In an ideal world if the pig herd could be established and maintained completely free of all pathogenic organisms then from a disease view point there would be few limitations to maximising production. Unfortunately we do not live in such environments and because of factors often out of the control of the farm, there will always be a variety of pathogenic or potentially pathogenic organisms present.

The purchased pig is the most important potential source of new infections and whilst the donor herd may be well monitored to determine its health status, an infectious disease could be incubating at the time of purchase.

The situation is further complicated by the variety of terms that have been used to describe the perceived health status of a herd. They are imprecise and open to wide interpretation. There are five categories of health status.

1. Germ free (axenic) - This is a pig which is thought to be totally free from infection with micro-organisms. Such pigs are produced by surgery (hysterotomy or hysterectomy) carried out on the pregnant sow near term. They are reared under completely sterile conditions usually in small containers enclosed in plastic balloons with filtered air and sterilised food and water. They can usually only be reared to a maximum of about six weeks because they get too big and unmanageable to maintain. They are for research purposes only. In fact, it is now realised that a germ free state is probably impossible to achieve. It is known that at least one retrovirus, which is inserted in the genetic DNA of every cell in the pigs' body, moves to the off spring from the parents. There are probably other as yet unknown viruses that behave similarly. Several viruses, for example, inclusion body rhinitis virus, although not inserted in the pigs' genes, also pass from mother to foetus before birth.

2. Gnotobiotic - This term means "known life". It describes a pig which has been produced and reared initially as a germ free pig but while continuing to be barrier-maintained is then deliberately infected with known micro-organisms. Its micro flora is thus clearly known and defined. Again they can only be reared to about six weeks when retrovirus and other unknown viruses may be present.

3. Specific pathogen free (SPF) - This term can be used to describe a pig or pigs, a herd or a disease control programme. It means that the herds are believed free from a short list of specified pathogens. Primary SPF piglets are usually produced in a similar way to germ free piglets and may be reared in isolators for about two weeks. During this time they may be given a probiotic flora (i.e. bacteria such as non pathogenic lactobacillus bacteria and streptococci which are meant to repress the growth of E. coli). They may also be given sterilised colostrum supplement. They are then removed from the isolator and placed in a very clean room under hygienic conditions. They become slowly contaminated with a simple bacterial flora derived from the attendants, dust in the air, and bacteria in their food and water but they remain free from specified pathogens (i.e. SPF). They can be used for research purposes or reared on to maturity and bred as the foundation stock of a new primary SPF herd.

4. Secondary SPF pigs are piglets born to the primary SPF mothers and secondary SPF herds are herds set up with secondary SPF breeding stock. Such programmes are run by SPF associations of which the largest is in Denmark with somewhat smaller ones in Switzerland and the USA. The associations draw up strict disease control regulations for their members to follow and regularly carry out a series of laboratory tests (mainly serological) to test for the presence or absence of specified pathogens. These usually include toxigenic Pasteurella multocidia (atrophic rhinitis), Mycoplasma hyopneumoniae (enzootic pneumonia - EP), certain serotypes of Actinobacillus pleuropneumoniae, Brachyspira hyodysenteriae (swine dysentery), Sarcoptes scabiei (mange) and lice. The herd breakdown rate with EP is usually high and there are also breakdowns with Actinobacillus pleuropneumoniae but at a lower rate. The other diseases are generally more consistently kept out.

5. Minimal disease (MD) - This term was ill defined and often became confused with SPF. There was an expectation by the pig farmer that there would be no more disease and the term also has negative connotations. This term was introduced in the early days of the development of multi-herd breeding companies to escape from the proscriptive rigidity of the SPF approach, which was found too inflexible for expanding international pyramids. It was realised in such pyramids that it was impossible to standardise health across all the herds using SPF and not necessary to do so. Furthermore it was realised that at a commercial level herds required compatibility of clinical freedom from disease rather than specified organisms per se. For example, Actinobacillus pleuropneumoniae comprises different strains varying in their virulence, some highly pathogenic and some non-pathogenic. It is an advantage for a commercial herd to contain non-pathogenic strains because they provide a level of immunity.

High health status (HHS)
Because of this and the problem with the term "minimal disease", the author proposed in the early 1970s a more positive term "High Health Status". This term has been widely taken up by pig breeders in the UK and North America and "Minimal Disease" has been largely discarded. It avoids some of the problems of minimal disease and gives a better indication of what the breeders are trying to achieve and what the commercial producer wants, but it is still imprecise and difficult to produce a specific definition and common understanding. The author therefore qualified it by adding "defined" i.e. "Defined High Health Status"(DHHS).

Defined high health status (DHHS)
This describes "a herd of recognised health status through the absence of the major infectious diseases". The diseases thought to be present or absent would be defined for that particular herd by the consulting veterinarian based upon clinical history and the results of pathological tests. The object of this approach is to ensure that each herd provides evidence of the absence of given diseases. It is important to emphasise that the term does not imply that animals are free of any infectious agent but that the observations and tests for the disease were all negative. This is important in legal terms since it is impossible to guarantee freedom from an organism or disease because at any one point in time the herd could unknowingly be incubating it. Furthermore an organism may be present, for example App that was non pathogenic i.e. the herd infected but no disease. It is also important that no antibacterial medicines are used routinely that would mask a virulent infection.

Using the term DHHS therefore, it becomes possible to describe the precise health profile of a particular herd with periodic veterinary documentation to that effect. It would be expected that most of the major infectious diseases would be absent under this definition and in Fig.2-20 an example of such a declaration for one farm is shown. This should be used for reference purposes and at a veterinary level only, where health compatibility can be discerned and assessed relative to the requirements of the recipient herd.

Such a declaration would be made by a veterinarian following clinical examination of the herd and receiving the results of any pathological tests. Examinations would be carried out every 2-3 months.

The history of the herd supported by other tests may allow some of the above to be declared absent and added to the defined health status. An example would be a long term herd history of freedom from piglet dysentery caused by Clostridium perfringens type C infection. Similarly, the absence of any clinical evidence of exudative epidermitis over a long period of time could allow a declaration based upon these observations. This however could not be based on specific bacteriology. It is emphasised that the assessment of DHHS must always be carried out by a competent pig veterinarian so that the information and criteria upon which a declaration is made accords with current accepted practices.