Parasitism is an interaction where one organism, the parasite, benefits by deriving nutrients at the expense of the host. Parasites occur across all biological kingdoms, from protozoa to vertebrates. They have evolved numerous strategies to exploit their hosts, ranging from temporary external feeding to long-term internal residence and sophisticated manipulation of host behavior.
External Parasites and Immediate Impact
Parasites that live and feed on the exterior surface of a host are classified as ectoparasites. These organisms, such as ticks, fleas, and mites, are adapted to accessing the host’s resources through the skin. Ticks and fleas utilize specialized mouthparts to penetrate the skin and siphon blood, which can lead to anemia and intense irritation for the host.
The impact of ectoparasites often extends beyond simple feeding damage, as many serve as vectors for disease transmission. Ticks transmit pathogens like bacteria and viruses during blood-sucking; they ingest the pathogen from one host and introduce it into a subsequent host’s bloodstream. Mosquitoes are also considered temporary ectoparasites, feeding on blood for brief periods and acting as vectors for diseases like malaria and West Nile virus.
Internal Parasites and Nutrient Theft
Endoparasites are organisms that take up long-term residency inside the host’s body, often residing in the digestive tract or major organs. These parasites execute resource theft by siphoning off the nutrients and energy the host takes in for its own metabolic processes. Tapeworms, for example, attach to the intestinal wall, absorbing digested food directly across their body surface.
This internal nutrient theft often causes chronic issues for the host, including significant weight loss and nutrient deficiencies. Other endoparasites, like liver flukes, establish themselves in the bile ducts or liver tissue, causing inflammation and disrupting organ function. The life cycles of many endoparasites are complex, requiring the host to ingest contaminated food or water containing eggs or larvae that migrate to a target organ to mature.
Parasites That Manipulate Host Behavior
Parasitism can involve the manipulation of the host’s behavior or reproductive system to benefit the parasite’s life cycle. This strategy ensures the parasite can complete a complex life stage, often by moving to a new host or receiving care it cannot provide for itself. One example is a fungal infection that causes “zombie-like” movements in insects, forcing the host to climb high so the fungus can release its spores for dispersal.
Avian brood parasitism, exemplified by cuckoo birds, involves the female cuckoo laying her egg in the nest of a host species, outsourcing parental care. The cuckoo chick often hatches first and may eject the host’s eggs, manipulating the foster parents into dedicating all provisioning effort to it. The protozoan Toxoplasma gondii provides another example, altering rodent behavior by reducing their fear of cats, increasing the likelihood of transmission to the definitive feline host.