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Respiratory Diseases and Control Strategies

See also chapter 3: Segregated weaning.

(448) Of all the diseases that affect growing and finishing pigs, chronic respiratory disease is the most economically important. It is extremely common and can be difficult to prevent and control. Growth rates and feed-intake are depressed together with poor feed efficiency and some herds there is heavy mortality. The control of respiratory disease requires an understanding of the complexities and interaction between the organisms that are present, the pig and the management of the environment.

The prevalence of respiratory disease is affected by the following:

  • The presence of respiratory pathogenic organisms.
  • The virulence of the pathogens present.
  • The level of the pathogens in the house environment.
  • The immunity of the pig and the time of exposure to the organisms.
  • The presence of secondary opportunistic bacteria.
  • The interactions between management, environment, the diseases and the pig.

The presence of respiratory pathogenic organisms

The infectious agents that damage the respiratory system of the pig and the type of disease they cause are shown in Fig.9-29.

The severity of the disease will depend in part upon the number of infective agents present and the weights of infection that challenge the pig. PRCV is a relatively mild almost non clinical infection.

PRRS likewise on its own has little clinical effect on the respiratory systems, but if it is combined with swine influenza severe respiratory disease will be experienced.

Under good husbandry conditions enzootic pneumonia caused by Mycoplasma hyopneumoniae on its own is usually a relatively mild disease, but when complexed with PRRS, SI or Pasteurella multocida or combinations, serious chronic disease syndromes can develop.

Of the bacterial infections there are at least 12 serotypes (serovars) of Actinobacillus pleuropneumoniae but disease is dependent in part upon the virulence of the strains present, some causing no problems, others causing severe disease. Virulent serotypes vary from country to country. Individual strains within serotypes also vary. The chronic respiratory disease syndrome is summarised in Fig.9-30. The greater the variety of virulent organisms in the population of pigs the more complex the picture. It is helpful in investigating a problem to determine which of these organisms are present.


See chapter 3 for further information.

The piglet at birth receives antibodies and immunity via the colostrum. Not only do these antibodies provide protection against enteric diseases but also against all other infections endemic on the farm. These levels of passive or maternal immunity reach their maximum in the piglet a few hours after birth and they then gradually disappear over the next 3 to 14 weeks. (Fig.9-31). The actual time of this disappearance differs from one disease to another, a vital piece of information that is used to manipulate the management of the system and control disease.

When investigating a respiratory disease problem it is essential to identify the point at which disease first appears, because this highlights where management changes could reduce the effects of disease.

This biological fact can be used to control not only respiratory diseases but also many others because if the piglet is removed from the sow at weaning to a totally clean environment the disease cycle may be broken. This is now adopted as the principle of segregated early weaning. See chapter 3. Fig.9-32 suggest the ages at which piglets would have to be weaned from the sow into isolated accommodation to prevent them becoming infected with different respiratory diseases. However in some herds these ages may be less because of the herd size and increased numbers of gilts which might be actively excreting organisms. Fig.9-33 also illustrates the similar ages for enteric disease for sake of completeness. The ages given assume that the source herd contains only breeding females and sucking piglets.

Management control and prevention

Fig.9-34 shows the options that are available to control respiratory and other diseases. Obviously if the organism is not present on the farm there will be no disease. Disease freedom can be maintained at a national, country or at a farm level and it is therefore an important part of the control system.

Freedom from disease

  • Whilst the piglet is protected by maternal antibody it will not become affected with many of the infectious diseases endemic in the herd.
  • At a national level major diseases such as FMD, ASF, CSF, SVD, Aujeszky's (PR) and Brucella suis, may be eradicated either by slaughter policies or testing and slaughter. Some countries declare freedom from some diseases, for example, Ireland is free from TGE and PED. The UK and parts of the EU from aujeszky's disease and Brucella suis.
  • At a farm level herds can be established and maintained free of App, AD, PAR, EP and PRRS but in dense pig areas breakdowns are common.
  • All respiratory diseases can be successfully eradicated by total depopulation of the herd and its replacement by disease free stock. (See chapters 2 and 3). In practice the siting of the herd relative to sources of infected pigs needs careful consideration. PRCV and SI will at some stage re-infect the herd. EP and PRRS will be transmitted if infected pigs are within 2 to 3km (1 to 2 miles) and in such cases it may be advisable to restock with exposed animals and vaccinate them.
  • PRRS can be controlled by selective depopulation of first and second stage nurseries provided the disease is stabilised in the breeding and finisher herds.
  • Pseudorabies or aujeszky's disease can be successfully eradicated through the use of gene deleted vaccines and testing for field virus carrier pigs and slaughtering them.
  • By using the techniques of medicated early weaning a new disease free population can be established on another site, from an existing diseased herd.
  • Atrophic rhinitis can be eliminated by vaccination and segregated diseased control.
Control on the farm
  • Medication and vaccination - Most if not all farms have at least one or more respiratory organisms present that require continual control procedures. The use of medication strategies for respiratory disease have been dealt with in chapter 4 and you are referred to this for further information. The use of vaccines can be limited by cost, the practical problems of administration to every pig and often two doses are required. However vaccine programmes to control AD, EP and AR have given excellent results in the field. The use of PRRS and SI vaccines are more problematical. Some vaccines are effective through the sow alone to control disease and the overall strategies that can be adopted are shown in Fig.9-35.

Management - This involves the manipulation of the environment, the pig and nutrition, giving attention to detail and identifying the important areas and making adjustments.

  • Identify the time of commencement of the disease. This is absolutely crucial to understanding the problem on the farm. If the onset of disease is identified and the incubation period calculated, then the time the pig first meets disease and the environmental factors contributing can be determined.
  • Keep the pig above its lower critical temperature. Creating an environment where the pig becomes catabolic (using more energy than it is consuming) is the ideal method to trigger off respiratory disease. The LCT is commonly breached by faulty ventilation systems, a change in feed from one house to another or a change in stocking density. Draughts, shortage of water, wet floors, dirty pens, low stocking densities will have a similar effect. Look at the weight of the pig entering the house. Is it heavy enough to cope with the environment, the type of housing and floor surfaces.
  • Assess the lying patterns of the pigs early in the morning and last thing at night.
  • Avoid rapid changes in the environment in the house. This is particularly important where high speed ventilation systems may cause chilling. The pig will experience similar changes whenever it is moved from one house to another particularly if this is associated with a change in floor type, from a solid to a slatted floor. Contrary to popular opinion a high air flow will have little effect on the transfer of organisms between the nose of one pig and another.
  • A floor space < 0.5 sq.m. per pig is associated with more pneumonia. Allow 0.7 sq.m. of total area per pig.
  • Pneumonia levels are less in environments that provide more than 3.6 cu.m. of air space per pig.
  • Bacterial levels above 100,000 per cu.m. will predispose to pneumonia.
  • Keep faeces contamination in pens at a low level because gut organisms produce endotoxins that can damage lung tissue and predispose to pneumonia.
  • Avoid low temperature and low humidity environments, because the particles in the air are very small and remain suspended. Such particles are inhaled to the very bottom of the respiratory tract where the organism can attack the lung tissues.
  • Maintain either a high temperature low humidity, or low temperature high humidity environment. Here the particles of water vapour are large, they attract the organisms from the air and prevent them penetrating the deep parts of the lung.
  • As dust levels increase in the house so does the severity of both pneumonia and rhinitis. Dust levels of more than 4mg/cu.m.start to have an effect on the lungs.
  • There is less pneumonia in extraction type ventilation systems than in ones that create positive pressure.
  • Natural ventilation, within large cubic capacity air spaces, is ideal, because the organisms rise away from the nose of the pig by natural convection.
  • Adapt all-in all-out housing systems that accommodate each week's production. This prevents the development of endemic disease.
  • After houses have been washed out use a space heater to bring the temperature back to normal. This is essential for pigs from 7 to 14 weeks of age. It may be necessary in certain climates to provide heat for 7-10 days when pigs move from a solid to a slatted floor and when there is a change from dry to wet feeding.
  • Keep -ammonia levels at less than 25ppm, carbon monoxide at less than 30ppm, hydrogen sulphide at less than 5ppm and carbon dioxide at less than 2000ppm
  • Fogging - this has been used in an effort to control respiratory disease within a populated house but the results are not convincing. Fogging with water periodically will increase the humidity but it also has the disadvantage of cooling the pig by evaporation. It requires fogging at least every 5 to 10 minutes to maintain sufficient water vapour in the house.
  • Make sure health control procedures are in operation that eliminate the risk of introducing new pathogens into your herd.
  • Avoid purchasing pigs from multiple sources, particularly finisher pigs.
  • The larger the herd the greater is the problem.
  • The more pig movement there is around the farm the more likelihood there will be of pneumonia.
  • As stocking density increases there will be a greater incidence of pneumonia and this is certainly true once more than 200 pigs occupy a common air space.
  • Avoid continual throughput and the mixing of different ages of pigs. The best way to maintain pneumonia on the farm is to mix pigs into a house that contains older ones that have already incubated disease and are passing out large amounts of infectious agents.
  • Study the complexity of organisms on the farm in relation to the time when pneumonia occurs. This can be a sequential phenomena, that is, three or four diseases affecting the pig over a range of 4 to 12 weeks of age.
  • Does the housing satisfy the environmental requirements of the pig at the different ages and weights?
  • Identify the time when disease first becomes evident.
  • Note when nutritional changes take place. When pigs move from one house to another there is often a drop in the nutrient density of the diet. This also coincides with a drop in intake for the first 2 to 7 days.
  • This reduced feed intake and change can result in catabolism and the pig dropping below the LCT.
  • Maintain high energy diets at critical times. Do not change the feed for at least 5 days when pigs move from one house to another.
  • Provide adequate water at all times. Look at the type of nipple drinkers, their availability and accessibility to the pig, particularly when it moves from one house to another.
  • Look at the methods of feeding. Are there any features of design or feeder placement that inhibit access to the feed.
  • Is there sufficient hopper space?
  • Make sure there are adequate levels of vitamin E in the diet. Add an extra 50-100iu to the tonne if there is a disease problem.
Segregated weaning and segregated disease control
  • When pigs are moved from one building to another already containing pigs they are exposed to a range of viruses and bacteria many which they may not have encountered before. Some of these may produce clinical disease and others may result in sub-clinical infection. The end result is a large uptake of protein and energy diverted to satisfying the demands of the immune system. This significantly depresses daily gain and food conversion efficiency. Segregated weaning to all-in all-out buildings on a week by week basis, eliminates or reduces such a challenge. The improved performance is likely to far exceed that of weaning into a continuously operated nursery containing different ages of pigs but in addition it is of great benefit in the control of respiratory disease. These procedures are discussed in detail in chapter 3.
The costs of respiratory disease are proportional to the numbers of infectious agents present on the farm, the management and environmental conditions that allow endemic disease to develop and high levels of exposure to susceptible pigs.

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