Work Package 1: Bacterial Disease Control

PHILIPPINES   

For 2002 and 2003, monitoring activity of problems in large-scale hatchery rearing of crab larvae was done to identify windows of opportunity for probiotic application. These led to identification of various microbial fouling and disease-causing organisms in spawned eggs and hatchery-reared larvae.

Microbial populations in eggs and larvae

Disease problems in mud crab larval rearing include severe fouling of eggs and larvae with filamentous algae, protozoan infestation, and sporadic fungal infection in the egg masses. Mortality of larvae in the hatchery is due to systemic bacterial infection and occasionally, fungal infection. Based on several runs, the main sources of bacterial pathogens in the hatchery are the following: developing eggs, untreated water supply, and natural food, mainly Brachionus and Artemia. Although the water supply can be effectively rid of bacterial pathogens by chlorination (10-15 ppm active chlorine), such technique does not remove pathogenic bacteria from other sources. 

Severe fouling of eggs with filamentous alga	Protozoan infestation in eggs

Fungi that were isolated from mud crab eggs and larvae are of two general types, vesicle-formers (Lagenidium-like) and non-vesicle-formers (Sirolpidium-like). For both fungal types, spore formation and release were inhibited by 10 ppm formalin bath exposure for 24-40 h, although the fungi remained viable and grew when replanted on peptone yeast extract agar. Fungal hyphae exposed to 50 and 100 ppm formalin for the same period did not produce nor release spores and were not visible after exposure.

Crab egg with fungal infection	Zoe infected with fungi

White spot syndrome virus (WSSV) in megalopae and pond-reared juveniles

The WSSV carrier status of hatchery-reared crab megalopae was determined using one-step and nested PCR tests using the WSSV-specific primer pairs designed by Kimura et al. (1996). Two batches of hatchery-reared megalopae were negative for WSSV after one-step and nested PCR tests. However, moribund samples of pond-reared crabs were positive after nested PCR test. Pooled samples representing hatchery and wild-sourced crabs obtained from ponds during harvest were negative for WSSV by one-step and nested PCR.

return to top