Fish Diseases and Management

Examination of larvae and post larvae

Examine the specimen under dissecting microscope. Look for activity, gut contents, faecal strands, surface fouling, deformities, broken and missing appendages etc. for larvae and postlarvae. Select abnormal animals and transfer with a pipette to a glass slide and observe. Examine under low magnification objectives first, then with sub stage condenser set at reduced light for:

•  surface fouling of appendages and cuticle for filamentous bacteria and protozoans,particularly stalled  
• oral region for bacterial "plaques" (presumptive for Vibrio spp.).
• larval mycosis - diagnosed by demonstration of hyphae, discharge tube, and motile zoospores.
• muscle atrophy in abdomen, especially in 6^th abdominal segment (muscle should fill half of space available).
• hepatopancreas atrophy, vacuolisation, lipid droplets and BP inclusion bodies.
• "bolitias blancas" (white balls) - sloughed hepatopancreas or midgut epithelial cells which gives a presumptive diagnosis of vibriosis.
• "bolitias negras" (black balls) – poorly digested algae which indicates poor algae quality or possible vibriosis.
• gregarine trophozoites in hepatopancreas or midgut cuticular deformities: bent or broken setae or spines
• melanized appendages tips or foci.
• Motile rod-shaped bacteria in hemocoel: vibriosis in most cases.

Direct examination of the shrimp tissue for BP, MBV or BMN

Dissect out hepatopancreas using fine pointed dissecting needles and:

• place hepatopancreas in a drop of clean seawater or saline on slide.
• add small drop of 0.01% malachite green.
• squash with coverslip, and examine over next 5 min for baculovirus occlusion bodies and / or hypertrophied hepatopancreas cell nuclei. BP and MBV occlusion bodies take up green stain rapidly compared to other cellular materials. MBV infection is characterised by single or multiple spherical occlusion bodies, which are intranuclear, and range in diameter from 0.1 to 20 μ. BP infection on the other hand is demonstrated by single tetrahedral occlusion bodies. BMN infection is identified by the presence of hypertrophied nuclei with single eosinophilic inclusion body that fills the nucleus.