Causes of the disease
Etiology is the study of the causes of disease. It gives a precise causal diagnosis of any disease. Broadly, the cause of diseases can be divided into three types:
a. Hypoxia and Ischemia
b. Intrinsic causes
c. Extrinsic causes
HYPOXIA AND ISCHAEMIA
Cells of different tissues essentially require oxygen to generate energy and perform metabolic functions. Deficiency of oxygen or hypoxia results in failure to carry out these activities by the cells. Hypoxia is the most common cause of cell injury. Hypoxia may result from the following:
The most common mechanism of hypoxic cell injury is by a reduced supply of blood to cells due to interruption i.e. ischemia.
However, hypoxia may result from other causes as well e.g. disorders of oxygen-carrying RBCs (e.g. anemia, carbon monoxide poisoning), heart diseases, lung diseases, and increased demand for tissues.
Pathophysiology of cell death by ischemia/ hypoxia
Those causes, which determine the type of disease present within an individual over which he has no control. These causes are further divided into the following subgroups:
Specific diseases occur in a particular genus or species of animals. e.g. Hog cholera in pigs, Canine distemper in dogs
Diseases do occur in a particular breed of animals such as Dairy cattle are more prone to mastitis. Brain tumors common in Bulldog/ Boxer.
Genetic relationship plays a role in the occurrence of diseases in animals. e.g. some chickens have resistance to leucosis; hernia in pigs due to weak abdominal wall.
Age of animal may also influence the occurrence of diseases such as:
- Young age- diarrhoeal pneumonia.
- Old age- Tumor
- Canine distemper- Young dogs
- Strangles- Young horse
- Prostatic hyperplasia- Old dogs
- Coccidiosis- Young chickens
Reproductive disorders are more common in females.
- Milk fever, mastitis and metritis in females.
- Nephritis is more in male dogs than female
- Bovine nephritis is more common in females.
Colour may also play role in the occurrence of diseases. e.g. Squamous cell carcinoma in white coat color cattle, melanosarcoma in gray and white horses
An unusual reaction of the body to some substances such as:
- Drug reaction, small dose of drug may produce reaction.
- Individual variations.
The etiological factors are present in the outside environment and may cause/ influence the occurrence of disease. These are also known as exciting causes/ acquired causes. Majority
of the cause of diseases falls under this group which is further classified as physical, chemical, biological, and nutritional causes.
The traumatic injury occurs due to any force or energy applied to the body of an animal e.g. During control, restraining, shipping, or transport of an animal.
Contusions or bruises arise from the rupture of blood vessels with the disintegration of extravasated blood.
Abrasions are circumscribed areas where epithelium has been removed by injury and it may indicate the direction of the force.
Partial loss of surface epithelium on skin or mucosal surface is termed erosion.
Incised wounds! cuts
Incised wounds are produced by sharp-edged instruments. They are longer than deep
Stab wounds are deeper than longer produced by the sharp-edged instruments.
Severance of tissue by excessive stretching and is common over bony surfaces or are produced by a cut through a dull instrument.
Compression injury is produced as a result of force applied slowly e.g. During parturition.
Force of compression waves against surfaces followed by a wave of reduced pressure. It can rupture muscles/ viscera.
Hitting at 90° by firearms to produce uniform margins of abrasion. Exit wounds are irregular and lacerated.
High voltage currents induce tetanic spasms of respiratory muscles and hit the respiratory center of the brain. It also produces flash burns. Lightning causes cyanotic carcass, postmortem bloat, congestion of viscera, tiny hemorrhage, and skin damage.
I degree burns
There is only congestion and injury to the superficial layers of the epidermis e.g. sunburn on hairless parts or white-skinned animals.
II degree burns
The epidermis is destroyed; hair follicles remain intact and provide a nidus for healing of the epithelium.
III degree burns
Epidermis and dermis both are destroyed leading to fluid loss, local tissue destruction, laryngeal and pulmonary edema, renal failure, shock, and sepsis: Till 20 hrs of burn, the burning surface remains sterile then bacterial contamination occurs. After 72 hrs millions of bacteria enter the affected tissue. Bacteria such as staphylococci, streptococci, and Pseudomonas aeruginosa invade the deeper layers of skin and cause sepsis. There is a state of immunosuppression in severe burns leading to impaired phagocytosis by neutrophils.
Hyperthermia means increased body temperature due to high environmental temperature e.g. Pets in a hot environment without water. Hyperthermia leads to increased respiration (hyperpnoea), rapid heartbeat (tachycardia), and degeneration in the myocardium, renal tubules, and brain.
Hypothermia means decreased body temperature and includes freeze induced necrosis of tissues at extremities
Radiation as a result of exposure to X-rays, Gamma rays, or UV rays leads to cell swelling, vacuolation of endoplasmic reticulum, swelling of mitochondria, nuclear swelling, and chromosomal damage resulting in a mutation. The impact of radiation is more on dividing cells of the ovary, sperm, lymphocytes, bone marrow
tissue, and intestinal epithelium. It is characterized by vomiting, leucopenia, bone marrow atrophy, anemia, edema, lymphoid tissue, and epithelial necrosis.
Viruses are the smallest organisms, which have only one type of nucleic acid DNA or RNA in their core covered by a protein capsid.
Sub viral agents
• Prion proteins are infectious proteins without any nucleic acid. e.g. Bovine spongiform encephalopathy.
• Viroids are having only nucleic acid without proteins. They do not cause any disease in animals. However, they are associated with plant diseases.
Coxiella burnetti causes Q-fever
Mycoplasma mycoides is responsible for pneumonia, joint ailments, and genital disorders.
Chlamydia trachomatis, C. psittaci cause abortions, pneumonia, and eye ailments.
Leptospira sp. causes abortion, icterus. Borrelia ansarina causes fowl spirochetosis in chickens.
Bacteria are classified as Gram-positive and Gram-negative on the basis of Gram’s staining. Gram-positive bacteria include Staphylococci, Streptococci, Corynebacterium, Listeria, Bacillus Clostridia. Gram negative bacteria are E. coli, Salmonella, Proteus, Klebsiella, Pasteurella, Pseudomonas, Brucella, Yersinia, Campylobactor etc. Besides, there are certain organisms stained with Zeihl Nelson stain and are known as acid-fast bacilli e.g.Mycobacterium tuberculosis and M. paratuberculosis.
Fungi pathogenic for animals mostly belong to fungi imperfecti. e.g. Histoplasmosis
Fungi cause three types of disease-
- Mycosis e.g. Actinomycosis;
- Allergic disease e.g. Ringworm;
- Mycotoxicosis e.g. Aflatoxicosis.
Parasites are classified mainly into 3 groups:
- Protozoan Parasites: Trypanosoma evansi, Theileria annulata, Babesia bigemina, Toxoplasma goodii, Eimeria Spp.
- Nematodes- Roundworms e.g. Ascaris,
- Trematoda– Flatworms e.g. Liver fluke
- Cestodes– Tapeworms e.g Taenia spp.
- Arthropods: Ticks, Mites, Flies, Lice