etiology

1. Intrinsic and extrinsic
causes of disease
Dr. Jeny K John
Assistant Professor, Clinical Pathology
Department of Veterinary Clinical Pathology
COVAS, SVPUAT, Meerut

2. Introduction
• Etiology: Study of the cause of disease
• It can be intrinsic or extrinsic
• Intrinsic factors act from within the individual
eg. Genus, breed, species etc
• Extrinsic factors are the influences from outside
• Eg. Physical, chemical, biological etc

3. Intrinsic causes of disease/Predisposing /Internal factors/endogenous
1. Genetic or inherited causes
Transmitted to the offspring through the germplasm
a. Lethal characters: inherited from parents and invariably cause death of the
animal, either in-utero or after birth
Eg. Atresia coli in horses, parrot beak in cattle
b. Sub lethal factors: Inherited and interfere with the function of the body but
do not cause death of the animal
Eg. Imperforate anus, infertility in cattle, scrotal hernia in pigs,
deafness in white cats
c. Errors in structure or function: inherited but not lethal only affect the structure
and function
Eg. Cryptorchidism, webbed digits

4. Conti….
2. Non genetic or non inherited defects
Abnormalities not transmitted via the germplasm
a. Anomalies : development defect affecting an organ or part of the body
These may be
1. Disturbance in development
a. Arrest of development
b. Excessive development

5. A. Arrest in development
1. Agenesis or aplasia
2. Hypoplasia
3. Atresia
4. Fissure
5. Fusion of paired organs: Horse shoe Kidneys
B. Excessive development
1. Congenital hypertrophy
2. Increase in number

6. Other intrinsic causes
1. Genus
Certain diseases are peculiar to certain genera of animals while others are not affected
Eg. Swine fever is the disease of pigs and no other animal are infected
2. Breed
Certain breeds either due to selective breeding or inherent characteristics, are more susceptible to some diseases than others
Eg. German Shepherd, Great Danes are more prone to bone diseases, Bulldog breeds and boxer suffer more brain tumors
3. Age:
Different diseases affect different age groups
Eg. Strangles affects younger horses, canine distemper affects pups, tumors in general are found more frequently in older
age groups, deficiency disease more in young animals, caecal coccidiosis in young birds

7. Conti…
4. Sex:
Male and female differ in the susceptibility to disease
Eg. Male are more susceptible to nephritis (dog) and in cattle more
common in females, Reproductive diseases are more common in the
females than in the male
5. Colour
Eg. Grey and white horses suffer from malignant melanoma more
frequently than other coloured horses
Animal with white skin or non pigmented skin more susceptible to
photodynamic diseases

8. Extrinsic causes/Exciting causes/exogenous
1. Physical agents
1.Physical influences
1. Mechanical trauma: perforation, laceration, concussion, sprain, luxation or
dislocation, fracture
2. Injuries due to pressure: atropy, bed sores
3. Injuries due to obstruction
4. Injuries due to malposition

9. 4.Injuries due to malposition
Volvulus: Rotation of the small intestine around its mesenteric
attachment
Torsion: Twisting of the organ upon itself , seen in uterus, large
intestine
Intussusception: Telescoping or invagination of one portion of
the intestine into immediately posterior portion of the gut
Prolapse: Appearance of an organ or a portion of an organ at a
natural or artificial opening

10. Excess heat:
• Lesion produced from excessive application of heat is called a thermal burn- surface burns or
inhalation burn injuries
• Excessive heat retention results in heat stroke
Excess cold
• Local effects- Frostbite- Local freezing is usually confined to the extremities eg feet, teats,
scrotum, tails, fetlocks, pasterns, coronary bands, combs, wattle and toe of fowls
• Systemic effects
3. Light and UV rays- sunburn/photosensitation
4. Electricity: Electrical burns and electrocution
2. Thermal injury:

11. 4.Injuries from change in atmospheric pressure
a. Decreased atmosphere pressure: Brisket disease
Animal moved from lower altitudes to an altitude of 8000 feet or more, they suffer from brisket disease
b. Increased atmosphere pressure: Caisson Disease Also called bends
Sudden decrease in pressure, free gaseous bubbles ,may be released in the blood and act as emboli

12. 5. Those that injure by radiation
1. X-rays
2. Gamma rays
3. Particulate radiation
Important target in living cell is DNA

13. 2. Chemical causes:
• Most toxic chemicals are poisons
• Inorganic poisons: acids (sulphuric acid), base (potassium hydroxide) and salts
(mercuric chloride)
• Organic poisons are derived from plants, molds, fungi and bacteria and certain
parasites and venomous animals
• Toxic agents exert corrosive or caustic action or act as organ poisons or nerve
poisons or they alter the blood

14. • Bacteria
• Viruses
• Helminths
• Trematodes
• Cestodes
• Arthopodes
• Fungi
• Rickettsiae
• Protozoa etc
3.Biological causes

15. 4. Immunological reactions:
Although the immune system serves in the defense against infectious agents,
immune reactions may cause cell injury
Eg. Autoimmune reaction, Anaphylactic reaction to drugs or foreign protein,
immunodeficiency

16. 5. Nutritional imbalance
Nutritional deficiencies and excess of nutrition
• Excess-Hypervitaminosis A, D
• Deficiency-Hypoproteinaemia, hypovitamonosis (e.g. Xerophthalmia in
vitamin A deficiency, star gazing in chicken in vitamin B 1 deficiency),
Hypocalcaemia (Milk fever in high yielding cows)

17. Thank you