A Winning Strategy of a Liver Worm: Keeping a Low Profile and Playing the Long Game

Many parasites have severe effects on host health and fitness, which may lead to the rapid death of the carrier if not treated promptly. However, other parasites play a waiting game, new research suggests. By studying host gene regulation, Swedish and Estonian scientists showed, for the first time, that the best game plan for a cyst-forming flatworm, Triaenophorus nodulosus, is to make no fuss, be patient inside the liver of the intermediate host, Eurasian perch, and wait to be eventually eaten by a final host, a northern pike. This is the end of the road for perch but the ultimate destination for a parasite, who develops into an adult worm to lay eggs and continue its complex life cycle.
By screening the activity of over 15,000 genes in infected and parasite-free perch using a technique called next-generation sequencing (NGS), known as RNA-Seq, the researchers found only a handful of genes that differed in their expression between the two types of fish. The few genes triggered by parasite infection in the spleen were linked to immune defence, while those in the liver were associated with metabolic functions. 'Interestingly, several identified genes that responded to infection have also been found to be associated with parasites in other species, suggesting their potential common role,' said the first author of the study, junior researcher Konrad Taube from the Estonian University of Life Sciences.
'Earlier work has suggested that in order to maximize the likelihood of reaching its final host, the parasite may benefit from weakening the perch, making it more susceptible to predation,' said Taube. 'However, if the perch is unable to sustain the parasite burden and dies before being eaten by the pike, it does not represent an optimal reproductive outcome for T. nodulosus. Therefore, it is in the parasite’s interest that the intermediate host can cope with the infection and live long enough to be preyed upon by the pike. The key to a successful life for parasite is therefore finding a balance between exploiting the host and exhausting it. Our findings also illustrate that not all conspicuous infections have severe effects on host physiology and gene regulation,' said Taube.
The study was conducted by a team led by Prof. Anti Vasemägi from the Department of Aquatic Resources, SLU, and Dr. Kristina Noriekiene from the Chair of Aquaculture, Estonian University of Life Sciences, and is published in the International Journal for Parasitology: Parasites and Wildlife. The project was supported by Estonian Research Council grant, Swedish Research Council grant, and the Kristjan Jaak Foundation scholarship.
Taube K, Noreikiene K, Kahar S, Gross R, Ozerov M, Vasemägi A (2023) Subtle transcriptomic response of Eurasian perch (Perca fluviatilis) associated with Triaenophorus nodulosus plerocercoid infection. International Journal for Parasitology: Parasites and Wildlife, 22, 146-154.