Tilapia farming, Oreocromis spp., It is one of the most widespread production activities in the world and refers to species of African origin grow and China produces 46% of such production. Outside their place of origin is indicated as introduced and as all species species has its own parasitic helminths and different cultivation sites, have registered the presence of these, in conditions of confinement and free life (^{Jimenez-Garcia et al., 2001}; ^{Zanolo et al, 2009}; ^{Le Roux and Avenant-Oldewage, 2010}; ^{Aguirre-Fey et al., 2015}). The presence of helminths in culture organisms, has been identified as a nonlethal parasitoids, but these monogeneans are among the most problematic helminths in fish production in production ponds, by viral and microbial infections that develop in the binding sites parasites (^{Gonzalez-Fernandez, 2012}). This study analyzes the prevalence of monogeneans Oreochromis spp., culture for point of sale.

As part of the pilot project for outlets live tilapia (PVTV), in november 2012 they were grown on the premises of Aquaculture Research Laboratory Graduate Applied- ITBOCA, 120 tilapia from aquaculture farm Medellin, Veracruz. They were kept under controlled conditions of temperature 28-30 °C and oxygen saturation (5 mg/l) and fed with food of choice (Purina fattening) in a vat of 10,000 liters in closed system and constant filtration, and by siphoning unused food and debris were removed tilapia.

They were examined to study helminthological 18 tilapias divided into two groups, nine fish were examined on day 1 of the culture and 9 more than 15 days, maximum time of sale of live tilapia in retail outlets. Morphometric data length in cm and weight in grams were taken, and helminthological test was applied to all tissues in the light of the stereomicroscope. Helminths recovered were fixed with 4% formaldehyde warm and preserved in 70% alcohol. Total subsequently stained preparations Mayer carmine or Gomori Trichrome were prepared, and mounted with Canada balsam (^{Lamothe-Argumedo, 1997}). For some preparations monogeneans semipermanent with Malmberg technique to observe sclerotic pieces were prepared (^{Vidal-Martinez et al., 2001}). The parasitic parameters prevalence (percentage of infected fish) and average intensity (average helminth infected for each species of helminth fish) and interval following criteria ^{Bush et al. (1997)} were calculated and behavior noted between two groups of sampling.

The 9 for the host examined one day, showed total length of 25.5±0.99 (24 to 27) and 353.6±48.8 weight g (290- 442), while for the 9 day fifteen fish presented total length of 21.8±7.6 (16-39) with weight of 273.4±344.6 (87-1118). Among both groups, worms 1158 located in 5 species of monogeneans were collected, four gill parasites species: Scutogyrus longicornis, Cichlidogyrus dossoni, Cichlidogyrus sclerosus, Cichlidogyrus tilapiae and a kind of intestinal monogenean Enterogyrus malmbergi. No species of other groups of parasites were recovered.

The highest prevalence and mean intensity was for C. sclerosus host both groups (Table 1), the prevalence for S. longicornis and C. dossoni increased significantly between the first and second sample and E. malmbergi only average increase in intensity. The observed changes are associated with favorable conditions for the development of tilapia farming, as also favorable for parasites. However, these infections do not cause fatal injures themselves, but under conditions of confinement increases the abundance recruitment. In addition monogeneans are a risk factor for the development of infections on which should be carried out preventive actions (^{Gonzalez-Fernandez, 2012}). The difficulties of handling these monogeneans infections are common in cultured tilapia Veracruz (^{Aguirre-Fey et al., 2015}) and around the world (^{Boungou et al., 2008}).

Table 1 Prevalence and mean intensity of helminth parasites Oreocromis spp., in point of sale.