October 2023. Module: U0005 Agro-Bioscience

Indirect or predisposing 
causes of disease are referred to frequently as 
“stress”
factors. Stress factors include 
chilling, poor ventilation, overcrowding, inadequate 
feeding and watering space, overmedication, etc. 
Times to be particularly aware of 
stress are at shipping and weaning.
What is disease?

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Infectious diseases are the greatest threat to livestock health. They are 
caused by 
bacteria, viruses, rickettsia, 
and
fungi
. Some 
protozoan
diseases behave much as infectious diseases and often are considered as 
such. Correctly, they should be classed with other 
parasitic
illnesses.
What is disease? Livestock diseases.
Veterinarians
- who 
are responsible for 
the livestock’s 
health 
and controlling the 
welfare condition.

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The triad of 
epidemiology and 
determinants of 
disease.
What is disease?

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Direct causes 
of disease are: 
1. bacteria
2. viruses
3. parasites
4. fungi
5. nutritional deficiencies
6. chemical poisons
7. unknown causes.
Virus
What is disease?
Bacteria
Fungi
Parasite
Chemical poisons

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Some common ways infectious diseases are introduced into and spread within herds 
or flocks are:
1. Introduction of diseased animals.
2. Introduction of healthy animals that have recovered from disease but are still 
carriers or can still transmit the disease to other animals.
3. Contact with inanimate objects that are contaminated with disease organisms 
(trucks and trailers, feeders, waterers, etc.).
4. Contact with carcasses of dead animals that have not been disposed of properly.
5. Impure water, such as surface drainage water.