"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

November 22, 2012

Pseudanisakis sp.

As has been discussed in a number of previous posts, most parasites don't get the whole host to themselves and often have to compete with other parasites for resources. In the case of gastrointestinal parasites, this can mean jockeying for the best real estate along the highway of pre-digested food that is the intestine. In some cases, the ideal position might already be occupied and the parasite needs to shift elsewhere to what is known in ecology as the "realised niche width". How this pans out depends on both what host they happen to be in and what other parasites happens to be around.

A researcher from University of Otago investigated how competition affects intestinal worms in different species of skates and how they are distributed within the gut. In the lower intestinal tract of elasmobranchs (sharks, skates, and rays) is the spiral valve - a series of folds and whorls that increases the surface area (and thus nutrient absorbent surface) of the intestinal wall. Different species have different number of whorls and this is where most intestinal worms of elasmobranchs live.
image modified from here

The most common types of tapeworms found in elasmobranchs are the tetraphyllideans (last year we featured a species which lives in the Great White shark) - this name translates roughly into "four leaves", so-called because their scolices (plural for scolex - the attachment organ of tapeworms) consists of four intricate lobes that fold out almost like a flower (you can see some of them here). These elasmobranch tapeworms are very specialised, and the shape of their scolex fits perfectly into the intestinal folds of their host and no other species (see this for example).

But the parasite we are focusing upon today is actually a nematode (roundworm) - Pseudanisakis sp. (photo on the right) - it infects three species of skates and shares them with a number of other parasites. When Pseudanisakis shares the spiral valve of the little skate (Leucoraja erinacea) with two species of tetraphyllidean tapeworms, its presence causes one of the tapeworms - Pseudanthobothrium purtoni - to shift from its usual position in the spiral valve and move more towards the anterior whorls. Contrast this with what happens in the smooth skate (Malacoraja senta) where Pseudaniskis simply lives alongside two other species of parasites (both also tetraphyllidean tapeworms) without anyone pushing anyone else out of place. But when Pseudanisakis is confronted with a different type of tapeworm, as is the case in the gut of the thorny skate (Amblyraja radiate), the nematode becomes the one that is forced to compromise, and the worm that causes Pseudanisakis to submit is Grillotia sp.

Grillotia belongs to a different group of tapeworms called the trypanorhynchs. Instead of four intricate lobes that fit snugly into the folds of the intestinal wall, it has four tentacles lined with hooked barbs that upon contact with the intestinal wall of its host, shoot out and embed themselves in the host's tissue (the photo on the left shows the scolex of a larval trypanorhynch with the tentacle just slightly protruding, see also this photo of a worm with one of its tentacles more fully extended). For whatever reason, in the presence of Grillotia, Pseudanisakis is compelled to move.

There appears to be a pecking order amongst the intestinal worms of skates, with trypanorhynchan tapeworms on top, followed by nematodes, then tetraphyllidean tapeworms trailing behind. Note that this kind of competition between these species only seems to occurs between worms that live in the spiral valve of skates. Similar worms living in the spiral valves of sharks seems to just leave each other alone. At this point, it remains uncertain why that is the case.

Randhawa, H.S. (2012) Numerical and functional responses of intestinal helminths in three rajid skates: evidence for competition between parasites? Parasitology 139: 1784-1793

November 13, 2012

Amblyomma nodosum

The parasite being featured today is Amblyomma nodosum (image on the right - male top, female bottom) - a species of specialised tick that happens to be one of only three species of parasite that were found while examining three roadkilled giant anteaters from Minas Gerais, Brazil. There are 100 species of Amblyomma from around the world (33 of which are from Brazil) and they have been described from a variety of hosts from amphibians and reptiles to birds and mammals, but A. nodosum is a specialist that lives exclusively on the giant anteater (Mymercophaga tridactyla) and the collared anteater (Tamandua tetradactyla). It was also the most abundant of all the parasites found on the anteaters in the study we are featuring today, occurring in moderately high numbers (average of 58 ticks per anteater).

The second parasite that was found is the chigoe flea Tunga penetrans. Unlike A. nodosum, this ectoparasite infects a wide range of hosts, and while most fleas simply hop onto a host, drink up some blood and jump away, T. penetrans females burrow into the skin and *stay* there, feeding on blood and laying eggs. They only occurred in low numbers on giant anteaters and were found burrowing into the footpad and nowhere else on the body (see image below).

The third parasite found was also the sole internal parasite in the anteaters, the tapeworm Oochoristica tetragonocephala. Tapeworms from this genus are known to infect a range of hosts including lizards, snakes, and a variety of mammals. The larva needs to infect an invertebrate host, specifically an arthropod, before reaching the gut of a reptilian or mammalian host (by the said reptile or mammal eating the infected arthropod) and maturing into an adult worm. In the case of this species infecting the anteater, ants and termites are the most likely candidates for where the larval stages reside, given the host's specialised diet.

Relatively speaking, the giant anteater has very a sparse parasite fauna. Usually, a mammal of its size would be infected with a dozen or more different species of parasites. But because of its specialised diet and solitary life style, there are very few opportunity for most parasites (except specialists or very abundant generalists) to infect the giant anteater (as reflected by its paltry parasite fauna). Such an example shows how the ecology of the host organism can often shape what parasites it is infected with.

Photos from figures in the paper.

Frank R, Melaun C, Martins MM, Santos AL, Heukelbach J, Klimpel S. (2012) Tunga penetrans and further parasites in the giant anteater (Myrmecophaga tridactyla) from Minas Gerais, Brazil. Parasitology Research 111:1907-1912