A couple weeks ago I blogged about scabies as a neglected tropical disease that plagues the underprivileged in tropical and sub-tropical climates.
Remember the epidemiological triad?[1] Well, today I’m going to focus on one corner of it: the agent.
I discussed already how many factors influence the incidence of scabies infections in different areas, and how a multifactorial approach would be best to control the mites. The current best strategy to rein in infestations is the mass administration of scabicidal drugs. However, if you will remember my blog from last week,[2] resistance to drugs is a natural process of evolution and develops rapidly. We’ll need another strategy if we want long-term control of this parasite. But what can we do against a disease that thrives among poverty and overcrowding, two problems that are not easily solved? Perhaps knowing more about the parasite itself will help us better control it.
A female scabies mite, once she’s ready to lay her eggs, will slowly make a long burrow underneath the skin, laying her eggs all the while. Once she’s finished, she dies within her burrow.
Her eggs take less than a week to hatch, and usually only around 10% will grow to adulthood. They hatch as larvae, which make shallow burrows to hide in while they molt into mature adults. A fully grown male will then find a female and mate. Since the female will only live a couple months after that, just long enough to lay all her eggs, mating only occurs once.[3] If a person is going to be infested with scabies, it will be by these fertilized females crawling on them from the previous host.
Upon reading about the scabies life cycle, I was interested to find that they only mate once. This reminded me of another parasite which similarly mates only once, and how researchers used that to their advantage.
Cochliomyia hominivorax, otherwise known as the screwworm (named for the spiral, screw-like pattern on the larvae), used to be a dangerous pest that greatly threatened an endangered species of Florida Key deer. Unlike most flies, which only feed on dead flesh, screwworm larvae will actually burrow into and consume healthy tissue in their unfortunate host, and eventually kill them if left untreated. They can lay their eggs in any open wound—even tiny ones made by ticks—as well as the navel of infants, and are attracted to any warm-blooded animal; that includes humans.[4]
Screwworms were first successfully eradicated from the United States in 1984[5] using a rather unique aspect of their physiology: they only breed once. When this was discovered, researchers grew hundreds of male screwworms and sterilized them, then released them into the wild. Many female screwworms laid unfertilized eggs because they mated with a sterile male, and within a few generations the flies were eradicated from U.S. soil.[6]
Perhaps a similar method could be used to control scabies mites in humans. This would be an interesting area of study, however there are certain ethical questions that would need to be addressed: with the screwworms, the males were just released into the environment where the females were; with scabies, which spend their entire life on a host, this would require purposely infesting a human being with more scabies. Sterile or not, this isn’t exactly an acceptable practice. One may argue that the benefit would outweigh the ethical cost, but that is straying into dangerous territory. It may be a viable option for the infested dogs and cats that spread the mites, however.
A less compromising option for human beings would be to find
a way to sterilize the scabies population already infesting a person, without
bringing harm to the host. This would not be easy, and maybe impossible;
radiation was used to sterilize the screwworms, but that obviously cannot be
used on a person. If one thing could be said for researchers, however, it’s
that they find new, creative solutions to pretty much any problem.
[1] https://vetmedone.health.blog/2019/01/21/hows-about-the-epidemiological-triad/
[2] https://vetmedone.health.blog/2019/02/11/its-the-resistance/
[3] https://www.cdc.gov/dpdx/scabies/index.html
[4] https://www.cdfa.ca.gov/ahfss/Animal_Health/pdfs/Screwworm_Fact_Sheet.pdf
[5] https://www.ars.usda.gov/news-events/news/research-news/2002/usda-celebrates-research-that-eradicated-the-screwworm/
[6] Ibid.
Vet Med and One Health 
