Parasites can be defined as organisms that live in or on another organism called a host. In most situations, the parasite benefits from this relationship, often at the expense of the host organism. Traditionally, parasites include protozoans and helminths. However, today, the term parasite is sometimes used to describe the multitude of viruses, bacteria, fungi, plants, and animals, including ticks, mites, and lice, that act in a parasitic fashion. Traditional parasites (protozoans and helminths) are responsible for many diseases in animals and humans and are transmitted to their host most often through the ingestion of contaminated food or water or arthropods, which act as intermediate hosts and vectors. Parasites pose health risks and economic costs in livestock and in humans and are often associated with epidemics when a disease occurs at a higher rate than would be expected within a defined area. The high prevalence of parasitic disease in humans provides opportunity for epidemiological studies that examine parasite pathogenicity, hosts, environment, and social conditions that may play a role in the spread of disease.
Research has shown that parasites existed in ancient civilizations as evidenced by written records and the discovery of eggs of parasites in ancient Egyptian mummies. In 1875, Fedor A. Lösch demonstrated that the causative agent of dysentery was the protozoan Entamoeba histolytica. Protozoa are single-celled, heterotrophic eukaryotes, most of which are freeliving. The discovery of E. histolytica as a pathogen led to the identification of other species of pathogenic protozoa. The flagellated protozoa Trypanosoma rhodesiense that is transmitted to humans through the bite of infected tsetse flies causes sleeping sickness (African trypanosomiasis). The organisms reproduce rapidly, avoiding recognition by antibodies in the blood and possibly outnumbering red blood cells. They travel through the bloodstream and eventually reach the spinal cord and brain, leading to coma and death. In a healthy human infected with T. rhodesiense, the disease may become a chronic condition, with the organism later becoming opportunistic if the immune system is weakened. The protozoa of the Leishmania species are transmitted to humans by infected sand flies and infect macrophages that attempt to engulf and digest the foreign pathogen. Eventually, the macrophages and immune defenses become overwhelmed causing leishmaniasis. This is a debilitating and fatal disease and epidemics have occurred in India, China, Africa, and Brazil.
Descriptions of malarial disease have dated back to ancient Chinese and Greek civilizations; however, the actual cause of malaria, protozoans of the genus Plasmodium, was not discovered until 1898, when it was found that humans could become infected through the bite of an infected mosquito. Once in the bloodstream, the Plasmodium travels to the liver where it infects and replicates within cells. The burden of organisms within a single cell will cause it to burst, releasing the Plasmodium, and allowing it to infect red blood cells, where it replicates rapidly, again causing the cell to rupture. The infection cycle into red blood cells may happen several times, resulting in a large quantity of Plasmodium and the symptoms of infection, such as intermittent fever.
Helminths include roundworms, also called nematodes, and flatworms, such as flukes and tapeworms. Filarial disease may be caused by one of several species of helminth nematodes. The filarial nematode Wucheria bancrofti is transmitted to humans by arthropods and is commonly found in Africa, the Middle East, Mexico, and Brazil. Humans and mosquitoes are the only suitable hosts in which Wucheria is able to complete its life cycle. Once injected into the bloodstream, the immature worms make their way into a lymph duct and mature into adults, which may take between 6 months and a year and result in a worm about 3 to 4 inches in length. Multiple adult females will exist in streams of clusters within a lymph duct where they reproduce, shedding thousands of microfilia every day, sometimes remaining in the lymph duct for 5 to 10 years. The accumulation of microfilia in the lymph ducts blocks the flow of lymphatic fluid causing swelling in affected body parts. Microfilia eventually migrate into the bloodstream and are drawn into the proboscis of a mosquito when it bites the host. In general, the immune system is able to defend against and kill the majority of microfilia, resulting only in minor illness associated with the lymphatic system and a low rate of morbidity.
Flatworms, such as the tapeworm, can cause infection and disease in humans. The tapeworm species Diphyllobothrium latum or one of several different species in the genus Taenia, in addition to several other genera, can be infectious. Tapeworms are highly specialized worms that attach themselves to the lining of the human intestinal wall using hooks that keep them firmly in place, or burrow into tissues such as muscle, the spinal cord, or the brain. Adult tapeworms tend to stay within their host as long as possible, with just one adult tapeworm per host growing and reproducing continuously, shedding eggs that are excreted in the feces. Juvenile tapeworms pose the greatest health risks to humans because of their tendency to burrow through the intestinal wall, migrating to internal organs where they interfere with normal tissue function. Infection with adult tapeworms may be asymptomatic; sometimes abdominal pain or diarrhea occurs but is not immediately known to be the result of a tapeworm infection. Infection with juvenile tapeworms can cause cysticercosis, typified by the formation of cysts under the skin, inflammation, mental disorientation, and seizures.
All parasites have a life cycle that involves a period of time spent in a host organism and the phases of which can be divided into growth, reproduction, and transmission. Life cycles of parasites can be divided into two categories: direct (monoxenous) and indirect (heteroxenous). Parasites with direct life cycles spend most of their adult lives in one host, known as the parasitic stage, with their progeny transmitted from one host to another, known as the free-living stage. Direct parasites often lack an intermediate stage and must leave their host. To do this, they must be able to survive in an environment outside their original host and then locate and establish in a new host. Parasites that depend on the host stage are called obligate parasites, whereas parasites that can skip the parasitic stage for several generations are called facultative parasites. Roundworms, trypanosomatids, and Cryptosporidium are examples of parasites with direct life cycles. Parasites with indirect life cycles are characterized by two host stages, which require a definitive host and an intermediate host. The definitive host stage is required for reproduction and the adult life phase. Within the intermediate host, parasite development occurs, after which it can be transmitted to a definitive host. Multiple developmental stages may take place in an intermediate host, which plays an important role in facilitating disease transmission in the form of vectors, such as mosquitoes, which pass immature parasites through their proboscis directly into the bloodstream of the definitive host. Filarial nemotodes, Plasmodium, and Leishmania are examples of parasites with indirect life cycles. Reservoir hosts typically tolerate parasites with no ill effects; however, the introduction of a new host into a population of reservoir hosts will often result in severe disease in the newly introduced host.
Close to 3 billion people worldwide are infected with parasites. Parasites are often endemic and sometimes epidemic in certain regions of the world. For instance, the pathogenic parasite Plasmodium that causes malaria is a constant concern in Africa and often occurs in endemic and epidemic proportions, and sleeping sickness, caused by a species of Trypanosoma, has resulted in epidemic disease in Uganda. The emergence of diseases resulting from pathogenic parasites is always an issue of concern in many tropical regions around the world. Studies of infected populations and of pathogenic parasites have provided, and continue to provide, insight into ways to prevent, treat, and control disease. Following simple sanitation procedures, such as washing hands, cooking meat, and keeping human waste separate from humans, food supplies, pets, and livestock can prevent many diseases caused by parasites. However, these sanitary guidelines are not so easy to follow in Third World countries, which may lack monetary support to provide clean water sources or proper medications, or in cultures where humans maintain intimate dwellings with their livestock and their coexisting parasites. A more difficult problem to overcome is the presence of arthropod hosts in numbers sufficient to infect large number of people over a short period of time, especially in tropical climates. These diseases can be maintained indefinitely in human populations that have a lack of access to medical relief to break the parasite infection cycle.
Unsanitary conditions are often the underlying cause of parasitic disease, especially in areas that are overpopulated or have poor water quality, or among populations that lack knowledge of parasites. In addition, parasites have evolved in ways that enable them to avoid antibody recognition and elimination by the immune system by entering into the cells of the body. This results in a cell-mediated immune response, including activation of helper and cytotoxic T cells, cytokines, and interleukins. Protozoa such as Toxoplasma gondii, Leishmania, and Plasmodium have each found ways to avoid or use the human immune system to their advantage, facilitating their replication and increasing their pathogenicity. Antiparasitic drugs can be divided into antiprotozoan agents and antihelminthic agents. Antiprotozoans are typically designed to be effective in disrupting a specific stage in a parasite life cycle. Drugs against Plasmodium can be taken as prophylactics in the case of oral chloroquine or as treatment for acute attacks in the case of oral chloroquine in combination with sulfadoxine or oral quinine in combination with tetracycline. Other commonly used antiprotozoal drugs include metronidazole, amphotericin B, and suramin.
Antihelminthic drugs cause physical damage to parasitic worms, in most cases targeting adult worms, and inhibit their metabolism, inhibit their ability to lay eggs, or facilitate their excretion from the host. The class of benzimidazole antihelminthics, which includes mebendazole and albendazole, causes degeneration of microtubules that inhibits glucose uptake. These drugs are used as first line therapy for most roundworm and some tapeworm infections. Ivermectin, synthesized from a group of naturally occurring substances, is often used to treat leishmaniasis by causing paralysis of the infecting worms and is effective against W. bancrofti. Other antihelminthic agents include diethylcarbamazine and praziquantal.
Epidemiology in Developing Countries; Malaria; Waterborne Diseases
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