In breeding herds the ideal situation is to have no highly virulent strains present but only mild or avirulent strains which then naturally immunise the herd. A naive herd, (i.e. one that has never been immunised by any natural infection) is a potential time bomb. However in some countries this method of control i.e. a totally naive herd is being advocated. If a virulent strain gets in it will create havoc. Fairly effective vaccines are commercially available in most countries but they only immunise against homologous serotypes (i.e. the serotypes that are incorporated in the vaccine) and not against other serotypes. In contrast, natural infection tends to immunise against all serotypes.

In a breeding herd that is free from clinical disease (including absence of characteristic lesions in the lungs) it pays to try to keep virulent strains out. In pig disease areas where herds are close together and the level of infection is high, it may prove impossible to do so on a permanent basis. In more isolated herds it may be possible to maintain freedom from the disease for long periods (although even if extreme measures are adopted, breakdowns may occur, the sources of which are often unknown.

The following measures should be adopted:

• Prevent entry of virulent strains by checking that the herds which supply you with replacement breeding stock are screened on the basis of herd history, clinical inspections and absence of clinical signs and regular lung examinations at slaughter.
• Provide all visitors, including your veterinarian, with a hat, clean coveralls and boots and insist that they wear them.
• If you have a large valuable herd, install a shower and make all visitors wash their hair, hands and beard if they have one.
• Check that they have not come direct from another diseased herd.
• Build a loading bay in such a way that when lorries collect pigs the driver does not have to enter your building and you do not have to go on the lorry.
• Avoid loading your pigs onto lorries which already have pigs on board from other farms. All vehicles should be empty and disinfected before arrival.
• Hold incoming breeding pigs in isolation, segregated from your herd for a minimum of three and optimum of six weeks and not only inspect them daily, but check that the source herd is still healthy before you bring them into your herd.
• Some people advocate testing the pigs in isolation serologically or micro-biologically (i.e. collecting nasal swabs and culturing for the bacterium) but these may be counterproductive because of false positive results and because the laboratory cannot always tell you whether the pigs have virulent or avirulent strains.
• In grower-finisher units which purchase 25 to 30kg pigs from weaner producers it is difficult to maintain freedom from virulent strains of this organism unless you are purchasing pigs from a single known source or limited number of known sources. The practice of all-in all-out by building or preferably site may also help.
• The organisation of a multi-site system in which the three-week-old piglets are weaned immediately from the breeding sow site into an all-in all-out nursery before coming to the grower/finisher is also likely to result in freedom from this disease.
• Consider adopting SEW or SDC techniques.

• In an infected breeding herd the most likely time to get clinical disease is in pigs over 15kg. If it does occur consider the following:
• Vaccinate for EP and control PRRS.
• Consider routine vaccination of sows and/or incoming gilts with App vaccine.
• New vaccines are coming on the market periodically including a live attenuated genetically modified one which causes no disease but immunises pigs against its bacterial toxins.
• Operate all-in all-out, at least by room, rather than continuous throughput production.
• Avoid stress and overcrowding.
• Avoid rapid temperature fluctuations.
• Avoid low humidities and low temperatures.
• Try fogging to decrease the numbers of organisms in the air. 1% Virkon S can be of value.
• Increase the levels of vitamin E by 50-100g/tonne.
• Maintain good ventilation and a warm air flow.
• Keep pigs warm, dry and draught free.
• Provide a plentiful supply of easily obtainable water. Temporary water deprivation will trigger off disease.
• Consider strategic feed medication in advance of and during the likely time of onset of disease.
• Keep injectable antibiotics in a refrigerator ready for prompt treatment of sick pigs.

• Avoid introducing pigs from multiple sources.
• Do not mix pigs from herds with the disease and pigs from herds which are free from the disease.
• Practice all-in all-out and not continuous flow.
• Consider prophylactic medication for a period after entry.
• Assess the results of vaccination.

• Large airborne particles >10µm are retained in nasal passages.
• Particles of 0.5 - 3µm penetrate deep into lung tissue. (Bacteria, App and mycoplasma)
• Low temperatures and high humidity produce large droplets that sediment quickly with less exposure.
• High temperatures and low humidity produce small droplets that sediment quickly with less exposure.
• Low temperatures and low humidity produce small droplets that stay airborne. A dangerous environment.