eChapters

Definition

The animal reacts to its own tissue (endogenous antigen) to incite production of antibodies or sensitized lymphocytes. There is breakage of tolerance to the self-proteins.

• Autoimmune diseases are prevented by elimination of sensitized T and B lymphocytes by the process of apoptosis in the thymus and bone marrow during development (Central tolerance; clonal deletion), in the peripheral tissues (Peripheral tolerance) and clonal anergy (Clonal avoidance) by defective presentation of cells.

• The tolerance of CD4+ TH cells is critical in preventing autoimmunity.

• Two major autoimmune diseases are thyroiditis and haemolytic anemia. Other conditions are rare in animals

Definition

Hypersensitivity is an altered reaction to a specific antigen that results in pathologic reactions upon the exposure of a sensitized host to that of specific antigen.

Explanation

Hypersensitivity reactions are inappropriate or misdirected responses to a specific antigen that result in harmful reactions upon exposure of a sensitized host to that specific antigen.

Affected animals require a sensitization phase in which the animal must have had either a previous exposure or a prolonged exposure to the antigen so that it can develop an immune response to the inciting antigen.

The harmful effects resulting from hypersensitivity reactions occur in the effector phase and are most commonly manifested through inflammation or cell lysis.

Definition

IDD (immunodeficiency diseases) occur when there is a failure of the immune system to protect the host from infectious organisms or the development of cancer.

IDD are disorders of immune system that result in immunological deficiency. Immunodeficiency could be from defects in

i. B and T- lymphocytes

ii. Granulocytes

iii. Complement

Tissue – Proliferating Potential of Cell Types
1. Labile Cells
• Continuously dividing cells
• e.g. epidermis, epithelial cells, bone marrow cells
2. Stable Cells - Quiescent cells
• Undergoes division occasionally
• Liver, kidney    pancreas, fibroblasts,
• Endothelial cells
3. Permanent Cells - Non–dividing cells: Neurons, muscle cells (cardiac, skeletal)

Definitions

Reaction of vascularised living tissue to local injury caused by various agents like microbes or necrotic tissue

Inflammation − Reaction of blood vessels − Accumulation of fluid and leucocytes in extra vascular tissues Inflammation and repair always go hand in hand

Beneficial effects of Inflammation

To destroy / dilute the injurious agent (microbes; toxins ) and to protect cell from injury (necrosis)

Harmful Effects of Inflammation

The Disturbances of Growth

• The disturbances in growth cover a broader spectrum of changes from no growth (Aplasia) to uncontrolled growth (Neoplasia).

• While uncontrolled growth (neoplasm) is not dealt here, the other forms of growth disturbances are considered in this chapter.

Cellular Adaptation to Injury

• The cells respond to altered physiological or pathological stimuli by adapting themselves.

• These changes are reflected as atrophy, hyperplasia, hypertrophy, metaplasia and dysplasia besides aplasia and hypoplasia.

Photosensitization is activation of photodynamic chemicals on the skin by long wave length UV or occasionally by visible light

• Necrosis and edema are produced in the exposed areas of skin of animals.

• The cellular damage by photosensitization is due to release of reactive oxygen species leading to mast cell degranulation and production of chemical mediators of inflammation.

Factors Necessary for Photosensitization in Animals

• Oxygen

• Sunlight

• Photodynamic chemicals

• Skin devoid of hair or wool and lacking pigments

Calcification

• Calcification is abnormal deposition of calcium salts in tissue other than bone.

• Calcium is normally present in blood and deposited in bones.

• Calcium if deposited in an abnormal tissue with normal or abnormal blood calcium level is considered as pathological condition.

Pathological calcifications are

1. Metatstatic calcification

2. Dystrophic calcification

Colouring Agents
Colouring agents are called as pigments. Tissues may be discoloured (e.g. 
Jaundice, tattoo) or excessively coloured (e.g. Melanosis) in diseases.
Origin
• External or exogenous pigments
• Internal or endogenous pigments
Exogenous Pigmentations
In exogenous pigmentations colouring substances can enter the body by three different routes.
• Respiratory route by inhalation
• Alimentary route by ingestion
• Cutaneous route by injection

Definition
Gangrene is a necrotic area invaded by saprophytic organisms leading to putrefaction.
Types of Gangrene
There are three types of gangrene
1. Dry gangrene
2. Moist gangrene
3. Gas gangrene
1. Dry Gangrene
Dry gangrene represents an area of coagulation necrosis resulting from  infarction followed by mummification. The extremities of the body like tail,  ears, legs and udder are affected.

Causes and Mechanism
I. External Causes/ Extrinsic Causes
1. Physical Causes
1. Trauma by cutting objects and blunt objects
2. Electrical – Lightning, high frequency current
3. Heat -Sun stroke, burns, fever
4. Cold Local tissue freezing, cold shock
5. Radiation
UV / X /cosmic radiations
6. Pressure
Increased or decreased pressure

Amyloid

• Amyloid (G. Amylon; Amyl(o) - STARCH) means starch-like.

• Amyloid is a pathologic glycoprotein deposited in the extracellular spaces and forms fibrils on polymerization.

Histological characteristics

• Amyloid is specially stained with Congo Red.

• Under polarized light, green birefringence is noticed because of alignment of fibrils.

• Amyloid fibrils are 7.5 to 10 nm in diameter, rigid, non-branching hollow-cored tubules of unknown length.

• β-pleated sheet configuration is seen in X-ray diffraction.

• The P-component which is a glycosa-amino-glycan (GAG) facilitates polymerization of amyloid.

Hyperaemia and Congestion
Definition
Hyperaemia is increased volume of blood in affected tissue or part.
Hyperaemia (Active Hyperaemia)
Occurs in arterioles or arteries
Increased blood flow in capillaries
Congestion (Passive Hyperaemia)
Occurs due to impaired venous drainage
Stasis of blood in veins
classification of hyperaemia

Definition

• Aetiology is defined as study of causation of disease.

Classification of Causes

• There are several agents or factors that can produce diseases in animals and can originate from within the body (intrinsic) or outside the body. (extrinsic).

Causes are classified into two categories

1. Predisposing causes (Intrinsic/Endogenous factors)

2. Definitive causes (Extrinsic/Exogenous factors)

Predisposing Causes

• Make the animal susceptible to definitive causes.

Introduction and Scope of Veterinary Pathology

• The aim of study of veterinary and animal sciences is to produce a competent veterinarians. The veterinarians are involved in the diagnosis of diseases of livestock, poultry, wild animals and captive animal species. The study of diseases is essential component of pathology.

• Pathology (Gr. Path(o) - Disease; logos: science, treatise, sum of knowledge in a particular subject) • Pathology is the subject linking basic subjects of anatomy, histology, embryology, physiology and biochemistry to clinical subjects of medicine, surgery and obstetrics and gynaecology.

• Pathology is a subject of terminology.

• It is important to familiarize with different terminologies used in describing events/ disease conditions. The definition of each terminology is the gateway to understanding the particular disease process.

• General Consideration

• Post-mortem examination of large animals

• Post-mortem examination of poultry

• Steps in post-mortem examination

• Writing of post-mortem report

• Collection, preservation and dispatch of specimens for laboratory diagnosis

General Consideration

Necropsy is examination of animal after death also known as post mortem examination. It helps in diagnosis of diseases and their control. It is said that “Necropsy is a message of wisdom from dead to living”. Necropsy include systemic examination of dead animal, recording of pathological lesions, their interpretation to make diagnosis of disease. Sometimes it is difficult to arrive any conclusion merely based on gross examination of dead animal. Then one should seek the help of laboratory examinations such as Histopathology, Microbiology, Immunology and Toxicology for confirmation.

• Neoplasm General

• Neoplasm in cattle and buffaloes

• Neoplasm in sheep and goat

• Neoplasm in equines, swine and camels

• Neoplasms in pet animals

• Neoplasms in poultry

• Neoplasms in wild animals

• Neoplasms in laboratory animals

Concretions

Concretions are solid, compact mass of material, endogenous or exogenous in origin, found in tissues, body cavities, ducts or in hollow organs. Concretions are stone-like bodies commonly occur in urinary system, gall bladder and gastrointestinal tract. Concretions of endogenous origin are known as calculi while those formed from exogenous material are known as piliconcretion (Hair), phytoconcretion (plant fibres) and polyconcretion (polythenes).

Calculi

Calculi are formed due to deposition of salts around the nucleus/nidus consisting of either fibrin, mucus, desquamated epithelial cells or clumps of bacteria. Due to the gradual and repeated precipitation of salts, calculi become laminated. In the process of calculi formation, the inner structural arrangement gets shrink, producing a rough superficial surface. Calculi formation is more common in urinary system and in gall bladder of man and animals; however, they may also occur in salivary gland, pancreas and intestines.

Aplasia/Agenesis

Aplasia or agenesis is absence of any organ (Fig. 9.1).

Hypoplasia

Hypoplasia is failure of an organ/tissue to attain its full size (Fig. 9.1).

Etiology

• Congenital anomalies e.g. hypoplasia of kidneys in calves.

• Inadequate innervation.

• Inadequate blood supply.

• Malnutrition.

• Infections e.g. cerebral hypoplasia in bovine viral diarrhoea.

Calcification

Calcification is the deposition of calcium phosphates and calcium carbonates in soft tissues other than bones and teeth. It may be classified as dystrophic and metastatic calcification.

Dystrophic Calcification

Dystrophic calcification is characterized by the deposits of calcium salts in necrosed tissue of any organ (Fig 8.1).

Etiology /Occurrence

• Necrosis.

• Parasitic infections.

• Tuberculous lesions.