Pleistophora sp. (a microsporidian) infection in an ornamental fish, swordtail, Xiphophorus helleri ? the first record of the parasite in Sri Lanka

Abstract

An aquarium owner in the Western Province of Sri Lanka reported that a stock of swordtail fish that he received from a farm at Ginigathhena area in July, 2005 showed heavy mortality when the fish were kept in two holding facilities in his aquarium. An investigation was carried out to find out the possible cause/s of mortality. Moribund fish were observed for behavioural changes and external clinical signs and then 3 random samples each consisting of 15 fish from each holding facility were obtained. Each fish was euthansiated and necropsy was performed using standard procedure. Tissue samples from affected areas of the body of fish were preserved, histological sections were obtained and observed under the microscope after staining. Affected fish were lethargic and solitary and showed restless swimming at times. The normal reddish orange colour of the fish has been lost at certain places of the skin that covers the dorsal musculature leaving, white patches; these white patches did not have any excess slime formation. Wet mounts of teased muscle from the area of white patches, revealed masses of sporophorous vesicles (pansporoblasts) and typical egg-shaped spores (3.5 µ x 7.5 µ ; with the prominent posterior vacuole which is a diagnostic feature of microsporidia ) were liberated when the sporophorous vesicles were ruptured. Histological sections of the muscle tissue also showed gram positive spores of a Pleistophora sp. in large numbers. There was no other infection / infestation apart from this in moribund fish indicating that the primary cause of mortality was Pleistophora infection in muscles. As there is no proven treatment for microsporidian infections, the owner was asked to destroy the whole stock (few hundreds of remaining fish) and disinfect the holding facilities. This is the first record of Pleistophora infection in Sri Lanka and this is the first record of Pleistophora in swordtail fish.