Date: 09.03.2020

Which Way to the Brain? A Parasite’s Favorite Entry Portals Into Fish


Parasites are found all over the world and there are many different types. However, they are often overlooked due to their small size. Parasites rely on hosts to survive. Hosts are organisms that are needed for completing the parasite’s life cycle. Therefore, parasites are constantly moving from one animal to another. How does this transmission between animals happen? We study how a parasite called Cardiocephaloides longicollis moves between animals and enters into their fish host. This parasite infects fish through their skin, to eventually develop in their brains. By visualizing C. longicollis with a fluorescent and glowing stain, we observed that its favorite entry portals into fish were areas close to the brain or with an easy connection to it, such as the eyes, the gills, or the back and belly areas. Our study helps us to understand one of the strategies that some parasites use to complete their life cycles.

Parasites are living organisms that rely on another organism to survive. The word is derived from the Greek word “parásitos,” which means “one who eats at the table of another.” The parasite usually benefits from this relationship by feeding off nutrients from the chosen organism, called the host, and sometimes also uses the host as a place to take refuge and reproduce. In parasite-host interactions, hosts evolve defense mechanisms to fight the parasitic infection, but parasites, in turn, come up with new strategies to find or survive in their hosts [1]. Some parasites have a simple life cycle, in which they need only one host species to develop and reproduce. This host, in which the parasite develops into its adult stage and reproduces, is called the definitive hostOrganism that the parasite infects and where it develops into the adult stage, which reproduces producing eggs that are released into the environment through the definitive host’s feces.. In other cases, parasites have a complex life cycle that involves more than one host species, sometimes even three (Figure 1). In a typical complex life cycle, the parasite reproduces in the definitive host, usually a vertebrate, such as a bird or a fish, which then releases parasite eggs into the environment through its feces. The eggs hatch to release larvaeYouth stage before transformation into an adult. For example, before becoming a butterfly, the larva was a caterpillar. that can infect the first intermediate hostOrganism that the parasite infects and where it develops as a larval stage. Intermediate hosts are needed for some parasites to complete their life cycles., usually an invertebrate, such as a snail. These larvae reproduce within this intermediate host and thousands of immature free-living and swimming stages, called cercariaeThe name for the larval stage that some parasites go through during development. Some cercariae can freely swim in the water to infect their hosts., are released into the environment, where they infect a second intermediate host, usually a fish or another invertebrate. Cercariae grow and mature within this host, developing into another stage called a metacercariae, and the cycle is completed when the definitive host eats the second intermediate host. Does this make you wonder how these parasites succeed in completing their life cycle when it seems so challenging?

The rest of the article:

van Beest G., Born-Torrijos A. 2020: Which Way to the Brain? A Parasite’s Favorite Entry Portals Into Fish. Frontiers For Young Minds 8: 25.

DOI: 10.3389/frym.2020.00025

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Biology Centre CAS
Institute of Parasitology
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