Foundations of Public Health  © Dennis Dantic


A- Pathogenic Parasitic Infections

Parasitic diseases in humans are mainly caused by single-celled organisms and worms. The cysts and eggs of these organisms are generally found in faeces which aids in the detection of the parasite in the human host while also providing a means for the parasitic species to exit the current host and enter other hosts

Common symptoms of a parasite infection include constipation, diarrhoea, gas and bloating, joint aches, chronic fatigue, excessive hunger, and immune dysfunction. Some parasites block up the large intestinal tract causing constipation while others create sodium and chlorate deficiencies which cause diarrhoea. Several other parasites suppress the immune system in order to continue existing in the host, causing immune dysfunction.

In general, parasites are much smaller than their hosts, show a high degree of specialization for their mode of life, and reproduce more quickly and in greater numbers than their hosts. Classic examples of parasitism include interactions between vertebrate hosts and diverse animals such as tapeworms, flukes, the Plasmodium species, and fleas. Parasitism is differentiated from parasitoidism, a relationship in which the host is always killed by the parasite such as moths, butterflies, ants, flies and others.

The harm and benefit in parasitic interactions concern the biological fitness of the organisms involved. Parasites reduce host fitness in many ways, ranging from general or specialized pathology (such as castration), impairment of secondary sex characteristics, to the modification of host behaviour. Parasites increase their fitness by exploiting hosts for food, habitat and dispersal.

Although the concept of parasitism applies unambiguously too many cases in nature, it is best considered part of a continuum of types of interactions between species, rather than an exclusive category. Particular interactions between species may satisfy some but not all parts of the definition. In many cases, it is difficult to demonstrate that the host is harmed. In others, there may be no apparent specialization on the part of the parasite, or the interaction between the organisms may be short-lived. In public health, only eukaryotic organisms are considered parasites, with the exclusion of bacteria and viruses.

Parasites are classified based on their interactions with their hosts and on their life cycles. Those that live on its surface are called ectoparasites (e.g. some mites) and those that live inside the host are called endoparasites (e.g. hookworms). Endoparasites can exist in one of two forms: intercellular (inhabiting spaces in the hostís body) or intracellular (inhabiting cells in the hostís body). Intracellular parasites, such as bacteria or viruses, tend to rely on a third organism which is generally known as the carrier or vector. The vector does the job of transmitting them to the host. An example of this interaction is the transmission of malaria, caused by a protozoan of the genus Plasmodium , to humans by the bite of an anopheline mosquito.

An epiparasite is one that feeds on another parasite. This relationship is also sometimes referred to as hyperparasitism which may be exemplified by a protozoan (the hyperparasites) living in the digestive tract of a flea living on a dog.

Parasitoids are organisms whose larval development occurs within another organism's body, resulting in the death of the host. Thus, the interaction between the parasitoid and the host is fundamentally different from true parasites and their host, and shares some characteristics with predation.

Parasites inhabit living organisms and therefore face problems that free-living organisms do not. Hosts, the only habitats in which parasites can survive, actively try to avoid, repel, and destroy parasites. Parasites employ numerous strategies for getting from one host to another, a process sometimes referred to as parasite transmission or colonization

Some endoparasites infect their host by penetrating its external surface, while others must be ingested. Once inside the host, adult endoparasites need to shed offspring into the external environment in order to infect other hosts. Many adult endoparasites reside in the hostís gastrointestinal tract, where offspring can be shed along with host excreta. Adult stages of tapeworms, thorny-headed worms and most flukes use this method.

Among protozoan endoparasites, such as the malarial parasites and trypanosomes, infective stages in the hostís blood are transported to new hosts by biting-insects, or vectors. Larval stages of endoparasites often infect sites in the host other than the blood or gastrointestinal tract. In many such cases, larval endoparasites require their host to be consumed by the next host in the parasiteís life cycle in order to survive and reproduce. Alternatively, larval endoparasites may shed free-living transmission stages that migrate through the hostís tissue into the external environment, where they actively search for or await ingestion by other hosts. The foregoing strategies are used, variously, by larval stages of tapeworms, thorny-headed worms, flukes and parasitic roundworms.

Some ectoparasites, such as monogenean worms, rely on direct contact between hosts. Ectoparasitic arthropods may rely on host-host contact (e.g. many lice), shed eggs that survive off the host (e.g. fleas), or wait in the external environment for an encounter with a host (e.g. ticks). Some aquatic leeches locate hosts by sensing movement and only attach when certain temperature and chemical cues are present.

Some parasites modify host behaviour to make transmission to other hosts more likely. For example, in California salt marshes the fluke Euhaplorchis californiensis reduces the ability of its killifish host to avoid predators. This parasite matures in egrets, which are more likely to feed on infected killifish than on uninfected fish. Another example is the protozoan Toxoplasma gondii , a parasite that matures in cats but can be carried by many other mammals. Uninfected rats avoid cat odours, but rats infected with T. gondii are drawn to this scent, a change which may increase transmission to feline hosts.

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