Balance of Health and Disease

When an animal is exposed to an infection (by for example a virus), it may be able to fight off the infection without showing any sign of disease. Alternatively, it may succumb to the infection, become ill and possibly die.

Whether the animal fights off the infection or becomes diseased depends partly on the nature of the infectious agent (bacteria, virus, etc.) and largely on the animal and the way that it is managed. Some infectious agents (for example, the virus that causes foot and mouth disease) are so virulent that no matter how healthy and well fed the animal was before, it is very likely that it will develop foot and mouth disease if it is exposed to the foot and mouth disease virus.

Most infectious agents, however, are not so virulent. Exposure to small doses of most bacteria and viruses will not cause disease unless the animal is already weak (as a result of poor management or prolonged under-nutrition). Exposure to large doses of bacteria and viruses, however, will cause disease even in well-fed animals, but the severity of disease may be lower in well fed animals compared with underfed ones.

The severity of an infectious disease therefore depends on:

The dose of infection received

The state of health of the animal being infected.

While livestock keepers won’t be able to keep away all infection, they can try and manage their herd or flock in such a way that the size of infectious dose is kept low. They can also aim to feed and manage their livestock in such a way that the livestock’s abilities to fight off disease are as great as possible. Good feeding is a key part of keeping the animal’s balance towards health rather than disease.

The balance between health and disease, and the role that feeding and management can play in preventing disease, is illustrated in the figure below that uses pneumonia in goats as an example.

If does are well fed, then they will produce good amounts of colostrum (the first milk produced just after the kid is born). It is important that kids get a good feed of colostrum in the first six hours of life, because the colostrum contains a number of antibodies that will help the kid fight off infection in the first few weeks of life before it is able to make antibodies of its own. Goats that are well fed and provided with enough protein, vitamins and minerals will also be much better able to fight off infection than goats that are thin, weak and suffering from deficiencies of particular nutrients.

In addition to good feeding, though, good management of the goats' housing will also help to reduce the incidence of a disease like pneumonia. Wet bedding chills the goat, and wet bedding often has a high concentration of ammonia (from the goats’ urine) that damages the windpipe (trachea). Damage to the trachea means the goat is less able to prevent infectious agents that they breathe in getting into their lungs. Ensuring the goats have a dry bed will prevent chilling and reduce the amount of ammonia that they breathe in. Frequent mixing of the goats with goats from other flocks (by communal browsing or by frequent buying in of new goats) will also expose goats to a range of new infections for which they won’t have antibodies. It is probably not possible for a smallstock keeper to keep their goats away from everyone else’s, but if mixing can be reduced, exposure to disease will be reduced as well.

The dose of infectious agents that cause pneumonia will also be reduced if the shed in which the goats are kept is well ventilated. If the air in the shed is constantly being changed, this should avoid a build-up in the concentration of bacteria and viruses being breathed in by the goats. Poorly ventilated houses, however, mean that bacteria can stay, and multiply, in the air that is being breathed in by the goats so that the goats’ infection dose is much greater. The more goats there are, potentially the more sources of infection there are. If a large number of goats are kept together in a very confined space, then again the amount of infectious agent coming from other goats is much greater. Keeping stocking densities down (by decreasing flock size or increasing shed size) will help reduce the dose of infection being breathed in by any one goat. When an animal is sick, it should be isolated. This allows it to recover without the stress of competing with other goats for feed and water. It also removes its infection from the other goats so that they are less likely to catch the same disease.