Hi All! This is the first installment for my monthly column, “Parasite Selfies”, and I hope you find it both entertaining and informative. I am fascinated by the obsession with “selfies” on social media, so in keeping with this explosive trend and in an effort to combat all of those annoying car driving, bathroom taking, gym going, food eating, and “just because I’m awesome” selfies inundating the Internet, I thought a parasite selfie might just be the breath of fresh air that the rest of us non-selfie taking individuals need. Not to say that there is anything wrong with taking a selfie. If it makes you feel good, then hey man, by all means, do what makes you happy. However, for me, I feel like I need a selfie with a little more substance.
Since I am a biologist, I figured a selfie based on the work I do might just be the type of “selfie- with-substance” that I needed to jump on board with this trend. Hopefully you guys all find this as entertaining as I do, and enjoy reading my column. My plan is to take a selfie with a parasite of interest once a month, then enthrall you with some fascinating information on the life history of the organism and its effect on fishes, particularly in Florida. Enjoy and embrace the weirdness that is to follow!
With that said, my first selfie is entitled “Cestode Pasta” and in the image above, you can enjoy a selfie of me eating a delicious bowl of cestode spaghetti! Before you freak out, this image is completely disproportionate, and I (well not me, I had help from a friend who is the equivalent of a Photoshop wizard) drastically enlarged the size of this cestode solely for entertainment value. Anyway, I thought this would be a good first topic to get the ball rolling, since all animals can acquire these critters, including humans.
Cestodes are commonly referred to as tapeworms and there is a plethora of old wive’s tales and superstitions surrounding these infamous parasites in humans. Some of these include sayings such as:
“If children pick their noses and eat what they pick, they will have worms”
“Take onion for worms as they do not like the smell of onions and will leave”
“If you have a tapeworm, do not eat for a long time; this will starve it. Then chew a piece of fried beefsteak and hold it in your mouth. The hungry tapeworm will smell the steak and put its head up in your mouth where you can grab its head and pull it out”
My personal favorite is the last one since tapeworms can grow very large in humans, and I can imagine pulling one out through your mouth would be especially invigorating; like a massage for your organs and esophagus…mmm. Jokes and fears aside, these parasites do not generally become pathogenic in fish, but some can certainly become pathogenic in humans, especially in developing countries. In fish, they are fairly abundant, both as adults and juveniles, and I come across them often while performing necropsies on a wide variety of species. The abundance of cestodes will vary depending on the species, and some species, such as the ocean sunfish (Mola mola) can acquire a high abundance of these individuals and still be considered “healthy”. In fact, although we normally associate parasites with a negative image, it is sometimes quite normal and healthy for a fish to have parasites, and this is frequently indicative of a healthy environment.
Life History: Cestoda
The class Cestoda falls under the phylum Platyhelminthes, and is further sub-divided into two sub-classes, Cestodaria and Eucestoda. Cestodes within Eucestoda are primarily differentiated by the morphological characteristics of their scoleces. The scolex is the attachment organ they use to anchor themselves in the host. Unlike the trailing chain of proglottids (the simple, segment-like reproductive units that make up most of the “body”), which are relatively uniform among species; the scolex is reliably different in different worms. Scoleces come in a variety of shapes and sizes. Structures that may be present or absent and differ in shape and size include grooves, suckers, hooks and/or spines, as reflected in the image below.
These parasites are endo-parasitic, meaning they infect the internal organs and are transmitted primarily via ingestion of infected prey. They have neither mouth nor intestine, and uptake nutrients via their tegument. They are generally hermaphroditic and therefore capable of fertilizing their own eggs. Cestodes have an indirect life cycle, which includes a free-living egg or larval stage (coracidia), one or two intermediate hosts and a final host. The first intermediate host is usually a copepod and the second intermediate host may either be a fish or another copepod-eating organism. The final host may be a higher trophic level organism, such as a piscivorous bird, shark, or mammal.
Disease and Treatment
Generally, parasites from marine fish do not cause disease in humans, but the presence of cestodes in fish filets may decrease the price at market and some species may also cause spoilage of the meat. Also, if a fish filet is eaten raw, there are some species which can infect humans, but this is a rare occurrence.
Although most adult cestodes are non-pathogenic, the Asian tapeworm, Bothriocephalus acheilognathi, is a rare exception. This species affects a wide range of freshwater hosts including minnows, golden shiners, carp species, channel catfish and some ornamental aquarium species such as Discus. In the US, this is a non-native parasite which was introduced with grass carp and has since caused serious mortality in production facilities for bait minnows, grass carp and juvenile common carp. This species is characterized by a large body size and a scolex with two long bothria, or longitudinal grooves (see image below). It will accumulate along the anterior intestine of the host, and may eventually lead to obstruction or rupture of the intestine, which usually ends in mortality. These parasites proliferate in an aquaculture setting and there are treatment options in place if they do become a problem. An aquaculturist can use Praziquantel, which is a drug commonly used to treat cestodes, monogeneans, and possibly larval digenean parasites. It can be administered as a bath, in which the fish absorbs the drug directly across the gills from the water, or it can be administered as an oral supplement or injection, but these last two methods are much more time consuming and expensive.
Overall, even though cestodes may look scary, they are generally harmless and very common in many species of fish. It is rare that humans will contract them from eating fish, especially if you are consuming cooked fish. They can be seen macroscopically sometimes, so next time you are filleting your dinner, have a look through the muscle for any white cysts, which look similar to white bumps. If you are buying your fillets, the fishmonger will normally take these out before they go to sale, but if you are catching your own fish, you are likely to encounter a couple of these. Take them out and reply back to this post with your very own parasite selfie! Hope you enjoyed the post, happy fishing =)