Hosts
A host is a living organism that provides essential resources for the survival of a parasite, which depends entirely on the host for food, warmth, protection, and conditions for reproduction. Parasites are generally much smaller than their hosts and are known to reproduce quickly, allowing them to thrive without necessarily killing their host. While the relationship is primarily detrimental to the host, as parasites can cause illnesses and nutrient absorption issues, they often avoid killing their host to maintain their life support. Common transmission routes for parasites include contaminated food and water, insect bites, and direct contact with infected animals or environments. Preventative measures against parasitic infections include proper handwashing, cooking food thoroughly, and using insect repellents. In the United States, several significant parasitic diseases are monitored by health authorities, with millions of cases reported annually. Diagnostic approaches for detecting parasitic infections involve examining fecal and blood samples, with treatment options including specific antibiotics and antimalarial medications. Understanding this host-parasite dynamic is crucial for both public health and personal well-being.
Hosts
Definition
A host is a living thing upon which another organism, a parasite, depends for survival. A parasite lives on or in the body of the host. To live, the parasite relies on the host to provide food, water, warmth, protection, and conditions for reproduction. The parasite does not provide anything beneficial in return and may cause the host to become ill.
Parasitic Infection
Parasites target a specific species of host for optimum survival. They are much smaller than their hosts and reproduce at a faster rate, so they are able to survive and procreate without killing the host.
Skin parasites attach themselves to a host and feed on the host’s blood. They typically lay their eggs on the skin surface, where their offspring may also feed on the host or be released into the environment to find other hosts.
Other parasites may be found in uncooked or undercooked meat, raw vegetables, and contaminated, dirty drinking water. They may also be carried on hands that have not been washed after handling animals, soil, or feces. Thus, parasites may be inadvertently ingested. The eggs and offspring of ingested parasites are shed into the environment in the host’s feces. Some parasites are deposited into hosts through insect bites. For example, mosquitoes are carriers of the protozoa that cause malaria.
Adverse Effects on the Host
If the host were to die, the parasite would lose its life support, so the parasite does not deliberately kill its host. However, its presence affects the host’s health. Parasites in the intestinal tract may prevent the host from absorbing nutrients. Parasites traveling in the bloodstream may clog blood vessels, lymph vessels, and bile ducts. Parasites residing in tissues and organs may cause damage by producing toxins that destroy the cells.
Prevention
One can prevent a parasitic infection through adequate handwashing technique, including the use of soap and water, after handling animals and raw meats, after working in soil, and before handling food. Cooking raw meats to sufficiently high temperatures also helps prevent infection. In addition, one can use insect repellent to ward off parasites.
Impact
According to the World Health Organization (WHO), in 2020, there were about 156 million reported cases of trichomoniasis. The Centers for Disease Control and Prevention (CDC) reported that there were about 1 million cases of giardiasis each year and an estimated 823,000 of cryptosporidiosis. Toxoplasmosis, caused by a food-borne parasite, leads to the death of more than 750 persons annually, according to the CDC.
Diagnostic tests for parasitic diseases include direct examination of fecal samples for parasites and their eggs (ova), endoscopy or colonoscopy to directly observe parasites in the gastrointestinal tract, and examination of blood samples under a microscope. Treatments include antibiotics such as metronidazole and antimalarial drugs such as chloroquine.
Bibliography
"Giardia." Centers for Disease Control and Prevention (CDC), 3 May 2024, www.cdc.gov/giardia/about/index.html. Accessed 4 Feb. 2025.
Hajare, Sunil Tulshiram, et al. "Assessment of Prevalence of Giardia lamblia Infection and Its Associated Factors Among Government Elementary School Children from Sidama Zone, SNNPR, Ethiopia." PLoS One, vol. 17, no. 3, 15 Mar. 2022, DOI: 10.1371/journal.pone.0264812. Accessed 4 Feb. 2025.
Marquardt, William C., ed. Biology of Disease Vectors. 2d ed. New York: Academic Press/Elsevier, 2005.
Reidl, Joachim, et al. "Vibrio cholerae and Cholera: Out of the Water and Into the Host.” FEMS Microbiological Reviews 26 (June 2002): 125-139.
Sasse, Amber. “A Lousy Reason for Asthma and Allergies: Parasites May Reduce Their Hosts’ Risk of Developing Immune Dysfunctions.” Popular Science, April 22, 2009.
Tolan, Robert W., Jr. “Infections in the Immunocompromised Host.” Available at http://emedicine.medscape.com/article/973120-overview.
"Travelers' Health, Cryptosporidiosis." CDC Yellow Book 2024, Centers for Disease Control and Prevention (CDC), 1 May 2023, wwwnc.cdc.gov/travel/yellowbook/2024/infections-diseases/cryptosporidiosis. Accessed 4 Feb. 2025.
"Trichomoniasis." World Health Organization (WHO), 25 Nov. 2024, www.who.int/news-room/fact-sheets/detail/trichomoniasis. Accessed 4 Feb. 2025.