What are parasitic diseases?

Types of Parasites

Parasites are organisms that take up residence, temporarily or permanently, on or within other living organisms for the purpose of procuring food. They include plants such as bacteria and fungi; animals such as protozoa, helminths (worms), and arthropods; and forms such as spirochetes and microscopic viruses.

The study of parasitism is a study of
symbiosis. Symbiosis occurs when two organisms, known as symbionts, live in close association with each other, usually with one organism living in or on the body of the other. Such a living arrangement is called a symbiotic relationship. In a symbiotic relationship, the symbionts are usually, but not always, of different species, and the relationship need not be beneficial or damaging to either organism.

Often, two symbionts will exist together merely as traveling companions. Such a symbiotic relationship is called phoresis. In such cases, neither partner is physiologically dependent on the other, but the smaller of the two organisms has simply attached itself for the ride. Examples of phoresis are bacteria carried on the legs of a cockroach and fungus spores on the feet of ants and beetles.

If a situation exists in which both symbionts benefit from an association, the partnership is referred to as a mutual relationship. In most cases, mutualism is obligatory because both symbionts have evolved to a point at which they are physiologically dependent on each other and the survival of both symbionts requires a continuous interrelationship. Such a relationship exists between termites and the protozoan fauna that live in their guts. Termites are unable to digest cellulose fibers, because their bodies cannot produce the proper enzyme, but protozoa that live in a termite’s gut synthesize the ingested cellulose fibers and excrete a fermented product that nourishes the host termite. The protozoa benefit by living in a stable, secure environment, with a constant supply of food, and the termite is supplied with sustenance.

Another form of mutualism that is not obligatory is called cleaning symbiosis. In this instance, certain animals, called cleaners, remove other parasites, injured tissues, fungi, or invading organisms from a cooperating host. Examples of cleaning relationships include birds that groom the skins of rhinoceroses and the mouths of crocodiles, and tiny shrimp that remove parasites from the body surfaces of fish.

When one symbiont benefits from its relationship with its host but the host neither benefits nor is harmed, the condition is called commensalism. Examples of commensals are pilot fish and remoras, which attach themselves to turtles or other fish, using their hosts as transportation and scavenging food left over when the hosts eat; in no way, however, do they harm their hosts or rob them of food. Another example of commensalism exists between humans and the amoeba Entamoeba gingivalis. This amoeba lives in the human mouth, feeding on bacteria, food particles, and dead epithelial cells, but it never harms its human host. The amoeba is transmitted from person to person by direct contact and cannot exist outside the human mouth.

When one member of a symbiotic relationship actually harms its host or in some way lives at the expense of the host, it is then a parasite. The word parasite is derived from the Greek parasitos, which means “one who eats at another’s table” or “one who lives at another’s expense.” A parasite may harm its host by causing a mechanical injury, such as boring a hole into it; by eating, digesting, or absorbing portions of the host’s tissue; by poisoning the host with toxic metabolic products; or by robbing the host of nutrition. It has been found that most parasites inflict a combination of these conditions.

The majority of parasites are obligate parasites—that is, they must spend at least a portion of their lives as parasites to survive and complete their life cycles. Most of these obligate parasites have free-living stages outside their hosts in which they exist in protective cysts or eggs. Certain symbionts are referred to as facultative parasites, which means that the organism is not normally parasitic, but if the proper situation arises, it becomes a parasite. The most common facultative parasites are those that are accidentally eaten or enter a host through a wound or body orifice. One facultative parasite whose infection of humans is almost always fatal is the amoeba Naegleria, which is responsible for amebic meningitis. Many obligate parasites are also hyperparasites. Hyperparasitism exists when parasites play host to other parasites; for example, the malaria-causing parasite Plasmodia is carried in mosquitoes, and juvenile tapeworms live in fleas.

Parasites that live their entire adult lives within or on their hosts are called permanent parasites. Other parasites, such as mosquitoes and ticks, are called temporary parasites because they feed on their hosts and then leave. Temporary parasites are actually micropredators that prey on different hosts, or on the same host at different times, as the need for nourishment arises. There are many parallels between parasitism and predation in that both parasites and predators live at the expense of their hosts or prey. Parasites, however, do not normally kill their hosts, because to do so results in their own death. It is the mark of a well-adapted parasite to produce as few pathological conditions in the host as possible.

Despite the knowledge that parasites are a major cause of disease, it is wrong to assume that an animal hosting a parasite must ultimately be in danger. The healthiest human or wild animal is probably harboring some type of parasite, and while the host and parasite may live for years without interfering in each other’s existence, at any given time, the healthy host can fall victim to a disease brought on by the parasite, or some change in the host may destroy the parasite.

Whether the host reacts to its symbiotic partner with indifference, annoyance, or illness is the result of many factors. The most important is how many parasites are being hosted. A single hookworm takes approximately 0.5 milliliter of blood per day from its host. This is about the same amount of blood lost when one pricks oneself with a needle. This amount of blood loss is so low that the host will never miss it and, in most instances, will not even know that the parasite is there. If a host harbors five hundred
hookworms, however, the blood loss per day becomes 250 milliliters, approximately one half pint, and the result is physically devastating to the host.

Causes and Symptoms

Medical parasitology is the study of human diseases caused by parasitic infection. It is commonly limited to the study of parasitic worms (helminths) and protozoa. The science places nonprotozoan parasites in separate disciplines, such as virology, rickettsiology, and bacteriology. The branches of parasitology known as medical entomology and medical arthropodology deal with insects and noninsect arthropods that serve as hosts and transport agents for parasites, as well as with the noxious effects of these pests. The study of fungi (molds and yeasts), including those that cause human disease, is called mycology.

Throughout history, human welfare has suffered greatly because of parasites. Fleas and bacteria killed one-third of the human population of Europe during the seventeenth century, and malaria, schistosomiasis, and African sleeping sickness have killed additional countless millions. Despite successful medical campaigns against yellow fever, malaria, and hookworm infections worldwide, parasitic diseases in combination with nutritional deficiencies are the primary killers of humans. Medical research suggests that parasitic infections are so widespread that if all the known varieties were evenly distributed among the human population, each living person would have at least one.

Most serious parasitic infections occur in tropical, less modernized regions of the world, and because most of the planet’s industrially developed and affluent populations live in temperate regions, many people are unaware of the magnitude of the problem. On an annual basis, 60 million deaths occur worldwide from all causes; of these deaths, half are children under five years of age. Fifty percent of these, 15 million child deaths per year, are directly attributable to a combination of malnutrition and intestinal parasitic infection. It must be noted that less than 15 percent of the world’s present population is served by adequate clean water supplies and sewage disposal programs, and that almost all intestinal parasitic infections are the result of ingesting food or water contaminated with human feces.

The transmission of parasitic diseases involves three factors: the source of the infection, the mode of transmission, and the presence of a susceptible host. The combined effect of these factors determines the dispersibility and prevalence of a parasite at a given time and place, thus regulating the incidence of a parasitic disease in a population. Because of host specificity, other humans are the chief source of most human parasitic diseases. The various manifestations of any human parasitic disease are a result of the particular species of parasite involved, its mode of transport, the immunological status of the host, the presence or absence of hosts, and the pattern of exposure.

Humans transmit parasitic diseases to one another through the intestinal tract, nose and mouth, skin and tissue, genitourinary tract, and blood. It is fecal discharge, however, that offers the most convenient and common means for a parasite or its ova and larvae to leave its host, since the majority of parasites inhabit the gastrointestinal tract. For this reason, the proper disposal of fecal material is the most important method of preventing the spread of parasitic disease. Since most parasites inhabit the intestinal tract, food and water are also important means of transmitting parasitic infections. The infective organism may be present in contaminated drinking water, in animal and fish flesh used as food, in human feces used as fertilizer, or on the hands of food handlers.

Arthropods are one of the main sources of parasitic diseases in humans. Arthropods act as both mechanical carriers of and intermediate hosts to many diseases—bacterial, viral, rickettsial, and parasitic—which they transmit to humans. In most tropical countries, basic preventive medicine for many devastating parasitic diseases depends on the control or eradication of insects and arachnids.

There are four major groups of parasites that most often invade human hosts: nematodes, trematodes, cestodes, and protozoa. Most nematodes, or roundworms, are free-living, and nematodes are found in almost every terrestrial and aquatic environment. Most are harmless to humans, but some parasitic nematodes invade the human intestinal tract and cause widespread debilitating diseases. The most prevalent intestinal nematodes are Ascaris lumbricoides, which infect the small intestine and affects more than a billion people; the whipworm Trichuris trichiura, which infects the colon and is carried by an estimated 500 million individuals; the human hookworms Necator americanus and Ancylostoma duodenale, which suck blood from the human small intestine and cause major debilitation among undernourished people; and Enterobius vermicularis, the human
pinworm, which infects the large intestine and is common among millions of urban dwellers because it is easily transmitted from perianal tissue to hand to mouth.

Nonintestinal, tissue-infecting nematodes are spread most often by hyperparasitic bloodsucking insects such as mosquitoes, biting flies, and midges. The most common tissue-infecting nematode is Trichinella spiralis, the pork or trichina worm, which is the agent of
trichinosis. Other important parasitic nematodes include Onchocerca volvulus, which is transmitted by blackflies in tropical regions and causes blindness, and the mosquito-transmitted filarial worms that are responsible for
elephantiasis.

Trematodes, or
flatworms, are commonly called flukes. Flukes vary greatly in size, form, and host living location, but all of them initially develop in freshwater snails. The human intestinal fluke, the oriental liver fluke, and the human lung fluke are all transmitted to humans by the ingestion of raw or undercooked aquatic vegetables, fish, or crustaceans. An important group of trematodes consists of the blood flukes of the genus
Schistosoma, which enter the body through skin/water contact and are responsible for schistosomiasis.

Cestodes, commonly called tapeworms, are parasitic flatworms that parasitize almost all vertebrates, and as many as eight species are found in humans. The two most common cestodes–Taenia saginata, the beef tapeworm, and Taenia solium, the pork tapeworm—are transmitted to humans by infected beef or pork products obtained from livestock that grazed in fields contaminated by human feces, or by contaminated water. The resulting disease, cysticercosis, which is potentially lethal, develops mostly in the brain, eye, and muscle tissue. Another animal-transmitted cestode is the dog tapeworm, Echinococcus granulosus, which dogs ingest by eating contaminated sheep viscera and then pass on to humans, who ingest the parasite’s eggs after petting or handling an infected dog. The human infestation of E. granulosus results in hydatid disease. Probably the most dramatic of the cestode parasites is the gigantic tapeworm
Diphyllobothrium latum, which may
reach a length of 10 meters and a width of 2 centimeters. This tapeworm is transmitted to humans by the ingestion of raw or undercooked fish. This tapeworm, like most cestodes, can be effectively treated and killed by drugs, but if the worm merely breaks, leaving the head and anterior segments attached, it can regenerate its original body length in less than four months.

Protozoa that can infect human hosts are found in the intestinal tract, various tissues and organs, and the bloodstream. Of the many varieties of protozoa that can live in the human intestinal tract, only Entamoeba histolytica causes serious disease. This parasite, which is ingested in water contaminated by human feces, is responsible for the disease amebiasis, also known as amebic dysentery. A less serious, though common, waterborne intestinal protozoan is Giardia lamblia, which causes
giardiasis, a common diarrheal infection among campers who ingest water fouled by animal waste.

Another group of protozoa parasites specializes in infecting the human skin, bloodstream, brain, and viscera. Trypanosoma brucei, carried by the African tsetse fly, causes the blood disease trypanosomiasis (African sleeping sickness). In Latin America, infection by the protozoa Trypanosoma cruzi results when the liquid feces of the reduviid bug is rubbed or scratched into the skin; it causes
Chagas disease, which produces often fatal lesions of the heart and brain. Members of the protozoan genus Leishmania are transmitted by midges and sandflies, and their parasitic infestation manifests in long-lasting dermal lesions and ulcers; the destruction of nasal mucous, cartilage, and pharyngeal tissues; or in the disease kala-azar, resulting in the destruction of bone marrow, lymph nodes, and liver and spleen tissue.

Two other types of protozoa are parasitic to humans. The first is the ciliate protozoa, which are mostly free-living, and of which only a single species, Balantidium coli, is parasitic in humans. This species is responsible for balantidiasis, an ulcerative disease. The second is the sporozoans, all of which are parasitic. Many species of sporozoans are harmful to humans, the most important being Plasmodium, the agent of malaria. The sporozoan parasite Toxoplasma gondii, the agent of
toxoplasmosis, is responsible for encephalomyelitis and chorioretinitis in infants and children and is thought to infect as much as 20 percent of the world’s population. Pneumocystis
pneumonia, a major cause of death among persons with Acquired immunodeficiency syndrome (AIDS), was formerly considered a result of sporozoan infection but is now thought to be fungal.

Perspective and Prospects

Because of their size, the large parasitic worms were among the first parasites to be noted and studied as possible causes of disease. The Ebers papyrus, written about 1600 BCE, contains some of the earliest records of the presence of parasitic worms in humans. In early Egypt, trichinosis, cysticercosis, and salmonellosis were all likely to be acquired from pigs. This knowledge is reflected in the law of Moses, later reinforced in the Qur'an, which forbids the eating of “unclean swine” or the touching of their dead carcasses—a clear indication that people knew of the relationship between parasitic worms and human disease. Persian, Greek, and Roman physicians were also familiar with various parasitic worms, and many of their early medical writings describe the removal of worm-induced cysts. The Arabic physician and philosopher Avicenna (979–1037 CE) was the first to separate parasitic worms into classifications: long, small, flat, and round.

The modern study of parasites began in 1379 with the discovery of the liver fluke, Fasciola hepatica, in sheep. During the eighteenth century, many parasitic worms and arthropods were described, but progress was slow prior to the invention and widespread use of the microscope. The microscope made possible the study of small protozoan parasites and allowed for detailed anatomic and lifecycle studies of larger parasites.

In 1835, Trichinella spiralis, the parasite responsible for the disease trichinosis, was described, and quickly thereafter knowledge concerning the parasitic worms of humans began to accumulate. Many new species were discovered, prominent among which were the hookworm and the blood fluke.

Between 1836 and 1901, the first protozoan parasites of humans were recognized and described; among the most important of these were the parasites responsible for giardiasis, gingivitis, vaginitis, trichomoniasis, kala-azar, and Gambian trypanosomiasis (sleeping sickness).

Although arthropods had been recognized as parasites since early times, their role in transporting other parasites and in spreading disease was not noted until 1869, when the larval stages of the dog tapeworm were found in the dog louse. Further investigations led to the identification in 1893 of ticks as the transmitting agents of Texas fever in cattle. By 1909, parasitologists had observed the development of the malarial parasite in mosquitoes, had proved the transmission of yellow fever by the mosquito Aedes aegypti, and had linked the tsetse fly to African sleeping sickness, the tick to African relapsing fever, the reduviid bug to Chagas disease, and the body louse to the transmission of typhoid fever.Human Experience: Philosophy, Neurosis, and Elements of Everyday Life

In 2013 the American Academy of Neurology (ANN) published new guidelines for the treatment of neurocysticerosis, a tapeworm infection of the brain that can trigger epileptic seizures. The AAN, concerned with what appears to be an increase in the incidence of the disease, based their recommendations on a review of ten studies published between 1980 and 2010. An estimated 40,000 to 160,000 cases of neurocysticerosis occur annually in the United States

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