Parasites are organisms that live in or on a host organisms and get their nutrients from the host, usually at the hosts expense. Parasites are extremely prevalent in nature, and there are many different species of parasites. As prevalent as they are, there is still much that we do not know about them.
Typically, most of the science and study of parasites is focused on how to get rid of parasites. This is certainly important, as parasites can be a major threat to our health. However, less attention has focused on understanding parasites and their ecological role. Parasitology is the field of science that studies parasites and tries to get a better understanding of these creatures.
What Is Parasitology?
Parasitology is the study of parasites, their hosts, and the relationship between them. As a discipline of biology, the scope of parasitology is not determined by the organism or environment in question but by their way of life. This means it forms a synthesis of other disciplines, and draws on techniques from fields such as cell biology, bioinformatics, biochemistry, molecular biology, immunology, genetics, evolution and ecology.
The study of these diverse parasitic organisms and their relationship to their host means that the subject of parasitology is often broken up into simpler, more focused units, which use common techniques, even if they are not studying the same organisms or diseases.
Different Types of Parasitology
Medical parasitology is the field of parasitology that deals with human parasites and their effect on human health. The parasitologist F.E.G. Cox noted that “Humans are hosts to nearly 300 species of parasitic worms and over 70 species of protozoa, some derived from our primate ancestors and some acquired from the animals we have domesticated or come in contact with during our relatively short history on Earth.”
One of the largest fields in parasitology, medical parasitology is the subject that deals with the parasites that infect humans, the diseases caused by them, clinical picture and the response generated by humans against them. It is also concerned with the various methods of their diagnosis, treatment and finally their prevention & control.
Veterinary parasitology is the study of parasites that cause economic losses in agriculture or aquaculture operations, or which infect pets and companion animals. Examples of some of the species studied in veterinary parasitology are:
- Lucilia sericata, or blowfly, which lays eggs on the skins of farm animals. The maggots hatch and burrow into the flesh, distressing the animal and causing economic loss to the farmer.
- Otodectes cynotis, the cat ear mite, responsible for Canker.
- Gyrodactylus salaris, a monogenean parasite of salmon, which can wipe out populations which are not resistant.
Structural parasitology is the study of structures of proteins from parasites. Determination of parasitic protein structures may help to better understand how these proteins function differently from the proteins in humans. In addition, protein structures may inform the process of drug discovery.
Quantitative parasitology is the quantitative study of parasitism in a host population. Parasites exhibit an aggregated distribution among host individuals, thus the majority of parasites live in the minority of hosts. This feature forces parasitologists to use advanced biostatistical methodologies. Quantitative parasitology involves the use of statistics to draw meaningful conclusions from observations of the prevalence and intensity of parasitic infection.
Parasites can provide information about host population ecology. For example, in fisheries biology, parasite communities can be used to distinguish distinct populations of the same fish species co-inhabiting a region. Additionally, parasites possess a variety of specialized traits and life-history strategies that enable them to colonize hosts. Understanding these aspects of parasite ecology can illuminate parasite-avoidance strategies employed by hosts.
Conservation Biology of Parasites
Conservation biology is concerned with the protection and preservation of vulnerable species, including parasites. A large proportion of parasite species are threatened by extinction, partly due to efforts to eradicate parasites which infect humans or domestic animals, or damage human economy, but also caused by the decline or fragmentation of host populations and the extinction of host species.
Taxonomy and Phylogenetics
The huge diversity between parasitic organisms creates a challenge for biologists who wish to describe and catalogue them. Recent developments in using DNA to identify separate species and to investigate the relationship between groups at various taxonomic scales has been extremely useful to parasitologists, as many parasites are highly degenerate and great at disguising relationships between species.
History of Parasitology
The history of parasites is as old as humans. Actually, much older. Human evolution and parasitic infections have run hand in hand. Scientific evidence suggests that sometime, about 150,000 years ago, Homo sapiens emerged in eastern Africa and spread throughout the world, inhabiting virtually the whole of the face of the Earth, bringing some parasites with them and collecting others on the way.
The development of settlements and cities facilitated the transmission of infections between humans, and the opening up of trade routes resulted in the wider dissemination of parasitic infections. In more recent times, weakened immune systems from toxins, poor health, and immunodeficiency viruses like HIV and AIDS, along with the large increases in travel, have allowed a number of new opportunistic parasitic infections throughout the world.
The first written records of what are almost certainly parasitic infections come from a period of Egyptian medicine from 3000 to 400 BC, particularly the Ebers papyrus of 1500 BC discovered at Thebes.
Later, there were many detailed descriptions of various diseases that might or might not be caused by parasites, specifically fevers, in the writings of Greek physicians between 800 to 300 BC, such as the collected works of Hippocrates, known as the Corpus Hippocratorum, and from physicians from other civilizations including China from 3000 to 300 BC, India from 2500 to 200 BC, Rome from 700 BC to 400 AD, and the Arab Empire in the latter part of the first millennium.
As time passed, the descriptions of infections became more accurate and Arabic physicians, particularly Rhazes (AD 850 to 923) and Avicenna (AD 980 to 1037), wrote important medical works that contain a great deal of information about diseases clearly caused by parasites.
The Italian man Francesco Redi (1626 – 1697), considered to be the father of modern parasitology, was the first to recognize and correctly describe details of many important parasites. He is also referred to as the father of experimental biology, and he was the first person to challenge the theory of spontaneous generation by demonstrating that maggots come from eggs of flies.
Redi was the first to describe ectoparasites in his Esperienze intorno alla generazione degl'insetti, with notable illustrations of ticks, including deer ticks and tiger ticks. It also contains the first depiction of the larva of Cephenemyiinae, the nasal flies of deer, as well as the sheep liver fluke (Fasciola hepatica). His next treatise in 1684 titled Osservazioni intorno agli animali viventi che si trovano negli animali viventi (Observations on Living Animals, that are in Living Animals) recorded the descriptions and the illustrations of more than 100 parasites. In it he also differentiates the earthworm (generally regarded as a helminth) and Ascaris lumbricoides, the human roundworm.
Antonie van Leeuwenhoek observed and illustrated Giardia lamblia in 1681, and linked it to "his own loose stools". This was the first protozoan parasite of humans that he recorded, and the first to be seen under a microscope.
A few years later, in 1687, the Italian biologists Giovanni Cosimo Bonomo and Diacinto Cestoni published that scabies is caused by the parasitic mite Sarcoptes scabiei, marking scabies as the first disease of humans with a known microscopic causative agent.
Modern parasitology developed in the 19th century with accurate observations by several researchers and clinicians. In 1828, James Annersley described amoebiasis, protozoal infections of the intestines and the liver, though the pathogen, Entamoeba histolytica, was not discovered until 1873 by Friedrich Lösch.
James Paget discovered the intestinal nematode Trichinella spiralis in humans in 1835. James McConnell described the human liver fluke in 1875. A physician at the French naval hospital at Toulon, Louis Alexis Normand, in 1876 researching the ailments of French soldiers returning from what is now Vietnam, discovered the only known helminth that, without treatment, is capable of indefinitely reproducing within a host and causes the disease strongyloidiasis.
Patrick Manson discovered the life cycle of elephantiasis, caused by nematode worms transmitted by mosquitoes, in 1877. Manson further predicted that the malaria parasite, Plasmodium, had a mosquito vector, and persuaded Ronald Ross to investigate. Ross confirmed that the prediction was correct in 1897–1898.
In the past century, with the advancement of microscopes, research on parasites has provided more conclusive evidence and a much greater understanding of these creatures. Though there is still much more to learn.
Modern research on parasites has shown that many of the important parasites encountered today not only existed but were widespread in their distribution before written records began, and our early ancestors must have been aware of the presence of the largest and most common worms and of some of the diseases caused by parasites.
Today, the field of parasitology continues to grow, especially as public awareness and health concerns regarding parasites are increasing. Parasitologists are learning about new and effective ways to diagnose, prevent and treat parasitic infections, and are understanding much more about parasites in general and the roles that they play in the environment, and in relationship to their host.
While it is arguable that the majority of parasites are harmful to humans, research suggests that some may actually be beneficial to us. Parasites appear to have played a significant role in shaping our immune systems, and may continue to pose a necessary challenge to our immune system that strengthens and protects us.
While there is still much more research needed, one area of interest for many scientists is a better understanding of the positive roles that parasites might play. As we begin to understand these creatures more, there will be less fear around the subject, and this will allow for more open investigations into the positive roles they could have, as opposed to focusing purely on the harm they cause and how we can get rid of them.
Parasitic infections do underlie many health issues, so it is certainly important to know how to eliminate parasites (link opportunity), but it is also important that we grow to understand more about parasites and the benefits that certain parasites may provide to their host organisms and to the total environment.