Introduction to General Pathology

Introduction

The term pathology is derived from Pathos, or suffering and 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, and can be observed in:

• The live animal
• Tissues surgically removed from the live animal during biopsy or excision
• Necropsy or post-mortem examination.

Description of Lesions

Descriptions of lesions are 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

Pathological changes to cells

Generally, disruptions at a cellular level usually involve changes or damage to the cells themselves, or they involve the presence of unexpected material within the cell, such as pigments or calcified material. When cells begin to degenerate or become infiltrated by foreign material then cellular metabolism has been disrupted. The alteration in the structure of the cell can be suggestive of the metabolic abnormality that has occurred, and result in the cellular parameters extending beyond the normal physiological range for the cell.

There are several changes in the size of cells and their respective organs that are commonly seen in pathological states:

• Atrophy is a decrease in the size of the cells and respective organ(s), occurring after the organ has reached it's normal size.
• Hypertrophy is an increase in the size of an organ due to an increase in size of the individual cells.
• Hypoplasia is a reduction in the size of cells and tissues due to a failure to grow to the normal size which ranges from mild hypoplasia to almost complete absence. Almost complete absence is also called vestigial or rudimentary.
• Aplasia and agensis refer to complete absence of tissue.
• Hyperplasia is an increase in the size of an organ due to an increase in the numbers of cells present within it. Hypertrophy and hyperplasia may occur concurrently.
• Metaplasia is a transformation of one type of tissue into another and occurs solely in connective tissue and epithelium.
• Dysplasia is abnormal growth within a tissue
• Anaplasia is a marked and irreversible loss of cellular differentiation with return to a more primitive state, which can be a feature of highly malignant tumours.
• Neoplasia is a serious disturbance of growth of tissues

The final pathological process that occurs at a cellular level is necrosis, which is the death of cells within the living body. Two things happen when necrosis occurs:

1. Further changes can take place in the tissue itself.
2. The surrounding unaffected living tissue can react to the necrotic tissue.

Disease

Definition and Type

Disease is a morbid (illness producing) process, having a characteristic train of symptoms or signs. It may affect the whole body or any of its parts, and 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, which compromises function 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, which may be modified by recent or concurrent disease, and/or previous exposure to the agent(s) responsible.
2. The disease-causing agent(s). Most do not cause a uniform pattern of disease. The host defences are important in determining the presentation of the disease. An agent's capacity to produce disease depends upon the dose and the virulence of the agent. Bear in mind that several agents may be involved - often one agent debilitates, allowing others to exert a greater effect that normal within the body. The presence of an agent does not necessarily mean it is the cause of the disease! On occasions, a pathogenic agent may be absent from the tissues, due to either clinical therapy or the action of the host defence systems
3. Environment, for example: in the overcrowding of animals, mixing animals from differing origins and interaction with 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 may also affect susceptibility to disease.

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 whilst 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 refers to how much of the disease is present, where the disease is found and in what species is the disease seen?
2. Aetiology refers to the causal agent(s) and includes predisposing factors
3. Transmission describes how a disease is spread between individuals, and includes the important question 'is the disease zoonotic'?
4. Pathogenesis describes how the causal agent(s) exert their effect within the body.
5. Diagnosis - this is reached on considering the clinical history and findings with reference to the clinical examination and the presence of any pathology. Confirmation is reached by monitoring the response to treatment or by biopsy or post-mortem examination
6. Prognosis and Treatment
7. Control and Prevention

Post-Mortem Examination

Post-mortem examination (PME) investigates the observable structural changes in the animal. Information relating to the disease within the body or specific tissue is gained from PME, including information on the disease's aetiology and pathogenesis.

Several types of changes are encountered at post-mortem examination.

• Those due to the disease, including visible lesions
• Those occurring immediately prior to death, known as agonal changes
• Those occurring after death, or post-mortem

Techniques Involved in Pathological Examination

• Fluid examination e.g. blood, urine, discharges.
• Cytology - collect and examine cells in smears, aspirates and fluids.
• Necropsy, which is a 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 such as testing for specific antibody activity in tissues and fluids.
  • Examination of serum can show prior exposure to a particular infectious agent (i.e. specific 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 which reveals the 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.