Author: Mark White BVSc LLB DPM MRCVS
Published: 2002

Disease caused by infectious agents (bacteria, viruses etc) is the result of an imbalance between the level of challenge and the degree of resistance or immunity that the pig may have.

The ability of the pig to resist infection will be influenced by a wide range of factors such as its age, its environment, its nutritional state, previous exposure to infection and its genotype.  However, in all cases, the level of challenge will influence the severity of disease in the animal.

If challenged for the first time by a novel primary infectious agent e.g. PRRS virus, there is a minimum number of viral particles needed to establish infection.  This is called the "Minimum Infective Dose".  Provided this is exceeded, the organism will colonise the pig, replicate in the body and then be able to go on and cause disease, unless the immune system can fight it off.  Clearly, the higher the level of challenge, the quicker the virus will reach a level in the body that will overwhelm the immune system.

In a similar way, organisms that are present within the environment of the pig, to which it may have some degree of immunity as a result of maternal colostrum or earlier low level challenge, can go on to produce disease if a higher challenge is met, i.e. the system becomes over-run.

Thus, it can be seen that all infectious disease is one of a "numbers game".  The higher the level of challenge, the more likely disease will result and the more severe it will be.

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Sources of Infection

1) Other Pigs.  The 2 principle groups of pigs that form a source of infection for others are:-

a) Older pigs

b) Sick pigs

Following replication of the organism in the body, there is a period of excretion.  The greater the initial challenge, the more the replication and, hence, level of excretion.  Sick pigs in particular act as "bug factories".

2) The Environment.  Excreted organisms build up in the environment and, in some cases, (e.g. bacteria) actually replicate in the environment if conditions are suitably warm and wet.

Given these basic facts, it is easy to understand why disease control recommendations are made:-

1) Operate all in all out system with pigs of similar age kept together.

2) Never move pigs backwards in the system.

3) Always remove sick pigs to a hospital area separated by air and dunging space from the mainstream system.

4) Regularly empty pens/buildings clean and disinfect.

5) Avoid scrape through systems.

6) Maintain vaccine programmes to specific agents.

7) Provide clean air, clean food and clean water.

A few comments about disinfectants are pertinent:-

1) If a disinfectant is claimed to be "harmless", it probably won't work!

2) Disinfectants are formulated to work at specified dilution rates, which are declared on the label.  For general use, always operate at the "General Orders" rate of dilution and measure out accurately.  Too weak, it will not work; too strong is wasteful and may damage the environment or pigs subsequently entering.  The standard "Glug in a bucket" is not good enough.

3) Allow disinfectants time to work - they are not instant.

4) Never try to disinfect a dirty area - most disinfectants are inactivated - to a greater or lesser extent - by the presence of organic matter.

5) Disinfectant efficacy is influenced by temperature - the warmer the environment, the better the effect.  In some cases, disinfectants cease to work at very low temperatures - check with your supplier.


NADIS hopes that you have found the information in the article useful. Now test your knowledge by enrolling and trying the quiz. You will receive an animal health certificate for this subject if you attain the required standard.

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