Tilapia Topic: Disease Vector

Ichthyophthirius multifiliis

Ichthyophthirius multifiliis or the equivalent marine form Cryptocaryon irritans can cause significant losses of cultured tilapia, especially if fish are raised under crowded conditions. The optimum temperature for "ich" is between 20 and 25'C. Young tilapia are particularly susceptible to infection by this protozoan ectoparasite.

There are three stages in the life cycle of ich. These are the adult stage or trophont, a cyst stage where asexual multiplication of the parasite occurs and the tomite stage which is the infective form of the parasite. The trophont stage is embedded in the skin of the host fish. The trophont stage causes damage to the fish, the extent of which is related to the number of trophonts present (e.g. the more present, the more damage is done). In particular, the abundance of these parasites on the gills causes the greatest threat to survival of the host. Trophonts are covered with short cilia and while on the host move between the layers of the skin and feed on cells of this tissue. These trophonts increase in size and can be recognized as the þwhite spotsþ that are characteristic for fish heavily infected by ich. Once mature, trophonts drop off of the fish; fall to the bottom of the container as a cyst stage and multiple to form 500 to 1000 tiny, motile tomites, the infective stage of the parasite. After three days the cysts rupture releasing the tomites. Tomites must contact a host fish within three days or the organisms will perish. When a tomite makes contact with a fish, it burrows into the skin and grows to form the adult or trophont stage.

Diagnosis

Tilapia infected with ich scrape on the bottom of the container (e.g. "flash") and have forced and increased rate of breathing. Small 1 mm sized white spots will be visible on the skin of the affected fish. As the infection progresses the number of these white spots dramatically increases often abruptly due to release of massive numbers of tomites from cysts.

Ichthyophthirius multifiliis or Ich is a large ciliated protozoan with a distinctive comma shaped nucleus. Usually, on wet-mounts these organisms will vary in size from 0.02 mm to nearly 1 mm. These different sizes reflect younger to older parasites. The cilia along the perimeter of the organism beats and slowly propels the protozoan forward. Under transmitted light in wet-mount preparations, Ich appears brownish in color.

Prevention

Prevention of ich is by blocking entry of the parasite into cultured tilapia populations. This is particularly important if the fish are being raised at high density. Elevated temperature (e.g. ò 30øC) will inhibit ich, thus increasing the temperature for a week or more can prevent the organism from multiplying rapidly.

Because the cyst stage is formed, the infection cycle of ich can be broken by transferring fish in an infected group to a new clean container every third day for four or more transfers. In this schedule, the fish are essentially moved away from the exposure to new batches of infective tomites. Once fish are removed the container should be disinfected with chlorine and completely scrubbed and cleaned before new fish are added to the tank.

Treatment

Ich infections can also be treated in formalin and malachite green baths (see treatment module of this program for more information). Two points should be made about chemical control. The first is that these treatments have not been approved by the FDA for use with tilapia intended for human consumption. The second point is that the chemicals affect the swimming, infective stage (tomite), but have little effect on the trophonts embedded in the skin of the host. Thus, chemical treatments will slow down or eliminate re-infection, but do little to affect the parasites that have already invaded the skin of the host.

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