Infection by Austrodiplostomum compactum metacercariae in fish from the Nova Avanhandava reservoir , Tietê river , São Paulo State , Brazil

This study aimed to evaluate the infection by the Austrodiplostomum compactum metacercariae in fishes from the Nova Avanhandava Reservoir, low Tietê river, São Paulo State, Brazil. The parasites were collected from eye (aqueous and vitreous humor), fixed in AFA solution and stained with carmine. The morphometric analysis was performed using a computerized system for analysis of images QWin Lite 2.5 (Leica). Prevalence, mean intensity of infection and abundance of infected fish were calculated. Of the 22 species of fish registered, five were infected by metacercariae: Hoplias malabaricus, Metynnis maculatus, Plagioscion squamosissimus, Satanoperca pappaterra and Schizodon nasutus. Of the 627 fish evaluated, 34% were infected. A higher prevalence was observed in P. squamosissimus and S. pappaterra. Schizodon nasutus and M. maculatus are new hosts reported for A. compactum metacercariae.


Digenetic
helminthes of the family Diplostomidae are responsible for 'wormy cataract', also called diplostomiasis (MARTINS et al., 1999).The genus Austrodiplostomum Szidat and Nani, 1951 have been reported infecting several species of vertebrates (KOHN et al., 1995;RAMOS-RAMOS, 1995;YAMAGUTI, 1971;SCHOLTZ et al., 1995;NIEWIADOMSKA, 1996;NIEWIADOMSKA;LASKOWSKI, 2002).These parasites, when adults, are able to infect birds and mammals, and the metacercariae stage are found infecting fish and amphibians, which are intermediate hosts in their life cycle (AMATO et al., 2001;FLOWERS et al., 2004;OSTROWSKI-NÚÑEZ, 1982;RIETSCHEL;WERDING, 1978).According to Eiras (1994), the presence of this parasite in the eyes can cause blindness or reduced vision, making the fish susceptible to the predation that facilitates the transmission of the parasite to the definitive host.

Material and methods
The study was conducted in the Santa Bárbara river, municipality of Buritama, São Paulo State, Brazil, a major tributary of the Nova Avanhandava reservoir, the penultimate hydroelectric plant of the mid Tietê river, whose dam is located at the geographic coordinates 21º07'S and 50º17'W (Figure 1).Fish were collected for this study on July 31, September 9, October 15 and December 18, 2003 using a gillnet of nylon monofilament, installed in several parts of the study area.Fish were identified and conferred on the basis of Reis et al. (2003) and Graça and Pavanelli (2007).For the determination of the hosts for A. compactum, each sampled fish was macroscopically examined for detection of the metacercariae in the ocular globe (aqueous and vitreous humor).When a parasite was observed in a fish species, the eyes of all specimens of this species were analyzed in a stereomicroscope.
The parasites collected were processed according to Amato et al. (1991).The morphometric analysis was performed using a computerized system for analysis of images QWin Lite 2.5 (Leica) in a sample of 21 metacercariae from P. squamosissimus.The morphometric results (in micrometers) are presented as minimum-maximum (mean) values.
The identification of the metacercariae was according to Niewiadomska (2002a;2002b), Kohn et al. (1995) Prevalence, mean intensity of infection and abundance of infected fish was calculated according to Bush et al. (1997).the highest prevalence, mean intensity of infection and mean abundance (Table 1).The metacercariae (Figure 2, Table 2) of A. compactum presented a foliaceous body, slightly concave in the ventral face and with a small conical segment in the posterior region; a small subterminal oral sucker, two lateral pseudosuckers in the anterior region; oval pharynx, short esophagus, intestinal caeca ending near the posterior region; oval holdfast (tribocytic organ).Gland cells occupying most of anterior region, extending from the beginning of intestinal caeca to the anterior region of the tribocytic organ and small gonads in the posterior region, after the tribocytic organ, were observed.

Discussion
Among the species found infected by A. compactum in the present study, P. squamosissimus, S. pappaterra and H. malabaricus had previously been recorded as hosts for this digenetic metacercariae (PAVANELLI et al., 2000;MACHADO et al., 2005;PAVANELLI et al., 2006).However, S. nasutus and M. maculatus are new hosts recorded for A. compactum metacercariae.
It is emphasized that, in P. squamosissimus, parasitic levels were considerably higher.This species, from the Amazon Basin, was introduced by the Energy Company of São Paulo (CESP), in reservoirs under its responsibility from 1966 to 1973 (TORLONI et al., 1993).According to Pojmanska and Chabros (1993), the prevalence of diplostomid in introduced fish is higher when compared with that of indigenous fish.This finding was also observed by Machado et al. (2005), who found higher rates of infection in fish introduced into the floodplain of the upper Paraná river, and P. squamosissimus species with the highest prevalence and intensity of infection.These authors suggest that probably A. compactum was introduced along with P. squamosissimus, which justified its higher prevalence and intensity of infection.However, native species may have been acting as intermediate hosts, to which the adaptation of metacercariae of A. compactum would thus justify the low prevalence rates and intensity of infection in these fish.
The introductions, depending on the fish species, can promote reductions of the native stocks or local extinctions, as result of several factors including dissemination of pathogens and parasites (AGOSTINHO; JÚLIO JUNIOR, 1996).The infection with A. compactum metacercariae in several fish species shows that the low specificity of the parasite favors its spread, infecting other species of native fish, and the parasite appears to have found appropriate ecological conditions for its parasite cycle (snails, fish and birds) in many aquatic ecosystems (MACHADO et al., 2005;RUIZ;AGUILAR, 2005).
In this reservoir, the elements necessary for the biological cycle of the parasite are present.Studies have demonstrated the occurrence of gastropod mollusks Biomphalaria sp.Preston, 1910(FRANÇA et al., 2007) and also birds of the genus Casmerodius Gloger, 1842 and Phalacrocorax Brisson, 1760.In this environment, H. malabaricus, M. maculatus, P. squamosissimus, S. pappaterra and S. nasutus serve as an intermediate host for A. compactum.However, we postulate that this helminth adapted better to P. squamosissimus and S. pappaterra, and that S. nasutus and M. maculatus are sporadical secondary intermediate hosts or even an accidental infection case, also suggesting a low specificity of this parasite.

Figure 1 .
Figure 1.Map of Brazil highlighting São Paulo State and the place of study at the Santa Bárbara river, in the Nova Avanhandava reservoir, low Tietê river, municipality of Buritama, Brazil.

Table 1 .
Parasitism by Austrodiplostomum compactum in the eyes of fish from the Nova Avanhandava reservoir, low Tietê river, municipality of Buritama, São Paulo State, Brazil.