Written by Rena Zhu and Edited by Sorina Long
Parasites have coexisted alongside human beings and other organisms for all of history, with the earliest records identifying infections in Egyptians between the 4th and 5th centuries BCE [1]. A parasite is an organism that lives on or in a host organism, and survives off the nutrients at the expense of its host [2]. Without a host, a parasite cannot live, grow, and multiply, so it is usually in the parasite’s interest to not kill the host. As medical technology advances, more information is discovered about various diseases, including parasite-induced diseases.
Parasites and their hosts engage in a back-and-forth arms-race; parasites evolve alongside their hosts, exploiting ways to guarantee their survival while hosts develop defense mechanisms in response. Currently, it is accepted that there are three classes of parasites: protozoa, helminths, and ectoparasites. Protozoa are single-celled organisms that are able to multiply in their hosts that maximize their probable chances of survival. Their reproduction leads to serious infections that can cause tissue swelling, and even tissue death. Parasitic protozoan eggs are typically transmitted from one host’s intestine to another host through a fecal-oral route, such as contaminated food or sexual encounters [2]. Examples of protozoa include freshwater amoeba and infectious spores. The next category of parasites are helminths, which are multicellular organisms that are usually visible to the naked eye. Unlike protozoa, helminths cannot multiply within the body and must lay eggs from which their offspring hatch before infecting a host. Examples of helminths are tapeworms, flatworms, or roundworms. The last category is ectoparasites – blood sucking arthropods such as mosquitoes, lice, and ticks. Ectoparasites are unique in a sense that they are also able to transmit diseases such as malaria to different hosts.
Parasitic infection of the central nervous system is rare, but it can occur. Even if the individual survives, the infected person may suffer from a disability of motor and sensory reflexes, delayed cognitive functions, and higher risks of seizures and epilepsy [3]. Helminths are usually the culprit of these neurological disorders as they damage the tissues they move through and induce an inflammatory response that can inadvertently cause severe hemorrhaging, especially in the brain [3]. The most common parasitic infection of the central nervous system is neurocysticercosis, which is caused by larval cysts when a human being consumes undercooked infected pork meat [4]. If left untreated, larvae are able to burrow through brain tissue and create large cysts. Infected individuals are likely to experience increased intracranial pressure, mental disturbances, and decreased cognitive functions such as visual acuity. Neurocysticercosis is one of the leading causes of adult onset epilepsy worldwide, and is costly to treat, averaging $37,600 per hospitalization. Unfortunately, cases are estimated to significantly increase in the coming years [4].
Alterations of host phenotype by parasitic infections are a commonly used infection technique by viruses, fungi, and protozoans. These can cause physical changes to the body and behavior that will benefit the parasite by maximizing their chances at survival and reproduction. These parasite-induced changes may include changes in visual appearance like color changes and abnormal behavior like crickets jumping into water. Parasitic-induced multidimensional manipulation characterizes severity of disease, and changes in at least two different phenotypic traits are noted as severe adaptations by parasites [5]. It is unknown whether multiple changes in host phenotype are mechanistically related or independent as relatively few studies explore the multidimensional characteristics of parasites, but the link between parasitic manipulation and changes in phenotypic trait expression allow for multifactoral exploration of our current understanding.
References:
- Cox, F E G. (2002). History of human parasitology. Clinical Microbiology Reviews. 15:595- 612.
- “About Parasites,” CDC, 18 Sept. 2020. https://www.cdc.gov/parasites/about.html. Accessed Oct. 31, 2020
- Carpio, A., Romo, M. L., Parkhouse, R. M. E., Short, B., Dua, T. (2016). Parasitic diseases of the central nervous system: lessons for clinicians and policy makers. Expert review of neurotherapeutics. 16:401-14.
- “Neglected Parasitic Infections in the United States: Neurocysticercosis,” Parasites – Cysterosis, CDC, 14 May 2020. https://www.cdc.gov/parasites/resources/pdf/npis_in_us_neurocysticercosis.pdf. Accessed Oct. 31, 2020.
- Thomas, F., Poulin, R., Brodeur, J. (2010). Host manipulation by parasites: a multidimensional phenomenon. Oikos. 119:1217-1223.