What causes disease?

Knowing why an animal gets ill can help us understand how to
prevent disease and treat sickness.

Blue ticks in various stages of development.
Photo: Greg Miles

Disease, or ‘sickness’, is any process that changes the appearance of parts of the body or the way in which they operate. It affects the ‘inner workings’ of the body; external injuries such as broken legs and cuts are not diseases.
Many diseases can make an animal sick. Each needs to be treated in a certain way, while some can be prevented with vaccines. The following are common causes of diseases:

Parasites
A parasite is a tiny organism that has to live on or in other animals in order to survive. Those living on the exterior are called external parasites or ectoparasites. They include flies, lice, fleas, ticks and mites, all of which can bring about serious diseases in animals or carry diseases such as redwater and heartwater. Internal parasites or endoparasites live inside the animal, typically in the stomach, intestines, lungs or liver. They include roundworms, flukes and tapeworms.

Microbes
A microbe is also a type of parasite as it lives in an animal’s body. But unlike most parasites, it is microscopically small. Some are beneficial, helping to digest food, for example. Others can cause disease, and include:

Viruses
A virus ‘hides’ inside a cell, which is the basic unit – or ‘building block’ – of all living creatures. Viruses, the smallest of all microbes, cause about 60% of diseases in animals and humans. Examples of diseases caused by viruses are rabies and Newcastle disease. Treating a disease caused by a virus can be difficult. Because viruses live inside cells, any medicine that kills the virus could harm the cell, weakening the animal.

Bacteria
Not all bacteria cause disease. But harmful ones – called ‘germs’ – can cause anthrax, black quarter and tuberculosis.
These bacteria can also get into wounds and infect them, making them worse. This is why fresh wounds should be treated as soon as possible.

Fungi
There are many kinds of fungus, from yeast to the green mould found on stale bread, to mushrooms. A fungus grows and feeds on organic material, and this can include animals. An example of a fungal disease in livestock is ringworm.

Poisoning
This is also a form of disease. In fact, the word ‘virus’ comes from an old English word meaning ‘poison’. An animal can be poisoned by chemicals, such as insecticides and dips, or by eating certain plants, or by being bitten by snakes, scorpions or spiders.

Cancer
Cancer occurs when some of the cells in the body become ‘sick’ and grow in an unnatural way. In turn, this stops the other parts of the body from working properly.

Source: Causes of Disease in Animals by Jenny Turton, compiled by the department of agriculture’s Directorate Communication in co-operation with the ARC-Onderstepoort Veterinary Institute.