Species diversity of the genus Thelohanellus Kudo, 1933 (Myxozoa: Bivalvulida) parasitizing fishes in Indian subcontinent.

A synopsis of 52 nominal species of the genus Thelohanellus Kudo, 1933 reported to infect freshwater fishes in India is presented. These species infect various organs such as gills, fins, scales, skin, muscles, gall bladder, kidney etc. The diagnostic features of the genus include tear-shaped myxospores with smooth shell valves and single polar capsule. The members are histozoic in their nature of parasitism infecting fresh water fishes. The present manuscript is aimed to compile the list of reported species in India along with essential information on morphometrics of vegetative stage, host, organ preference and pathogenic effects if any. Maximum number of species are recorded from state of West Bengal (25) followed by 23 in Punjab (out of which 8 were already reported from other states of India), Andhra Pradesh (7), Orissa (3), Manipur (1) and Haryana (1). Majority of the species have been recorded to infect major and minor carps (43 species) and 9 species in cat fishes. Out of 43 species infecting Indian major carps, T. batae infected the epithelial lining of gill filament of Labeo bata and also the wall of the duodenum and pectoral fin of Wallago attu in Harike Wetland (Punjab). Organ distribution of the species belonging to the genus Thelohanellus indicated that the plasmodia of 27 species were located in the gills, 14 species in the fins, 7 species in the gall bladder, 4 species in muscles and 3 species in kidney, 2 each in scale and skin on the head region and 1 species each in brain, eye, spleen, liver and wall of the duodenum. Recent studies conducted by the authors and also by few earlier studies in West Bengal have documented many species of Thelohanellus to cause serious gill haemorrhagic disease and dermal lesions. Histological studies on the gills of cultured carps in Punjab infected with members of Thelohanellus have indicated that these parasites cause 15-20 % damage to respiratory surface leading to localized loss of function.

Histological location of myxosporean plasmodia in fish tissue with Luna's method.

In the present study, a staining technique, Luna's method, has been used for detection and tissue location of myxosporean spores/plasmodia in histological sections of gills and liver. The Luna's staining was performed on the sections of the tissue fixed in Bouin's, embedded in paraffin wax. The mature plasmodia were stained in bright red color and pansporoblasts and gill tissue stained in blue. This staining helped to detect and locate even a single myxospore of less than 15 µm in size located within the tissues of the host.

Prevalence, site and tissue preference of myxozoan parasites infecting gills of cultured fish in Punjab (India).

Native carp species cultured in Indian farms in Punjab (catla Catla catla, rohu Labeo rohita, mrigal Cirrhinus mrigala, exotic carps such as silver carp Hypophthalmichthys molitrix, grass carp Ctenopharyngodon idella, common carp Cyprinus carpio and a catfish Sperata seenghala) were examined for the presence of myxozoan parasites infecting gills. Firstly, the gills were examined under a zoom-stereomicroscope for the presence of plasmodia. The number of plasmodia per gill was counted to determine the index for the intensity of infection. Infected tissues were processed for histology, and 3-4 µm sections of infected gills were stained with haematoxylin & eosin and Luna's method. A total of 19 species of myxosporean were found infecting various cell types in the gills. Of these, 14 species belonged to the genus Myxobolus, 3 species to the genus Thelohanellus and 2 species to the genus Henneguya. Species belonging to the genus Myxobolus formed the interlamellar and intralamellar vascular (LV) type plasmodia, and species belonging to the genus Thelohanellus and Henneguya formed intrafilamental vascular (FV) type plasmodia. Mixed infections comprising 2, 3 or 4 different myxozoan species were noted in individual fish. The most common type of parasitism was polyparasitism due to 4 myxobolids co-occuring in fish with an infection rate of 23.16%. All species caused mild to severe haemorrhagic gill disease with little clinical symptomatology.

Myxobolus nanokiensis sp. nov. (Myxozoa: Bivalvulidae), a new pathogenic myxosporean parasite causing haemorrhagic gill disease in cultured Indian major carp fish, Labeo rohita (Hamilton 1822) in Punjab, India.

The plasmodia of Myxobolus nanokiensis sp. nov. were found infecting gills of Labeo rohita (Hamilton 1822) The infection rate was found to be 36.67 % (30 fishes were examined and 11 fishes were infected) in the Nanoki pond in Patiala district Punjab. Numerous minute plasmodia each filled with 150-200 spores were detected. Smear of scrapped blood-tinged mucous from gills exhibited millions of spores. Histological sections also indicated numerous plasmodia measuring 38.33-40.33 µm in diameter in the blood vessels of gill filaments. Spores of M. nanokiensis sp. nov. were elongate pyriform in shape and morphologically unique in having sharply pointed beak-like anterior end. Spores measured 9.28 µm × 5.71 µm in size. Polar capsules were equal, pyriform, 5.71 × 2.73 µm in size, each having polar filament with 7-9 coils. The present species has been proposed as new on the basis of its peculiar shape and morphometrics. This is the first report of any myxobolid infection in the farmland fishes in Punjab (India). The plasmodia in the gill filaments were of intralamellar vascular type (LV) and were present within the entire length of the filament. These plasmodia caused hemorrhage, necrosis of the blood vessels and cellular infiltration.