Tag: scabies

Barely Scabing By

https://en.wikipedia.org/wiki/Sarcoptes_scabiei#/media/File:Sarcoptes_scabei_2.jpg

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.

Featured

How’s About the Epidemiological Triad?

An epidemiological triad is a tool used by One Health professionals to not only identify and categorize a health issue, but to also develop a control plan.

This is a general example of an epidemiological triad.

https://qph.fs.quoracdn.net/main-qimg-6622d9b9a55f6372dbd3e0f293d06676

It has three corners: host, agent and environment.

The ‘agent’ stands for whatever it is that causes the disease. This can be a virus, a bacterium, a toxin, etc. We need to make a distinction between the disease itself and the agent that causes it. The flu disease consists of the common flu symptoms, but the agent that causes the disease is the Influenza virus. Just because someone is infected by the Influenza virus, doesn’t necessarily mean that they will get the flu disease; for example, if they are healthy and have gotten a flu shot, they will be less likely to get sick. In the ‘agent’ section of the triangle they list all the features of the agent that may cause disease.

The ‘host’ is whatever the agent can cause disease in. Some species are resistant or immune to certain agents, and will not come down with the disease even if they are infected. Some animals are naturally resistant to disease by the agent unless certain conditions are fulfilled; for example, if the animal is young, old, stressed, starved, or otherwise weakened. In this section of the triangle they list all the conditions that could predispose a possible host to contracting the disease when infected by the agent.

The ‘environment’ section stands for the environmental conditions that could increase chances of a host coming into contact with the agent, and make the host more susceptible to contracting the disease. In this instance, if the geographical area is favorable to the agent’s survival, if the host is constantly in close contact with infected individuals, has limited access to medicine or sanitation, or other conditions that may make disease more likely.

Here’s an example of an epidemiological triad for an actual disease, Scabies:

This disease is caused by the agent Sarcoptes scabei, a microscopic mite. The unique thing about this agent is that cases of disease are very uncommon in developed countries where most inhabitants have access to healthcare, and are not overcrowded. In developing countries, however, it is one of the most common causes of skin diseases, and is most prevalent in tropical areas where the mites survive best.[1] For this reason, scabies is considered a neglected tropical disease: a disease which disproportionately affects “populations living in poverty, without adequate sanitation, and in close contact with infectious vectors.” [2] Young children and the elderly are particularly susceptible to contracting scabies, as well as secondary complications.

A triad like this is used to find possible control points in the disease cycle. The agent could be controlled by administering scabicide, a medicine that kills the mite. Prevalence of the disease could be lessened by controlling the disease in the stray dogs that can transmit the mite to people.

The triad could also be used to identify areas and situations at high risk for scabies outbreaks, and allow surveillance and preparations to react in case one actually takes place. Right now the World Health Organization recommends mass administration of scabicidal drugs as the most feasible and effective method of controlling the scabies disease.

The epidemiological triad is a good example of what One Health is about: taking into account the whole picture of what leads to disease, by listing and investigating all the factors. Looking at ways to intercept and stop the disease cycle, and not just focusing on treating those who are already infected. Working for the improved health of all people, as well as animals and the environments they live in.

[1] https://www.who.int/neglected_diseases/diseases/scabies/en/

[2] https://www.who.int/neglected_diseases/diseases/en/