Current state of knowledge of helminth parasites of amphibians in Argentina: a taxonomic and ecological approach

HAMANN M. I.

São Paulo

Mesa redonda; XXII Congresso Brasileiro de Parasitologia; 2011

Studies related to the parasite fauna of amphibians, and especially taxonomic and ecological studies of helminth parasites, have been conducted in Argentina for several years. Of the amphibians known to be present in the country, 20% have been analyzed; about 70 helminth species come from different families of Trematoda (Echinostomatidae, Diplostomidae, Proterodiplostomidae, Stigeidae, Plagiorchiidae, Macroderoididae, Brachycoeliidae, Diplodiscidae, Derogenidae, Gorgoderidae and Allocreadiidae), Nematoda (Rhabdiasidae, Onchocercidae, Molineoidae, Kathlaniidae, Hedruridae, Dictyocaulidae, Cosmocercidae, Atractidae, Ascarididae, Rhabdochonidae, Physalopteridae, Pharyngodonidae, Kathlaniidae, Gnathostomatidae and Anisakidae), Cestoda (Nematotaeniidae) and Acanthocephala (Centrorhynchidae). The larval stages and adults of the trematodes and nematodes have the highest species number and show low host specificity; that is, they are predominantly generalist species. Most of the nematodes found in these amphibians have direct cycles that enter the host through skin penetration, while the remaining groups of helminths require intermediate hosts to complete their biological cycles. Ecological research suggests that various aspects related to the amphibians (e.g. body length, sex, ontogenetic changes, and trophic and reproductive behaviour), to the parasites (e.g. maturity, life cycle, etc.), to the climatic factors (e.g. rain, season, etc.) and to the type of habitat (e.g., land, water, pristine or disturbed by agricultural activity) have an impact on the occurrence and abundance of helminths at the population and community levels. The main conclusion reached at the population level is that infections do not have a marked seasonal pattern in relation to the maturity of the parasites, since the temperature in our region is not a limiting factor in parasite seasonality, but these infections may be associated with dry and rainy seasons. The research shows that in most cases there are no significant differences in the number of infected female anurans compared to male anurans. However, the host body size are important in determining the parasites abundance of helminths since greater body size of host provide more surface area for colonization of parasites, i.e., greater probability of skin penetration by cercariae and larval nematodes, and so this factor can favour higher values of parasitism. Additionally, most species of helminth show an aggregated pattern of distribution; that is, only a few host individuals are heavily infected. At the component community level, richness can vary from 6 to 24 helminth species per infected host, while at the infracommunity level, the highest mean richness value is no more than 5 helminth species per infected host. Research on intestinal helminths indicates that the dominant species in fossorial and terrestrial amphibians are nematodes (e.g., species of Cosmocerca), while in aquatic and arboreal amphibians, trematodes (e.g., Catadiscus inopinatus) are dominant. When considering the total helminths (larvae and adults), larval trematodes (Bursotrema tetracotyloides and Lophosicyadiplostomum aff. nephrocystis) dominate. Analyses of helminth parasite community richness in different environments show that terrestrial, fossorial, arboreal and aquatic amphibians have fewer helminth species than semiaquatic amphibians, which may indicate that these hosts, present in both aquatic and terrestrial environments, are exposed to a greater diversity of parasites, as they have a higher rate of exposure to a wider range of prey species, and, hence, to diverse infective states. Thus, the composition of helminth fauna in these vertebrates is related to diet, feeding strategies, vagility and the habitats these amphibians frequent. At the infracommunity level, helminth species richness is significantly affected by host body size, and associations and covariations between different groups of helminths have also been observed. Possible interactions between congeneric species in natural infections have been analyzed, in which both species use the same microhabitat and presumably the same food resource. This result suggests the possibility that the two species employ different feeding mechanisms, determined by morphological differences. The coexistence of both species in space may be associated with the differentiation of niches and also with an intraspecific aggregation that is stronger than interspecific aggregation, thus preventing competitive exclusion. With respect to extrinsic factors, the two species show high infection levels in different seasons (temporal heterogeneity), and this seasonal difference may also facilitate the coexistence of these congeneric populations. Analyses of the metacercariae found in the amphibians show that the movement of these hosts in different aquatic environments allows the entry of infective larvae throughout their ontogenetic development (tadpole, juvenile and adult stages). These infections suggest that they are suitable second intermediate hosts for these metacercariae. In fact, these results may be alternatives to ensure transmission because amphibians occupy an intermediate position in the food web, being easy prey for potential definitive host, such as snakes, birds and mammals. On the other hand, anthropogenic disturbances — in agricultural areas — may have both positive and negative effects on parasite infracommunity richness and abundance in the host amphibians. Helminth richness is lower in amphibians studied in agricultural areas compared to those in pristine areas. Finally, studies concerning the structure and organization of helminth communities show patterns that are not entirely isolationist, meaning they can be considered to occupy a position on a continuum between the extremes of isolated and interactive communities. For these reasons, the results found so far represent a breakthrough in the understanding of parasitic helminths in amphibians, as these studies have identified several intrinsic factors that influence the structure and the ordering of the parasite fauna at different organizational levels (population and community). However, more extensive studies on helminth communities of amphibians in the region are needed to make solid predictions about the patterns and processes that regulate this host-parasite-environment system. This is even more important if we consider the high host diversity in our region, on one hand, and, on the other, the effect of habitat fragmentation on biodiversity due to strong anthropogenic pressure, mainly in tropical ecosystems.