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

• See Recognition and Description of Lesions

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.
• Histochemistry
  • Microscopic visualisation of enzymatic activity in tissues.
• Immunological methods
  • Specific antibody activity can be detected in tissues and fluids.
    • Examination of serum can show prior exposure to a particular infectious agent (i.e. specifice antibodies).
  • Specific antigens can be detected in tissues.
    • When linked to a marking agent (e.g. a fluorescent dye), an antibody can localise its antigen in the tissue.
• Electronmicroscopy
  • Electronmicrosopcy shows fine detail of the surfaces or internal structures of cells.
• Bacteriology/ Virology/ Parasitology
  • These techniques allow the isolation and identification of pathogenic bacteria, viruses and parasites.
• Toxicology
  • Analysis of tissues for particular poisons and toxins.