Parasitic Diseases

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Parasitic Diseases

Introduction

Disease History, Characteristics, and Transmission

Scope and Distribution

Treatment and Prevention

Impacts and Issues

BIBLIOGRAPHY

Introduction

A parasite is an organism that lives on or in a host organism. It is dependent upon the host for food and protection. For millions of years, parasites and humans have co-existed. Many parasites do no damage, particularly protozoa in low numbers, but some can cause significant harm. Parasitic infections, such as toxoplasmosis, malaria, and Guinea worm, strike millions of people annually in every region of the world. These infections are often painful, debilitating, or deadly.

There are three main classes of parasites that can cause disease in humans: protozoa, helminths, and ectoparasites. Protozoa are microscopic, one-celled organisms. A serious infection can develop from just a single organism that then multiplies. Helminths are flatworms, thorny-headed worms, and roundworms. Ectoparasites are ticks, fleas, mites, and lice that burrow into the skin. Arthropods, including mosquitos, serve as the vectors of many different pathogens (disease-causing organisms).

Disease History, Characteristics, and Transmission

Transmission of protozoa typically occurs by a fecal-oral route through contaminated food or water or by person-to-person contact. Arthropod vectors, such as ticks, transmit protozoa that thrive in human blood or tissue. Helminths are spread by ingestion, usually through contaminated meat or water.

Scope and Distribution

Parasitic diseases occur most often in the topics and subtropics, but can also occur in more moderate climates, or anywhere the parasite and its host or vector resides. Worldwide, parasitic diseases, of which malaria is the leader, kill more than two million people annually. The World Health Organization estimates that over one person in four harbors some sort of parasitic helminth (worm). In the United States, trichomoniasis is the most common parasitic infection, with over seven million cases diagnosed per year.

The increased movement of people from region to region has also spread parasites. Chagas, an insect-borne parasitic disease, once rarely appeared in the United States. Immigration from Central and Latin America is a contributing factor in an increased incidence of Chagas parasite, Trypanosoma cruzi, in the United States. The American Red Cross now screens blood donors for the parasite to prevent transmission. In the Los Angeles area, the parasite was found in one in 3,800 donors by 2006.

Another disease, cysticercosis, caused by tapeworm larvae, is also on the rise in the United States. However, the disease disproportionately affects foreign-born residents, especially Hispanics who immigrated from Mexico and Central America, most of whom contracted the disease in their home countries.

WORDS TO KNOW

ARTHROPOD: A member of the largest single animal phylum, consisting of organisms with segmented bodies, jointed legs or wings, and exoskeletons.

ECTOPARASITES: Parasites that cling to the outside of their host, rather than their host's intestines. Common points of attachment are the gills, fins, or skin of fish.

HELMINTH: A representative of various phyla of wormlike animals.

HOST: Organism that serves as the habitat for a parasite, or possibly for a symbiont. A host may provide nutrition to the parasite or symbiont, or simply a place in which to live.

PATHOGEN: A disease causing agent, such as a bacteria, virus, fungus, etc.

PROTOZOA: Single-celled animal-like microscopic organisms that live by taking in food rather than making it by photosynthesis and must live in the presence of water. (Singular: protozoan.) Protozoa are a diverse group of single-celled organisms, with more than 50,000 different types represented. The vast majority are microscopic, many measuring less than or 5 one-thousandths of an inch (0.005 millimeters), but some, such as the freshwater Spirostomun, may reach 0.17 inches (3 millimeters) in length, large enough to enable it to be seen with the naked eye.

VECTOR: Any agent, living or otherwise, that carries and transmits parasites and diseases. Also, an organism or chemical used to transport a gene into a new host cell.

Treatment and Prevention

The first step in treating a parasitic disease is to identify it but this is complicated by the number of diseases with similar symptoms that exist in the world. In 2006, a device known as GreeneChip became generally available to detect parasites. It is a microscope slide covered with bits and pieces of genetic information from nearly 30,000 different viruses, bacteria, fungi, and parasites. First used by the World Health Organization (WHO), the $125 slide has subsequently been delivered to the Centers for Disease Control and Prevention (CDC).

If a parasite is detected in a host early enough, strong anti-protozoal drugs such as nifurtimox can bring the parasite to undetectable levels or eliminate it entirely. In other cases, such as the common Latin American and Middle Eastern blight leishmaniasis, medication derived from the chemical element antimony and anti-fungal drugs have stopped the parasite.

Patients with severe infections must also undergo weekly blood tests and electrocardiograms to make sure that their kidneys, liver, and heart remain healthy throughout treatment. Some parasites, such as the one that causes Chagas, may have multiplied over years and decades. In these cases, infected individuals may require treatment with heart-regulating drugs or regulation via an implanted pacemaker.

As of May 2007, there were no vaccines licensed to prevent parasitic diseases. Research and development of potential vaccines focus on three distinct ways to stop the spread of parasitic diseases. Anti-disease vaccines target blood forms and parasite-produced toxins. Transmission-blocking vaccines prevent the development of the parasite within a host. Anti-infection vaccines target parasites in stages where they are most likely to infect.

Several ongoing animal studies are testing possible vaccines against helminth and arthropod-borne parasitic diseases. As of 2005, at least one potential malaria vaccine was ready for clinical testing in humans. However, researchers have already noticed that some forms of vaccination may have unintended consequences. In some studies, parasites that survived in immunized animals displayed increased virulence. Such virulent strains of parasitic diseases such as malaria could be resistant to existing anti-malarial drugs or thwart emerging vaccines. Thus, many health officials assert that more must be done to identify and combat sources of parasites.

Impacts and Issues

Many parasitic diseases no longer have geographic borders. Physicians must consider that parasites which are common in distant regions may be seen in immigrants or travelers.

Regular access to potable water for drinking, cooking, and washing can virtually eliminate water-borne parasitic diseases. However, the WHO estimates that one billion people worldwide live without access to clean water. Improved access to uncontaminated water, personal hygiene and food safety education, and construction of sewer-sanitation systems—even as basic as proper latrines—dramatically reduce incidence of parasite related illnesses.

Parasitic disease prevention can have environmental consequences. Pesticides are often spread over large areas to reduce parasite-transmitting insect populations. Pesticides are powerful chemicals that can contaminate drinking water and soil. People who work around pesticides must take precautions to avoid exposure.

Use of some pesticides, such as dichloro-diphenyl-trichloroethane (DDT) remains controversial, but pesticides are an important part of parasitic disease control that save thousands of lives each year. To minimize environmental damage, researchers are working to develop better pesticides, many directed at killing parasite-transmitting insects, or the parasites themselves, while reducing toxicity to humans and animals. Health workers also promote responsible use of pesticides. For example, in the fight against mosquito-borne diseases, health workers advocate the use of insecticide-treated nets to minimize the need for topical insecticides applied directly to the skin.

Research indicates that global climate change may impact the incidence of parasitic infections in humans. Recent studies in Kenya indicate a strong correlation between rising temperatures, more variable rainfall, and a greater incidence of mosquitoes spreading malaria into highland areas previously protected from the parasite. A 2007 report by the Intergovernmental Panel on Climate Change (IPCC) advised that millions of people may be affected by similar shifts in the spread of parasitic diseases. Other studies dispute these findings, asserting that increased migration of people and animals has a more significant impact on parasite distribution.

Malaria remains the top parasitic killer in the world, causing from one to two million deaths around the world each year, mostly among children. On June 30, 2005, U.S. president George W. Bush announced the President's Malaria Initiative (PMI), a cooperative program with the goal of cutting malaria deaths in half by 2008 in target African nations. The PMI funds research, anti-malarial drugs, and treatment as well as distributes insecticide-treated mosquito nets. Many other nations, international agencies, and nongovernment organizations have similar anti-malaria campaigns. Among non-government charitable entities, the Bill and Melinda Gates Foundation is one of the world's leading sponsors of anti-malaria programs and malaria vaccine research, thus far granting over $400 million.