Scientists from the USA have become the first to provide experimental evidence to show the difference in virulence among different strains of the whitespot parasite, Ichthyophthirius multifiliis.
A team of parasitologists from the Department of Infectious Diseases at the University of Georgia's College of Veterinary Medicine studied two forms of the whitespot parasite, Ichthyophthirius multifiliis, and found that one of them killed all of the fish infected while the other only killed half.
The different parasite strains studied, known as serotypes or serovars, differ in the presence of a particular type of membrane protein found on their surface called an immobilisation antigen or i-antigen.
So far, five different serotypes of whitespot have been described and can be identified from their characteristic i-antigens. The function of i-antigen proteins is unclear, however, when they are purified and injected into fish they give the fish immunity against whitespot parasites expressing that i-antigen protein in their membranes.
Alton Swennes, Jane Noe, R Craig Findly and Harry Dickerson studied the NY1 and G5 isolates of whitespot that contain the A and D serotypes, which previous studies have suggested differed in their virulence.
Lead author of the paper, Harry Dickerson, told Practical Fishkeeping: "The major "take home" message from our virulence paper is that it showed experimentally (for the first time, I believe) that there are in fact, differences in virulence between isolates of I. multifiliis. The fact that isolates of I. multifiliis can be classified based on immobilization serotypes opens the way for more studies of this type."
Much more deadlyThe results of experimentally infecting fish with the different strains of parasite show that the NY1 strain is much more dangerous and virulent to fish than the G5 variety. Without treatment, all of the NY1 infected fish had died in less than three weeks, while more than half of the G5 infected fish survived.
The study revealed that the two forms of whitespot differ both in manner in which they infect the fish and in their ability to overcome the fishes' immune system. For some reason, fish seem considerably more susceptible to infection from the NY1 strain of whitespot.
Five days after the fish were intentionally infected one of the strains of Ichthyophthirius they were examined to see how many trophont-stage parasites were present on the tail fin. The G5-infected fish had about 11 parasites on their tails, but the NY1-infected fish had more than 80 trophonts on the caudal fin.
Whitespot typically undergoes a number of infection cycles in which parasites attack fish, form the characteristic white cysts and then reproduce - the offspring then go back and attack the fish again. However, fish can develop some immunity to the parasite if they survive an initial infection, making them more resilient to further infection cycles.
Although the team managed to identify some antibodies produced by the fish to the virulent NY1 strain, the sheer number of parasites produced in subsequent infection cycles by this strain was so much higher than the G5 variety that it overwhelmed the fish.
Killer aquarium strain?Many amateur and industry-based fishkeepers believe that a new strain of whitespot, or a drug-resistant strain, is currently causing problems and proving harder to eradicate. Dickerson told Practical Fishkeeping: "I have not heard any reports of the parasite developing resistance to commonly used medications, but it's possible that drug pressure could select for resistant strains of I. multifiliis.
"One cannot unequivically determine differences in virulence or drug resistance unless parasites from the field are isolated, cloned, and tested under laboratory conditions. Your question underscores a fundamental issue regarding the natural history of I. multifiliis, namely, the dearth of predictive (rather than merely descriptive) information on the epizootiology of the disease."
I asked Dickerson whether the apparent drug resistance that fishkeepers have been reporting in the "new" aquarium whitespot could in fact be due to it being a similar form to the NY1 strain covered in this paper, and that the treatments were failing because of the differences in the speed of the NY1 strain's life cycle.
Said Dickerson: "It is possible that the timing of drug treatment regimens could select for isolates that grow and replicate at variable rates, because many drugs are only effective on the parasite while it is in the water and not within the fishes' epithelium. The differences in virulence between isolates G5 and NY1 described in our paper appeared to be related to growth rate and replication. Again, this is a very difficult question to answer unless field isolates are characterized and tested."
"To address the important issues and questions that you raise regarding epizootiology, there are two important technologies that need to be developed in my opinion.
"The first is a means to cryopreserve isolates of I. multifiliis so that viable reference strains can be frozen and retrieved for further study and comparison. This has not yet been achieved, but I believe it can be done.
"The second is the development of reference antibodies for serotype characterization of isolates. We have begun to do this, and currently have monoclonal antibodies for two immobilization serotypes (A and D). These two serotypes appear to occur fairly frequently in nature, but many more serotypes exist and it would be great to find out how many there are and begin to develop a knowledge base.
"If the two technologies I mention above were currently available, comparative studies on drug resistance, virulence and other aspects of the natural history of infection could begin to be resolved. Needless to say, this information would also be important for the development of new diagnostic tests, therapeutics, and vaccines."
For more details on the study see the paper: Swennes AG, Noe JG, Craig Findly R and H Dickerson (2006) - Differences in virulence between two serotypes of Ichthyophthirius multifiliis. Dis Aquat Organ. 2006 Apr 6; 69(2-3): 227-32