Animal Disease – SS3 Agricultural Science Lesson Note (Second Term)

An animal disease is any condition in which an animal departs from its normal state of health, so that its body cannot function normally.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define livestock disease, susceptibility and disease resistance;
2. classify animal diseases according to the organisms that cause them;
3. state the factors that predispose animals to diseases;
4. list the factors that determine an animal’s resistance to disease;
5. state the economic importance of animal diseases;
6. explain the methods of preventing animal diseases.

What is animal disease?

Animal disease is any deviation from the normal state of health, or any interference with body processes that stops the body working normally.

• Effect on the animal: it causes ill health.
• Effect on production: it lowers live weight gain, milk, egg and wool production, and work done by work animals.
• Causes: viruses, bacteria, fungi, protozoa and malnutrition.
• Pathogen: any organism that causes disease, such as a virus, bacterium, fungus or protozoan.

What are the classes of animal diseases?

There are five classes of animal diseases based on their cause: viral, bacterial, fungal, protozoan and malnutrition (metabolic) diseases.

Viral diseases

Viral diseases are caused by viruses.

• Newcastle disease: affects poultry.
• Foot-and-mouth disease: affects cattle, sheep, goats and pigs.
• Rabies: affects dogs and other mammals.
• Fowl pox: affects poultry.

Bacterial diseases

Bacterial diseases are caused by bacteria.

• Anthrax: affects cattle, sheep and goats.
• Brucellosis: causes abortion in cattle.
• Tuberculosis: affects cattle and other animals.
• Fowl cholera: affects poultry.

Fungal diseases

Fungal diseases are caused by fungi.

• Ringworm: causes patches on the skin of cattle and other animals.
• Aspergillosis: affects the breathing organs of poultry.

Protozoan diseases

Protozoan diseases are caused by single-celled animal-like organisms.

• Coccidiosis: affects poultry and rabbits.
• Trypanosomiasis: affects cattle and is spread by the tsetse fly.
• Babesiosis (redwater): affects cattle and is spread by ticks.

Malnutrition or metabolic disorders

These disorders come from lack or imbalance of nutrients, or from faulty body processes. They are not spread from one animal to another.

• Milk fever: low blood calcium in cows after calving.
• Rickets: weak bones from lack of vitamin D, calcium or phosphorus.
• Anaemia in piglets: lack of iron.

What is susceptibility to disease?

Susceptibility is a condition in which an animal is easily attacked or infected by disease.

What is disease resistance?

Disease resistance is the ability of an animal to withstand the attack of a pathogen and remain healthy.

What is the difference between susceptibility and resistance?

What factors determine the resistance of an animal to disease?

Six factors determine how well an animal resists disease.

• Development of immunity: antibodies in the blood fight pathogens. Calves get early immunity from colostrum, the first milk.
• Species of animal: some species resist diseases that attack others.
• Inoculation: vaccination gives the body immunity before infection.
• Level of feeding: well-fed animals resist disease better.
• Age of animal: very young animals have weak defences.
• Physiological condition: a healthy, unstressed body resists better than a weak or stressed one.

What factors predispose animals to diseases?

Predisposing factors are conditions that help start and continue a disease.

• Health status of the animal: unhealthy animals get infected more easily than healthy ones.
• The farm environment: a dirty, unhygienic environment encourages disease.
• Nutrition: animals not fed properly in quality and quantity are easily infected.
• Management: poor management, such as wrong use of drugs and vaccines, invites disease.
• Unfavourable climatic conditions: extreme rainfall, temperature and wind weaken animals.
• Poor housing: animals exposed to heavy rain and high temperature fall ill, and parasite infestations can break out.
• Stress conditions: pressures such as overcrowding and transport lower resistance.
• Breeds of animals: poor breeds are more prone to disease than good breeds.

What are the common symptoms of animal disease?

Symptoms are signs that show an animal is sick.

• Loss of appetite: the animal eats little or nothing.
• Dullness: the animal is weak and inactive.
• Weight loss: the animal becomes thin.
• Fever: body temperature rises above normal.
• Diarrhoea: the animal passes watery dung.
• Coughing or hard breathing: the animal struggles to breathe.
• Discharge: fluid runs from the nose, eyes or mouth.
• Rough coat or ruffled feathers: the coat or feathers look dull and untidy.
• Swellings or skin sores: lumps, wounds or patches appear on the body.
• Lameness: the animal limps or cannot stand well.
• Fall in production: milk, eggs or growth drop.
• Staying apart: the animal separates from the group.

What is the economic importance of animal diseases?

Animal diseases cause losses to farmers and to the nation.

• Poor growth: sick animals gain weight slowly.
• Poor food utilisation: sick animals use feed badly.
• Low yield: milk, egg, meat and wool output falls.
• Low income to the farmer: less produce and money spent on drugs reduce profit.
• Poor quality of animal products: products from sick animals may be unfit for sale.
• Death of livestock: the farmer loses the animals and the money invested.

How can animal diseases be prevented?

Animal diseases are prevented by good management that keeps animals healthy and keeps pathogens away.

• Quarantine: new stock is kept apart from the original animals and checked before joining them.
• Vaccination: animals are vaccinated at the right time and in the right doses to give immunity.
• Hygiene: clean pens, watering and feeding equipment, dispose of waste and disinfect regularly.
• Good feeding: well-fed animals resist disease better.
• Breeding: keep disease-resistant stock on the farm.
• Rotational grazing: moving animals between paddocks prevents the build-up of parasites.
• Isolation: sick animals are separated from healthy ones.

Evaluation

1. Define livestock disease.
2. What is the difference between susceptibility and disease resistance?
3. List four classes of animal disease, with one example of each.
4. List three factors that determine resistance to disease.
5. Explain quarantine and rotational grazing.

Assignment

Reading assignment

• Essential Agricultural Science for Senior Secondary Schools by O.A. Iwena, Chapter 30, pages 383 to 386. Answer revision questions 11 and 14 on pages 396 and 397.

Weekend assignment

1. An effective way of ensuring built-in resistance to disease in calves is to:
   • a) vaccinate calves at weaning
   • b) allow calves access to colostrum
   • c) ensure regular veterinary check-ups
   • d) observe proper quarantine
   • e) give a balanced diet
2. Disease-causing organisms are referred to as:
   • a) vectors
   • b) pathogens
   • c) pests
   • d) insects
3. Animal diseases are generally caused by the following:
   • a) protozoa
   • b) nematode
   • c) virus
   • d) fungi
4. An animal is said to be resistant to disease if:
   • a) the animal easily succumbs to infection by the disease
   • b) the presence of the disease pathogen does not cause the expression of disease symptoms
   • c) the animal can endure the effects of the disease
   • d) the animal has no antibodies in its blood
5. The factors that can predispose animals to diseases include the following except:
   • a) poor housing
   • b) immunity
   • c) malnutrition
   • d) unfavourable weather

Theory

1. What is livestock disease?
2. Briefly explain four factors that could predispose animals to diseases.
3. List ten common symptoms of animal disease.