INTRODUCTION TO PARASITOLOGY
Parasitology is a study of relationships; more specifically, it is the scientific study of the biologic relationship known as parasitism.
A close relationship between two living organisms is known as symbiosis. Organisms involved together in symbiosis are referred to as symbionts or symbiotes. Symbiotic relationships range from being very superficial (simple) to extremely intimate (complex). Symbiotic relationships may be as simple as one symbiont being transported by another; such a transportation-based relationship is referred to as phoresy. A simple symbiotic association may evolve into a more complex physiologic dependence of one species on another species. At least one of the two symbionts will receive some measurable benefit from the relationship. When one symbiont receives benefit from the other, but causes no harm or produces no measurable benefit, the relationship is referred to as commensalism. If both symbionts receive some benefit from the relationship, it is known as mutualism. Parasitism is the term describing relationships where one symbiont, the parasite, receives some benefit from the relationship, while the other symbiont, the host, is harmed.
The terminology used in describing different symbiotic relationships seems very precise when defined in a text, yet the boundaries of these associations can be blurred in "the real world". Commensalism may exist while the host's nutrition is adequate or as long as the host has a properly-functioning immune system. Most authorities agree that morbidity and mortality constitute harm, thus illness and death as consequences of a physiologic relationship between two organisms are evidence of an obvious parasitic relationship. Upsets in some of the delicate balances necessary to maintain commensalism between two organisms can result in benefit for one organism in the relationship at the expense of the other, i.e., parasitism. A liberal definition of harm may be necessary to accept some relationships as belonging under the heading of "parasitism". Some parasites inflict obvious structural and/or metabolic damage, while many others may injure by simply increasing the energy needs of the host. A host may need to increase food intake to compensate for the effects of a parasite. Cells and proteins will be directed toward maintaining an immune response against a parasite, expending resources that could be needed to fight a more serious pathogen. Increased needs for nutrients may force the host to modify its behavior patterns in order to obtain more food, thus exposing the host to other potential threats. Parasitism over extended periods of time can be an expensive circumstance for the host.
The harm to hosts caused by parasites can be grouped into seven basic categories:
Parasites that feed on skin or live most of their lives in association with the integument of the host, thus producing an infestation, are known as ectoparasites. Parasites that dwell primarily in the internal tissues of the host, thus producing an infection, are known as endoparasites.
Parasites and hosts are individuals living in large, complex populations, communities and ecosystems that include many other organisms. The parasite and the host interact with each other, but they must interact with the other species (predators, scavengers, etc.) around them, too. Adaptation is the key to parasite success. The adult parasite (or the most well-differentiated form of the parasitic organism) feeds and reproduces sexually in or on an organism specifically referred to as the definitive host. Success of the parasitic species depends on, among other things, the success of its various host species. Some parasites develop a relationship with only one definitive host species; such parasites are described as being very host-specific. Other parasites are not very host-specific; they are able to proliferate and reproduce sexually in several species of definitive hosts. Parasitism is full of risks for both the parasite and the host. If the host becomes extinct, the extremely host-specific parasite will probably disappear as well. A parasite that has adapted the ability to survive in several definitive hosts increases its chances for success, over evolutionary time, in at least one species.
Survival of a parasite requires that the parasitic species must be able to spread from one susceptible host to another susceptible host. Transmission stages are forms of parasites that, when in appropriate contact with a susceptible host, are capable of survival and/or development in such hosts. Parasites often produce, in definitive hosts, transmission stages (eggs, larvae, cysts, etc.) that are released into the bloodstream, intestinal contents, urine, etc.; these stages exit the definitive host and are intended to survive the threats posed by the external environment, for the purpose of infecting another definitive host. Some transmission stages are protected from environmental extremes of temperature and humidity only by structural design; upon release from the definitive host, or perhaps after some required time outside of the definitive host has passed, these transmission stages can passively or actively infect another definitive host. Parasites that spread directly from definitive host to definitive host have a direct life cycle. Some parasites have adapted to life by producing transmission stages that live, develop, and persist in other organisms in the surrounding community. The parasite does not undergo sexual reproduction in these organisms, but required development, or transformation from one life cycle stage to another, occurs. Organisms that are required for the development of immature or asexual stages of parasites are known as intermediate hosts. Asexual reproduction is possible with certain parasites in their respective intermediate hosts. Intermediate hosts become infected with immature or asexual stages of parasites by ingesting transmission stages, such as eggs, larvae, and cysts, produced by parasites in the definitive host, by ingestion of post-transmission stages inhabiting other intermediate hosts, etc. Definitive hosts acquire parasite transmission stages by eating various types of infected intermediate hosts, or by being fed on by hematophagous arthropod intermediate hosts. Other organisms, known as paratenic hosts, may harbor transmission stages of parasites acquired in various ways; paratenic hosts however are not required for the completion of the parasite's life cycle. Parasites do not undergo substantial change in form inside a paratenic host, although some parasites may increase in size in a paratenic host. In the known parasite-paratenic host relationships that have been studied, the parasitic stages can persist for long periods of time in the paratenic host. Parasites may be transmitted between intermediate and paratenic hosts, and likewise transmission may occur between multiple paratenic hosts. Paratenesis is one way that parasite transmission stages can greatly extend their lifespans in an ecosystem. Paratenic hosts are not physiologically required by a parasite, but ecologically they may be very important for the regular completion of the parasite's life cycle. Parasites that use intermediate and paratenic hosts have indirect life cycles.
Parasites succeed, biologically-speaking, by surviving and reproducing. In a perfect world (from the parasite’s perspective), parasites adapt to environmental conditions and/or they infect definitive hosts, intermediate hosts, and paratenic hosts. In a complex ecosystem, some of these randomly-distributed parasite transmission stages may take paths that will not lead to success. Many transmission stages succumb to unsuitable environmental extremes of temperature, humidity, exposure to sunlight, etc. Parasites may enter aberrant hosts, in whom conditions are not adequate for survival, further development, or sexual maturity. Aberrant hosts are a dead-end for both the individual parasite and the parasite species. Almost every occurrence of a parasite in an aberrant host results in the failure of aberrant parasites to either survive or to produce viable transmission stages.
These are a few of the most basic concepts of parasitology. There are exceptions to any rules, and the definitions used in parasitology are constantly tested by new discoveries about parasites, hosts, and parasitic relationships.