The animal body is made up of large numbers of different types of cells. When these cells break down and die, the process is called necrosis. The death of a cell cannot be reversed, but while it is breaking down and dying, the process can be stopped or even reversed by removing the disease factor or factors which are causing the breakdown. Sometimes the degeneration or breakdown of the cells takes place gradually and when a number of the cells in any organ have been affected, that organ will cease to function properly and the animal will show symptoms of this. If these symptoms are ignored by the farmer and a further breakdown takes place, the animal will die. If a post-mortem is carried out on such an animal, i.e. if the dead animal is opened up and the organs examined, those organs affected by the disease will appear different from a normal, healthy organ. Sometimes this difference can be seen quite clearly and at other times, the organs have to be examined under a microscope, and with the use of special techniques.


An injury can cause disease in one of two ways; firstly the animal may be directly injured so that it cannot function properly. The injury may be local, for instance a bruise or lameness which can be treated fairly easily or the injury may be severe such as a broken leg, in which case the animal may have to be put down. In general the treatment of serious injuries is a matter of economics, with the cost of treatment being balanced against the value of the animal.

Secondly, an injury may be the cause of subsequent illness because the tissues exposed by a wound can became infected by bacteria or other organisms. Examples of diseases caused by organisms invading a wound are blood poisoning, septic wounds, gangrene and tetanus or lockjaw. Farmers should make sure that any wounds sustained by their animals are quickly and properly treated.


Poisons are substances which cause chemical processes inside the animal that lead to the degeneration and death of the body cells. Poisons can be put into two groups, those that are present in plants which are eaten by the animal, and those which are chemical and usually eaten by mistake. There are a number of weeds growing in the veld in Africa which can cause poisoning in grazing animals, and in certain circumstances, star grass pastures and jack-beans can be poisonous. Examples of chemical poisons are some herbicides and insecticides, and substances such as lead which can affect an animal which licks an old paint tin. In some cases an excess of a useful mineral can cause poisoning. Examples of this are excesses of iron, fluorine, sulphur and selenium.


Sometimes inherited characteristics, those passed on from the parents to their offspring, can cause certain cells of the body to degenerate. An example of this is dwarfing in calves.


Feeding is a factor which has a great effect on the health of farm animals. There are four ways in which the diet of an animal can affect its health. These are:

  • Feeding the animal too little or too much. Too little food will cause a loss of production from the animal and in severe cases can lead to malnutrition and death from starvation. Over- feeding can lead to the animal’s becoming overweight and unhealthy;
  • Feeding a ration which is not balanced for the animal’s requirements. Deficiencies of protein,

major minerals, trace elements and vitamins, together with water, can lead to a great number of deficiency diseases;

  • Feeding a ration containing a specific toxic substance or poison in the food; and
  • Allowing the animal to eat food or drink water which has been contaminated by bacteria or by parasites such as worms.

The health of the animal can he affected positively by feeding a diet which increases its resistance to bacterial disease.


This is the most common cause of ill health in animals, and the organisms involved are:


Bacteria, Fungi, Viruses, Protozoa.


Worms, Ticks, Lice, Insects


These are small organisms belonging to the plant kingdom. They consist of a single cell, but when they multiply, cells often remain joined together so that they appear to be multi-cellular. Reproduction is asexual and is carried out by a process known as binary fission. This is a very simple process whereby the cells of the bacteria become larger, a cell wall develops across the middle, and the two halves separate to become two new bacteria. This process can be completed in 20 minutes, so that if conditions are good and plenty of food is available, thousands of millions of bacteria can be produced from a single cell in 24 hours. This very rapid multiplication accounts for the rapid course of a disease and the way in which a disease can spread throughout a herd in a short time.

Bacteria can be identified in a laboratory by the following methods:

  • By looking at the cells through a microscope after they have been stained by means of special dyes. The shape of the cells means that they can be classified into different groups. Cocci are spherical shaped bacteria, and if they are in the form of chains they are called Streptococci, and if in groups, are called Staphylococci. Bacilli are rod-like in shape, Vibrio look like the letter V, and the Anthrax Bacilli have a special skin which identifies them. Some of these shapes are shown in the diagram below.

Figure 1: Shapes of Bacteria

  • The forms of bacteria shown in the diagrams are called vegetative forms, which are fairly easily destroyed by heat. However, some bacteria can form round spores and these are very difficult to destroy. Spores are formed when conditions are poor, as in a lack of moisture, and when conditions improve the spores return to the vegetative shape and begin to multiply.
  • By allowing the bacteria to grow on special plates and under ideal conditions of food and temperature. Infected material is placed in a small dish containing a nutritious jelly called agar and the dish is incubated in an oven. The bacteria multiply very rapidly and form characteristic patterns on the agar.
  • By injecting a solution of the bacteria into an animal to see if any disease is produced and then identifying the disease.

These are plants, some large like mushrooms and others so small they can only be seen under a microscope. They start from a single cell and spread by simple division to form long threads, and they also produce spores which are released and spread the disease over a wide area. Ringworm is an example of a disease caused by a fungus.

Figure 2: Fungus: Ringworm in a Calf


These are the smallest organisms at present known, and they can be seen only through an electron microscope which can magnify an object up to 400 000 times. Some idea of the very small size of viruses is shown in the diagram below:

Figure 3: A Comparison of Sizes between Bacteria and Viruses

Viruses can be separated from other organisms in a liquid by passing the liquid through a porcelain filter which will hold back the bacteria, fungi and other micro-organisms and allow the viruses to pass through. As they are so small, viruses are difficult to identify and the methods used for identifying bacteria are of little use. They can be multiplied in living tissue, and one method is to inject the virus into a fertilised hen’s egg and place it in an incubator. Viruses can also be multiplied in an artificial

culture of human or animal tissue cells.

Viruses are the most highly infectious causes of disease, and examples of very infectious diseases caused by them are the common cold, human influenza, cattle plague, foot and mouth disease, swine fever, rabies, distemper in dogs and fowl pest.


These organisms belong to the animal kingdom and are composed of single cells that can only be seen under a microscope. There are many different kinds of protozoa and some of them cause disease in both humans and animals. In many cases, the disease is transmitted by ticks, biting insects or flies. Examples of diseases caused by protozoa are malaria in humans which is transmitted by the mosquito, Redwater and East Coast Fever in cattle, both transmitted by ticks, Sleeping Sickness transmitted by the tsetse fly, and Coccidiosis in poultry and rabbits. The organisms causing these diseases are shown below.

Figure 4: Protozoa Organisms causing Diseases



These are invertebrate, multi-cellular animals which are divided into three classes; the Roundworms

or Helminths, the Tapeworms or Cestodes, and the Flatworms or Platyhelminthes.

The Roundworms (Phylum Aschelinthes) are round, cylindrical, pointed at either end and have a smooth white, semi-stiff skin. They vary in size from 1mm to 500mm in length, and each individual is either a male or a female. Only one class is of importance to agriculture and that is the Nematodes.

The Flatworms (Phylum Platyhelminthes) are divided into the Trematodes (Class Trematoda), an example being Liver Fluke, and the Cestodes (Class Cestoda) which are the Tapeworms.

      Abscesses: a swollen area within body tissue, containing an accumulation of pus.

Many of the worms have complicated life cycles which are spent partly in the infested animal and partly in an organism of another species or in the outside world on herbage. When in the animal, they may be found in one spot only, or they may travel around the body, passing through several different organs. In some cases e.g. the Liver Fluke, the worm spends most of its life inside the animal host and in one particular organ, in this case the liver.

Wherever the worms may be situated in the body they are a foreign system competing for the available nutrients, and while they do not reproduce as rapidly as the bacteria and fungi, they can multiply into large numbers in a relatively short space of time. When they are present in an animal in large numbers, they usually produce ill health, either by using up food which the animal requires, or by doing direct damage to an organ or

tissue. This damage can be a site for bacterial infection with abscesses being formed. All the worms are classed as Endoparasites because they live inside their animal host.


These are the parasites which live outside the animal host and which either bite the animal or suck its blood. They belong to the Phylum Arthropoda and are divided into the Class Insecta and the Class Arachnida. The Class Insecta includes all the parasites commonly called insects, and the Class Arachnids are the ticks and mites. Insects have three pairs of legs and a body divided into head, thorax and abdomen. Some have two pairs of wings, others have only one pair and some have no wings at all. The ticks and mites have four pairs of legs in the adult stage and a body in a single piece with well- developed mouth parts used for biting and sucking.

Figure 5: Some of the Worms, Insects and Ticks are shown in the illustrations below.