Fish can be immunized against Ich, the dreaded "white-spot" disease that is the bane of home aquarists and commercial fish farmers, government scientists have shown. Although the team still has many obstacles to overcome, the study presented Friday at a Boston meeting of the American Chemical Society indicates for the first time that a protective vaccine is within reach.
Ichthyophthirius multifiliis, commonly known as Ich, is the most common protozoan parasite of fish. It is characterized by the appearance of white spots, about the size of salt or sugar granules, on the fishes' skin, and is especially common when fish are grown in crowded conditions. Symptoms include loss of appetite, rapid breathing, hiding or resting on the bottom of tanks or ponds, and rubbing or scratching against objects. The disease kills 50% to 100% of those infected.
Development of a vaccine against Ich, like that of one against malaria, has been stymied by the parasite's complex life cycle. The parasite in its feeding stage, called the trophont, lives in nodules on the skin. After the parasite feeds, the nodules fall off and the protozoan enters an encapsulated dividing stage, the tomont, which adheres to plants and gravel. The tomont divides up to 10 times by fission, producing large numbers of infective theronts, which attack fish and start the cycle anew.
Marine biologist Dehai Xu of the U. S. Department of Agriculture's Agricultural Research Service in Auburn, Ala., and his colleagues used two approaches to developing a vaccine. Xu noted that theronts "infect the skin, but if you inject it into the fish they do not become infected." They found that vaccinating channel catfish with live theronts resulted in survival as high as 90% among immunized fish.
The primary drawback of this approach is the need to individually inject fish, many of which are quite small. Fish owners would prefer something simpler, such as a bath the fish could be immersed in for vaccination.
To address this possibility, Xu and his colleagues used high-frequency sound waves to kill the trophonts, then showed that bathing the catfish with the dead parasite induced immunity. Xu reported that 60% of the fish vaccinated this way survived exposure to the parasite, compared with only about 10% of unvaccinated fish.
The problem now, he added, is that "we do not have the way to [grow] large quantities of the parasite. It's very difficult. I hope that someday, someone can solve this problem."