Why Would Parasites Kill the Host?

Parasites are life-forms, such as bacteria, fungi and insects, that survive on or inside another animal, feed on the nutrients within the host and are sheltered from the outside environment. Parasitic life-forms often kill their hosts following their growth to adulthood and after the parasites have taken all the nutrients from the host.

  1. Types

    • Parasites take various forms. Humans can be affected by parasites such as fleas and ticks that feed on the blood of human hosts, according to the University of Georgia. Other forms of parasites invade the cavity of the host and survive inside, either killing the host quickly or waiting until the parasite has reached adulthood before doing so. Parasites can take the form of bacteria, viruses, fungi, protozoans, insects and worms.

    Parastoids

    • Parastoids are types of insects that lay their eggs on the bodies of other insects in order to allow their young to grow within the body of the host and feed on the food that the host has eaten. This process takes place on insects such as the monarch butterfly, which are infected by tachinid flies and braconid wasps. The tachinid fly lays eggs on the monarch caterpillar that hatch into larvae, then enter the host through the mouth or anus and survive inside the body cavity. When the adult flies and wasps reach adulthood, they kill the host and emerge into the environment. Braconid wasps lay around 32 eggs that grow into adult wasps within the monarch butterfly.

    Nematodes

    • Parasitic nematodes differ from parastoids in that the nematode kills the host insect quickly after entering the body in order to feed on the body from the inside and breed many nematodes over a short time. Entering the body through natural openings, the nematode kills its host insect, such as turf insects, within two or three days, Colorado State University reports. The nematode then uses chemicals to halt the decomposition process in the host, and feed and breed within the animal until it expands and bursts, releasing thousands of nematodes into the surrounding area.

    Fatal

    • The majority of parasites do not kill their host, as in the case of sharks and parasitic fish and crustaceans. Instead they feed on nutrients and food from the host in small quantities, which causes energy loss in the host but does not prove fatal. Where parasites rely on a host for survival, it is in their interest to keep the host alive and in turn keep themselves alive.