Aquatic Animal Diseases Significant to Australia: Identification Field Guide 4th Edition
Signs of disease
Important: Animals with disease may show one or more of the signs below, but the pathogen may still be present in the absence of any signs.
Diseases caused by any of the microcell species are similar, with few or no clinical or gross signs present with light infection. Identification of the Bonamia or Mikrocytos species requires histological laboratory examination and molecular diagnostic techniques.
Disease signs at the farm, tank or pond level are:
Gross pathological signs are:
Infection with species of Bonamia exitiosa rarely results in gross pathological signs of disease in oysters; often the only sign is increased mortality.
Microscopic pathological signs are:
B. exitiosa is an intrahaemocytic protist in the phylum Haplosporidia that causes lethal infection of certain oysters. Some evidence suggests that B. exitiosa in New Zealand is similar but not identical to the species of Bonamia known to infect southern mud oysters in Australia. Based on current information they are considered to be separate species.
Species known to be susceptible to infection with B. exitiosa are listed below.
a Naturally susceptible (other species have been shown to be experimentally susceptible)
Presence in Australia
EXOTIC DISEASE—not present in Australia.
The list of similar diseases below refers only to the diseases covered by this field guide. Gross pathological signs may be representative of a number of diseases not included in this guide, which therefore should not be used to provide a definitive diagnosis, but rather as a tool to help identify the listed diseases that most closely account for the gross signs.
Infection with B. ostreae, Bonamia spp. and Mikrocytos mackini
There are few or no visual cues to the presence of this disease other than poor condition, shell gaping and increased mortality. Consequently, it is impossible to differentiate between Bonamia species based on gross signs alone; any presumptive diagnosis requires further laboratory examination.
Light microscopy can contribute diagnostic information, but further laboratory examination and molecular diagnostic techniques are required for a definitive diagnosis.
Due to the uncertainty in differentiating diseases using only gross pathological signs, and because some aquatic animal disease agents might pose a risk to humans, only trained personnel should collect samples. You should phone your state or territory hotline number and report your observations if you are not appropriately trained. If samples have to be collected, the state or territory agency taking your call will provide advice on the appropriate course of action. Local or district fisheries or veterinary authorities may also provide advice regarding sampling.
Emergency disease hotline
The national disease hotline number is 1800 675 888. This number will put you in contact with the appropriate state or territory agency.
The accepted procedures for a conclusive diagnosis of infection with B. exitiosa are summarised in the World Organisation for Animal Health Manual of diagnostic tests for aquatic animals 2011, available at www.oie.int/en/international-standard-setting/aquatic-manual/access-online.
Further information can be found on the website of the Sub-Committee on Animal Health Laboratory Standards at www.scahls.org.au/procedures/anzsdps2.
These hyperlinks were correct and functioning at the time of publication.
(1) Heavy B. exitiosa infection in New Zealand dredge oyster (Ostrea chilensis), by histology
Source: B Diggles
(2) Heavy B. exitiosa infection in New Zealand dredge oyster (Ostrea chilensis), by in situ hybridisation with a molecular probe
Source: B Diggles
(3) Leydig tissue with circulating haemocytes, many of which have Bonamia exitiosa in them (the small pink spheres in the haemocyte cytoplasm, smaller than a nucleus)
Source: B Jones
(4) Inflammatory response in a section through the digestive gland of New Zealand dredge oyster (Ostrea chilensis)
Source: B Jones
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