Parasitism Fundamentals

Parasitism Fundamentals

Living organisms rarely live in isolation. Many species have evolved close, long-term associations with other species, and one of the commonest of these associations is parasitism — a relationship in which one organism (the parasite) lives in or on another organism (the host), draws its nourishment from the host, and usually harms the host in the process without killing it outright (at least not immediately). Understanding the vocabulary of parasitism is the key to following the four life cycles that make up the bulk of this chapter.

Facultative and obligate parasites

A facultative parasite is an organism that is capable of living an independent, free-living existence, but which will opportunistically adopt a parasitic mode of life if it happens to gain entry into a suitable host. It does not need a host to complete its life cycle — parasitism is an option, not a requirement, for such an organism.

An obligate parasite, in contrast, cannot complete its life cycle without spending at least one obligatory stage inside or upon a host. Every parasite studied in detail in this chapter — Entamoeba histolytica, Plasmodium vivax, Ascaris lumbricoides and Wuchereria bancrofti — is an obligate parasite: none of them can grow, multiply or reproduce to the next generation without a host.

Types of hosts

• A definitive host is the host in which the parasite either attains sexual maturity or undergoes its sexual phase of reproduction. For Plasmodium, this is the female Anopheles mosquito, because sexual fusion of gametes occurs in the mosquito's gut.
• An intermediate host is the host in which the parasite passes its larval stage(s) or its asexual phase of multiplication. For Plasmodium, humans serve as the intermediate host, since only asexual schizogony occurs in the human body.
• A vector is an organism, usually an arthropod, that actively transports a parasite (or pathogen) from an infected host to a new, susceptible host. A reservoir host is an alternative host — often a domestic or wild animal — that harbours the parasite and continues to act as a source of infection for humans even when no human cases are immediately apparent.

Mechanical versus biological vectors

Not every vector treats the pathogen the same way:

• A mechanical vector merely carries the infective stage of a pathogen passively on its body surface or mouthparts, without the pathogen undergoing any development or multiplication inside the vector. The common housefly, which picks up bacteria and cysts from filth and deposits them on food, is the classic example.
• A biological vector is one inside which the parasite actually develops further or multiplies before becoming infective again. The Anopheles mosquito (for Plasmodium) and the Culex mosquito (for Wuchereria) are biological vectors, because part of each parasite's life cycle is completed inside the insect's body.

Hyperparasites and parasitic castration

A hyperparasite is a parasite that itself lives upon or within another parasite — in other words, a parasite of a parasite. This shows that the parasitic mode of life can be layered.

Parasitic castration describes a special, non-lethal form of host damage in which a parasite (or its larval stage) destroys, atrophies or otherwise functionally disables the reproductive organs of its host. The host is not killed; it may even continue to grow larger than an uninfected individual, because the energy that would normally go into producing gametes and offspring is diverted instead towards general body maintenance — which incidentally also benefits the parasite living off that same energy pool.

Neoplasia, hypertrophy and hyperplasia

Some parasites and pathological states also cause abnormal tissue growth, called neoplasia — the uncontrolled, purposeless proliferation of cells that produces a new mass of tissue (commonly called a tumour or growth) that serves no useful function for the body. Two distinct cellular mechanisms can enlarge a tissue or organ:

• Hypertrophy — an increase in the size of individual cells, without any increase in the number of cells.
• Hyperplasia — an increase in the number of cells within a tissue, through active cell division, which secondarily enlarges the tissue or organ.

Health and disease

Finally, this chapter is built around the working definitions of health and disease. Health is best understood not merely as "the absence of disease" but as a state of complete physical, mental and social well-being. Disease is any deviation from the normal structure or functioning of the body that produces discomfort, and diseases are broadly grouped into communicable (infectious) diseases — caused by pathogens and capable of spreading from an infected individual to a healthy one — and non-communicable diseases, which do not spread from person to person. The parasitic infections and other infectious diseases surveyed in this chapter are all communicable diseases of great public-health importance in India.