Spherical baculovirosis

Signs of diseases

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.

Disease signs at the farm level

• lethargy

Clinical signs of disease in an infected animal

• emaciation
• secondary gill and surface fouling by ectoparasites

Gross signs of disease in an infected animal

• white hepatopancreas (digestive gland) and midgut

Disease agent

The causative agent is a Penaeus monodon-type baculovirus. Various strains of P. monodon-type baculovirus are considered enzootic in wild penaeid stocks throughout the Asia–Pacific region. Based on the wide geographical and host species range, the existence of different strains of the baculovirus is likely. Polymerase chain reaction (PCR) tests designed for East and South-East Asian isolates of the baculovirus have been shown to give false negative test results for P. monodon-type baculovirus infected P. monodon from Africa, further suggesting that the baculovirus is made up of more than one strain. The virus is known to occur in fresh and brackish water.

Host range

Crustaceans known to be susceptible to the virus:

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

Presence in Australia

Spherical baculovirus has been officially reported from New South Wales, Queensland and Western Australia.

Epidemiology

• Transmission is horizontal, direct from the water column or through cannibalism. It is believed, but not proven, that transmission can also be vertical from broodstock to offspring.
• Other species of prawns exposed to the virus may act as carriers.
• Up to 90 per cent mortality occurs in postlarval hatchery prawns, especially at high densities.
• With good culture practices, mortalities from even severe spherical baculovirus infections in cultured populations can be negligible.

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

Baculoviral midgut gland necrosis, tetrahedral baculovirosis

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.

Further reading

The currently accepted procedures for a conclusive diagnosis of spherical baculovirosis are summarised at www.oie.int/eng/normes/fmanual/A_00052.htm