Parasitology

Gametocito of one of the species Plasmodium of malaria, in a blood smear.

Parasitology is the branch of biology that studies the phenomenon of parasitism. On the one hand, it studies living parasite organisms, and their relationship with their hosts and the environment. Conventionally, it deals only with eukaryotic parasites such as protozoa, helminths (flukes, cestodes, nematodes) and arthropods; the rest of the parasitic organisms (viruses, prokaryotes and fungi) are traditionally considered a matter of microbiology. On the other hand, it studies parasitism or diseases caused in humans, animals and plants by parasitic organisms.

By definition, a parasite is an organism that lives at the expense of a host, although the field of Parasitology is limited to those eukaryotic organisms, both unicellular and multicellular, that have chosen this way of life. Even so, it may come as a surprise that there are many more parasitic organisms than free-living organisms, even excluding viruses and many groups of bacteria and fungi that are also strict parasites in their way of life. Therefore, it must be concluded that parasitism is a successful way of life and as such has emerged in all eukaryotic evolutionary groups: protists, animals and plants.

Parasitology was born as a discipline within zoology, and in its origins it was essentially descriptive. Consequently, the first parasites described were metazoans, and with the later use of the microscope it was extended to the field of protozoology. The European colonial expansion and the confirmation of the serious problems for human and animal health, caused by parasites, especially in tropical areas, led to an increase in medical interest in parasitology (see below). As a consequence, parasitology began to be studied from an etiological-pathological perspective, in which the parasite-host relationship plays a key role. The striking adaptive mechanisms present in these amazing organisms soon stimulated further study. As a result of the interest in these organisms, it is worth mentioning that many advances in basic science have been produced from research with parasites.^{[citation required]}

The importance of parasites from a health perspective is indisputable. Estimates from the World Health Organization indicate that there are more than 260 million people with malaria or malaria, 200 million with schistosomiasis, 500 million with amoebiasis, 700 million with ascariasis and more than 40 million with pathologies caused by trypanosomatids (the disease of sleep, Chagas disease or leishmaniasis).^{[citation required]}

History

Aristotle (37 4 – 322 B.C. C.) described and classified a group of intestinal worms (helminths). Others such as Pliny the Elder and Galen studied human and animal parasites.

In the Middle Ages, the wise Avicenna wrote in Persia a complete treatise on helminths and nematodes and methods to combat and cure them.

Francesco Redi (1686) and later Lázaro Spallanzani. (1729-1799) used parasites as evidence to refute the theory of spontaneous generation. Since then each parasite has its anecdote; at the end of the 19th century, for example, malaria and its vector were discovered.

Many parasitologists began observations under a microscope to discover different protozoa, but really the first parasitic protozoan that was observed was by the Swiss doctors Malmsten (1857) and Stein (1862) discovering Balantidium coli, which is one of the largest protozoa and inhabits the intestines of the pig.

Branches of Parasitology

For a more specific study, parasitology is further divided into three branches:

• Medical parasitology or clinical parasitology: Study human parasites.
• Zooparasitology: Study the parasites of animals.
• Plant physiology or parasitology: Study plant parasites.

Relations with other disciplines

Parasitology is a branch of biology, and specifically ecology, although due to its important repercussions on human and animal health, much of the research in this science focuses on its implications in medicine, veterinary science, and pharmacy, since parasites cause diseases in humans, animals and plants of great health or economic interest and one of the key objectives is to learn how to diagnose them (for example, through a stool or immunological analysis), cure and eradicate them. Within this branch of medical and veterinary health parasitology is also the study of the epidemiology of these parasitic diseases, within what can be classified as environmental parasitology, since it studies the factors that explain the distribution and frequency of parasites..

The main importance of this branch is that many of the "tropical diseases" that we know are of parasitic origin and are largely due to lack of hygiene and favorable environmental conditions in underdeveloped countries (approximately 75% of the world population).

Epidemiology

Example of the global distribution of parasites (malaria).

The effect of a parasitic infection is closely related to geographical, social, and economic factors, so that another of the objectives of parasitology falls into the field of epidemiology when studying the incidence, morbidity and mortality, as well as control methods and fight against parasites and their vectors (parasitic organisms that are more or less innocuous "per se", but that can be transmitters of other disease-causing organisms). The objective would be to control the populations of these vectors or provide guidelines that allow solving sanitary and epidemiological problems. As they are both very simplified organisms and with interesting mechanisms to circumvent their host's defenses, parasites have often received attention from genetics or molecular biology. They have also provided data to interpret the evolution of species.

Parasitology and tropical medicine

In general, the history of parasitology is strongly linked to the History of Tropical Medicine, which is nothing other than the study of typical diseases of underdeveloped countries that are generally found in the tropics.

The history of tropical medicine begins when the great European powers began the colonization of tropical areas in Asia and Africa in the 19th century, mainly with this, many Europeans observed that their colonists upon returning from the colonies to Europe presented very strange diseases (mostly parasitic). Naturally, England, France and Germany began to create institutes and hospitals to treat these rare diseases, a typical case is the "London School of Tropical Medicine" Founded in 1899, in one of London's central neighborhoods.

The WHO currently classifies the 11 main tropical diseases into 3 groups:

• Emerging: African tryanosomiasis, leishmaniasis and dengue.
• Persistents: malaria, tuberculosis and schistosomiasis.
• Controlled: Chagas disease, oncocercosis, filariasis and amebian dysentery.

Although it may not seem like it, these diseases are still very common in underdeveloped countries, especially in children and the elderly.