Yellow Head Virus (YHV) Disease
Rhabdovirus (40-50 x 150-170 nm)
Subadults and broodstock of P. monodon, P. aztecus, P. duorarum, P. merguien-sis, P. setiferus, Palaemon styliferus, Acetes spp.
Infected shrimps show light yellowish, swollen cephalothorax. The gills appear whitish, yellowish or brown.
EFFECT ON HOST:
Before the appearance of clinical signs of disease, the shrimps develop an ab-normally high feed intake and rapid growth. Thereafter, there is marked reduc-tion in food consumption prior to cessation of feeding and the onset of rapidly accelerating mortality. Moribund shrimps swim slowly near the surface at the edge of the pond. Acute epizooties occur in juvenile to sub-adult shrimps about 20 days post stocking especially during the 50-70 days grow-out culture period. The occurrence of this disease may be associated with unstable phytoplankton bloom, bad pond bottom, high stocking density or exposure to pesticides.
Systemic infection is associated with virus assembled in the cytoplasm of ecto-dermal and mesodermal cells (gills, lymphoid organ, hemocytes and connec-tive tissues). Massive necrosis is attributed to cytoplasmic replication of the virus. The virus can cause a total crop loss within 3 to 5 days of onset of clini-cal signs with incubation period of 7-10 days. The virus in water remain infec-tive up to 72 h. Shrimp reservoirs include Palaemon styliferus. About 4% broodstock are infected. In the Philippines, a recent sampling of 250 shrimps reported positive for YHV in 16% of specimens.
Signs of disease and phase contrast microscopy of fresh hemolymph stained with Wright/Giemsa stain. Histopathological analyses show the presence of basophilic usually spherical, perinuclear cytoplasmic inclusions in the hemocytes, lymphoid organ, hematopoietic tissues, pillar and epithelial cells in the gills, spongy connective tissue cells in the subcutis, muscle, gut, antennal gland, gonads, nerve tracts, ganglia and other cells of ectodermal and mesoder-mal origin. Electron microscopy, Western blot, RT-PCR, and infection bioassay are confirmatory diagnostic tests.