Sarcocystis spp.

Student guest post time! One of the assessments that I set for students in my ZOOL329 Evolutionary Parasitology class is for them to write about a paper that they have read, in the manner of a blog post. The best blog posts from the class are selected for re-posting (with their permission) on here. So from the class of 2025, here is a guest post by Hayley Doggen titled "Sarcocystis: You can’t see me" about a hidden danger in wild deer.

Wild deer and feral pigs are found pretty much everywhere in Australia, which is what makes them such great game animals for hunters. It also makes them really good at spreading parasites to livestock, other wild animals or even to humans. But how much do we really know about the parasites carried by deer and pigs in Australia? Well, compared to other places like the UK, we know very little. In fact, in 2020, when hunters found white cysts in the rump of a wild deer shot in Taree, New South Wales, this was the very first report of a Sarcocystis-like infection in Australian wild deer, even though we already knew that Sarcocystis parasites can live in Australian livestock (including sheep) and wild deer found in other countries.

A) Image of cysts in a deer caught in Taree which prompted the study; (B) to (D) microscopy images of Sarcocystis in animals caught in the study. All from Figure 3 of the paper.

Sarcocystis is part of a group of parasites known as Apicomplexa. When these guys are eaten up by their intermediate hosts - usually herbivores (e.g. deer) or omnivores (e.g. pigs) - they pierce the animal’s intestinal wall and, using blood vessels like highways, are able to crawl into the animal’s muscles. Inside the muscles, the Sarcocystis can form special cysts, known as sarcocysts. The sarcocysts begin as a single parasite which then multiplies, expanding the sarcocyst. Inside the cyst, the parasites mature and become infective, ready for when a carnivore eats the parasite’s current host. Often, the cysts are big enough to see with the naked eye, which is why visual inspection of meat is an important part of Australia’s food safety policies. Alas, not all the 200+ species of Sarcocystis form cysts big enough to see. As the researchers in this study found out, some species have cysts which are microscopic.

This post focuses on a study conducted in southeastern Australia, where researchers examined muscle tissues of wild deer and feral pigs for Sarcocystis parasites. Previously, two other studies examining the carcasses of deer in Australia found no signs of infection from Sarcocystis, but after the unique discovery in Taree, these researchers thought the lack of results might be due to the way the meat were examined for parasites. The researchers of this study decided to use different techniques which included mashing the muscle tissue and straining it to find parasites, which were then looked at under a microscope, while their DNA was examined to confirm if the parasite was a species of Sarcocystis.

The researchers confirmed that seven of the animals that they examined had been infected by three different species of Sarcocystis, one of which hasn’t been described before. Five of the animals didn’t have any sarcocysts that were visible to the naked eye, instead they had microscopic cysts. Of the microscopic species found in the feral pigs in this study, they have also been reported in domestic pigs in other countries.

Since this parasite is comfortable in both types of pig, there is the possibility that feral pigs will (or have already) spread the parasite to domestic pigs, especially in the increasingly common free-range farms. This is particularly concerning since the only protection we have in place in Australia against Sarcocystis is visual inspection of meat. So, it is likely that infected meat may make its way into unsuspecting homes.

On top of this, we already know of three species of Sarcocystis that attack humans, causing vomiting and diarrhea among other symptoms. Because of this, it is reasonable to consider the possibility that other species of Sarcocystis, including the one discovered in this study, could leave anyone unfortunate enough to eat infected meat with a really bad day ahead of them.

Reference:

Shamsi, S, Brown, K, Francis, N, Barton, D. P., Jenkins, D. J. (2024) First findings of Sarcocystis species in game deer and feral pigs in Australia. International Journal of Food Microbiology 421: 110780.

Kamegainema cingula

Dracunculidae is a family of worms with a nasty reputation. Most of this is due to Dracunculus medinensis, also known as the Guinea Worm which causes its human hosts such painful suffering that the World Health Organisation has set out to eradicate it. As of now, Guinea Worm has mostly been eliminated from the human population, though it persists in dogs. But there are also many other species of dracunculid worms out there, infecting various animals including other mammals, as well as reptiles. In fact the majority of known dracunculid worms are parasites of snakes.

Left: Lesion caused by Kamegainema cingula on the skin of a Japanese Giant Salamander (from graphical abstract of the paper). Right: Larval K. cingula  (indicated by the red arrow) in a copepod (from Fig. 4 of this paper)

Most species in the Dracunculidae family are in the Dracunculus genus, but Kamegainema cingula is one which stands out from the crowd. Aside from belonging to its own distinct genus, it also lives in a very distinct host - the Japanese Giant Salamander. Usually, K. cingula dwells underneath the salamander's skin, hidden out of sight. But when it comes time for the parasite to release its larvae, it produces a lesion on the salamander's skin from which it unloads its offspring.

Researchers in Japan studying these parasites examined salamanders from rivers systems in the Hyōgo and Kyoto prefectures. They did so by walking through the streams at night, examining the salamanders they came across for skin lesions associated with K. cingula, and extracting the parasite whenever it was possible to do so without injuring the salamander. The worms were only visible during April to June, between spring and early summer, when they would poke out of those skin lesions to release their larvae. 

Like the infamous Guinea Worm, the larval K. cingula uses tiny copepods as its intermediate host - which scientists have previously confirmed via experimental infection. From there, it is possible that the juvenile worms might infect fish as paratenic (transport) hosts first before reaching the salamander, since adult giant salamanders won't usually eat speck-size copepods, but they would definitely eat a fish.

Genetic analyses of those worms revealed that evolutionary speaking, Kamegainema occupies a key position in the Guinea worm family tree, having split off early during its evolution from the Guinea worms and the rest of its relatives. In a way, K. cingula and its life cycle could possibly represent the ancestral condition for dracunculid worms, where the parasite is able to release its larvae freely into the surrounding waters whenever it wants.

There are two other families of roundworms which are related to the dracunculids - Micropleuridae which includes worms that infect crocodiles, turtles, and sharks, and the Philometridae, a family of fish parasites that squeezes themselves into awkward places in a fish's body including their muscles and gonads. It is notable that all those worms mostly infect aquatic hosts or at least animals that spend extended periods of time in the water. In this way, by infecting an amphibian, Kamegainema is sort of at an evolutionary crossroad between worms like the fish-dwelling philometrids and those that infect land-dwelling animals like the Guinea worm.

So the ancestor of the Guinea worm might have started out infecting aquatic animals, but when the dracunculid worms started infecting land-dwellers, it needed some drastic adaptations to ensure that it can still reach the water to complete its life cycle. And that adaptation ended up being the ability to coax their hosts into the water by inflicting fiery pain upon them. 

The idea that a successful parasite is one that lives in harmony with its host and inflicts minimal harm has no basis in nature - sometimes, the solution to completing a life cycle includes causing the host agony and suffering.

Reference:

Tsuchida, K., Urabe, M., & Nishikawa, K. (2025). Seasonal change and phylogenetic position of Kamegainema cingula (Nematoda: Dracunculidae) parasitic in Japanese giant salamanders. International Journal for Parasitology: Parasites and Wildlife 26:101052.