Diseases of shrimps

Diseases

With the expansion and intensification of shrimp culture, and consequent increased research efforts, a number of diseases that affect shrimps in captivity have been identified.

Many of them are associated with sanitary conditions in larval and fry rearing facilities, and some are caused by nutritional deficiences. Among the infectious diseases of shrimps are virus, bacterial, fungal and protozoan diseases; the more important non-infectious diseases are those caused by epibionts.

The virus disease caused by Baculoviruspenaei has been reported in P. duorarum,

P. aztecus, P. setiferus, P. vannamei and P. stylirostris. Penaeus monodon has been foundto be infected by a baculovirus, referred to as the Monodon baculovirus (MBN) and P. japonicus by one that causes mid-gut glandnecrosis (baculoviral midgut gland necrosis virus – BMNV). Though the virus infects adult shrimps as well, mortality occurs mainly in the post-larval or early juvenile stages.

In the baculovirus diseases the hepatopan-creatic tubule epithelium is affected. In the larval stages the anterior mid-gut epithelium may also be affected. Mortality seems to result from the loss of the infected epithelium. There appear to be no external signs and diagnosis requires histological examination to detect tetrahedral inclusion bodies in the hypertro-phied nuclei of affected cells. Diagnosis of BMNV in P. japonicus is made by the greatly hypertrophied nuclei within the hepatopancreatic epithelial cells undergoing necrosis. In MBV the polyhedral inclusion bodies tend to be multiple and spherical.

A recently discovered virus disease is the infectious hypodermal and haematopoietic necrosis (IHHN) in P. stylirostris (later also found in P. monodon), diagnosed by the small particles (16–28mm) of cubic symmetry in affected tissues. Fry weighing 0.5–2g are affected most seriously, and it is now known to affect older shrimps as well. In acute cases resulting in death, massive destruction of the cuticular hypodermis and often of the haematopoietic organs, glial cells in the nerve cord and loose connective tissues can be observed. Penaeid shrimps surviving IHHNV infections become carriers of the virus and transmit them to their offspring.

Another new viral infection caused by a hepatopancreatic parvo-like virus (HPV) has been diagnosed in P. merguiensis, P. monodon, P. orientalis and P. semisulcatus. The symptomsare poor growth, anorexia, reduced preening capacity, increased surface fouling and occasional opacity of the tail musculature. In all the affected species, necrosis and atrophy of the hepatopancreas can be observed. Heavy mortality occurs during the juvenile stage.

Bacterial diseases in shrimps may occur as ‘shell disease’, characterized by localized pits in the cuticle, or as localized infections and generalized septicaemias affecting all life stages. In all reported cases, motile, gramnegative, oxidase-positive, fermentative rods, mostly of vibrio species, have been found. Successful therapy includes addition of antibiotics to the tank water in hatcheries and the incorporation of antibiotics in the ration at the grow-out stage. Disinfection of all culture facilities helps to reduce the incidence of the disease.

Systematic non-inflammatory mycoses of larval stages and the generally localizedmycoses of the juvenile and adult stages accompanied by inflammation caused by fungus infection are common among most Penaeid species. Lagenidium and Siropidium are the best known phycomycetes affecting shrimps. Infected individuals become immobile as a result of the profuse growth of the mycelium in the host, replacing most of the muscle and other soft tissues. By using only pre-treated or filtered sea water in hatcheries, the entry of zoospores into the water supply system can be prevented. A multiple six-hour application of Treflan^® in the parts-per-billion range is also reported to be effective in preventing the disease. Malachite green oxylate at 0.006ppm concentration is useful in arresting or preventing epizootics, if added prior to their establishment. A single application of 0.01ppm trifuralin has been reported to be adequate to kill Lagenedium and Siropidium zoospores.Another fungal disease of cultured shrimps is caused by Fusarium solani. The infection may occur through pond bottom muds and detritus or sea water. Wounds or abrasions on the host can easily be infected. Lesions may occur in the gills or at the bases of appendages or on the cuticle, and well-developed lesions are darkly melanized. The black gill disease of P. japonicus is caused by this fungus. Mortalities of the whole stock can occur in highly susceptible species, and no effective methods of prevention or cure are known at present.

Among the non-infectious diseases caused by epibionts, Leucothrix disease and ciliate gill diseases are important. Leucothrix disease, caused by the bacterium Leucothrix mucor, occurs in juvenile and adult shrimps. The bacteria attach themselves to the body of the host, particularly the gills and accessory gill structures. Larval and post-larval shrimps may become covered by the filaments of the pathogen, affecting respiration, feeding, locomotion and moulting. Severe losses may occur sporadically and, if not controlled, can also cause continuous low-level losses. Treatment with a sea-water-soluble copper compound (available commercially as Cutrine–Plus^R) at concentrations of 0.2–0.5mg Cu/l for four to six hours in static conditions has been found to be effective in preventing and curing the disease.

The ciliate gill diseases are caused by protozoans of the genera Zoothamnium, Epistylis and Vorticella. When the surfaces of the gills are covered with these organisms, hypoxia and death occur in the same manner as in Leucothrix diseases. Formalin is reported to be effective in controlling the infection.

The cotton or milk shrimp disease denotes a group of diseases caused by at least four species of microsporidians (Protozoa). Infected shrimps have opaque musculature and ovaries and dark blue or blackish discoloration due to expanded chromatophores in the cuticle. Multiple infections have been reported. There are no proven methods of cure for this disease.

Three types of environmental diseases have been reported in Penaeid shrimps, namely muscle necrosis (spontaneous necrosis), cramped tail and gas-bubble disease. Muscle necrosis is characterized by whitish opaque areas in the striated musculature, especially of the distal segments of the abdomen. It is believed to be caused by severe stress from overcrowding, sudden temperature and salinity changes and low dissolved oxygen, or rough handling. If large areas are affected, the disease may prove fatal. The chronic and typically septic form of the disease is known as ‘tail rot’, when the abdomen, or its appendages, becomes completely necrotic, red in colour and begins to decompose. In the initial stages the disease can be controlled by reducing stress.

The ‘cramped tail’ condition generally occurs during summer. It is characterized by a dorsal flexure of the abdomen which is rigid and cannot be straightened. It is believed that this condition is brought about by elevated water and air temperatures and stress due to handling in warm weather.

Gas-bubble disease of shrimps, caused by supersaturation of atmospheric gases and oxygen, is very similar to the one described in finfishes. Early signs of the disease are rapid and erratic swimming, which may soon be followed by a state in which the shrimp floats near the water surface. If supersaturation is due to oxygen, the condition can be controlled by reducing the level, but if caused by nitrogen or other gases it is usually lethal.

Chronic soft-shell syndrome occurs in P.monodon in brackish-water ponds with poorsoil and water conditions. The affected shrimps show high levels of calcium and phosphorus in the hepatopancreas, and lower levels of phosphorus in the exoskeleton. Soft-shelling could be induced by exposure to pesticides, and reversed by improved diets containing 14 per cent mussel meat (see Chandrachakool, 2003).

The recently reported white spot syndrome virus (WSSV) affecting Penaeus monodon caused colossal damage to the shrimp industry.