Digenea , Heterophyidae , Centrocestus formosanus ( Nishigori , 1924 ) metacercariae : Distribution extension for Mexico , new state record , and geographic distribution map

Centrocestus formosanus (Nishigori, 1924) is a minute trematode species inhabiting the intestine of fish-eating birds and mammals (Scholz and Salgado-Maldonado 2000). This trematode was apparently introduced to Mexico as larval stage (sporocyst or redia) within its first intermediate host, i.e., the thiarid snail Thiara tuberculata (Müller, 1774) (Amaya-Huerta and AlmeydaArtigas 1994). The second intermediate host includes various species of freshwater fish; to date, metacercariae of C. formosanus has been recorded parasitizing 59 freshwater fish species, both native and introduced, in 12 states of central and southeastern Mexico (Pérez-Ponce de León et al. 2007). However, the occurrence of this trematode species in fishes of northern Mexico has not been documented even though the snail T. tuberculata has been reported in hydrological systems of Chihuahua and Coahuila (ContrerasArquieta 1998).

Centrocestus formosanus (Nishigori, 1924) is a minute trematode species inhabiting the intestine of fish-eating birds and mammals (Scholz and Salgado-Maldonado 2000). This trematode was apparently introduced to Mexico as larval stage (sporocyst or redia) within its first intermediate host, i.e., the thiarid snail Thiara tuberculata (Müller, 1774) (Amaya-Huerta and Almeyda-Artigas 1994). The second intermediate host includes various species of freshwater fish; to date, metacercariae of C. formosanus has been recorded parasitizing 59 freshwater fish species, both native and introduced, in 12 states of central and southeastern Mexico (Pérez-Ponce de León et al. 2007). However, the occurrence of this trematode species in fishes of northern Mexico has not been documented even though the snail T. tuberculata has been reported in hydrological systems of Chihuahua and Coahuila (Contreras-Arquieta 1998). Metacercariae (a total of 5485 cysts, prevalence = 100%, mean intensity = 182.2 cysts per infected host) were observed in vivo; some were isolated from cysts using preparation needles, fixed with 4% formalin and stained with Mayer's Paracarmin ( Figure 3). Morphology of metacercariae fully corresponds with those described by Scholz and Salgado-Maldonado (2000) for central and southeastern Mexico. Voucher specimens were deposited in the Colección Nacional de Helmintos (CNHE), Instituto de Biología, Universidad Nacional Autónoma de México, Mexico City, with the accession #6253. Additionally, some shells of Thiara tuberculata (deposited at the Colección Nacional de Moluscos, Instituto de Biología, Universidad Nacional Autónoma de México, accession # CNMO 2763) were collected from the bottom of the river, but not alive specimens were found during our sampling and we could not examine the first intermediate host for larval stages of C. formosanus. Even though, the presence of the snail in the locality is important to correlate the presence of the trematode species and it explains its high infection levels.
Northern Mexico is predominantly an arid region, with important hydrological systems inhabited by numerous native freshwater fish species, including an endemic component (Miller et al. 2005). However, a comparison with central and southeastern Mexico show a few number of studies conducted on the helminth fauna of freshwater fishes. These few studies do not report the presence of metacercariae of Centrocestus formosanus, although its occurrence should be correlated with previous records of the gastropod intermediate host in the region, and the high capacity of the digenean to disperse and infect a large number of second intermediate host species. Our findings represent the first record of the metacercariae of C. formosanus in freshwater fish of northern Mexico, where this invasive parasite could represent a potential risk factor to the survival of native fish fauna because its high prevalence and pathological effects over the host (Vélez-Hernández et al. 1998;Alcaraz et al. 1999;Ortega et al. 2005).
The snail Thiara tuberculata serves as the first intermediate host of Centrocestus formosanus in Mexico (Scholz and Salgado-Maldonado 2000). This snail species has been purposely introduced into some localities to control other snail populations that act as vectors for human schistosomiasis, a control mechanism that has been used for other snails (Pointier et al. 1991;Scholz and Salgado-Maldonado 2000). Instead, it was apparently introduced to Mexico as a food source for the black carp Mylopharyngodon piceus (Richardson, 1846), a species that was imported from China with aquaculture purposes (Amaya-Huerta and Almeyda-Artigas 1994).

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The high dispersal capability of C. formosanus and its occurrence in a wide range of freshwater fish species throughout Mexico represents a potential problem not only for aquaculture, but also for establishing proper conservation practices in wild native fish. Knowledge about distributional range of the metacercariae of this trematode species, along with further research in other stages of its life cycle, will contribute with relevant data that, added to information derived from the host biology and the deterioration of the ecosystems they live in, will allow conservationists to propose proper management strategies to preserve biological resources in the area.
This project was funded by the Consejo Nacional de Ciencia y Tecnología (CONACyT 83043), and Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica (PAPIIT-UNAM IN 209608) to G.P.P.L.