Parasitic Diseases

More than 100 parasitic or organisms are know to be causes of serious diseases in human beings. Such diseases present a particular danger to people living in subtropical and tropical areas, as well as to children everywhere. A  substantial percentage of the world's population is permanently debilitated to some degree by parasitic diseases.

Strictly speaking, any infection organism could be considered a parasite, but the term is conventionally applied only to those larger organisms which cause diseases, as contrasted to bacteria and viruses. Three major groups of parasitic organisms then, referred to collectively as helminths, and a number of ARTHROPODS.

Protozoan parasites include Amoeba, Giardia and other flagellates, ciliates, and sporozoans, Helminths include trematodes (Flukes), cestodes (Tapeworms), and nematodes (roundworms, including filariids). In the case of protozoans and helminths, infections by the organisms and their resulting effects on the body constitute the parasitic disease involved. In the case of arthropods, however, including arachnids (mites and ticks) and insects (for example, the bedbug, flea, louse, mosquito, tsetse fly), some of the organisms may in fact live as parasites on the body and produce various debilitating effects and disorders. They are of more concern to epidemiologists, however, as the vectors for a wide range of serious human illness, including many of the diseases caused by protozoans and melmints.

Among the major diseases caused by protozoan infestations are amebiasis, belantidiasis, amoebic dysentery, kala-azar, leishmaniasis, malaria, toxoplasmosis, trichomoniasis, and trypanosomasis. Among the helminths, schistosomiasis, also known as bilharziasis or snail fever, is the only human disease known to be caused by a fluke. Other major helminth infections include ascariasis varioous forms of filariasis, including eye worm and river blindness; and trichinosis. The many diseases transmitted by bitting arthropods include trypanosomiasis (sleeping sickness) carried by tsetse flies, malaria and filariasis carried by mosquitoes, leishmaniasis and kala-azar carried by sand flies, Chagas's disease carried by tritomid bugs, and river blindness carried by black-flies (Other, nonparasitic diseases transmitted by such arthropods are described in individual entries on those llife forms.)

Other routes of infection for parastic diseases vary according to the parasite. Food transmitted human parasites include. Entamoeba histolytica, the cause of trichinosis. Soil and water transmitted diseases include ascariasis, schistosomiasis, and hookworm. Human parasitic diseases classified as zoonoses are diseases transmitted to humans from other animals. Toxoplasmosis, for example, is an infection caused by a protozoan normally parasitic in felines and rodents, and trichinosis normally occur in pigs and rats.

Immune responses are part of the human body's internal defense mechanisms against parasitic invasion. A successful parasite must overcome host immunity. The antigen antibody interactions involving metazoan parasites are more complex than those involving bacteria and viruses of the multiplicity of the metazoan parasite's antigen system. Production of parasits antigens resembling host antigens, or the binding of host antigens to the parasite body surface, may prevent the host body from recognizing the parasite as an invader. This may explain why parasite can be successful.

Growth Requirement of Bacteria

The growth requirement of microorganism can be grouped on the basis of their need for oxygen to grow. Facultatively anaerobic bacteria can grow in high oxygen or low oxygen content and are among the more versatile bacteria, In contrast, structly anaerobic bacteria grow only in conditions where there is minimal or no oxygen present in the environment. Bacteria such as bacteroides found in the large bowel are examples of anaerobes. Strict aerobes only grow in the presence of significant quantitie of oxygen. Pseudomonas bacteria grwo under conditions of reduced oxygen and sometimes also require increased levels of carbon dioxide. Neisseria species (e.g. the cause of gonorrhea) are examples of micraerophilic bacteria.

Biochemical reactions: Clinical microbiology laboratories typically will identify a
pathogen in a clinical sample, purify the microorganism by plating a single colony of the
microorganism on a separate plate, and then perform a series of biochemical studies
that will identify the bacterial species.

Serologic systems: Selected antisera can be used to classify different bacterial
species. This may be based on either carbohydrate or protein antigens from the
bacterial cell wall or the capsular polysaccharide. (Group A streptococcal M proteins or
O and H polysaccharide antigens of salmonella).

Environmental Reservoirs: When considering likely pathogens it is also important to
know which of the different species are found in different locations. Environmental
reservoirs are generally divided into those that are endogenous (i.e., on or within the
human body) and exogenous (somewhere in the environment). When considering the
likely cause of an infection the likely source of the infection is important in your differential diagnosis. For example, an anaerobic organism resident in the large bowel is
the likely cause of an abdominal abscess that develops after large bowel surgery. A skin
rash developing in a hiker with a history of multiple tick bites is more likely to be borrelia, the agent of Lyme disease. An outbreak of food poisoning traced to imported
unpasteurized cheese might be due to listeria.

Endogenous reservoirs account for a large proportion of human infections. Many parts
of the body have their own normal flora. S. epidermidis is found on the skin. Viridans
streptococci are a part of the normal oropharyngeal flora and S. aureus is a commensal
of the anterior nares.