Opalina sp. is a genus of protozoa with over 200 different species. Opalina is a leaf shaped ciliate look alike because of its many rows of flagella. These organisms lack a mouth and contain many nuclei. Opalina infections are most commonly found in the intestines and rectum of frogs and toads as well as other amphibians, fish, mollusks, and reptiles.
Opalina has a very interesting and unique life cycle because it undergoes both sexual and asexual reproduction. Sexual reprouction is much more costly to a parasite than asexual reproduction. Why does the Opalina parasite invest so much energy in sexual reproduction?
A common example of a life cycle of Opalina is with frogs or toads.
Usually the feces of a frog or toad that carries cysts of Opalina will settle to the bottom of a body of water. Here, the encysted Opalina are eaten by tadpoles. When excysted in the intestine of the tadpoles, they begin to divide to uninuclear male and female gametes. These gametes fuse and become large adult trophozoites. That is their sexual reproduction stage. After, they will continue to reproduce asexual by binary fission within the tadpole and frog or toade. Opalina will encyst in the intestine and be excreted in the feces of the frog and the cycle will repeat itself each spring.
If Opalina is excreted before encysted, it will die outside of its host. If excreted outside of its natural aquatic environment, it will generally not be ingested and the life cycle will halt.
Several studies have been done to understand the life cycle of Opalina species. Many suggest that Opalina undergoes binary fission during most of the year. In the spring, when the hosts of the parasite are mating, this is when Opalina will encyst. This is convenient for the Opalina parasite because its hosts breed in water and its young begin their lives in water and there fore become infected.
Several papers have examined the physiological and chemical pathway that endues cysts and the sexual reproduction cycle to begin. It is important to understand why Opalina species encyst during the time of year so that tadpoles are able to ingest cysts and the cycle can persist. It is hypothesized that certain hormones are involved in mating of frogs and toads are involved with signaling Opalina to encyst while in their intestines and rectum.
One study injected toads that were infected with a species of Opalina with adrenaline, ACTH and testosterone in the off breading season. After the injections they examined the toads behavior and the number of encysted parasites. They found that injections of adrenaline or ACTH in male and female toads caused parasites to encyst. The authors suggest that adrenaline cause the secretion of ACTH so they elicit the same response. In male toads, injections of testosterone were also found to induce parasites to encyst. They hypothesize this is because in male toads, testosterone in linked to that same metabolic pathway and breakdown chemicals of ACTH signaling the parasite to encyst.
ACTH and adrenaline did not induce sexual reproduction in the toads. This is an important finding to understand the life cycle of Opalina and how it relies on the hosts physiological process to survive.
Perhaps this is an example of parasite manipulation on its host. This could be a "boring byproduct" or coincidence that the Opalina parasite requires this environment to survive. Or it could be an "extended phenotype" of the organism on its host. More research is necessary to understand what induces these metabolic pathways to understand if Opalina is manipulating its host or if it is specialized to its host.
There is no pathology of Opalina that is commonly recognized. It is suggested that Opalina has a commensalism type relationship with its hosts.
One study states that the prevalence of Opalina in amphibian populations may skew our understanding of its pathology. it poses the question: If 70% of amphibian populations are affected by this parasite, are we really sure that it is not pathogenic?
Other studies suggest that individuals with heavy infections of Opalina, does impact the growth rate of individual tadpoles, sometimes inhibiting metamorphosis, because of the resource competition with the parasite. If this is the case then the control and prevention of Opalina is critical to the security of the ecology of many lakes, rivers and ponds.
Some research has been done to investigate the possibility of controlling and eliminating the parasite. The study suggests that the antibiotic metronidazole is able to clear infections of Opalina in toads with one single dose in 1 week.
It is estimated that Opalina could be infecting up to 70% of toads in North America and is known to be world wide. The rate of infection is, however, generally unknown.
Opalina is most often found in hosts with other infections suggesting that they may establish easier when the host is immunocompromised by fighting off other infections.
This suggests that Opalina follows an "assembly rule" where once it is infected by a more pathogenic parasite, Opalina has the ability to also infect this host. Without the primary infection by another parasite, Opalina may not be able to become established as an infection as easily. This was demonstrated in a study of one species of green frogs. Tadpoles were infected in a lab setting without any other parasites present and by the time the tadpoles metamorphosized, the Opalina infections were eliminated by the green frogs immune system.
This also is an explination of why Opalina invests in sexual reproduction. If the Opalina parasite has difficulty establishing within a host, it would invest in meiosis at the cost of fewer offspring and the risk of not finding a mate inorder to generate unique genotypes. These unique genotypes would result in better, more competitive Opalina that can perhaps evade the hosts immune system and become established more easily.
Resources and Papers cited
1) Bieniarz J. 1950. Influence of vertebrate gonadotropic hormones upon the reproductive cycle of certain protozoa in frogs. Nature, Lond 165: 650.
2) Nickol R., Tufts, D. Single-dose Metronidazole Clears Opalina sp. from Juvenile Bufo woodhousii. J. Parasitol., 99(3), 2013, pp. 573–575.
3) Hegner, R. W. 1923. Observations and experiments on Euglenoidina in the digestive tract of frog and toad tadpoles. Biological Bulletin 45: 162–180.
4)H. Sandon. The Species Problem in the Opalinids (Protozoa, Opalinata), with Special Reference to Protoopalina. Transactions of the American Microscopical Society, Vol. 95, No. 3 (Jul., 1976), pp. 357-366
5)C. P. Goater. 2001. Ecological monitoring and assessment network (EMAN) protocols for measuring biodiversity: Parasites of amphibians and reptiles. Parasitology Module Steering Committee, Parasitology Section. Canadian Society of Zoologists, p. 1–59.
6) M.M. El Mofty, I.A. Sadek. Mechanism of action of adrenaline in the induction of sexual reproduction (encystation) in opalina sudafricana parasitic in bufo regularis. International Journal for Parasisology. Vol 3pp425-431. 1973.
7) Dr. Schall's Class notes