Yellowhead disease

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Yellowhead disease
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Signs of disease

Important: animals with disease may show one or more of the signs below, but disease may still be present in the absence of any signs.

Yellowhead disease in giant black tiger prawn
(Penaeus monodon). Note yellow heads of infected
prawns on left. Prawns on right are normal.
Source: DV Lightner

Disease signs at the farm level

moribund prawns aggregate near surface at pond edges
infected 5–15 gram prawns begin feeding at abnormally high rate for several days and then cease feeding entirely
ass mortality 2–4 days after cessation of feeding

Clinical signs of disease in an infected animal

white, yellow or brown gills
yellowing of the cephalothorax and general bleaching of body
yellow, swollen digestive gland makes head appear yellow

Disease agent

The causative agent of yellowhead disease is yellowhead virus (YHV), a corona-like RNA virus in the genus Okavirus, family Ronaviridae and order Nidovirales. Yellowhead virus (genotype 1) is one of six known genotypes in the yellowhead complex of viruses and is the only known agent of yellowhead disease. Gill-associated virus (GAV) is designated as genotype 2. GAV and four other known genotypes in the complex (genotypes 3–6) occur commonly in healthy Penaeus monodon in East Africa, Asia and Australia and are rarely or never associated with disease. Comparison of DNA sequences of reverse-transcription polymerase chain reaction (RT‑PCR) products from YHV and GAV suggests that they are closely related, but distinctly different viral strains or species.

Host range

YHV is highly infectious for most known species of cultivated penaeid prawns. Natural and/or experimental infections have been reported to occur in the following species of penaeid and palaemonid shrimps, prawns and krill:

black tiger prawn* (Penaeus monodon)—primarily
Gulf banana prawn* (Penaeus merguiensis)
mysid shrimp* (Palaemon styliferus)
northern white shrimp* (Penaeus setiferus)
red endeavour prawn* (Metapenaeus ensis)
paste prawn* (Ascetes sp.)
Antarctic krill (Euphasia superba)
barred estuarine shrimp (Palaemon serrifer)
brown tiger prawn (Penaeus esculentus)
greentail prawn (Metapenaeus bennettae)
Kuruma prawn (Penaeus japonicus)
northern brown shrimp (Penaeus aztecus)
northern pink shrimp (Penaeus duorarum)
Pacific blue shrimp (Penaeus stylirostris)
Pacific white shrimp (Penaeus vannamei)
Sunda river prawn (Machrobrachium sintangene)

* naturally susceptible (other species have been shown to be experimentally susceptible)

There are variations in the susceptibility of different species to disease. Laboratory trials have shown that YHV can cause high mortality in Penaeus monodon, P. vannamei, P. stylirostris, P. aztecus, P. duorarum, Machrobrachium sintangene, Palaemon styliferus and P. serrifer.

Until proven otherwise, it should be assumed that most penaeid prawns worldwide are susceptible to infection with yellowhead disease.

Presence in Australia

EXOTIC—not present in Australia.

Epidemiology

The tiger prawn suffers acute epidemics, with mortality reaching 100 per cent within 3–5 days from first appearance of the gross signs.
YHV can be transmitted horizontally by injection, ingestion of infected tissue, immersion in membrane-filtered tissue extracts, or cohabitation with infected shrimp. Transmission has also been demonstrated by injection of extracts of paste prawns (Ascetes sp.) collected from infected ponds.
Tiger prawns younger than 15 days postlarvae (PL-15) are fairly resistant to yellowhead disease compared to those from PL-20–25 to subadult, which are highly susceptible.
Massive mortality usually affects early to late juvenile stages in rearing ponds.
There appear to be at least four types of virus in the YHV group. Only yellowhead virus has been reported to cause yellowhead disease.
Vectors may include asymptomatic carrier crustaceans.

Differential diagnosis

The differential diagnostic table and the list of similar diseases appearing at the bottom of each disease page refer only to the diseases covered by this field guide. Gross signs observed might well be representative of a wider range of diseases not included here. Therefore, these diagnostic aids should not be read as a guide to a definitive diagnosis, but rather as a tool to help identify the listed diseases that most closely account for the gross signs.

Similar diseases

Taura syndrome

Further images

Sample collection

Because of uncertainty in differentiating diseases using only gross signs, and because some aquatic animal disease agents might pose a risk to humans, you should not try to collect samples unless you have been trained. Instead, you should phone your state or territory hotline number and report your observations. If samples have to be collected, the agency taking the call will advise you on what you need to do. Local or district fisheries/veterinary authorities could advise you on sampling.

Emergency disease hotline

For your state or territory emergency disease hotline number, see Whom to contact if you suspect a disease.