Signalment:  

Adult blacklip abalone (Haliotus rubra).Approximately twenty sick and dying abalone were found off Tyringa, near Bairds Bay on the west coast of South Australia. Post-mortem changes were significant and the cause of death was unclear. Due to an outbreak of abalone viral ganglioneuritis (AVG), a notifiable disease of abalone in South Australia occurring in Tasmania, the case was regarded as urgent and treated as a possible emergency disease.

Samples of mouthparts and a part of the foot and surrounding mantle from 3 affected blacklip abalone, 1, 2, and 3 were submitted in 10% buffered formalin for microscopic examination as well as a part of the foot with pedal nerves in alcohol for referral to Australian Animal Health Laboratory for PCR for abalone ganglioneuritis (ABG).

Gross Description:  

Multiple randomly scattered, firm, yellow, raised or centrally cystic nodules, ranging from 2 mm to 8 mm were seen within and along the edges of the foot of all three abalone. The cystic nodules contained flocculent yellow or yellow-brown watery fluid. Lesions were most numerous in abalone 2 (from which these sections have been prepared). 

Histopathologic Description:

Pedal tissue: Moderate post mortem changes are present. There are single (in some sections) or multiple rounded cystic lesions containing large numbers of ovoid vacuolated or dark-staining unicellular organisms 10-17 μm in diameter and multicellular rounded organisms 12-30 μm in diameter amongst moderate numbers of infiltrating haemocytes. The lesions are well delineated but non-encapsulated. In occasional sections, similar lesions are seen abutting onto, but not infiltrating into, pedal nerves.

There were no histopathological lesions of ABG in the mouthparts (sections not presented).

Morphologic Diagnosis:  

Multifocal necrosis and cavitation with numerous intralesional protistal organisms consistent with Perkinsus spp. and haemocyte infiltration, pedal tissue.

Lab Results:  

PCR was negative for ABG. 

Condition:  

Perkinsus sp.

Contributor Comment:  

Perkinsus spp. is a group of protists in the phylum Perkinsozoa, in the infra-kingdom Alveolata, but there remains uncertainty about the status of the genus and there are divergent views regarding its taxonomy. Susceptible hosts include abalone: Haliotis rubra, Haliotis laevigata, Haliotis cyclobates and Haliotis scalaris, but a wide range of other molluscs are also susceptible. The known range of P. olseni includes Australia, Europe and eastern Asia. 

ODonoghue et al linked P. olseni to widespread kills of abalone in the Gulf of St. Vincent side of the Yorke Peninsula, South Australia, and abalone stocks in this area have not recovered (S. Mayfield personal communication).(5) In the early 1990s, a significant proportion of H. rubra along approximately 500 km of the NSW coastline between Port Stephens and Jervis Bay died in association with P. olseni infections.

The route of entry of Perkinsus olseni is unknown. It proliferates in tissues but is eventually sequestered by the host and killed or ejected. During sequestration the parasite becomes surrounded by abalone pigment, causing the yellow/brown appearance of the nodules. These reduce market value, causing losses in fishery value. In some severe infections, death occurs without nodule formation, suggesting that the host is unable to mount an effective immune response in some cases. Infection is not always lethal to H. rubra and expression of disease is widely associated with environmental variables, primarily temperature, with infections becoming more severe at higher temperatures or when food availability diminishes.(4) Transmission occurs directly; prezoosporangia develop into zoosporangia in seawater which then release hundreds of motile, biflagellated zoospores (about 3 by 5 μm) which are infective to abalone and other mollusks.(3)

JPC Diagnosis:  

Pedal tissue: Rhabdomyositis, necrotizing, focally extensive with numerous protistal trophozoites and schizonts. 

Conference Comment:  

The contributor provides a good summary of Perkinsus olseni infections in mollusks. Conference participants discussed terminology for the life stages of these protists. The life cycle of Perkinsus olseni consists of trophozoite, hypnospore and zoospore stages.(1,6) Hypnospores (prezoosporangia) are dormant, thick-walled cells that develop into motile, flagellated zoospores in aerated seawater. Once ingested by the host, the zoospore develops into a non-motile, multi-nucleated single cell trophozoite. Under anaerobic conditions, such as occurs in necrotic tissue, the trophozoites develop the vegetative hypnospores.(1,6) Participants also discussed another Perkinsus species, P. marinus, which causes Dermo disease in oysters. Dermo is a significant cause of mortality in the Eastern oyster, Crassostrea virginica.(6)

Although this animal did not have abalone viral ganglioneuritis (AVG), participants discussed this disease as well. AVG is caused by a herpesvirus that affects only abalone; other mollusks appear to not be affected. Mortality rates in farmed abalone are up to 100%. The disease has been found to be slowly spreading in wild abalone along the Victorian (Australia) coast as well; however, the exact mortality rate in wild populations is uncertain.(2)

There was moderate slide variation, with some sections exhibiting more significant necrosis, and others showing cavitation of the pedal tissue. 

References:

1. Choi KS, Park KI. Review on the protozoan parasite Perkinsus olseni (Lester and Davis 1981) Infection in Asian Waters. In: Ishimatsu A, Lie HJ, eds. Coastal Environmental and Ecosystem Issues of the East China Sea. Terrapub and Nagasaki University; 2010. http://www.terrapub.co.jp/onlineproceedings/fs/nu/pdf/nu2010269.pdf. Accessed online 26 January 2013. 
2. Corbeil S, McColl KA, Williams LM, et al. Abalone viral ganglioneuritis: establishment and use of an experimental immersion challenge system for the study of abalone herpes virus infections in Australian abalone. Virus Res. 2012;65(2):207-13.
3. Goggin, CL, Sewell KB, Lester RGJ. Cross-infection experiments with Australian Perkinsus species. Diseases of Aquatic Organisms. 1989:T:55-59.
4. Lester RJG, Hayward CJ. Control of Perkinsus Disease in Abalone. Final Report for FRDC Project no. 2005;2000/151.
5. O'Donoghue PJ, Phillips PH, Shepherd SA. Perkinsus (Protozoa: Apicomplexa) infections in abalone from South Australian waters. Transactions of the Royal Society of South Australia. 1991;115:77-82.
6. Petty D. Perkinsus infections in bivalve mollusks. University of Florida IFAS Extension. http://shellfish.ifas.ufl.edu/PDFs/Publications/Perkinsus%20Infections%20of%20Bivalve%20Molluscs.pdf. Accessed online 26 January 2013. 

Click the slide to view.

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3-1. Foot

3-2. Foot

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