Bacterial Diseases of Freshwater Crayfish
 
Bacteria are frequently considered to be secondary or opportunistic pathogens of freshwater crayfish. However, a number of species or strains have been associated with serious mortality, often in holding or purging tanks.
 
Rickettsioses
Bacteraemia
Enteric bacterial infection
Bacterial shell disease
Eye necrosis
Gill fouling by Gram-negative sp.
Nocardiosis
 
The method for managing bacterial diseases is dependant on the type of disease itself. In the past, antibiotics were frequently used to manage bacterial problems in crustacean aquaculture, but the emergence of resistant strains of bacteria has reduced their efficacy and curtailed their use. Generally, bacterial infections are controlled by environmental management. Bacteria tend to build-up in systems with high organic loads, so it is important to manage water and substrate qualities (eg. pond bottoms or filtration systems) by proper maintenance of systems, such as pond preparation between crops, and water quality monitoring. Excessive organic material should be removed from ponds or tanks via water exchange, and may be physically removed between crops. Antibiotics may be used for treating valuable broodstock, but it is not advisable to use antibiotics on a large scale (in ponds) and in some countries their use in aquaculture is strictly controlled.

 

Rickettsioses
 
Two forms of rickettsiosis have been reported in freshwater crayfish. One rickettsia-like organism (RLO) is systemic, whilst the other RLO infects only the hepatopancreatic tubule epithelium. The systemic RLO, recently named Coxiella cheraxi, has been associated with serious mortality in Cherax quadricarinatus (redclaw) in Australia. The hepatopancreatic RLO has been observed in just one specimen of C. quadricarinatus.

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Bacteraemia
 
Recent research has shown that bacteria frequently occur in the haemolymph of crayfish. However, opinion is divided as to whether this is a disease condition as the crayfish are often quite healthy. Clinical signs of bacteraemia in freshwater crayfish are non-specific and typically are limited to increasing apathy and paralysis immediately prior to death. Therefore, it is difficult for the most experienced of observers to detect any clinical signs until the disease is relatively advanced. Bacteria belonging to the genera Vibrio, Aeromonas and Pseudomonas are most frequently associated with severe bacteraemia.

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Enteric bacterial infection
 
Bacteria are normal inhabitants of the gut of freshwater crayfish. In stressed crayfish, and/or those infected with a virulent strain, the bacteria may proliferate in the foregut, midgut and in the hepatopancreatic tubules causing necrosis in the epithelium. Large areas of the hepatopancreas are often affected in such cases. Bacteria associated with enteric infections include Citrobacter, Pseudomonas, Acinetobacter, Enterobacter and Alcaligenes species.

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Bacterial shell disease
 
Shell disease of freshwater crayfish has been associated both with bacteria and fungi. The disease is characterised by progressive erosion of the exoskeleton and is fatal when large areas of the exoskeleton have been eroded. Bacteria associated with shell disease include Aeromonas, Pseudomonas and Citrobacter species.

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Eye necrosis
 
This syndrome has only been reported in Cherax quadricarinatus. Eye necrosis typically occurs in warmer months and is characterised by complete erosion of one or both eyes by Aeromonads. The condition is usually fatal presumably due to secondary bacteraemia.

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Gill fouling
 
Filamentous Gram-negative bacteria occasionally attach to the cuticle of the gills and often are accompanied by fouling protozoans. The condition is usually associated with poor water quality. The condition is of little significance. However, if measures are not taken to address the water quality, the bacteria will proliferate and impair water movement within the gill cavity which is vital for proper gill functioning leading to hypoxia (suffocation).

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Nocardiosis
 
A putative Nocardia sp. was observed in one case in England.

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References
 
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