#cestodes

Eggs of Dipylidium caninum, the common tapeworm of dogs and cats.

Tapeworm eggs leave the host’s body in protective little segments, called "proglottids," that look like grains of rice. The proglottids can often be seen crawling from the animal’s anus, caught in the fur under the tail, or in the stool. If the proglottid is crushed on a slide and viewed under the microscope, you see tons of these eggs. Since the eggs of this species are too heavy to float well in our fecal flotation solutions, we don’t usually see them in a fecal flotation.

The good news is that while they’re super gross, they can’t infect you or other animals directly. First the eggs have to be eaten by a flea (the intermediate host), and then the flea needs to be eaten by the dog, cat, or person (final host). The bad news is if we see this kind of tapeworm, we know we need to treat for fleas, too, or the pet will just keep being reinfected.

Endoparasites: Trematodes and Cestodes

We did an article about Nematodes not so long ago that covered the most important intestinal and non-intestinal species regarding companion animals. This article completes the chapter on Endoparasites where we will describe the general characteristics of Trematodes and Cestodes, as well as the characteristics of their most relevant representatives. Intestinal Trematodes Trematodes, or also known…

We have been studying parasites this week and it’s a subject I’ve already developed a real interest in. I think it’s the crazy array of life-cycles and inherent cheekiness of the idea of revolving your life around feeding off other’s resources that make these little organisms intrigue me. I’m going to write a few posts outlining the key features of different parasites. First up is the cestodes – more commonly known as tapeworms.

Structure Adult tapeworms consist of a chain (strobila) of progressively maturing, independent reproductive units called segments or proglottids, anchored to the intestinal wall by a scolex and neck. The scolex is a useful way to identify certain species, and is covered in hooks and suckers, varying in composition per species.

The two major groups of tapeworms are the Pseudophyllidean and Cyclophyllideans. Pseudophyllideans have a scolex with four longitudinal grooves and central genital pores on each segment. There are no Pseudophyllideans of importance in the UK. Cyclophyllideans have four circular suckers around the scolex. Some also have hooks. Most of these tapeworms live in the small intestine. They absorb nutrients over their body surface which is covered in a layer called the tegument, which aids with absorption and secretion and has minute finger like protections on the surface. New segments bud off from behind the scolex.

Reproduction Each proglottid has both male and female sex organs, producing both male and female sex cells. Their sex organs include a uterus, ovaries, testes and seminal vesicles as in humans. This means they produce by sexual reproduction, either from the same or a neighbouring worm. Cross-fertilisation with another worm is often the preference. The genital pores in Cyclophyllideans are lateral, so to the outer side. Gravid segments are formed at the end of the chain and contain over 100,000 eggs. Genrally one or two segments bud off daily and exit the animal through faeces or their own mobility. Unlike other parasites, each egg is immediately infective and contains a tapeworm larvae with six hooks, surrounded by a shell with a striated appearance under the microscope.  

Life-cycle The general Cyclophyllidean lifestyle is quite simple. The eggs are shed in the faeces and are immediately infective and infect an intermediate host, if it is a compatible animal. It is here that they develop in to the larval stage. Different species of tapeworm have different larval stages, ranging from a simplistic Cysticercus, a fluid filled bladder with one inverted scolex, to complex Hyatid and Alveolar cysts with thousands of inverted scolices. The larval stage in the intermediate host is called a metacestode. I should mention here that in parasite terms, a larvae is a stage of development that does not look like the adult parasite, whereas a nymph is an immature stage of the parasite that has similar morphological features to the mature adult.

The eggs are broken down in the intestine and the larvae use hooks to travel through the intestinal mucosa and enter the blood stream to reach their predilection site. This is the site where they are most commonly found and prefer to develop in. An example in the Taenia saginata which uses cows as an intermediate host and embeds cysticerci larvae in to the muscles of the cows, infected the meat. If humans eat this meat when it is undercooked, they become the final host and the adult worm develops in their intestines. Final hosts can become infected in a number of ways, including a predator eating infected prey (e.g. cat and mouse), accidental ingestion of infected mites on pasture, or consumption through irritation, for instance grooming an area and ingesting a flea acting as an intermediate host.

Cestodes can be diagnosed in animals by taking a faecal sample and examining it under the microscope for gravid segments and eggs. Popular treatments include oral medications that are toxic to the adult tapeworm. Weight loss, hunger, diarrhoea and lethargy are all signs that an animal has worms. Giving your animal wormers simply clears out the worms already in the system. Farm cats are often getting reinfected as they keep eating the intermediate hosts - mice!  I have attached a few pictures to outline what I’ve said. The first is a gravid segment, full of eggs and ready to pass in the faeces.  The second is a diagram of a cysticercus larval stage, the simplest of the larval stages. The third is of an entire tapeworm and the fourth is a mature segment with the reproductive organs on show. 

Hope this was a useful overview! There’s more parasites to come!