A microorganism that affects the human host sufficiently to cause disease or death is called a “pathogen.” Not all parasites, for example, are pathogens, and not all pathogens have a parasitic origin. An example of a non-parasitic pathogen is Clostridium botulinum. Some pathogens are the result of fungi, some of viruses, or protozoa, as well as bacteria. Anthrax infection, for example, happens only by certain strains of Bacillus anthracis, which contains plasmids that encode the anthrax toxin. It is also encapsulated, protecting the pathogen. Opportunistic pathogens are those organisms that are usually a part of the body’s natural flora and only become harmful after an invasion, as when surgery or accidental injury takes place.
Different pathogens act in different ways. Some produce toxins, while others invade cells or tissues and then produce toxins. Even when localized in the body, such infections can have systemic effects. Symptoms are often a result of the body over-reacting in its own defence. For instance, inflammation is an important part of the body’s natural reaction to pathogens and is a response to a signal that there is a problem in a certain area. Defences are sent to increase the exposure of a pathogen to the body’s own antimicrobial factors.
How does a pathogen cause disease? The following are just a few diseases to show how a pathogen works on the body.
Botulism causes muscle paralysis and death, mainly because of respiratory failure. The pathogen (strains of Clostridium botulinum) form a toxin that binds to nerve-muscle junctions, stopping the release of acetylcholine which stimulates the muscles.
>Cholera produces vomiting and profuse watery diarrhea. The pathogen causing this are strains of Vibrio cholerae, which multiply in the intestines, forming an enterotoxin which acts on the mucosal cells in the small intestine. Within the mucosal cells, the toxin stimulates an enzyme (adenylate cyclase), which causes an increase in cellular cAMP. This, in turn, leads to a massive outflow of electroytes (Na + Cl) and water into the lumen of the intestine, resulting in massive and rapid dehydration.
Cystic fibrosis is a hereditary disease characterized by severe bronchial congestion. In some cases, the respiratory tract becomes colonized by certain strains of Pseudomonas aeruginosa, which produce an abundant supply of mucusy slime that dramatically diminishes the probability of survival.
E. coli strains invade and destroy cells in the small intestine and colon, producing abdominal pain; profuse watery diarrhea; and ultimately, rapid dehydration. Some strains can even produce a toxin after adhering to the mucosal tissues of the intestine.
Oroya fever (Bartonella bacilliformis), occurring in some parts of South America, produces fever and anemia. It is transmitted through the bites of sandflies. The bacterium grows in and on erythrocytes (red blood cells) and in the endothelial cells (cells that line the cavities of the heart, blood, and lymph vessels, as well as some cavities of the body) of the host. As the bacteria invade the red blood cells, they cause the death of the cells, producing anemia.
Tetanus causes uncontrollable contractions of skeletal muscles, often leading to death from asphyxiation or exhaustion. The disease develops when wounds are contaminated with the pathogen Clostridium tetani, which produces a toxin called tetanospasmin. This toxin acts on certain cells in the CNS, preventing the release of glycine which permits muscles to move as they should.
Typhoid is caused by Salmonella typhi, which produces intestinal symptoms and septicaemia or blood poisoning. After ingestion, the pathogen invades mucosa in the small intestine. In some cases, the ileum becomes so inflamed that it causes necrosis or death of the tissue, producing hemorrhaging.
Bacterial pathogens and the diseases they cause:
Legionella pneumophila (pneumonia)
Neisseria gonorrhoeae (gonorrhea)
Mycoplasma pneumoniae (pneumonia)
Mycoplasma hominis (UTI’s, PID)
Mycobacterium tuberculosis (tuberculosis)
Mycobacterium avium (tuberculosis)
Chlamydia trachomatis (venereal syndromes, trachoma)
Listeria monocytogenes (Listeriosis)
Salmonellae spp. (GI disorders)
Shigella spp. (GI disorders)
Escherichia coli (enteropathogenic strains) (GI disorders)
Yersinia enterocolitica (GI disorders)
Staphylococcus aureus (purulent discharges – boils. Blisters, pus-forming skin infections)
Staphylococcus pyogenes (Scarlet/rheumatic fever, “strep” throat).
Viral pathogens and the diseases they cause:
Hepatitis A,B,C,D (hepatitis)
Human immunodeficiency virus – HIV (AIDS)
Cytomegalovirus (congenital viral infections, mononucleosis)
Epstein-Barr virus (Burkitts lymphoma and other lymphoproliferative diseases)
Herpes virus – types I and II (cold sores, genital herpes)
Human papilloma virus (genital warts, cervical cancer).
Protozoal pathogens and the disease they cause:
Leishmania donovanii (Leishmaniasis)
Plasmodium spp. (malaria)
Pneumocystis carinii (pneumonia)
Trypanosoma spp. (trypanosomiasis).
Infectious diseases, agents, and estimated yearly deaths:
Data, from WHO, displays the yearly worldwide deaths from each disease during the decade preceeding its publication in 1992. There are approximately fifty million deaths per year worldwide from all causes.
Acute respiratory infections (bacteria, viruses, protozoa, fungi) (6,900,000)
Diarrheal diseases (bacteria, viruses) (4,200,000)
Tuberculosis (bacteria) (3,300,000)
Malaria (protozoa) (1-2,000,000)
Hepatitis (viruses) (1-2,000,000)
Measles (virus) (220,000)
Meningitis (bacteria) (200,000)
Schistosomiasis (parasitic worm) (200,000)
Pertussis – whooping cough (bacterium) (100,000)
Amoebiasis (protozoa) (40,000-100,000)
Hookworm (parasitic worm) (50-60,000)
Rabies (virus) (35,000)
Yellow fever (virus) (30,000)
African trypanosomiasis – sleeping sickness (protozoan) (20,000+).
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