What is it?
Yellowhead is a positive-sense, single-stranded RNA (ssRNA) virus, originally thought to be related to rhabdoviruses but now known to be related to coronaviruses and arteriviruses.
Yellowhead disease has now been formally classified in the new genus Okavirus and new family Roniviridae within the viral order Nidovirales.
Where and when might it occur?
The appearance of the disease and mortality often occur within two to four days following an interval of exceptionally high feeding activity that ended in abrupt cessation of feeding.
Yellowhead disease can be identified by high and rapid mortality in mainly early to late juvenile stages.
The yellowing of the cephalothorax, bleaching of body colour and congregation at pond edges is also often seen.
Histologically, moribund shrimp show systemic necrosis of cells of ectodermal and mesodermal origin, with the formation of intensely basophilic cytoplasmic inclusions (by haematoxylin and eosin staining) that arise from phagocytosed nuclei and viral inclusions. However, gross signs of infection and histopathology are not entirely reliable indicators of YHD and the use of molecular methods for confirmatory diagnosis required.
Confirmation of yellowhead disease requires more detailed analysis by RT-PCR, Western-blot analysis, in-situ nucleic acid hybridisation and TEM.
Control and treatment
No effective vaccination methods have been developed so far.
Source: Cefas and OIE