Presented by
A. Fituri
MBRC/LIBFISH Aqua Component Group
1. INTRODUCTION
Towards the end of July 1994 sick shrimps were observed at the Ain Kaam aquaculture farm with some mortality being reported every day. The LIBFISH Aqua Working Group was requested to visit the farm and provide technical advice on the matter.
2. SIGNS AND SYMPTOMS
General appearance of the animals:
Eyes: whitish plaques and atrophy;
Gills: dark spots;
Body:
light grey colour.
Some of the animals had one eye missing due to the disease and were losing their balance (spiral swimming). They were expected to die within a few days.
3. WATER QUALITY
Temperature inside the tanks in August around midday was 29.4°C in shade-protected tanks and 30.4°C in unprotected tanks. Dissolved oxygen was around saturation (6mg/l), which is quite low for shrimps.
4. HYGIENE
Evidence of deficient hygiene could be observed in the following ways:
Poor cleanliness of the hatchery hail (presence of birds and insects).
Floor and drainage channel not washed very often.
Shrimps fed on minced fish of doubtful freshness.
Excess feed not siphoned out of the tanks.
5. PRELIMINARY ASSESSMENT
5.1 Visual Observation
Based on the symptoms observed and reference to relevant literature, it was diagnosed that the shrimps were suffering from black gill disease (Sindermann and Lightner, 1989).
5.2 Microscopic Examination
Microscopic examination was carried out with the following results:
Cone-shaped conidiospores of Fusarium could be observed through examination of wet-mounts of infected tissue prepared from lesions. This observation constitutes a definitive diagnosis of the occurrence of Fusarium sp.
The fungus was isolated through in vitro culture of extracts of lesions on Sabouraud dextrose agar (at room temperature) and stain (lactophenol cotton blue).
The diagnosis was further confirmed by the microbiology laboratory of the Faculty of Veterinary Medicine, El Fatah University, Tripoli. It should be noted out that the microbial pollution rate of the water from the tanks and of the specimens was very high.
Further histopathological examination is in process.
6. CONCLUSION
Superficial infection of the carapace and further internal contamination through abrasions by Fusarium sp. conidospores present in sea water and detritus left inside shrimp tanks may represent the main mechanism of infection.
High temperature of the water in the summer time (around 30°C in July-August) and low oxygen concentration are additional factors provoking stress on the shrimps, leading to decrease in their natural immunity to infections. Furthermore, these conditions are highly favourable to a proliferation of micro-organisms.
It has also been documented in the literature that Fusarium spp. are commonly isolated from farmed or wild Decapoda Crustacea such as the ones used to feed shrimps at Ain Kaam farm.
Poor hygiene conditions allow fast development of fungal infection throughout the farm.
* For a more complete account of this investigation see Fituri and Vallet (1994).
