When you hear “Kissing Bug” maybe your mind goes to something delicate and sweet, like gentle kisses from a colorful butterfly in a meadow. Today, however, I’m going to share with you a story that is a little rougher around the edges – a story of triatomine bugs and the protozoan parasite they can carry called Trypanosoma cruzi. Rather than the traditional butterfly kiss of fluttering your eyelashes on someone’s skin (also a little unsettling), the infected triatomine bug prefers to crawl onto your face once you are asleep at night, bite your face, defecate on you, and leave you to smush the infective feces into the bite when you touch your face from the irritation, which is totally rude and uncalled for. Then, adding insult to injury, once you rub your face, pressing the pathogen into the bite or mucous membranes (eye, nose, or mouth), you can become infected with T. cruzi which is now known as Chagas disease.
Chagas disease was named after a Brazilian physician and scientist, Carlos Chagas, who discovered the disease in 1909. The disease exists only in North and South America, with a majority of cases in rural, poverty-stricken areas of Latin America.
So, what is a method of reducing human infection of Chagas disease in predominant areas? Biodiversity has commonly been associated with the reduced risk of vector-borne diseases to humans, although this is not always straightforward. A very recent paper from Zahid et al. examines the possibility of utilizing zooprophylaxis, or the “decoy effect” in which animals that are poor or dead-end hosts to a pathogen are used to deter the vectors from afflicting humans. In this case, chickens were used in proximity to humans, finding that chickens at an intermediate (rather than short or far distances) distance from humans reduced the parasite prevalence in humans but only “under the condition that the vector birth rate from feeding on chickens is sufficiently low.”
Studies have shown that the presence of infected or at-risk animals within or closely around the household pose a greater risk for contracting the disease – Dogs, cats, guinea pigs, rodents, chickens, as well as some armadillos, skunks, foxes, etc. can serve as hosts for the pathogen and are commonly present within these households or nearby. Specifically, dogs and cats have been shown to be extremely infectious hosts of the pathogen as compared to humans, further stressing their role in the life cycle of the parasite. Education efforts remain in effect to utilize insecticides within the home to divert or kill insect vectors as well as rodent control methods and dog density control around homes.
The Dilution Effect Hypothesis states that greater diversity in potential host species (greater biodiversity) leads to a decrease in human transmissions. The following are assumptions associated with the hypothesis and my answers are based on the current literature regarding Chagas Disease:
- Must be variation in host competence for pathogen
Yes – There is documented variation in host competence among animals. Dogs, cats, rodents, and guinea pigs have been noted to be extremely competent, amplifying hosts as compared to other animals and humans.
2. The host species left in low biodiversity communities must be the highly competent species.
Yes – Regarding the highly competent triatomine vectors and the mammalian hosts, low biodiversity within the population does not appear to be a factor in competence as they also seem to be the only vector.
3. Transmission must be primarily or entirely horizontal, host to host, not from parent to offspring.
No/Yes – Chagas disease can also be transmitted to offspring by congenital transmission (from a pregnant woman to her baby by crossing the placental barrier) although cases are primarily horizontal.
4. The focal host species must interact with a range of other species.
Yes – The triatomine bugs take blood meals from a variety of mammalian hosts depending on the location and habitat.
5. Increased host diversity doesn’t also cause an increase in vector density, or if it does, the increase must be outweighed by the decrease in prevalence of infection among host focal species.
Apparently so – Gottdenker et al. find that, interestingly enough, environmental disturbances such as deforestation, actually increased vector infection with T. cruzi, possibly by altering host community structures to favor hosts that are short-lived with high reproductive rates (like rodents). Oda et al. found that host diversity alterations did not correlate with disease risk.
Due to the answers provided for the five assumptions, I would support that the Dilution Effect Hypothesis is a reasonable approach in Chagas disease relief efforts in that all of the outlined qualifications were positive with the slight exception that the disease can be transmitted vertically. However, seeing as though this is not common due to widespread screening programs in endemic areas, I did not stress the importance much in regard to the dilution effect. Also, to support this conclusion, there are peer-reviewed articles previously and currently examining this possibility and the implementation of properly distanced animals (along with other preventative measures) to reduce the occurrence of human Chagas cases.
Chagas disease is generally not prevalent with origin in the U.S., although the vector can survive in the southern U.S. However, in the U.S., at least 300,000 cases of chronic Chagas disease are estimated among people originally from Latin American countries. One of the signs of infection can sometimes be a swollen eye (seen above), with the eye sometimes serving as the site of entry for the infection, after the triatomine bug bit you and pooped on your face while you slept causing you to push their infected feces into your own eye. Tricking you into finish their dirty deed.
Sources
Gottdenker, Nicole & Chaves, Luis & Calzada, Jose & Saldana, Azael & Carroll, Ron. (2012). Host Life History Strategy, Species Diversity, and Habitat Influence Trypanosoma cruzi Vector Infection in Changing Landscapes. PLoS neglected tropical diseases. 6. e1884. 10.1371/journal.pntd.0001884.
Gürtler RE, Cécere MC, Petersen RM, Rubel DN, Schweigmann NJ (1993) Chagas disease in north-west Argentina: association between Trypanosoma cruzi parasitaemia in dogs and cats and infection rates in domestic Triatoma infestans. Trans R Soc Trop Med Hyg 87:12–15
Gürtler, Ricardo & Cardinal, Marta Victoria. (2015). Reservoir host competence and the role of domestic and commensal hosts in the transmission of Trypanosoma cruzi. Acta tropica. 151. 10.1016/j.actatropica.2015.05.029.
https://www.cdc.gov/parasites/chagas/
Montgomery, S. P., Parise, M. E., Dotson, E. M., & Bialek, S. R. (2016). What Do We Know About Chagas Disease in the United States?. The American journal of tropical medicine and hygiene, 95(6), 1225–1227. https://doi.org/10.4269/ajtmh.16-0213
Oda, E & Solari, Aldo & Botto-Mahan, C. (2014). Effects of mammal host diversity and density on the infection level of Trypanosoma cruzi in sylvatic kissing bugs. Medical and veterinary entomology. 28. 10.1111/mve.12064.
Orozco, M. M., Enriquez, G. F., Alvarado-Otegui, J. A., Cardinal, M. V., Schijman, A. G., Kitron, U., & Gürtler, R. E. (2013). New sylvatic hosts of Trypanosoma cruzi and their reservoir competence in the humid Chaco of Argentina: a longitudinal study. The American journal of tropical medicine and hygiene, 88(5), 872–882. https://doi.org/10.4269/ajtmh.12-0519
Sandra M. De Urioste-Stone, Pamela M. Pennington, Elizabeth Pellecer, Teresa M. Aguilar, Gabriela Samayoa, Hugo D. Perdomo, Hugo Enríquez, José G. Juárez, Development of a community-based intervention for the control of Chagas disease based on peridomestic animal management: an eco-bio-social perspective, Transactions of The Royal Society of Tropical Medicine and Hygiene, Volume 109, Issue 2, February 2015, Pages 159–167, https://doi.org/10.1093/trstmh/tru202 Zahid, M.H., Kribs, C.M. Decoys and Dilution: The Impact of Incompetent Hosts on Prevalence of Chagas Disease. Bull Math Biol 82, 41 (2020). https://doi.org/10.1007/s11538-020-00710-5
