates zweifeli) in Mexico (Mosqueda-Cabrera et al., 2010) using the compression method. Thus, if G. turgidum AL3 were encysted in the musculatures of fish, it might be not so difficult to detect them by the compression method. Related to this, previously Mosqueda-Cabrera et al. (2010) reported that G. turgidum AL3 were not encysted in experimentally infected fish hosts, such as guppies and Nile tilapia, and they disappeared from fish hosts rather quickly within 2 months post-infection. In Tecualilla, Sinaloa, Mexico, where practically 100 % of opossums, D. virginiana, were infected with G. turgidum AL3 (Diaz Camacho et al., 2009). We examined a large number of fish, Dormitator latifrons and Eleotris picta (Diaz Camacho et al., 2008) Although we found high infection rate of G. binucleatum AL3 in those two fish species, but were unable to detect any G. turgidum larvae from them. Thus, taking all those works together, fish in general might be not suitable intermediate hosts of G. turgidum. Further extensive survey for G. turgidum larvae in fish is required to elucidate this issue.
In terms of the size of G. turgidum AL3, the only one sample found from naturally infected fish by Mosqueda-Cabrera et al. (2023) was about 1.5 mm in length and 0.14 mm in width, which is compatible with those (average 1.7 x 0.14 mm) of G. turgidum AL3 found from the musculature of frogs by the same study group (Mosqueda-Cabrera et al., 2010), but is much smaller compared with the AL3 of other Gnathostoma species, which are about 3 – 5 mm in length. As for the size of G. turgidum AL3, Diaz-Camacho et al. (2010) found much larger G. turgidum AL3 of almost 1 cm length including one worm just molting to adult stage from the liver of naturally infected opossums, Didelphis virginiana. Almeida-Artigas et al. (2010) also found two such extremely large G. turgidum AL3 in the liver of a naturally infected opossum, Philander opossum pallidus, which is now known as Philander vossi Gardner & Ramírez-Pulido, 2020. Mosqueda-Cabrera et al. (2010) as well described the extraction of two large G. turgidum AL3, one of which is of 8.2 x 0.5 mm size under molting to the adult, from the liver of an experimentally infected opossum. Thus, G. turgidum AL3 have a potential to grow up further to gigantic AL3 in the liver of the definitive host opossums. Since the molting to adult stage was consistently observed in those three different studies, biological significance of the intrahepatic growth of G. turgidum AL3 in the definitive hosts needs further clarification. Also, whether such an additional growth of G. turgidum AL3 would occur in non-susceptible mammalian hosts including humans is of extreme interest in relation to the pathogenesis.
Not only the low infection rate and the smaller size of AL3 in fish, G. turgidum has many unique features compared with other Gnathostoma species: They mature in the liver before establish in the stomach of opossums (Díaz-Camacho et al., 2010) and the size of adults are extremely large (mature females are about 10 cm). Their host range seems to be extremely narrow, found only in opossums, with clear seasonality by spontaneous cure (Nawa et al., 2009; Torres-Montoya et al., 2014). Moreover, molecular phylogenetic study showed that G. turgidum is distinct from all other Gnathostoma species and is considered an oldest one (Nawa et al., 2015). Thus, G. turgidum is fascinating not only as a potential zoonotic pathogen, but also as a biological research target because of its unique and mysterious features.