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.

Necrosis

Necrosis The term necrosis means death of cells within the living body. Two things happen when necrosis occurs: a. further changes can take place in the tissue itself; and b. the surrounding unaffected living tissue can react against this necrotic tissue. Causes of Necrosis. There are three main causes of necrosis: 1. loss of blood supply - tissues depend upon their blood supply to remain alive, 2. non-living agents such as chemicals or physical injuries, 3. living agents such as bacteria, viruses, fungi or parasites. 1. Loss of blood supply .A diminished blood supply to a tissue is called ischaemia. This type of necrosis is called ischaemic necrosis, also called infarction - defined as necrosis of a portion of tissue due to an interruption (usually sudden) in the blood supply to that portion. The effects of ischaemia on a tissue will vary according to: a. the type of tissue affected - some tissues are more susceptible than others. b. the type of cell in the tissue - the general rule is that parenchymatous cells, the essential functioning cells, are more susceptible than the stromal supporting cells. c. the metabolic activity of the tissue - very active organs i.e. those that work continuously are more susceptible. d. whether or not there is a good or potential collateral blood supply 25 There are three ways in which tissue ischaemia can be brought about a. compression of the blood vessel from without e.g. too tight a bandage will cause tissue ischaemia. A common cause of tissue ischaemia is strangulation of the intestine by a twist upon itself, or a mass such a lipoma ( a relatively common growth of mature fat tissue ) forms in the mesentery, becomes pedunculated ( attached by a fine band of the mesentery) and can encompass a portion of intestine. Initially, the compression of the mesenteric veins will prevent outflow of blood leading to congestion and swelling of the affected portion. When the arterial supply becomes occluded or cannot supply sufficient blood to the tissue because of pressure in the swollen intestine and its vessels, the affected area undergoes an ischaemic necrosis with disastrous sequelae of rupture and peritonitis or gangrene and toxaemia due to absorption of toxic products of the necrosis and intestinal bacteria. b. a narrowing of the lumen e.g. thickening of the wall in arteriosclerosis. c. blocking of the lumen of the vessel, important causes are thrombi and emboli. Renal vessels are commonly affected. See the effects of emboli in Circulatory Disorders. 2. The action of physical or chemical agents. The physical agents include burns, cold, frostbite, X-rays, pressure, and actual pinching or crushing of the tissue. The necrosis is direct in the case of burns and indirect in the case of crushing or pinching which causes occlusion of the vessels supplying the tissue which will undergo necrosis. Chemical agents may be directly caustic or corrosive in action or exert their effects when absorbed and metabolised to a more toxic substance. 3. Living agents - their effect is either through their toxic effects on cells or their colonisation of the cells. Appearance of necrotic lesions In a typical necrotic lesion there are three zones. 1. Where the effect of the causal agent is maximal, there is a sphere of necrosis. 2. A little further away, the tissue will be damaged but not yet dead, and so there is a zone of degeneration short of death. 3. Still farther away, where the effect of the agent is insufficient to cause death or degeneration of cells, we have a zone where the body is reacting to the dead tissue 26 Gross and histological features of necrotic tissue. 1. Colour change in the tissue. In contrast to living tissue, dead tissue tends to be paler, partly because there is no circulation in dead tissue 2. Consistency (texture) of the tissue. The appearance of the centre of the necrotic lesion will vary according to the 1. type of agent responsible and 2. the tissue in which it is acting. This appearance may give a clue to the agent responsible, and the types of necrosis encountered are based on their gross description a. Coagulation necrosis Gross: The necrotic lesion will be firmer and dryer on the cut surface. The gross appearance still resembles somewhat the nearby living tissue. It is a feature of bacteria which produce toxins, infarction, and some foci of viral replication. Micro: the general architecture of the tissue is preserved. Certain changes present can be recognised in an ordinary H&E section, and are related to loss of cellular detail. a. the cells may appear somewhat larger and their outline may be lost b. the cytoplasm appears structureless and homogenous. c. there are nuclear changes - the most important 27 There are three types of nuclear change. i. Pyknosis - Greek for dense - is a condition in which the normal nuclear structure is replaced by a very dense, heavily staining, somewhat smaller angular mass of chromatin. ii. Karyorrhexis - Greek karyon = nucleus; rhexis = breaking up. This appearance is rather the reverse of the above. It appears as though the nucleus has exploded rather than condensed in the cytoplasm, and irregular-sized bits of dense nuclear material are found scattered throughout the centre of the cytoplasm. iii. Karyolysis - this means dissolution of the nucleus. The nuclear staining with haematoxylin becomes fainter and only the ghost outline of the nucleus remains. Apoptosis - is a term used to denote the programmed death of scattered single cells in living tissues. Unlike necrosis, there is no reaction to the death of the cell. It is thought that some cells are programmed to die - a form of cell regulation in a tissue - unless there is a change in circumstances in a tissue that require them to continue living. One such circumstance is the development of tumours in which cellular regulation is absent. Cellular apoptosis is thought to be important in deleting extra cells produced in embryogenesis, in cyclical physiological changes in the genital tract of females, in the death of lymphocytes, in graft rejection, and in cellular death by the same injurious substances that cause necrosis in higher doses. The cells undergoing apoptosis lose their connections with their neighbours and shrink; the nucleus becomes pyknotic; and the cytoplasm becomes eosinophilic. The cell breaks up into fragments that are engulfed by neighbouring cells or local macrophages. Their remnants can be seen in the neighbouring cells’ cytoplasm. 28 b. Liquefactive (Colliquative) Necrosis. Gross: In the brain where there is a lot of lipid, the intracellular enzymatic changes make it softer and more fluid in nature. This is known as 'malacia'. Initially it becomes swollen with a gelatinous, sometimes bloodtinged appearance due to disruption of the blood vessels but later becomes fluid. Micro: the microscopic appearance will not resemble the nearby living tissue as it may have already lost any semblance of the nearby living tissue because it is becoming a fluid. Another type of liquefactive necrosis, quite important is pus formation which occurs when the organism causing the initial necrosis in the tissue, is capable of attracting to the necrotic area large numbers of neutrophils and also capable of killing them. These are the so-called pyogenic bacteria. When the neutrophils die they release proteolytic enzymes which digest the dead tissue and also more incoming neutrophils. The liquid formed is called pus, and it is composed mainly of the dead and dying neutrophils together with the remnants of the necrotic tissue cells. 29 In favourable cases, the neutrophils may eventually kill the organisms, but in most cases the organisms persist, all the time producing more pus. This produces an expanding sphere of pus that is called an abscess. The pressure will build up and if near to the skin will cause pressure on the overlying skin, and when the pressure is sufficient or the surgeon lances it, the abscess will burst discharging the pus, and hopefully with it the organisms responsible. This is nature’s way of ridding the body of an injurious agent. Micro: In the case of an abscess, the necrotic area will show varying stages of degeneration of the neutrophils, ranging from nearly normal neutrophils, to pyknosis, karyorrhexis and karyolysis, and finally to a homogenous structureless admixture of remnants staining faintly bluish. As this sphere of pus is forming, there is a host inflammatory response directed against it. This is composed of a capsule of fibrous tissue in which there are many blood vessels on its inner surface, which transport the vast number of neutrophils into the necrotic centre. This is called the 'pyogenic membrane'. In superficial abscesses, when the abscess has discharged to the surface, this membrane can be viewed as a crater that has a reddish somewhat ragged lining. Where the abscess is deep within an organ such as the liver, there is nowhere to discharge to, and the fibrous capsule around the pus is markedly thickened. . c. Caseation necrosis. The necrotic tissue appears grossly like cheese. The colour varies from white to grey to yellowish. In sheep it appears whitish while in cattle there may be a yellowish tinge. The fluid content also varies giving a dry crumbling consistency in some cases to being more like cottage cheese in others. It is really a mixture of coagulation and liquefactive necrosis, and is a feature of necrosis caused by some specific organisms. 30 Micro: On histological section, there is a complete loss of the architecture, the necrotic material being purplish in colour due to random intermixing of the components that stain with haematoxylin and eosin i.e. bits of nuclear material interspersed with cytoplasmic fragments. This type of necrosis is a feature of granulomatous (tumour-like proliferation of chronic inflammatory cells) processes such as tuberculosis in some species, as in the ox, pig and sheep. The necrotic tissue is not derived principally from the organ in which it occurs, but from a special type of host inflammatory cell - the macrophage - which is sent into the tissue in large numbers to engulf the organism. The organism has defences against the enzymes of the macrophages and is quite willing to continue to grow and multiply within these macrophages eventually causing their death. In some granulomas, the macrophages will combine together and form giant cells. Fungi and parasites also cause granulomas. Calcification. In the condition of calcification, calcium salts are deposited within the necrotic tissue in an effort to make it more inert. It is seen quite commonly in necrosis in cattle and sheep, and is a common feature in lesions which show caseation necrosis, and usually indicates a lesion of long standing. Such calcified necrotic tissue can be appreciated grossly. The deposits of calcium salts can be palpated and on cutting into the necrotic portion, the calcified material may be both felt as a gritty substance and heard by a grating sound against the knife. The colour is usually chalk-white but may have a yellowish green tinge if the inciting cause is a parasite. Parasites attract a large number of eosinophils that are responsible for this colour. This type of calcium deposition in necrotic tissue is called dystrophic calcification. It is an attempt to make the tissue more inert. It does not result from elevated levels of calcium in the blood. Micro: In sections stained by H&E, calcium has a distinctive dark blue colour. You may also see some shattering of the calcium and adjacent tissue due to the effect of the microtome knife passing through it. It blunts the knife and there may be score marks throughout the rest of the section. Difficulty sometimes arises in distinguishing calcium from bacterial colonies that stain a similar colour. A definitive special stain for calcium is to stain the section with silver nitrate (the von Kossa method). The calcium stains black. 31 Sequel to necrosis These will vary in relation to the causes of necrosis, but by and large it is important to distinguish between dead tissue on the surface of the body and dead tissue in the depths of the body; a piece of dead skin as compared with a portion of dead liver. Dead tissue on the surface can be shed and is said to slough, whereas dead tissue in the centre of the liver cannot naturally be shed from the surface. Something else happens to it. It can either be absorbed or replaced by fibrous tissue, or it can be enclosed by fibrous tissue. This is part of the process of inflammation - the response of the body to local injury. It is a general rule that small areas of necrosis become absorbed and replaced by fibrous tissue (scars) while larger areas become encapsulated by fibrous tissue, the necrotic portion remaining in the centre. This effectively cuts off any contact with living tissue, and allows perhaps further changes to occur within the necrotic portion to make it more inert. The encapsulated portion is called a sequestrum. It can be, as mentioned before, be calcified to make it more inert. When necrosis occurs on an epithelial surface, two things may happen depending upon the depth of the necrosis. In a case like 'Foot and Mouth' where the necrosis is confined to the middle and outer layers of the epithelium, the remaining underlying germinal layer divides and replaces the shed portion. This type of necrosis confined to the epithelium is called an erosion. It leaves no scar. When the necrosis extends below the basement membrane of the epithelium as might be caused by an applied corrosive substance or a burn, the body reacts to this interruption in the integrity of the epithelium with an underlying inflammatory reaction attempting to repair the deficit by fibrous tissue. This type of necrosis is called ulceration, and the resultant contraction of the fibrous tissue leaves a scar. Fat necrosis This is confined to the fat depots of the body. It has a very distinctive appearance grossly: instead of the fat being semi- translucent and malleable, it shows areas of focal opacity and is very hard in consistency. This appearance is due to the intracellular fat after the fat cells have died, being broken down into fatty acids which combine with Ca++, Na+ and K+ ions to forms soaps. These soaps are substances foreign to the body and they provoke a host inflammatory response. Unlike fat, they do not dissolve out in routine preparation of sections, These areas of fat necrosis remain indefinitely, may show great scarring, and quite often calcify. It occurs principally in two ways in the body. 32 a. Enzymatic necrosis of fat. This happens when there is a release of pancreatic enzymes into the neighbouring mesenteric fat, the release being caused by a damaged pancreas e.g. due to an adjacent tumour. b. Traumatic necrosis of fat. This is seen in the subcutaneous tissue following trauma to the area. It is quite common in the brisket of recumbent animals due to the prolonged pressure on the area. It is worth mentioning here another condition of fat that can also undergo necrosis and calcify. This is the so-called lipomatosis that occurs for some unknown reason in Channel Island breeds of cattle. In this condition, there are focal areas of increased fat in the mesentery. They often surround several loops of the gut, and if they become necrotic, they may strangle the enclosed gut with disastrous consequences for the animal.' Gangrene Gangrene is a post-necrotic change, and in some cases is the ultimate degradation of necrotic tissue. The tissue is already dead. There are two main types a. Wet gangrene - life threatening b. Dry gangrene - non life threatening a. Wet gangrene can either be due to: a. the agent which initially kills the tissue, further putrefying it, or b. the gangrene may be due to dead tissue killed by some other means being invaded by organisms which putrefy it. In other words the former may be viewed as a primary gangrene, while the latter is secondary. An example of the former is gangrenous mastitis of the udder of the cow caused by Staphylococcus aureus, the organism killing the tissue and then putrefying it. An example of secondary gangrene is that which occurs when a portion of gut twists on its mesentery or when a lipomatous mass attached to a strand of mesentery, loops around a piece of intestine as depicted earlier with regard to tissue ischaemia. The blood supply to the gut is cut off, and the affected portion becomes necrotic. Wet gangrene supervenes when the putrefactive organisms that are normally present in the gut invade the dead tissue. 33 A further cause of wet gangrene is when a ligature around an extremity causes ischaemic necrosis of tissue distal to it and the necrotic tissue becomes invaded with putrefactive bacteria. The gross appearance is of a swollen puffy tissue cold to the touch and with a horrible smell owing to the hydrogen sulphide (the smell of rotting flesh) produced in the putrefying tissue. In comparison to other dead tissue the zone of inflammation between the dead putrefying tissue and living tissue is indistinct. This type of gangrene is overwhelming disastrous for the animal unless treated quickly and effectively, as the organisms produce potent toxins either themselves or in their breakdown of the dead tissue and the animal rapidly succumbs to toxaemia. A variation of wet gangrene is that produced by Clostridia organisms such as Clostridium chauvei and C. septicum whereby they also form gas. This is called gas gangrene. The conditions of Blackleg and Clostridia contamination of wounds produce this type of gangrene. Dry gangrene, on the other hand, is not life threatening. This is really mummification (like an Egyptian mummy) of an extremity, such as the tail, foot or ears of animals. There is an occlusion of the blood supply to the extremity. The tissue becomes necrotic. Because of air circulating around the extremity, water is drawn out of the tissue, drying and preserving it. There is little if any bacterial growth in the tissue and it eventually sloughs off. In small animals crushing of the tail may cause this by cutting off the blood supply. 34 In large animals, it is seen commonly following a septicaemic condition in which bacteria are growing and passing around in the circulation. An embolus blocks the blood supply. When seen in calves, the possibility of Salmonellosis must be kept in mind. Other causes are frostbite and ergot poisoning. Finally, necrosis is irreversible.