During a parasite survey of galaxiid fishes (Galaxiidae) from Patagonian Andean lakes, a new species of nematode, Hedruris suttonae n. sp. was collected from the stomach of the native Galaxias maculatus (Jenyns) and G. platei (Steindachner). Specimens were examined by light and scanning electron microscopy, especially head morphology, female caudal prehensile structure, and distribution of spines. The new species is distinguished by body and tail size, morphology and size of spicules, the arrangement of caudal papillae in the male, the female caudal hook, and size of eggs. Hyalella patagonica (Ortmann), a Neotropical species of Amphipoda, is reported as its natural intermediate host. Data regarding prevalence and mean intensity in the intermediate and definitive hosts are included. The diet and habitat of the hosts, the percentage of gravid females, the high values of prevalence, and mean intensity in galaxiid fishes, as well as the wide distribution of H. suttonae , collectively indicate that, in these oligotrophic Andean lakes, G. maculatus and G. platei are true definitive hosts of this nematode.

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A new species of Catatropis from a freshwater pulmonate snail of the family Chilinidae, which is endemic to South America, is described. Naturally infected Chilina dombeiana were collected from several localities in Andean Patagonia. The characteristics of the larval stages are presented. Experimentally reared adults, located in the distal portion of the intestinal caeca, were recovered from chickens and ducks. Adults of Catatropis chilinae n. sp. can be distinguished from all other species in having 9-11 (10) ventral glands, a cirrus-sac extending between the first third and the middle of the body, a metraterm slightly shorter or equal to the cirrus-sac, vitelline follicles reaching forward to the middle of the body, lobed testes, and a genital pore closely posterior to the caecal bifurcation. Eggs bear polar filaments only at the anopercular end. Rediae have only one or two cercariae. Shed cercariae are trioculate with a long tail and encyst in the environment, and metacercariae become infective 72 hours after encystment. This species is widely distributed between 40 degrees 10' S and 43 degrees 09' S and it is the first Catatropis species recorded for the Chilinidae and for Argentina.

Helminth infections within tissues tend to be subjected to a host response that can include encapsulation and melanization to isolate the parasite. The effectiveness of this response depends on the host species. During a survey of parasites of the native fish, Galaxias maculatus , we found conspicuous, strongly melanized exterior cysts located in the caudal peduncle of the fish. Dissection of these cysts exposed larval nematodes whose morphometrical features allowed their identification as Eustrongylides sp. Species of this genus are distributed worldwide. Galaxias maculatus was previously reported as second intermediate host to Eustrongylides sp. larvae, and the aquatic bird Podiceps major was cited as definitive host of Eustrongylides tubifex in Andean Patagonian lakes. The site of infection in the fish, and the host's response are unique among infections by larvae of Eustrongylides sp. in fishes, so the objective of this study was to describe the histological injury caused by larvae of the genus Eustrongylides parasitizing G. maculatus . Samples of fish were taken near the shore of Gutiérrez lake, in northwestern Patagonia. Some larval morphometric features were described to confirm the identity of larvae to generic level. Histopathological analysis (tissues sectioned at 5–7 μm, stained with Hematoxylin and Eosin, and Masson's Trichrome) of the caudal peduncle revealed almost complete disappearance of the epaxial musculature due to compressive atrophy and a chronic inflammatory response, associated mainly with a dense fibrotic capsule and an intense melanic deposit. This is the first description of the histopathology of an external cyst caused by Eustrongylides sp. larvae in fishes.

Galaxias maculatus (small puyen) is an abundant native fish distributed in lakes and rivers of the Patagonia, and it is the frequent prey of other fishes, fish-eating birds, and mammals. Previous studies have shown that it is parasitized by 33 metazoan species and that the richness and composition of the parasite communities vary between lakes. The aim of the present work was to analyze the relationship between the composition of fish assemblages and the helminth component community structure of G. maculatus . Ten environmentally similar, small, shallow lakes, belonging to the Nahuel Huapi Lake basin, were chosen because of the differences in the native fish assemblages. Parasite community structure in G. maculatus varied according to the fish assemblage of each lake. The presence of the piscivorous fish Percichthys trucha regularly produced variations in the composition and richness at the component and infracommunity levels, as well as the percentage of autogenic parasite species in G. maculatus .

A new species of Catatropis Oghner, 1905 from a freshwater Neotropical prosobranch snail, Heleobia hatcheri (Hydrobiidae), is described. Naturally infected snails were collected from Nahuel Huapí Lake in Andean Patagonia. The characteristics of the larval stages are also presented. Experimental adults were recovered from the distal region of the intestinal caeca of chicks and ducklings and natural adults from a wild duck Anas platyrhynchos. Adults of Catatropis hatcheri n. sp. can be distinguished from all other species of the genus in having 10-12 (11) ventral glands in each lateral row, the cirrus-sac extending back to between the first third and the middle of the body, the metraterm shorter than the cirrus-sac, a previtelline field of 1,258-1,544 (1,396), vitelline follicles reach back to the anterior border of the testes with some follicles extending slightly lateral to them, only external testicular margin lobed and genital pore in median line just posterior to the intestinal bifurcation. In addition, the eggs have one filament on each pole, the rediae contain one or two mature cercariae, and the cercariae are tri-oculate, with a long tail and encyst in the environment.

A new species of Notocotylus was found parasiting a freshwater pulmonate snail, Biomphalaria peregrina. Naturally infected snails were collected from two temporary ponds in the Nahuel Huapí National Park in Patagonia. The characteristics of the larval stages are presented. Experimental adults were recovered from the intestinal caeca of ducks and chicks. Adults of Notocotylus biomphalariae n. sp. exhibit an aspinose tegument, two lateral rows of 11 ventral glands and a median row of four, a uterus with 12-16 coils of which 2-4 are previtelline, a metraterm equivalent in size to 65-68% of the cirrus-sac length, a previtelline field which extends to the middle of the body, a lobed testis and a genital pore closely posterior to the intestinal bifurcation. The rediae have one to three cercariae. The cercariae, when shed, are trioculate and have a long tail; they encyst in the environment and become infective 12 days after encystment.

Two species of Digenea are recorded from the intestine of Percichthys trucha (Percichthydae) from Patagonian water bodies. Polylekithum percai n. sp. (Allocreadiidae) is distinguished by its body size, the relative size of the suckers, the anterior extension of the vitelline fields, the number of eggs in the uterus and the different species of fish host from the two previously described species of Polylekithum. Homalometron (= Austrocreadium) papilliferum (Szidat, 1956) n. comb. is redescribed. Due to its spined tegument and the absence of a cirrus-sac, this species is considered a member of the family Apocreadiidae.