Metacercariae of Strigeidae Parasitizing Amphibians of the Chaco Region in Argentina

The goal of the present study was to analyze the metacercariae found in adult amphibians collected in the Argentinean Chaco region. A total of 194 frogs, Leptodactylus chaquensis, Leptodactylus latinasus, and Lepidobatrachus laevis was examined for digeneans. Three different larval trematodes of the genus Strigea (Digenea, Strigeidae) were found infecting the liver, mesentery, body cavity, and muscle of frogs. These metacercariae are described and illustrated for the first time and provide new information on the composition of metacercariae in amphibians, thus contributing to the knowledge of larval trematodes biodiversity in the Neotropical region. We also present new information on the morphology and morphometry of strigeid metacercariae.


INTRODUCTION
The study of metacercariae in amphibians is significant because these hosts transmit metacercariae to terrestrial vertebrates, which are potential final hosts where these larvae can complete their life cycle.The identification of these larvae is not trivial, once they have few morphological features with taxonomic value.
However, metacercariae of the family Strigeidae Railliet, 1919 in amphibians of the Neotropical region have not yet been morphologically characterized or illustrated (see lunaschi and Drago 2007).In this context, our aim is to describe the morphological and metric features of strigeid metacercaria species in naturally infected frogs from the Argentinean Chaco Region.

MATERIALS AND METHODS
A total of 194 adult frogs, Leptodactylus chaquensis, Leptodactylus latinasus, and Lepidobatrachus laevis was collected in 3 localities of Argentina (Table I).All adult specimens were killed using a chloroform (CHCI 3 ) solution.The present study is in accordance with all the regulations and ethical and legal considerations for the capture and use  Yamaguti (1975) and Niewiadomska et al. (2002).Photographs were taken with a Leica DFC 295 camera mounted on a Leica DM 2500 microscope, Leica Microsystems, Wetzlar, Germany.For examination using scanning electron microscopy (SEM), some specimens were dehydrated through an ethanol series, acetone, and ether.The specimens were gold-coated and examined using a Jeol 5800LV, Jeol, Tokyo, Japan.

RESULTS AND DISCUSSION
Strigea sp. 1 Cyst (measurements based on 10 cysts): Tetracotyle cyst, egg-shaped, 760-1100 (949.5 ± 115.9) long x 540-650 (594.0 ± 37.5) wide; cyst inner wall of parasite origin with a hyaline layer, covered by a radiated layer, surrounded by an elastic layer apparently formed by the intermediate host as a reaction against the parasite (Figures 1a, 2a).larvae not easily released.
Here we found 3 strigeid metacercariae in the Chaco region of Argentina.These metacercariae are added to the larvae previously described parasitizing different amphibians of Corrientes Province, Argentina (Hamann and González 2009), and are new records of metacercariae for the Chaco region.The main morphological differences between these 3 larvae are shown in Table II.We are aware that this study presents many gaps, which, hopefully, will be supplemented by future molecular analyses and life-cycle studies.Specifically, we contribute new information on morphological and metric data, when they are not known more than adult stage in most of the strigeid species recorded from Argentinean wild birds (see Drago and Lunaschi 2015).