Worms in tropical fish

platy with worms

Roundworms, or nematodes, are internal parasites which live in the digestive system of fish. Although fish can carry small numbers of roundworms without any apparent ill effects, they can cause a variety of problems including loss of weight and even in severe cases they can kill fish. The main problem worm species are

  • Capillaria
  • Camallanus
  • Capillaria species


Life Cycle
Capillaria worms can infest a wide range of fish species including cichlids (Discus and Angelfish are commonly infected), carp, goldfish, gouramis and tetras. The worm life cycle is simple, and does not require any intermediate host. The adult female worms lay eggs which can take up to 3 weeks to develop depending on the water temperature. When the embryos hatch from the eggs, they can be eaten by susceptible fish and then the worms will develop in the fish, with the adult egg-laying stage being reached in about 3 months. However, sometimes the Capillaria worms can inhabit ordinary (tubifex) worms and then the fish can become infected by simply eating the infected worm.

Once inside the fish, the parasitic worm develops in the intestinal tract of the fish and can cause symptoms ranging from weight loss to intestinal blockage in heavy infestations and even death.

Definitive diagnosis is tricky in live fish – post-mortem findings can often demonstrate the presence of the adult females which can be identified by the brown barrel-shaped eggs within them, or even in some cases the eggs themselves can be seen in the intestinal tract of the fish. Immature larvae or adult males can occasionally be missed during post-mortem examination since they are almost transparent.

The worms can be simply treated by the administration of levamisole (Chanaverm) to the pond or tank water containing the affected fish. Also good hygiene will help – remove any organic debris or faeces as often as possible following treatment.
Take care with feeding the fish and be aware that certain types of live food may contain these nematodes and could re-infect your fish.

Camallanus species
Life Cycle

These worms infect freshwater fish such as guppies, swordtails, cichlids and other freshwater species. They reproduce by producing live offspring since the females incubate the eggs while they are in their bodies to produce the larvae. These larvae are passed out in the faeces where they are ingested by other creatures such as copepods. This means that the worm life cycle is ‘indirect’ – in other words they require to infect another host and partially develop in that in order to be able to infect fish. The stage of development within the copepod is that of second stage larva developing into third stage larva, and when the infected copepod is eaten by a susceptible fish the third stage larvae are released into the fish where they finally develop into adults, and the whole reproductive cycle begins again.

One of the first symptoms which may become evident is a reddish coloured worm protruding from the anal vent of an affected fish. Affected fish also lose weight and can become more susceptible to other diseases.

In the absence of any visible external symptoms diagnosis can be confirmed by post-mortem examination of the gastro-intestinal tract where the characteristic worms can be found. If any females are present it is often possible to find them with both eggs and first stage larvae within them.

These worms can be readily treated by the administration of Chanaverm to the tank water, and since they are confined to the gastro-intestinal tract treatment is usually very successful. Also since copepods or similar crustaceans are required for the successful transmission of this parasite, take care not to feed them to your fish as part of their live feed.

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