PARASITES AND PARASITOLOGY:

Human parasitology is the study of those organisms which parasitise humans.  According to the very broad definition of parasitology (see 'An introduction to symbiosis'), parasites should include the viruses, bacteria, fungi, protozoa and metazoa (multi-celled organisms) which infect their host species.  However, for historical reasons the first three have been incorporated into the discipline of Microbiology.  Parasitology claims those protozoa (single celled animals), helminths (worms) and arthropods whose existence depends on the availability of host animals - they are obligate parasites.  There are few facultative parasites; they survive and reproduce without a host - and within this 'grouping' there are very few that infect humans (other than some free-living amoebae).  It is also possible to argue about whether certain insects and mites are "temporary parasites" or "micro-predators", insects as a group belong to the discipline of Entomology, while ticks and mites are the concern of Acarology.  The insects that are of most interest in human parasitology are those that are vectors of several parasitic infections.

LIFE-CYCLES:

The key to understanding the dispersal and transmission of a parasite species is its life-cycle, in effect, a short description of parasite biology and epidemiology.

Protozoa, in their motile, feeding, growing and asexually-multiplying forms are known as trophozoites (tropho = feeding; zoite = small animal).  These stages are adapted to life inside the host and generally are unable to survive the 'external' environment.  Under appropriate conditions some trophozoites of gut protozoa coat themselves in a protective shell and shut down metabolically, to become cysts.  These are designed to survive extended periods outside the host and 'move' between hosts.  In the most highly evolved protozoans (apicomplexans), which are intracellular parasites, multiple division of the trophozoite nucleus (schizogony; schizo = split; -gony = form of reproduction) leads to the production of many merozoites (meros = piece, segment) which then invade other host cells.  Eventually, instead of undergoing schizogony, merozoites develop (gamogony) into either macrogametocytes or microgametocytes.  Fertilisation of the macrogametocyte by a microgamet, i.e. sexual reproduction, leads to the formation of an oocyst.  This stage is designed to survive the outside world and contains infective 'seeds' (sporozoites), which arise during its maturation.

The metazoan parasites (multi-celled, i.e. worms and arthropods) generally are dioecious, i.e. adults occur as seperate males and females.  Tapeworms and most flukes are the exceptions (hermaphrodites).  After copulation, females produce viable eggs, each containing an embryo.  This then develops into a juvenile, or larva (embryonation), which will hatch under suitable conditions.  The egg may be the infective stage or larvae may develop further, through stages or in one or more intermediate hosts, before they are able to reinfect their definitive host (see below).

Several host species may be involved in a life-cycle: the definitive host is that species in which the adult (or sexual) form of the parasite occurs; the intermediate host is the species which supports the development and/or multiplication of the non-sexual, or larval stages of the parasite.  Intermediate hosts which physically carry the infective stage from one host to another are termed vectors; they are mechanical vectors if they simply transmit the parasite (unchanged) and cyclical vectors if they also function as true intermediate hosts that support essential development of the parasite.  Intermediary hosts in which the parasite may not undergo essential development in them may be termed paratenic hosts; these hosts carry parasites through food chains to the definitive host.

Host-specific parasites are very 'particular' about which species they infect; this applies to definitive and intermediate hosts.  Specificity is determined by a complex of factors: ecological specificity - does the host share it's environment with the parasite; ethological specificity - does host behaviour expose it to the parasite; physiological specificity - does the parasite recognise the host and feel 'comfortable' within its surroundings.  An important consideration in human parasitology is that of zoonotic diseases - a human infection caused by an organism which occurs naturally in other animals and has 'crossed' into humans due to low host-specificity (it may be practical to define zoonotic organisms as a organisms that could survive in the absence of people). 

CLINICAL AND PATHOLOGICAL CONSIDERATIONS:

While a parasite should evoke a host reaction/response, there need not be any obvious adverse effects because in the majority of cases infected individuals exhibit little evidence of disease.  The determining characteristic is parasite virulence, i.e. capacity to induce disease, which depends on inter-related factors such as invasiveness, fecundity, means of egress from the host, stimulation and/or suppression of immunity and inflammation, production of exo- and endo-toxins and resistance to host defences. 

Within this field, indeed, within the broad area of infectious diseases it is conceptually important not to confuse aetiological agents with their effects on the host.  An infection occurs when an organism, i.e. the parasite is found in its host.  Infection is a host-organism interaction; it cannot exist without the host.  Presence of infective organisms in the environment (i.e. in food or water) is not an infection but contamination.  It is incorrect to refer to contaminated food as 'infected food supplies'.

Further, an infection is not the same as a disease, which is a pathological change in the host (abnormalities induced in tissues by direct mechano-chemical damage and/or release of toxins and/or inflammatory response).  Illness occurs when the host suffers the effects of the disease and becomes a patient; complains of (subjective, felt by the patient) which interfere with normal life and manifests (sometimes) as clinical signs (objective and detectable).  This is illustrated below: