2.9. Remarks

Tapeworms found by the present authors in common carp correspond in their morphology to K. japonensis as redescribed by Scholz et al. (2011a). All specimens were immature except a single worm bearing just two eggs in the uterus. The worms were 4.8–10.7 mm long and 0.64–1.23 mm wide at the level of the scolex or at the first third of the body. The scolex is cuneifimbriate, 0.64–1.01 mm wide (Fig. 3A and B). Anterior-most testes arrange in two longitudinal row, confluent more posteriorly; posterior-most testes anterior or anterolateral to the cirrus-sac (Fig. 3A, C). Anterior-most vitelline follicles begin at the same level as first testes and reach posteriorly as far as to the ovary, with a few follicles present lateral to the ovarian wings, but not continuous with postovarian follicles (Fig. 3A, C).

Khawia japonensis was described as Caryophyllaeus japonensis from the common carp, Cyprinus carpio, in Japan and occurred originally in East Asia (Yamaguti 1934; Protasova et al., 1990; Scholz et al., 2001, 2011a). With the import of common carp throughout the world, the parasite was introduced to North America (described there as Khawia iowensis by Calentine and Ulmer, 1961) and more recently to Europe (Scholz et al. 2011b, 2018).

Scholz et al. (2011a) considered K. japonensis to be a specific parasite of C. carpio, even though Hoffman (1999) reported non-cyprinid fishes such as bigmouth buffalo, Ictiobus cyprinellus, white sucker, Catostomus commersonii (Cypriniformes: Catostomidae), and black bullhead, Ameiurus melas (Rafinesque) (Siluriformes: Ictaluridae) as hosts of this parasite in North America. However, these fishes are apparently incidental hosts or other caryophyllidean tapeworms were misidentified.

In North America, K. japonensis occurs in Canada (Manitoba and Ontario) and USA (California, Illinois, Iowa, Kansas, Minnesota, Nebraska, North Dakota, Oklahoma, Oregon, South Dakota, Tennessee, Wisconsin – Scholz et al., 2018). The only previous record of K. japonensis (as K. iowensis) from Mexico was listed in an Appendix 1 by Pérez-Ponce de León and Choudhury (2005) from unspecified cyprinids from the Rio Grande basin in Mexico. However, no other details about this record were provided. The present specimens confirm the occurrence of K. japonensis in northern Mexico and most likely represent the southernmost record of the parasite (25.270833 S, 103.773056 W).

Khawia japonensis is distinguished from congeneric species, including Khawia sinensis Hsü, 1935, another parasite of common carp, by the shape of its body, with almost the same width throughout its length, a cuneifimbriate scolex slightly wider than the neck region, the first testes beginning very close posterior to the scolex, usually anterior to the first vitelline follicles (Fig. 3A, B), and a few vitelline follicles present lateral to the preovarian uterine loops, with a few isolated follicles lateral to the ovarian arms (Fig. 3A, C). Unlike K. sinensis, which is considerably larger (up to 12 cm) and has a wider distribution area (Oros et al., 2009), the pathogenicity and veterinary importance of K. japonensis for cultured carp are not known.