Diseases and control measures - L.vannamei

The major disease problems are included in the table below. There are no chemicals or drugs available to treat the viral infections listed but good management of pond, water, feed and health status of stock inputs can reduce their virulence. Outbreaks of the most serious virus (WSD) always occur after dramatic changes in water parameters such as temperature, salinity caused by heavy rain, DO2, hardness, and the stress to shrimp caused by deterioration in water quality and pond bottom environment. Pond preparation by proper bottom cleaning or regular scraping of the fouled layer is also a key factor for prevention of the shrimp stress caused by built up waste and toxic gases, and also for the elimination of virus carriers, particularly crustaceans. For confirmation, the Polymerase Chain Reaction (PCR) test for white spot disease or other viruses is widely used for screening of broodstock before spawning, nauplii before larval rearing, late PL before pond stocking, and shrimp in ponds for regular monitoring.

DISEASE	AGENT	TYPE	SYNDROME	MEASURES
White spot (WSD)Otherwise known as WSBV, WSSV	Part of the white spot syndrome baculovirus complex	Virus	Acutely infected shrimp show rapid reduction in food consumption; lethargy; high mortality rates with cumulative mortalities reaching 100 percent within 3 to 10 days of the onset of clinical signs; acutely infected shrimp often have loose cuticle with white spots (which represent abnormal deposits of calcium salts by the cuticular epidermis) of 0.5 - 2.0 mm in diameter that are most apparent on the inside surface of the carapace; in many cases moribund shrimp display a pink to reddish-brown colouration due to expansion of cuticular chromatophores & few if any white spots	Screening of broodstock, nauplii, PL & grow-out stages; avoiding rapid changes in water conditions; avoiding shrimp stress; avoid use of fresh feeds, particularly crustacean; minimising water exchange to prevent virus carriers entering the pond; treating infected ponds or hatcheries with 30 ppm chlorine to kill infected shrimp & carriers; disinfect associated equipment
Yellowhead (YHD) Also known as Yellow-head shrimp disease, Yellow-head virus (YHV), Yellow-head baculovirus (YBV), Yellow-head disease baculovirus (YHDBV)	Not yet described	Virus	Acute epizootics with high cumulative mortalities that may reach 100 percent within 3-5 days after appearance of clinical signs; infection is horizontally transmitted; PL 15 have been found to be resistant but PL 20-25 & on-growing juveniles through to sub-adults are highly susceptible; initially, feeding increases, followed by reduced feeding in later stages of the disease; pale body; yellowish swollen cephalothorax & hepatopancreas; whitish-yellowish-brownish gills; presumptive diagnosis can be made on basis of pond history, clinical signs, gross changes & histopathology	Screening of broodstock before hatchery operation & PL before stocking in pond; avoiding rapid changes in water pH, alkalinity, & dissolved O2; avoiding fresh aquatic feeds; proper cleaning of pond bottom before stocking; infected ponds & hatcheries must be disinfected similar to WSV (see above)
Baculoviral Midgut Gland Necrosis (BMN)Also known as midgut gland cloudy disease, white turbid liver disease, and white turbidity disease	Baculovirus	Virus	Generally infects larvae & early postlarval stages in which it can cause high mortalities; apparent white turbidity of the hepatopancreas caused by necrosis of tubule epithelium & possibly also the mucosal epithelium; larvae affected but later stages (late postlarvae) tend to show resistance; source of infection documented as wild-caught female spawners; larvae float inactively on the surface & exhibit a white midgut line through the abdomen	Wash fertile eggs through a soft gauze by running clean seawater to remove excrement or faeces of spawner; if infected, culture facility must be disinfected to avoid re-introduction of virus
Nuclear Polyhedrosis BaculovirosesAlso known as Monodon baculovirus disease (MBV)	Baculovirus	Virus	Lethargy, anorexia, dark coloured shrimp; reduced feeding & growth rates; often increased surface & gill fouling with various epibiotic & epicommensal organisms; severely affected larvae & postlarvae may exhibit a white midgut line through the abdomen; acute MBV causes loss of hepatopancreatic tubule & midgut epithelia &, consequently, dysfunction of these organs, often followed by secondary bacterial infections; linked with high mortalities (>90%) in late postlarvae & juvenile shrimp in many culture facilities; usually juvenile & adult P. monodon are more resistant to MBV than larval shrimp; MBV may predispose infected shrimp to infections by other pathogens;	Reduce stocking density, use of chemicals & environmentally induced stress; prevent contamination of fertilised eggs from spawner faeces by washing in formalin or iodophore treated seawater; if infected, culture facility must be disinfected & stock should be removed & sterilised