General Pathology Considerations Notes

General Considerations

Following general comments apply to most of the infectious di seases in humans:

There are several examples of certain infectious diseases which are not so common in the for example, Tuberculosis, Leprosy, Typhoid fever, Cholera, Measles, Pertussis, Malaria, Amoebiasis, pneumonia etc.

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• Vaccines have, however, been successful in controlling or eliminating some diseases all over the world Paediatric Diseases And Conditions Of Infancy And Childhood Diseases.
• Common Paediatric Malignant Tumour Smallpox, poliomyelitis, measles, pertussis, etc. Similarly, insecticides have helped in controlling malaria to an extent However, infections still rank very high as a cause of death in the world. Reasons for this trend are not difficult to seek:
• Development of newer and antibiotic-resistant strains of microorganisms; the classic example is that of methicillin-resistant Staph. Aureus (MRSA). Administration of immunosuppressive therapy to patients with malignant tumours and transplanted organs ] making them susceptible to opportunistic infections.
• Increasing number of patients reporting to hospital for different illnesses but instead many developing hospital-acquired infections.
• Human immunodeficiency virus (HIV) infection first detected in 1981 has added to the global burden of infectious diseases by AIDS as well as due to other related opportunistic infections.
• Besides microorganisms, a modified infectious host protein present in the mammalian CNS has been identified called prion protein.
• Prions are transmissible agents similar to infectious particles but lack nucleic acid.
• These agents are implicated in the etiology of spongiform encephalopathy (including kuru), bovine spongiform encephalopathy (or mad cow disease) and Creutzfeldt-Jakob disease or CJD (associated with corneal transplantation).
• (Dr Prusiner who discovered prion protein was awarded Nobel Prize in medicine in 1997).

Host-Microbe Inter-Relationship

While talking of microbial infective diseases, let us not forget the fact that many microorganisms may actually benefit mankind. Following is the range of host-organism interrelationship, which may vary quite widely:

1. Symbiosis i.e. cooperative association between two dissimilar organisms beneficial to both.
2. Commensalism i.e. two dissimilar organisms living together benefitting one without harming the other.
3. True parasitism i.e. two dissimilar organisms living together benefitting the parasite but harming the host.
4. Saprophytism i.e. organisms thriving on dead tissues.

Chain In Transmission Of Infectious Diseases

Transmission of infectious diseases requires a chain of events and is the consequence of interrelationship between disease-producing properties of microorganisms and host-defense capability against the invading organisms.

Transmission of infections occurs following a chain of events pertaining to various parameters as under:

• Reservoir of pathogen:  Infection occurs from the source of reservoir of the pathogen. It may be a human being (for example, Influenza virus), Animal for example, Dog for rabies), Insect (for example, Mosquito for malaria), or soil (for example, Enterobiasis).
• Route of infection:  Infection is transmitted from the reservoir to the human being by different routes, usually from breach in the mucosa or the skin, at the portal of exit from the reservoir as well as the portal of entry in the susceptible host.
  • In general, the organism is transmitted to the site where it would normally flourish for example, N. gonorrhoea usually inhabits the male and female urethra and, therefore, the route of transmission would be sexual contact.
  • Mode of transmission The organism may be transmitted directly by physical contact or by fecal contamination (for example, Spread of eggs in hookworm infestation), or indirectly by fomites (for example, Insect bite).
• Susceptible host: The organism would colonize the host if the host has good immunity but such a host can pass on infection to others. However, if the host is old, debilitated, malnourished, or immunosuppressed due to any etiology, he is susceptible to manifestations of infection. The key to the management of infection lies in breaking or blocking this chain for transmission and spread of infection.

Factors Relating To Infectious Agents

Microbial factors favoring transmission of infections are as under:

1. Mode of entry:  Microorganisms causing infectious diseases may gain entry into the body by various routes for example,
   • Through ingestion (external route)
   • Inoculation (parenteral method)
   • Inhalation (respiration)
   • Perinatally (vertical transmission)
   • By direct contact (contagious infection); and
   • By contaminated water, food, soil, environment or from an animal host (zoonotic infections).
2.  Spread of infection:  Microorganisms after entering the body may spread further through the phagocytic cells, blood vessels and lymphatics.
3. Virulence of organisms:  Many species and strains of organisms may have varying virulence for example,The three strains of C. diphtheriae (gravis, intermedius and mitis) produce the same diphtherial exotoxin but in different amounts.
4. Production of toxins:  Bacteria liberate toxins which have effects on cell metabolism. Endotoxins are liberated on lysis of the bacterial cell while exotoxins are secreted by bacteria and have effects at distant sites too.
5. Product of organisms:  Some organisms produce enzymes that help in spread of infections, for example, Hyaluronidase by Cl. welchii, streptokinase by streptococci, staphylokinase, and coagulase by staphylococci.

Factors Relating To Host

Microorganisms invade the human body when defenses are not adequate.

These factors include the following:

1. Physical barrier:  A break in the continuity of the skin and mucous membranes allows the microorganisms to enter the body.
2. Chemical barrier:  Mucus secretions of the oral cavity and the alimentary tract and gastric acidity prevents bacterial colonization.
3.  Effective drainage:  Natural passages of the hollow organs like respiratory, gastrointestinal, urinary and genital system provide a way to drain the excretions effectively. Similarly, ducts of various glands are the conduits of drainage of secretions. Obstruction in any of these passages promotes infection.
4. Immune defence mechanisms:  These include the phagocytic leucocytes of blood (polymorphs and monocytes), phagocytes of tissues (mononuclear-phagocyte system).

Methods of identification of microorganisms:

Methods Of Identification:

• The organisms causing infections and parasitic diseases may be identified by routine H & E stained sections in many instances.
• However, confirmation in most cases requires either application of special staining techniques or is confirmed by molecular biologic methods.
• In addition, culture of lesional tissue should be carried out for species identification and drug sensitivity.
• Generally, the organism is looked for at the advancing edge of the lesion in the tissue section rather than in the necrotic centre.
• In the following pages, common diseases produced by pathogenic microorganisms are discussed.
• Each group of microbes discussed here is accompanied by a Table listing diseases produced by them.

These lists of diseases are in no way complete but include only important and common examples:

• No attempts will be made to give details of organisms as that would mean repeating what is given in the textbooks of Microbiology.
• Instead, salient clinicopathological aspects of these diseases are highlighted.

General Considerations:

Certain infectious diseases continue to be major health problems in the developing countries

For example:

• • Tuberculosis
  • Leprosy
  • Typhoid fever
  • Cholera
  • Measles
  • Pertussis
  • Malaria
  • Amoebiasis
  • Pneumonia etc.
• Outcome of any microbial infection depends upon factors pertaining to the organism and the host.
• Microbial factors favouring transmission of infections are its mode of entry, spread of infection, virulence, production of toxins, and other products.
• Host factors favouring invasion by microorganisms are breach in physical and chemical barrier, block in drainage, impaired immunity.
• Identification of organism is done by routine H & E stain, special stains (Gram’s, Giemsa, AFB, PAS, GMS), culture and molecular methods

• A –  Gram’s stain.
• B –  Ziehl-Neelsen (ZN) or acid-fast bacilli (AFB) stain.
• C –  Giemsa stain.
• D –  Periodic acid Schiff (PAS) stain.
• E –  Mucicarmine stain.
• F – Gomori methenamine silver (GMS) stain.