Despite more research, sophisticated production systems, technologically advanced diagnostics, better nutrition, improved genetics, and, of course, smarter veterinarians, biology continues to humble the pork industry.
Today's disease complexes are likely related to changes induced both by producers and by pathogens.
Let's look first at pathogens. Front and center is porcine reproductive and respiratory syndrome (PRRS) virus, which continues to wreak havoc on the industry. Research has shown that it can increase the severity of other respiratory problems such as Streptococcus suis, Mycoplasmal pneumonia and possibly Haemophilus parasuis. PRRS has definitely been a major player in the porcine respiratory disease complex (PRDC) problem. In addition, this virus can change fairly readily and thus repeat its damage in herds immune to the original PRRS strain.
Porcine circovirus 2 (PCV2) is associated with postweaning multisystemic wasting syndrome (PMWS). Although experimental infections alone caused microscopic lesions and mild clinical signs, the severe clinical syndrome wasn't observed without exposure to parvovirus or PRRS. PCV2 is considered part of PRDC by some diagnosticians.
Then there is our old friend, swine influenza. Although this virus can cause disease alone, it can also be part of PRDC, and contributes to the rise in disease complexes with the recent appearance of new subtypes of flu. H3N2 is the most notable, but others are being diagnosed.
Improved diagnostics have enabled veterinarians to identify more viruses, bacteria and other microorganisms.
Some disease complexes may have been present all along — we've just defined them better with today's testing procedures.
Significant changes in production systems and pig flow by people have contributed. There are larger numbers of pigs on sites, often with a high percentage of the population naïve to several organisms. A few pigs in every group are likely to be carriers of pathogens. If shedding of (or outside exposure to) two or more infectious agents occurs close in time, it's an ideal setup for the disease complex.
With high breeding herd replacement rates and higher-health sow herds, maternal immunity may be lower than previous single-site, continuous-flow units. Separation of pig flow from sow herds means pigs are less likely to become exposed to infectious agents.
Genetic selection and nutrition programs encourage leanness, push for maximum pig performance and may contribute to disease.
Overall, the use of feed grade antibiotics in finishing pigs has been reduced. This reduces cost and residue concerns, but it may be a contributing factor to the disease complexes.
The first key to dealing with a disease complex is proper and complete diagnostics. This will usually entail postmortems and good sample collection. It may require multiple necropsies of different ages. Paired serology or a cross-section of different ages on the same day may be of some benefit. Routine serological monitoring may also have some merit.
Targeted vaccination programs may reduce one of the agents involved. Mycoplasma or flu vaccinations are good examples. Sow vaccination may be needed to reduce nursery or early finisher problems.
Elimination programs have been successful for removing PRRS. Besides depopulation, mass vaccination and uni-directional pig flow have helped eliminate PRRS from selected grow-finish sites. Pulse medication can reduce the impact of bacteria and mycoplasma.
When dealing with a syndrome such as PMWS, where we are still learning about the factors involved, and there is no vaccine for PCV2, experimentation may be needed.
A recent case of PMWS in a nursery was diagnosed by histopathology (tissue analysis). Pigs were being weaned offsite at 3 weeks old. Wasting increased to where mortality was 7-8%. PRRS was circulating in the nursery. The sow herd was vaccinated with a killed PRRS product, and the problem was occurring too early to vaccinate pigs for PRRS.
Therefore, based on some of the experimental data that showed more severe problems when parvovirus was involved with PCV2, the sow vaccination program with lepto-parvo-erysipelas was changed from weaning to pre-farrowing. Nursery signs declined and mortality is now 2-3%.
The case can be made that both pathogens and people have played a part in increased disease complexes and syndromes in today's pig populations.