Page 190 - Textbook of Pathology, 6th Edition

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Infectious and
Chapter 7
Chapter 7
Parasitic Diseases

SECTION I
INTRODUCTION (including kuru), bovine spongiform encephalopathy (or mad
cow disease) and Creutzfeldt-Jakob disease (associated with
Microorganisms, namely bacteria, viruses, fungi and corneal transplantation). (Dr. Prusiner who discovered prion
parasites, are present everywhere—in the soil, water, atmos- protein was awarded Nobel Prize in medicine in 1997).
phere and on the body surfaces, and are responsible for a Transmission of infectious diseases requires a chain of
large number of infectious diseases in human beings. Some events and is the consequence of inter-relationship between
microorganisms are distributed throughout the world while disease-producing properties of microorganisms and host-
others are limited to certain geographic regions only. In defense capability against the invading organisms. Briefly,
general, tropical and developing countries are specially chain in transmission of infections and factors determining
affected by infectious diseases than the developed countries. this host-microorganism relationship are given below:
There are several examples of certain infectious diseases which
are not so common in the developed world now but they Chain in Transmission of Infectious Diseases
continue to be major health problems in the developing
countries e.g. tuberculosis, leprosy, typhoid fever, cholera, Transmission of infections occurs following a chain of events
measles, pertussis, malaria, amoebiasis, pneumonia etc. pertaining to various parameters as under:
Vaccines have, however, been successful in controlling or i) Reservoir of pathogen. Infection occurs from the source
eliminating some diseases all over the world e.g. smallpox, of reservoir of pathogen. It may be a human being (e.g. in
poliomyelitis, measles, pertussis etc. Similarly, insecticides influenza virus), animal (e.g. dog for rabies), insect (e.g.
have helped in controlling malaria to an extent. However, mosquito for malaria), or soil (e.g. enterobiasis).
infections still rank very high as a cause of death in the world. ii) Route of infection. Infection is transmitted from the
Reasons for this trend are not difficult to seek: reservoir to the human being by different routes, usually from
Development of newer and antibiotic-resistant strains of breach in the mucosa or the skin at both— the portal of exit
General Pathology and Basic Techniques
microorganisms; classic example is that of methicillin- from the reservoir and the portal of entry in the susceptible
resistant Staph. aureus (MRSA). host. In general, the organism is transmitted to the site where
Administration of immunosuppressive therapy to the organism would normally flourish e.g. N. gonorrhoeae
patients with malignant tumours and transplanted organs usually inhabits the male and female urethra and, therefore,
making them susceptible to opportunistic infections the route of transmission would be sexual contact.
Increasing number of patients reporting to hospital for iii) Mode of transmission. The organism may be transmitted
different illnesses but instead many developing hospital- directly by physical contact or by faecal contamination (e.g.
acquired infections. spread of eggs in hookworm infestation), or indirectly by
Lastly, discovery in 1981 of previously unknown deadly fomites (e.g. insect bite).
disease i.e. acquired immunodeficiency syndrome (AIDS)
caused by human immunodeficiency virus (HIV). iv) Susceptible host. The organism would colonise the host
While talking of microbial infective diseases, let us not if the host has good immunity but such a host can pass on
forget the fact that many microorganisms may actually infection to others. However, if the host is old, debilitated,
malnourished, or immunosuppressed due any etiology, he
benefit mankind. Following is the range of host-organism inter- is susceptible to have manifestations of infection.
relationship, which may vary quite widely: Key to management of infection lies in breaking or
1. Symbiosis i.e. cooperative association between two blocking this chain for transmission and spread of infection.
dissimilar organisms beneficial to both.
2. Commensalism i.e. two dissimilar organisms living Factors Relating to Infectious Agents
together benefitting one without harming the other. These are as under:
3. True parasitism i.e. two dissimilar organisms living
together benefitting the parasite but harming the host. i) Mode of entry. Microorganisms causing infectious
diseases may gain entry into the body by various routes e.g.
4. Saprophytism i.e. organisms living on dead tissues.
Besides microorganisms, more recently a modified host through ingestion (external route);
protein present in the mammalian CNS has been identified inoculation (parenteral method);
called prion protein. Prions are transmissible agents similar inhalation (respiration);
to infectious particles but lack nucleic acid. These agents are perinatally (vertical transmission);
implicated in the etiology of spongiform encephalopathy, by direct contact (contagious infection); and