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.