Porcine respiratory disease complex (PRDC) is a multi-faceted disease challenge. It is the result of synergistic and additive effects from concurrent or sequential infection with viral and bacterial pathogens. Often initiated by Mycoplasma hyopneumoniae, porcine reproductive and respiratory syndrome (PRRS) virus, swine influenza virus (SIV), or Aujeszky's virus, typically it is compounded by opportunistic invaders such as Pasteurella multocida, Streptococcus species, Haemophilus parasuis, Actinobacillus pleuropneumoniae (APP), circovirus type 2 (PCV2), and Bordetella bronchiseptica. Economic losses accrue from increased mortality, culls, treatment costs, poor feed conversion, and reduced average daily gain.
PRDC has long been recognised, but some of its components are gaining in prominence and the worldwide disease situation is becoming more complex. In North America, for example, PRRS has become well established, and post-weaning multisystemic wasting syndrome (PMWS) is increasing. In the United Kingdom, PMWS is well established and PRRS is increasing. In the rest of the world, both syndromes are on the rise.
As a quick reminder, PRRS in naïve herds can have various results. These include reduced farrowing rate; early farrowing with stillborn, mummified and weak live pigs; increased mortality in pre-weaning and nursery pigs; increased numbers of unthrifty weaned pigs; and elevated incidence of bacterial infection. PRRS in established herds is expressed more in its respiratory form in neonatal and growing pigs, with fever, respiratory distress, failure to thrive, decreased appetite and increased mortality. In adult pigs, North American strains can cause major reproductive disorders and abortion storms.
The PRRS virus is transmitted primarily by pig-to-pig contact via infected faeces, urine and milk. Transmission through needles and via insects such as mosquitoes and flies also is possible. When the syndrome first surfaces in a country or locale, the level of disease can be high enough that multiple routes of transmission may occur.
Looking now at PMWS, its cause is unknown, but the most likely agent is the PCV2 porcine circovirus consistently associated with lesions of affected pigs. It is antigenically distinct from the highly contagious but non-pathogenic PCV1. PMWS affects pigs between 4-16 weeks of age, approximately 1-2 weeks after weaning. Affected pigs appear quite different from their rapidly growing herd-mates; they seem to waste away, due to a cessation of growth and/or loss of weight. It is theorised that a co-factor may exist to bring on the clinical disease, since PCV2 is routinely isolated from healthy pigs.
Clinical signs include high fever, listlessness, rapid wasting, hairy coat and gaunt appearance. Some pigs may become pale and yellow and/or develop dyspnea, a slight cough, and/or slight diarrhoea, which may be associated with secondary invaders. All clinical signs are exacerbated with secondary infections. Generally, 25% of a pen is affected and mortality rates can exceed 25%. The disease will run its course through a group of pigs in approximately 3 weeks.
Solutions? It is clear that just as viruses mutate, disease syndromes evolve. What can the global animal health community do to intervene? Virtually any multi-factorial syndrome requires a multi-solution approach. In the case of PRDC, interventions are needed for both bacterial and viral components.
Antibacterial solutions: The international pig industry is fortunate to have approved antibiotics to combat proliferation of bacterial organisms. Antibiotics are a resource in limiting bacterial disease, whether primary or secondary. Products designed exclusively for use in animals and with low resistance profiles such as florfenicol are available worldwide in formulations for both treatment and prevention. They are viable tools for judicious use in controlling the bacterial factors in PRDC.
Antiviral solutions: Since viruses do not respond to antibiotic therapy, vaccination is the most common approach to control. Many viral vaccines are available, although strain variation and maternal antibody interference must be carefully considered. Vaccination programmes must be customised to specific herds depending on viral load, pig flow and labour availability.
Management solutions: For complete PRDC control, sound management is essential. The following list reviews management options for reducing the incidence. Note that it is not a blueprint for a whole-herd health programme and certain practices may not be feasible in some countries.
All-in/All-out (AIAO) production is the practice of moving pigs as groups or batches and cleaning facilities between groups. This allows for breaks in pig flow, which is critical for breaking the cycle of re-infection found on most farms. AIAO can be practised by room, by barn or even by site. Keeping groups or batches of pigs together throughout the production cycle also allows for better management of nutritional and environmental needs.
Segregated Early Weaning (SEW) and Medicated Early Weaning (MEW) are processes that can be employed alone or as part of AIAO production. Both SEW and MEW require a facility separate from the existing operation and breeding herd.
SEW is based on the assumption that pigs are relatively immune to disease during the first 3 weeks of life due to maternal transfer of antibodies. When that passive immunity wanes, however, the pigs may be susceptible to pathogens shed by the sow and other older pigs. By weaning the pigs before 18 days of age, they obtain the benefit of maternal immunity and are removed from the sow before she is likely to infect them.
MEW involves weaning and isolating pigs at approximately 5 days of age to protect them from bacterial pathogens endemic in the herd. Typically, sows are medicated with antibiotic when they enter the farrowing house and until their pigs are weaned, and the pigs are medicated from birth through approximately 10 days of age.
Water supplies need careful maintenance, because any interference with water intake will raise stress, limit feed intake and dehydrate the mucociliary escalator. A compromised mucociliary escalator in turn compromises a pig's ability to clear its respiratory tract, increasing its susceptibility to respiratory pathogens.
Feed quality is a high priority in maintaining high-health pigs. Dust in feedstuffs can overwhelm a pig's respiratory tract defences. In addition, moulds or mycotoxins on feed can be a direct insult to the immune system.
Air must be clean. On average, a pig weighing 25kg will breathe in 2 litres of air every minute. Any factors affecting air quality may therefore affect the pig's respiratory tract. For example, ammonia gases slow the mucociliary escalator while low humidity results in excessive dust particles entering the lung and humidity over 75% can facilitate growth of bacterial pathogens. However, draughts may be the most important air quality issue influencing a pig's ability to resist respiratory disease. Draughts force pigs to constantly adjust their metabolism in attempts to maintain a thermo-neutral body temperature. This interferes with sleep and diverts energy from the immune system. Fans, curtains, flooring and insulation should be monitored constantly to assure proper ventilation and air flow control.
Hygiene is important. Every effort should be made to assure sanitation. Although many pathogens are quite hardy, many will succumb to disinfectants or even to the simple mechanical removal of wastes such as by hosing. Enzootic disease is often the result of pathogen build-up and may require depopulation as a last-resort control measure.
Other basic recommendations in connection with the control of PRDC also apply across wider health issues. Diligent attention is required in the proper storage and use of medicines, allocation of floor space appropriate to the stocking rate, provision of clean and dry bedding, maintenance of floor surfaces to help prevent foot trauma and actions to control insects and rodents.