Known diseases and parasites

[Ulli Bauer]

By Ollie/Beastie
Translated by Ulli Bauer

In general, practically only the diseases of marine species have been researched more in detail since numerous saltwater species serve as food crabs and are thus more in the focus of commercial interests than freshwater crabs.
This section will be amended by and by.

Burn spot disease/brown spot disease
Round, reddish-brown to black spots on the carapace with a crater-like perforation in the middle, often white or black in color, caused by diverse lower fungi. There are specific pathogens for each crayfish species, which however always cause the same symptoms. For the Chinese mitten crab Eriocheir sinensis, e.g., this pathogen is Septocylindrium eriocheir, for the noble crayfish Astacus astacus the filamentous fungus Ramularia astaci, and for the American cray Oronectus limosus it is Didymaria cambari. Some sources also give chitinoclastic bacteria as a cause.

It is possible that these pathogens make use of small ruptures in the carapace to enter the body. As far as we know there is no cure, but a light infestation is not critical. However, if the animals are weakened or very stressed, the fungus can spread as a secondary infection and lead to death.

Brown tree leaves, alder cones and Indian almond leaves are considered helpful.



Rust spot disease
Brown-orange spots on the carapace, also caused by a fungus. This is said to be clearly different from burn spot disease although the symptoms are quite similar. These diseases are difficult to keep apart.



Shell disease/Burn Spot/Brown Spot disease
Bacteria of the genera Vibrio, Pseudomonas and Aeromonas cause this necrotic spot disease in nearly all marine decapods, but exclusively as dangerous secondary infection when the animals are exposed to stress or environmental pollution. The infection only occurs when the carapace is injured. Highly contagious in animal cultures. Treatment with malachite green (lobsters) or antibiotic baths (penicillin-streptomycin, furanace, erythromycin, oxolinic acid, malachite green, formalin) for shrimp.



WSSV, White Spot Syndrome Virus
WSSV is caused by a DNA virus of the genus Whispovirus practically omnipresent in cultures as well as in nature. Crabs are not as often affected as shrimp and can be infected with the virus without showing any symptoms. The disease can be fatal and has to be reported (to the Office International des Epizooties (OIE) or the World Organization for Animal Health).



Crayfish plague
Crayfish plague is a dangerous infectional disease of European crayfish, caused by the filamentous fungus Aphanomycetes astaci, which was first observed in Europe in the 19th century. Since the 20th century it has spread further, mostly due to the introduction of American crayfish into European water bodies, and European crayfish species have been almost eradicated as this highly infectious disease causes them to die in high numbers.
American crays are partially resistant: They are carriers, but their immune system keeps the pathogen in check as long as there are no further infections, high stress levels or intoxications. Symptoms are daylight activity, no flight reflex, white discolorations of the muscular tissue, paralysis, scratching, lethargy and finally limb loss.

As far as we know, crabs are not infected by or are immune against crayfish plague.



Porcellain disease
When the tissue can be seen shimmering through the carapace whiteish and porcellain-like, the animal is probably infected with porcellain disease. It is caused by the microsporidium Thelohania contejeani, which infects only the crustaceans' muscular tissue.

When the parasite reproduces the animals get stiffer and stiffer and lose color. They also show a milky white coloration, first on the underside. Finally the crustaceans cannot move at all any more and die.

Infected animals have to be separated, as one way the pathogen spreads is by cannibalism. There are reports according to which malachite green oxalate ought to help against porcellain disease.



Cotton/Cooked crab disease
Cotton or cooked crab disease is very similar to porcellain disease and is also caused by microsporidia, in this case Ameson michaelis, that infest the cells and destroy them. Infected tissue looks whiteish. This parasite is also spread by cannibalism.

Other reports say that there are whiteish burs and lumps or oval to rod-shaped cysts in the exterior carapace, caused by sporozoans. There is no known treatment other than separating/removing infected or dead animals.



Milk disease
Milk disease in marine crabs is similar to porcelain disease and is also caused by microsporidia. Up to now there is no known treatment. The disease destroys blood cells and proteins. It makes the hemolymph look milky when the pathogen has reproduced in the phagocytes of the hepatopancreas and then infects the hemolymph. The pathogen is a hitherto undescribed alpha-probacterium with slight affinities to other Rickettsia.



Bitter crab disease
Bitter crab disease is caused by the dinoflagellate (single-cell microscopic algae) Hematodinium perezi. The parasite consumes oxygen from the crab's hemolymph and tissue and thus makes the animal weak and lethargic and often even leads to death. It has been mostly reported from relatively warm waters with a high salinity in Maryland, Virginia, Georgia and Florida (USA).

Once the crab is infected, the parasite reproduces quickly to up to 100 million cells per milliliter lymph within six weeks. The crab's blood turns milky white and loses its ability to clot. At an early stage the disease can only be detected by microscopical examination. At a later stage the carapace can turn pink as if the crab had been partially cooked.

The disease got its name from the fact that food crab meat turns bitter and tastes somewhat like aspirin.



Paramoebiasis/Grey crab disease
Paramoeba perniciosa infects Callinectes, Cancer, Carcinus and Homarus on the East Coast of the United States. Grey discoloration of the abdominal carapace, lethargy, loss of the hemolymph's coagulation ability and almost always fatal if the parasitic amoebae infect tissues and the disease becomes systemic. No known treatment.



Mold
External infections by mycoses (fungal infections) are visible to the naked eye, although not always easily. The symptoms are white cotton-like deposits, or greenish filaments that collapse outside of the water. They are mostly caused by the water molds Saprolegnica or Achyla, often as secondary infections, which spread into the animal's interior (internal mycoses).

According to various reports a treatment with Cilex, Fungol or Costapur, a malachite green medicinal product, is often successful.



Bell animalcules

Ceylon leopard crab (Ceylonthelphusa kandambyi) strewn with bell animalcules.

Plants and crustacean carapaces look as if they were mouldy, but in reality they are only used as substrate by bell animalcules. These are harmless protozoans that usually go away all by themselves with the next water change.




Microscopical view of bell animalcules on a plant.

Bacteria
Like fungal spores bacteria are omnipresent, and there are apathogenous (beneficial or harmless) as well as pathogenous (detrimental) species. Normally the immune system keeps them in check, and bacterial diseases only occur as secondary infections, i.e. if the immune response is weakened for other reasons and the pathogenous bacteria can reproduce at explosive rates.

Bacteria can cause external as well as internal infections as well as afflict organs. If the latter is the case there is no cure (and no diagnosis, as crabs have an opaque carapace), the first two can be treated with Chloramphenicole and sometimes also with Furanol, according to Crustakrankheiten.de.



Gaffkemia/Red tail disease
Aerococcus viridians var. homari is the best-researched bacterium in invertebrates. Lobsters with a heavy infection have pink blood and thus a red tail, and stop eating after two days. The bacteria reproduce in the hemolymph, just five of them are enough to cause a lobster to fall prey to the disease. This disease also afflicts diverse crabs (Cancer, Libinia, stone crabs). Treatment with antibiotics.



Parasites: Worms and leeches
Crustaceans can be infested with various flukes (trematods) and hookworms (nematods) as well as leeches. They are usually cast off when the animal molts - then it is important to quickly take out the exuvia of the tank together with the worms before they can infest the freshly molted crab again. The latter is most important with leeches.

According to the Bavarian regional association of fishermen a possible treatment is putting the crab into a burnt lime solution (200 g of burnt lime to ten liters of water) for a minute at the longest. This ought to kill off the leeches, but not their eggs. After this bath the animal has to be rinsed well in clean water in order to free their gills from lime rests.

Trematods and nematods can be treated with Flubenol and Gyrodactol - as these worms also live under the carapace, treatments have to be repeated several times.



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A good overview of possible and successful treatments (as well as dosages) is given by Michael Wolfinger at: http://www.crustakrankheiten.de/service-...osetabelle (unfortunately, only in German) – even though this diagnostic page is dedicated to shrimp, however, its content can also be applied to crabs and crayfish.