Science of Parasitology

Summary

Parasitology is the study of parasites and the relationship between parasites and host organisms. The primary interest in parasitology is to investigate the role of parasites in diseases that affect humans, livestock, and pets. Parasitologists work closely with medical professionals and pharmacologists to develop drugs that combat parasitic infections. Parasitology became a distinct branch of medicine and biology in the mid-nineteenth century after the development of microscope technology. Although medical parasitology dominates the field, some ecologists and zoologists study parasites to learn more about the role parasitism plays in evolution and ecology.

Definition and Basic Principles

Parasitology is the study of parasite organisms, their relationship with host organisms, and their role in disease. Parasitism is defined as a prolonged and intimate association between two organisms in which one organism (the parasite) benefits at the expense of the other organism (the host). Scientists have identified thousands of parasites from various groups, including bacteria, protozoa, animals, plants, and fungi. The study of parasitic bacteria is covered under bacteriology, while parasitologists concentrate on parasites from the eukaryotic group, which includes protozoa, animals, fungi, and plants.

Parasites infect their hosts for the purpose of obtaining food or completing a portion of their reproductive cycle. They may infect their host by either attaching to the outside of the host's body, entering an existing opening such as the mouth or anus, or tunneling through the host's tissues. Some parasites are specialized to infect a single type of host organism, and others can parasitize organisms from a variety of species. Some parasites display complex life cycles that may include infecting hosts from different species during specific parts of their life cycles. Humans are vulnerable to infection by hundreds of types of parasitic worms and more than seventy species of protozoa.

A key feature of parasitism is that it causes damage to the host. Some parasites reduce the host's ability to obtain or absorb nutrients, while others damage tissues directly. Some parasites are known to cause disease, such as malaria and Lyme disease.

Background and History

Early civilizations, including the ancient Greeks, kept records of patients suffering from infection by large, easily visible parasites such as the tapeworm. However, scientists were not able to examine most parasites in detail until the invention of the microscope in the mid-seventeenth century. The next major advance was the discovery of bacteria in the late nineteenth century, which precipitated the discovery of the link between parasites and disease.

In the early twentieth century, scientists conducted the first detailed studies into the nature of malaria, giardiasis, and many other diseases caused by parasites. Although researchers have been unable to develop vaccines for malaria, sleeping sickness, and many other types of parasite-related diseases, scientists have been able to drastically reduce instances of infection because of a greater understanding of the life cycles of the parasites and the vectors (dispersal organisms) they use. In addition, cooperation between parasitologists and pharmacologists led to the development of medications that can effectively reduce the intensity of parasitic infections.

Although the medical study of parasitism remains the most active facet of the field, ecologists and evolutionary biologists began studying the role of parasites in nature in the 1980s, ushering in a new age of parasitology research. Studying the role of parasites in nature has also helped further the study of medical and agricultural parasitology.

How It Works

Though various types of organisms can be classified as parasites, parasitology tends to be divided into three major branches of study: helminthology (parasitic worms), entomology (insect parasites), and protozoology (parasitic protozoa). Each field is further divided into specialties, the most important of which are medical, agricultural, and ecological parasitology.

Medical Parasitology. Medical parasitologists focus on parasites that cause disease and infection in humans. They work closely with pharmacologists to develop vaccines and antiparasite drugs and with physicians to develop new therapeutic techniques.

Parasitologists also collect information that is used to create guidelines for diagnosis. This includes documenting symptoms common to each type of parasitic infection and cataloging the life stages of parasitic species. Intestinal parasites may cause diarrhea, intestinal pain, and the development of granulomas, or tumorlike masses. Other parasites can cause muscle and joint pain, fatigue, and various skin lesions and rashes.

In most cases, patients with parasitic infections are treated with drugs that function by differential toxicity, meaning that the chemicals in question are more toxic to the parasites than to the host. However, most antiparasite medications are also somewhat toxic to the host and can cause a variety of side effects.

Agricultural and Veterinary Parasitology. The subfields of agricultural and veterinary parasitology are extremely important from an economic perspective. A 2009 study from India estimated $3.5 billion is spent combating animal parasites. Some parasites, such as the parasitic roundworm, are capable of infecting plants, animals, and humans. A 2020 study in the journal Agronomy estimated that nematodes, a class of crop parasites, caused losses amounting to $125 billion annually worldwide.

In many cases, parasites lead to the death of livestock or crop plants. Research has also shown that even minor infections can affect an animal's metabolism so that the animal will never obtain full growth or development. Therefore, parasites lead to lower yields from both crops and livestock.

Agricultural parasitologists perform a variety of laboratory experiments to develop antiparasite drugs and vaccines. Medications developed in agricultural laboratories are sometimes co-opted by medical parasitologists to create medications used to treat human patients.

Ecological Parasitology. Ecological parasitologists investigate the role of parasites in nature. This includes making field observations of the various methods parasites use to survive in their environments and their ultimate effect on the evolution of species.

Many evolutionary biologists believe that parasitism has had an important influence on the evolution of species. In a 2009 study of a species of snail from New Zealand, researchers found that in populations with high levels of parasites, the snails will switch from asexual reproduction (producing clones) to sexual reproduction, which produces genetically diverse offspring that are more resistant to infection. This discovery led the researchers to speculate that parasites may have been instrumental in the evolution of sexual reproduction.

Applications and Products

The development of antiparasite drugs involves research from all branches of parasitology, as well as organic chemistry, pharmacology, and infectious disease medicine. Some antiparasite medicines have been found to affect nonparasitic illnesses, leading to greater integration between parasitology and other branches of medicine.

Malaria. Malaria is one of the most common and widespread diseases caused by parasite infection. According to the Centers for Disease Control and Prevention (CDC), 229 million reported cases of malaria occurred worldwide in 2019 alone. Malaria has one of the highest fatality rates of all diseases: in 2019, over 400,000 people who contracted malaria died of the disease. In the 2020s, these numbers only rose further with the World Health Organization (WHO) reporting 249 million cases in 2023. Malaria is caused by plasmodium, a parasite that is typically spread through mosquito bites.

The typical strategy for treating malaria involves using an array of antimicrobial drugs designed to kill plasmodium as it develops. Treatment is difficult because plasmodia have tremendous genetic diversity and many strains are resistant to existing treatments. In 2009, a team at the Monash University ARC Center announced the results of a set of experiments indicating that it may eventually be possible to use a drug to deactivate an enzyme essential for the malaria parasite to digest nutrients. Deprived of this key enzyme, the malaria parasite will starve inside its host.

Parasite Prevention. In addition to treating parasites with drugs, researchers also focus on preventing infection through better hygiene and behavioral modification. In the case of malaria, controlling mosquito populations is an essential step toward preventing the spread of malaria. Parasitologists also contribute to the development of antibug sprays, mosquito netting, and other types of insect repellents to prevent parasite infection.

Another area of research in parasite prevention involves finding ways to prevent parasites from spreading through food or water. Ultraviolet (UV) light treatments have been effective in disinfecting food and water and killing parasites in various stages of life. Although UV radiation has often been used to disinfect foods, water treatment using UV radiation has just begun to become widespread.

In 2009, New York City became the first city in the United States to mandate UV light treatment for water processing to eliminate cryptosporidium, a parasitic microbe that causes a diarrheal infection known as cryptosporidiosis. Before the process was introduced in the United States, it had been used effectively in Great Britain to reduce instances of cryptosporidium infection.

Impact on Other Medical Fields. Discoveries from parasitology have filtered into other areas of research and development. For example, the physiological reaction to parasite infection gives important clues about the function of the immune system and the development of resistance. In some cases, medications developed to combat parasite infection have been found to have beneficial therapeutic effects for other types of diseases.

In 2008, for instance, it was discovered that miltefosine, a drug used to treat protozoan infections, could also be effective in treating people infected with the human immunodeficiency virus (HIV). Miltefosine works by preventing the development of macrophage reservoirs, cells that, because of their long lives, allow the virus to proliferate in secret before it becomes detectable.

Careers and Course Work

For those interested in pursuing a career in parasitology, a strong background in biology is essential. However, individuals from many fields may contribute to parasitology. Because parasites are such a common feature in nature, numerous career opportunities are available for those wishing to study parasitism.

Most professional parasitologists have backgrounds in biology, zoology, and medicine. Ecological parasitology involves specialists with backgrounds in evolutionary theory, taxidermy, and ecology. Medical parasitology invites contributions from those with backgrounds in infectious disease, public health, internal medicine, pharmacology, and medical statistics.

Alternatively, individuals may decide to study the effects of parasitism on society and culture. A full appreciation of parasitism involves economic analyses, sociological studies, and various social and medical activities.

Social Context and Future Prospects

With millions of people worldwide contracting parasitic diseases every year and many more living with the threat of infection, the study of parasitism is vital to public health. Given the tremendous economic and social effects of parasitic diseases, research in parasitology has the potential to create major changes around the world.

Many parasitologists believe that the future of parasitology depends on developments in genomics and genetic medicine. Scientists have been examining the genetic components of parasitic organisms with the goal of finding more effective treatments for parasite-related diseases. The next generation of antiparasite medications may be genetically tailored to combat parasite organisms, thereby reducing the unpleasant side effects caused by differential toxicity. Adapting advanced drug-delivery nanotechnology first used in humans may represent yet another innovative avenue for combating parasites. Innovations continue in the field of parasitology, with scientists also using artifical intelligence to aid their research.

Rising temperatures, disruption to moisture patterns, pollution, ongoing urbanization, and the habitat loss of host species may all affect parasite life cycles and geographic distribution in the future. Thus, preventing transmission while avoiding drug resistance and cascading ecosystem effects will be among the challenges future parasitologists face.

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