General Pathology

An Introduction to General Pathology

• The term pathology is derived from:
  • Pathos, or suffering
  • Logos, or reasoning/logic.
• Pathology is defined as the study of disease including:
  • Aetiology - causal factor(s)
  • Pathogenesis - the development of the disease within the body.
  • Lesions - the observable structural changes in the tissues and fluids of the body.
  • Pathophysiology - the functional changes in diseased tissues.
  • Sequel - the consequences of the disease in the body.
  • Remote effects - the effect of disease in one tissue on other tissues in the body.

Lesions

• Lesions are the abnormalities or changes seen in living tissues due to disease.
• Observed in
  • The live animal
  • Tissues surgically removed from the live animal
    • Biopsy/ excision
  • Animals soon after death
    • Necropsy, post-mortem examination.

Decription of Lesions

• Descriptions of lesions is very important
• Whole organs, tissues or individual lesions are described under headings such as
  1. Size
  2. Shape
  3. Colour
  4. Weight
     • Generally in relation to body weight
  5. Texture and Consistency
  6. Appearance of the cut surface
  7. Contents of hollow organs
  8. Position, relationships and effects on adjacent tissues

Disease

Definition and Type

• Disease is a definite morbid (illness producing) process, having a characteristic train of symptoms or signs.
  • May affect the whole body or any of its parts.
  • The disease's aetiology, pathology and prognosis may be known or unknown.
• There are two main categories of disease.
  1. Acute
     • Characterised by sudden onset and short duration.
     • The outcome of acute disease may be:
       • Death
       • Resolution due to host defence response or clinical therapy
       • Progression to chronic disease
  2. Chronic
     • Characterised by insidious onset and protracted course.
     • The outcome of chronic disease may be:
       • Progressive destruction of tissue
         • Compromises funtion and endangers life,
       • The halting of the course of disease, with tissue repair by scarring.

Factors Involved in the Development of Disease

• There are three factors which conspire with each other to produce disease.
  1. The individual animal.
     • In particular, the animal's nutritional and immune status
       • This is modified by:
         • Recent or concurrent disease
         • Previous exposure to the agent(s) responsible
  2. The disease-causing agent(s).
     • Most do not cause a uniform pattern of disease
       • Host defences are important in determining the presentation of the disease.
     • An agent's capacity to produce disease depends upon:
       • The dose
       • The virulence of the agent
     • Several agents may be involved.
       • Usually one agent debilitates, allowing others to exert a greater effect within the body
     • The presence of an agent does not necessarily mean it is the cause of the disease!
     • A pathogenic agent may be absent from the tissues, due to:
       • Clinical therapy
       • Host defence systems
  3. Environment, for example:
     • Overcrowding of animals
     • Mixing animals from differing origins
       • Carriers are allowed to infect susceptible animals.
         • Carriers are animals which harbour the pathogenic agent but do not show signs of disease.
     • Changes in management routine

Types of Agents Causing Disease

1. Infectious organisms
   • Viruses
   • Bacteria
   • Fungi
   • Parasites
2. Physical
   • Trauma
   • Pressure
   • Heat
   • Cold
   • Radiation
3. Chemical
   • Toxic organic and inorganic substances
   • Toxins produced by infectious organisms
4. Nutritional
   • Deficiencies of vitamins and trace elements
   • Excess vitamins and trace elements
5. Genetic defects
   • There is a very wide range of potential defects.
     • Some are incompatible with life
     • Others affect specific systems within the body

Aspects of Disease

• There are many aspects of a disease that must be considered in order to understand it in full.
  1. Incidence
     • How much of the disease is present?
     • Where is the disease found?
     • In what species is the disease seen?
  2. Aetiology
     • Causal agent(s)
     • Predisposing factors
  3. Transmission
     • How is the disease spread between individuals?
     • Is the disease zoonotic?
  4. Pathogenesis
     • How the causal agent(s) exert their effect within the body.
  5. Diagnosis
     • History
     • Clinical findings
       • Clinical examination
       • Clinical pathology
     • Biopsy or post-mortem examination
  6. Prognosis and Treatment
  7. Control and Prevention
     • The ideal situation

Post-Mortem Examination

• Post-mortem examination (PME) investigates the observable structural changes in the animal.
• Information relating to the disease withing the body or specific tissue is gained from PME.
  • This includes information on the disease's
    • Aetiology (cause).
    • Pathogenesis (development).
• Several types of changes are encountered at post-mortem examination.
  1. Those due to the disease
     • Lesions
  2. Those occuring immediately prior to death
     • Agonal
  3. Those occuring after death
     • Post-mortem

Techniques Involved in Pathological Examination

• Fluid examination
  • E.g. blood, urine, discharges from orifices and so on.
• Cytology
  • Examination of cells in smears, aspirates and fluids.
• Necropsy
  • Visual examination of the gross changes in the dead body.
• Histopathology
  • Microscopic examination of:
    • Tissues selected from the dead body after necropsy.
    • Biopsy/excision materials from lesions in the living animal.

5. Histochemistry - microscopic visualisation of enzymatic activity in tissues. 6. Immunological methods - detection of specific antibody activity in tissues and fluids. When linked to a marking agent i.e. a fluorescent dye or other substance, it can localise in the tissue the antigen for that particular antibody. Examination of serum can show prior exposure to a particular infectious agent. 7. Electronmicroscopy - fine detail of the surfaces or internal structures of cells. 8. Bacteriology /Virology/Parasitology - isolation and identification of pathogenic bacteria/viruses/parasites. 9. Toxicology - analysis of tissues for particular poisons and toxins.