"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

July 16, 2020

Neofoleyellides boerewors

Mosquitoes are mostly known for being blood-suckers. But despite that reputation, they actually spend most of their adult life feeding on nectar - it's only when female mosquitoes are ready to breed that she needs blood to fuel the development of her eggs. Because of this feeding habit, mosquitoes are used by a wide variety of blood-borne parasites to ferry them from one host to another.

In humans, mosquitoes are responsible for transmitting a variety of parasitic infections such as the malaria parasite and filarial worms, as well as a range of different viruses. This also extends to other animals that are fed upon by mosquitoes, which are host to their own array of mosquito-transmitted parasites. There are some species of mosquitoes that specialise in feeding on ectothermic ("cold-blooded") vertebrates such as frogs and toads, and accordingly those mosquitoes are also vectors for a range of parasites that infect those animals.

Left: Microfilaria larva from toad blood, Top left: Late L1 sausage-shaped stage from mosquito thorax, Top right: Adult female worm, Bottom right and left: Infected toad with adult worm in its right eye.
Photos from Figure 3, 5, and 7 of the paper
Neofoleyellides boerewors is a filarial nematode in the Onchoceridae family, a group of parasitic roundworms that includes the parasite that causes river blindness in humans, but the species that infect amphibians are not as well-studied. This paper describes one such species that has been found in the guttural toad, Sclerophrys gutturalis.

The adult worm mostly lives in the toad's body cavity or just under the skin, though some can end up in other parts of the body. For example, in one particularly heavily infected toad, the researchers found 52 adult worms, and one of those worms had even spilled over into the toad's right eye where they caused internal bleeding and blindness.

Inside the toad's body, the adult worm produces larval stages called microfilarials that circulate in the amphibian's blood vessels while waiting for a rendezvous with a hungry mosquito. When the mosquito slurps up a belly full of toad blood, they also end up ingesting a bunch of those baby worms.

Once inside the mosquito, these microfilarial transform into chubby, sausage-shaped worms (indeed, the species name of this parasite, boerewors, is named after a popular type of South African sausage), and proceed to congregate amidst the fat bodies in the thorax, where they can grow by feeding off the mosquito's nutrient reserves. After spending about ten days there, the larvae developed into the infective stage, ready to infect another toad. They migrate to the mosquito's head and move into position at the insect's mouthpart, preparing to disembark into the bloodstreams of another toad the moment that the mosquito begins feeding.

Anurans (frogs and toads) are host to a wide range of parasites, many of which have unique life cycles and life histories which are adaptations to the developmental history of their amphibian hosts. There is still a great deal we don't know about the diverse array of parasites that are found in frogs, toads, and other amphibians.

With many of those amphibians under threat from climate change, habitat destruction, and the dreaded amphibian chytrid fungi, it is highly likely that we may never fully learn about the wonderful adaptations of their associating symbionts - a hidden world of biodiversity that would tragically disappear along with their hosts.

Netherlands, E. C., Svitin, R., Cook, C. A., Smit, N. J., Brendonck, L., Vanhove, M. P., & Du Preez, L. H. (2020). Neofoleyellides boerewors n. gen. n. sp.(Nematoda: Onchocercidae) parasitising common toads and mosquito vectors: morphology, life history, experimental transmission and host-vector interaction in situ. International Journal for Parasitology 50: 177-194