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Abstract The present study used regression models to evaluate the existence of factors that may influence the numerical parasite dominance with an epidemiological approximation. A database including 3,746 fish specimens and their respective parasites were used to evaluate the relationship between parasite dominance and biotic characteristics inherent to the studied hosts and the parasite taxa. Multivariate, classical, and mixed effects linear regression models were fitted. The calculations were performed using R software (95% CI). In the fitting of the classical multiple linear regression model, freshwater and planktivorous fish species and body length, as well as the species of the taxa Trematoda, Monogenea, and Hirudinea, were associated with parasite dominance. However, the fitting of the mixed effects model showed that the body length of the host and the species of the taxa Nematoda, Trematoda, Monogenea, Hirudinea, and Crustacea were significantly associated with parasite dominance. Studies that consider specific biological aspects of the hosts and parasites should expand the knowledge regarding factors that influence the numerical dominance of fish in Brazil. The use of a mixed model shows, once again, the importance of the appropriate use of a model correlated with the characteristics of the data to obtain consistent results.

Abstract The variability in parasite abundance has an ecological basis; however, from an epidemiological point of view, the contribution of factors inherent to the host to the variability in parasite abundance remains an open question. A database consisting of 3,746 specimens of 73 fish species was used to verify the relation between the distribution of parasite abundance in fishes and a set of biotic factors inherent to the hosts. Classical and mixed Poisson regression models were constructed. Prevalence ratios (PR) and their respective 95% confidence intervals were estimated. The parasite abundance was significantly higher in female hosts, nonschooling species, species from benthopelagic and pelagic habitats, and fishes with greater body length. Overall, these results suggest that the variability in the abundance of infection is an attribute of the parasite species. Although the results are biologically plausible, important gaps may still exist and should be explored to better understand the variations in parasite abundance, which has great relevance in epidemiological studies. We reinforce the importance of choosing the statistical model most appropriate for the nature of the data to avoid spurious results, especially when the autocorrelation in the data is not taken into account.

Durante o estudo dos metazoários parasitos de Paroaria dominicana Linnaeus, 1758 (Passeriformes), quatro infrapopulações de Platynosomum illiciens (Brown, 1901) (Trematoda) foram coletadas nos ductos hepáticos, com intensidades parasitárias de 10, 11, 18 e 36 espécimes respectivamente. Diferenças entre os valores morfométricos médios e o número total de ovos das infrapopulações foram estatisticamente testadas. O peso do fígado não se correlacionou ao peso das aves, ao comprimento do corpo das aves e à abundância parasitária. Foram detectadas correlações positivas entre o tamanho das infrapopulações e o número de ovos produzidos e entre o tamanho do corpo dos parasitos e o tamanho dos ovos. Correlações negativas foram observadas entre a intensidade parasitária e o peso do fígado; entre o tamanho das infrapopulações e o tamanho dos indivíduos dessas infrapopulações; entre o número e o tamanho dos ovos e entre o tamanho do corpo dos parasitos e o número de ovos. Os resultados indicaram que as infrapopulações maiores se estabeleceram em aves com fígados menores e que quanto maiores as infrapopulações foram observados menor tamanho corporal e menor tamanho dos ovos, os que se apresentaram em maior quantidade.During the study of the metazoan parasites of Paroaria dominicana Linnaeus, 1758 (Passeriformes), four infrapopulations of Platynosomum illiciens (Brown, 1901) (Trematoda), parasitic in the liver ducts were collected, with intensity of infection 10, 11, 18 and 36 specimens respectively. Differences between the means of morphometrical values and the total number of eggs were statistically tested. The liver weigh were not significantly correlated with avian total weigh and length, and with the total parasitic abundance. Positive correlations were observed between the size of infrapopulations and the total number of eggs, and between parasite body size and the eggs. size. Negative correlations were observed between parasitic intensity and liver weigh; between the size of infrapopulations and the parasite body size; between total number and the size of eggs and between parasite body size and total number of eggs. These results suggests that host with smaller liver had greater infrapopulations. These greater infrapopulations showed smaller parasite body size and higher size and number of eggs.

Entre abril de 2002 e julho de 2003, foram necropsiados 231 espécimes de peixes: 39 Astyanax bimaculatus (Linnaeus, 1758), 79 A. fasciatus (Cuvier, 1819), 23 Hypostomus affinis (Steindachner, 1877), 26 Hoplias malabaricus (Bloch, 1794), 30 Loricariichthys castaneus (Castelnau, 1855) e 34 Trachelyopterus striatulus (Steindachner, 1876) provenientes do Reservatório de Lajes (22º42' - 22º50'S, 43º53' 44º05'O), Estado do Rio de Janeiro, Brasil, para estudo das suas comunidades parasitárias. A maioria dos espécimes de H. affinis (95,6%) e H. malabaricus (84,6%) estavam parasitados por pelo menos uma espécie de metazoário. Em A. bimaculatus, A. fasciatus, L. castaneus e T. striatulus 41%, 39,2%, 56,7% e 14,7% dos espécimes estavam parasitados, respectivamente. Foram coletadas oito diferentes espécies de metazoários parasitos: 2 em A. bimaculatus, 3 em A. fasciatus, 3 em H. affinis, 4 em H. malabaricus, 4 em L. castaneus e 1 em T. striatulus. As comunidades de metazoários parasitos das seis espécies de peixes estudadas apresentaram típico padrão de distribuição agregada. Foram observados dois casos de correlação negativa entre a abundância e a prevalência parasitárias e o comprimento total dos hospedeiros. A comunidade parasitária de L. castaneus apresentou os maiores valores de intensidade média, índice de dispersão e diversidade. As comunidades parasitárias dos peixes estudados apresentaram escassez de correlação entre a abundância, riqueza parasitária e diversidade com o comprimento total dos hospedeiros. Os baixos valores de riqueza e diversidade das comuni-dades parasitárias podem ser atribuídos as características oligotróficas do Reservatório de Lajes.From April 2002 to July 2003, 231 freshwater fishes from Lajes Reservoir in the State of Rio de Janeiro, Brazil (22º42' - 22º50'S, 43º53' - 44º05'W) were necropsied to study their community metazoan parasites: 39 Astyanax bimaculatus (Linnaeus, 1758), 79 A. fasciatus (Cuvier, 1819), 23 Hypostomus affinis (Steindachner, 1877), 26 Hoplias malabaricus (Bloch, 1794), 30 Loricariichthys castaneus (Castelnau, 1855) and 34 Trachelyopterus striatulus (Steindachner, 1876). The majority of specimens of H. affinis (95.6%) and H. malabaricus (84.6%) was parasitized by one or more metazoan species. In A. bimaculatus, A. fasciatus, L. castaneus and T. striatulus 41%, 39.2%, 56.7 and 14.7% of specimens were parasitized, respectively. Eight different metazoan parasites species were collected: 2 in A. bimaculatus, 3 in A. fasciatus, 3 in H. affinis, 4 in H. malabaricus, 4 in L. castaneus and 1 in T. striatulus. The parasites of the six host species showed the typical aggregated pattern of distribution. Two cases of negative correlation between host's total length and prevalence and parasite abundance were detected. The parasite community of L. castaneus showed the higher values of mean intensity, index of dispersion and higher values of diversity. The parasite communities of the studied fishes showed scarcity of significant correlations of parasitic abundance, species richness and diversity with the size of the host. The low species richness and diversity of parasite communities could be originated by the oligotrophic characteristics of the Lajes Reservoir.