Emergency Animal Disease Bulletin - No 112

​Author: Brett Herbert, Australian Government Department of Agriculture

Crayfish plague is an exotic disease and is listed in Australia’s National List of Reportable Diseases of Aquatic Animals. It is a serious disease of freshwater crayfish, and Australian crayfish are highly susceptible. Suspicion of crayfish plague must be reported to state veterinary or fisheries authorities immediately.

Background

Crayfish plague is caused by a water mould called Aphanomyces astaci. Its native hosts are North American freshwater crayfish and although it produces some minor pathology, does not usually cause death in American freshwater crayfish. However, in naïve crayfish (never exposed to the pathogen) the disease causes death and effectively exterminates native crayfish from infected waters in as little as a couple of weeks.

North American crayfish were first introduced into Europe in the late 1800s and carried crayfish plague with them. Crayfish plague travelled around Europe, with American crayfish that were transported for translocation into European waterways. However, once crayfish plague was introduced to a country, it could also spread rapidly via contaminated fishing gear and water. Crayfish plague continues to spread to previously unaffected areas of Europe.

The impact was severe in areas with a culture of consuming crayfish, particularly Scandinavia. National declines in crayfish populations vary from 50–80% and lakes where crayfish were eliminated became overgrown with aquatic plants.1

In Finland, the estimated cumulative direct losses due to crayfish plague over the past 90 years are €630M.2

Crayfish in Australia

Australia’s freshwater crayfish fauna is diverse, with over 135 described species including a large number that are rare and endangered. Crayfish play a crucial role as predators, herbivores and in breaking down detritus in freshwater ecosystems, and are often the largest animals in Australian mountain streams.

Australian crayfish can be split into two broad groups—spiny and smooth shelled crayfish. Spiny crayfish are slow growing, have very large claws and usually prefer clear, cool water. Due to their low meat yield and slow growth they are not farmed. They are important fauna of streams where habitat is suitable, and many have highly restricted distributions, have low fecundity and are listed as endangered. The best known of the spiny crayfish is the Murray River cray (Euastacus armatus). Smooth shelled crayfish include commercially and recreationally important species such as marron (Cherax cainii), redclaw crayfish (Cherax quadricarinatus), and the ubiquitous ‘yabby’ (Cherax spp.).

Marron, redclaw and yabby are farmed in Queensland, New South Wales, South Australia and Western Australia. Marron are primarily farmed in Western Australia and South Australia. Annual production varies but in 2011–12 it was valued at $1 444 000 and $343 000 respectively.3 Redclaw production in Queensland has declined over the past ten years and is now valued at $792 000. Yabby production for food is limited, mainly from farms in Western Australia ($377 000) and New South Wales ($325 000). Redclaw and yabbies have been translocated (legally and illegally) for recreational fishing. Many species have substantial cultural importance, and have been a source of food for aboriginal people for millennia.

Crayfish are ‘keystone species’ in aquatic environments, acting as major processors of organic materials facilitating the release of energy and nutrients, turning over substrates and aerating soil, and can reach very high biomasses in some systems. Some smooth shelled crayfish are remarkably persistent in dry environments and can survive drought in deep burrows for extended periods.

Cray fish plague transmission and spread

Aphanomyces astaci spreads by means of motile zoospores released from the mature filaments in infected crayfish. The zoospores are attracted to crayfish cuticle, and filaments penetrate immediately. Zoospores can remain motile for up to 3 days and cysts survive for 2 weeks (in distilled water).4 Zoospores can re-encyst three times if they do not encounter a host. It is recommended to wait three months before attempting to re-stock waters in which crayfish have been killed by crayfish plague to allow all zoospores to die out.

Historically, crayfish plague spread to Europe with infected American crayfish brought and then distributed for culture and fishery purposes. The American crayfish are tolerant of infection and can remain carriers for life. Crayfish plague is also spread via contaminated fishing equipment or zoospores in water.5

American crayfish, including red swamp crawfish (Procambarus clarkii) and signal crayfish (Pacifastacus leniusculus), can carry crayfish plague and may exhibit little or no sign of infection. Until recently, crayfish plague was only known from temperate to cool climates, although a Spanish strain is active at temperatures of 18–25°C. The strain that affects the invasive red swamp crawfish can sporulate at temperatures up to 29.5°C.4 Red swamp crawfish have established in Japan, China and Chinese Taipei, and populations are sometimes maintained in ornamental fish outlets in the region.6 There is therefore potential for spread of the pathogen throughout the region, either in contaminated water or with live or dead crayfish.

Freshwater crabs are susceptible to crayfish plague and may act as reservoirs for the pathogen. The Chinese mitten crab (Eriocheir sinensis) and the European crab (Potamon potamios) are both susceptible to infection with crayfish plague.7 The Chinese mitten crab is listed as one of the most invasive species on the planet and has established successfully in Europe and North America. Australia has a very rich and diverse freshwater crab fauna. If infected Chinese mitten crabs established in Australia, their extensive migration habits (migrating upstream from estuary spawning grounds) could spread crayfish plague inland to crayfish and crab populations.

Susceptibility of Australian species

Australian crayfish are known to be highly susceptible to crayfish plague.8 Eight species of four genera (including yabbies, C. destructor) of Australian crayfish were experimentally exposed to zoospores of Aphanomyces astaci. They showed limited or little effective host response to the invading filaments. The usual response of crayfish to infection is encapsulation and melanisation of invading filaments.

Redclaw crayfish and marron have been introduced into many countries for farming purposes. In late 2013 crayfish plague was detected in farmed redclaw crayfish in Taiwan.9 It was detected in five widely spread locations. Four of the five locations experienced 100% mortality while one experienced 5% mortality and 20% morbidity. The 2013 outbreak in Taiwan is the first reported case of Australian native crayfish being naturally infected by A. astaci and demonstrates the high susceptibility of this tropical crayfish to the pathogen. Pathology was not described in this report.

As demonstrated by its ability to infect Chinese mitten crabs and European crabs, crayfish plague is not very species specific and could potentially infect most or all Australian freshwater crab species. It can cause disease and death in crabs although the effects are not as immediate or dramatic as in crayfish.

Clinical disease

Often the first sign of crayfish plague is the sudden mass death of crayfish in a water body. However, some symptoms do appear, including behavioural changes and external signs. Affected crayfish may become active in the day and appear uncoordinated and lethargic. Whitish areas may be visible in the muscle through the cuticle under the tail. At very low temperatures (<5°C) effects may be slower to emerge, and the disease may be chronic rather than lethal in susceptible species.

American crayfish species are tolerant of infection but carry it. When they become infected the shell may develop dark patches, as the reaction by crayfish to damage is to lay down melanin around the pathogen. These patches are easiest to see on the underside along the abdomen.

The Identification Field Guide to Aquatic Animal Diseases of Significance to Australia provides details of clinical signs and diagnosis for crayfish plague.i
The AQUAVETPLAN Disease Strategy Manual for crayfish plague provides details on gross and clinical signs of the disease.ii

Diagnosis

Crayfish plague should be differentiated from other possible causes of mass death such as insecticide poisoning. PCR techniques for detection of the pathogen have been developed. There is an Australian and New Zealand Standard Diagnostic Procedure which details methods for diagnosis of crayfish plague.iii The OIE Manual of Diagnostic Tests for Aquatic Animals also provides details on diagnostics to confirm the presence of the pathogen.iv

Relevance to Australia

Australian freshwater crayfish are known to be susceptible to crayfish plague fungus. Freshwater crayfish are important and dominate in many Australian ecosystems, and many species are rare or threatened.10 Many species have restricted distributions so could become extinct very rapidly if exposed to the disease. Crayfish are the basis of an aquaculture industry, and support the recreational and cultural fisheries. The potential consequences of the entry of this disease into Australia could be severe.

The outbreak of crayfish plague in Taiwan indicates that tropical species can be affected. It caused very high mortalities, suggesting that a similar outbreak in Australia could have devastating consequences. Some strains of crayfish plague fungus are temperate but others transmit in temperatures up to 29.5°C, suggesting that crayfish plague could infect large areas of Australia.

American crayfish are widespread in Asia, and are regarded as pests in some countries. Until recently crayfish plague had not been reported from Asia. The report from Taiwan indicates that plague is present and could, potentially, be widespread in the region in feral populations of North American crayfish or Chinese mitten crabs.

Australia has import conditions to restrict the entry of freshwater crayfish or crabs for both environmental and biosecurity reasons. These conditions limit the risk of entry of crayfish plague into Australia. However, vigilance for diseases should always be maintained. If an exotic disease is suspected please call the disease watch hotline 1800 675 888 for advice and assistance.

References

Aquatic Animal Diseases Significant to Australia: Identification Field Guide 4th Edition

AQUAVETPLAN - Disease Strategy Manual - Crayfish Plague

Aquatic ANZSDPs

Manual of Diagnostic Tests for Aquatic Animals 2014

NOBANIS. Invasive Alien Species Fact Sheet – Aphanomyces astaci. http://www.nobanis.org. 2011. Accessed 10 July 2014.

Finnish Game and Fisheries. The effects of crayfish plague on Finland’s crayfish economy. Accessed 15 September 2014.

Skirtun M, Saklqvist P, Viera S. Australian Fisheries Statistics 2012. FRDC project 2010/208, ABARES, Canberra, ACT, 2013.

OIE. Manual of Diagnostic Tests for Aquatic Animals 2012. 2013. Crayfish plague (Aphanomyces Astaci). Accessed 10 July 2014.

NOBANIS. Invasive Alien Species Fact Sheet – Aphanomyces astaci. http://www.nobanis.org. 2011. Accessed 10 July 2014.

FAO. Cultured Aquatic Species Information Programme Procambarus clarkii (Girard, 1852). 2014. Accessed 10 July 2014.

Svoboda JIR , Strand DA , Alstad T, et al. The crayfish plague pathogen can infect freshwater-inhabiting crabs. Freshwater Biol 2014;59:918–929

Unestam T. Defence reactions in and susceptibility of Australian and New
Guinean freshwater crayfish to European-crayfish-plague fungus. Aust J Exp Biol Med Sci 1975;53:349–359.

OIE. Event summary: Crayfish plague (Aphanomyces astaci), Chinese Taipei. 2014. Accessed 10 July 2014.

Coughran J, Furse JM. Conservation of freshwater crayfish in Australia. Crustacean Research 2012;7 Special Number 7:25–34