For some time there has been a debate about the causes of the so-called “whirling disease” in discus fish (Symphysodon) and other tropical ornamental fish species. I hope via this article to inspire further research and discussion and highlight some of the facts and myths surrounding this problem. The illness itself is known to cause fish to whirl violently in a circular manner often with no apparent or obvious cause and usually die some short time later. Many theories have come about as to how and why this strange behaviour occurs ranging from infectious organisms to dietary deficiencies or even unsuitable food choices causing some level of overwhelming toxicity or poisoning; the answer undoubtedly lies with infection.
At some point a connection was found between the European cold water parasite Myxobolus celebralis and instances of whirling occurring in Salmonids and other similar species inhabiting temperate waters such as Onchorhychus mykiss (Rainbow Trout). The parasite M. celebralis undergoes a two-host life cycle whereby it requires the presence of both a Salmonid fish and a tubificid oligochaete worm in order to complete its development; the only known worm genus which can host this parasite is Tubifex. sp and is commonly imported worldwide as a food source for ornamental fish. The species M. celebralis has negligible activity and cannot reproduce very successfully above 20°C but yet, research by El Matbouli et al (1999) demonstrated that the spores of M. celebralis could re-infect their tubificid host even after the worm was cured of the infection by being held at 30°C for 3 weeks just by returning the worm to waters of 15°C or less; the same research showed that not all developmental stages of the parasite were destroyed even after 2 weeks at 20°C. This doesn’t prove anything relating to infection of species from more tropical waters, but what it does demonstrate is that the spores of M. celebralis are extremely tolerant of varying temperatures; previous research by El Matbouli et al (1991) even showed that M. celebralis spores could survive for up to 3 months at -20°C!
Now, the fact that Myxobolus celebralis is a cold water organism with an optimum temperature for development of 15°C does not explain how tropical fish species can be infected by this particular parasite so we need to look a little deeper. A second parasitic organism known as Ceratomyxa shasta is also known to infect Salmonids within a restricted range in American waters in much the same way as M. celebralis using a freshwater worm (Manayunkia speciosa) as a host. Research indicated that despite temperature increases promoting infection of Salmonids by this species, the parasite does not induce whirling. We also must take into account the degree of host-specificity that these organisms display e.g. how picky they are about which fish they infect. M. celebralis demonstrates a clear dependence on Salmonid species as indicated by the 1999 research by El Matbouli et al after attempting to infect several other fish types including Poecilia reticulata (guppy), Oryzias latipes (Japanese Rice Fish), a cyprinid species and even a tadpole. C. shasta infection also appears to be limited to Salmon and Trout (Hallett & Bartholomew 2012).
Furthering investigations into what the cause could be, I was able to find a research article entitled “Detection of Myxozoan Parasites in Oligochaetes Imported as Food for Ornamental Fish” (Lowers & Bartholomew 2003) from the Department of Microbiology and Centre for Fish Disease Research in Oregon, USA; this article outlined something very interesting. When examined for myxosporean parasites, namely M. celebralis, the researchers discovered seven different triactinomyxons, the organism at the stage which infects the fish host, none of which corresponded with the triactinospore, the spore which releases the triactinomyxon, of M. celebralis; in short, there were potentially seven other different Myxozoan parasites which had infected the tubificid worm! This opens up the debate for potential infection of other more tropical fish species and invokes the question of whether any of these other Myxozoans could survive, thrive or even reproduce at higher temperatures and are they specific to other hosts?; with much of the Tubifex supplied to the ornamental fish trade being cultured and supplied from Asia it is certainly up for consideration and should be investigated. For me, it creates the thought that even in a closed system where fish are not fed live foods, introduction of a new fish previously fed with Tubifex worms and harbouring the respective parasites or eggs of the parasites can cause infection to occur.
While on this subject and having done a great deal of reading I wondered about the best ways to eliminate problems like this. Should oligochaetes and other organisms of freshwater origin be cultured for freshwater fish? Is it overall much safer to feed marine live foods to freshwater fish and vice versa? What about euryhaline species? Or should these live foods be banned altogether to eliminate the risk of introduction to other habitats as stipulated in the Lowers & Bartholomew research? There are a lot of risks and yet there can also be a lot of benefits to using live foods in both commercial and domestic hatcheries. Perhaps the issue is more that some live food options need to be removed or replaced for those which do not possess the capacity to host infectious freshwater fish parasites, but it that feasible?
Having done some research on one of the more recent live food trends to move into the aquarium hobby, Lumbriculus variegatus (The Blackworm), an option often cited as the best replacement for risky worms such as Tubifex, I discovered another article entitled “Potential risk of fish-borne nematode infections in humans in Brazil” (J.C. Eiras et al 2016) which outlined the existence of the Diocotphyme renale parasite (Giant Kidney Worm) in the following tropical fish species in Brazil, Acestrorhynchus lacustris, Gymnotus silvius, Hoplosternum littorale. D. renale is a roundworm parasite with a varied list of teleost hosts, the final form of which infects fish eating mammals including humans resulting in destruction of the kidneys; surgical removal is the only way to prevent further infection. Research by Mace & Anderson (1975) showed that Lumbriculus variegatus (The Blackworm) is in fact the oligochaete host for the first developmental stage of D. renale and a final piece of research by D. Pedrassani et al (2009) examined the effect of temperature on the egg development of D. renale and determined that the optimum temperatures for successful development and hatching of this parasite were between 25°C and 30°C. Realising this, I begin to question whether it is indeed possible to find “safe” live food options for ornamental fish, it seems as though there are great risks involved, many of which have yet to be scientifically determined.
If ever there was an argument which supports the use of our prepared formulation Naturekind Advanced Fish Food which uniquely mimics natural diets of fish species without the risks of parasitic infection and is fantastic as an alternative for raising fry, this is it. But from an ecological standpoint the risks of introducing a non native species to a foreign ecosystem cannot be ignored and is probably the most important consideration that should come from this article. Anyway, there are a lot of factors to consider in such a complex issue but at least my hope is that we can open up this dialogue to a wider audience and explore the risks, benefits and ethics.