Question: How do I get an accurate diagnosis for the porcine respiratory disease complex?

Answer: The etiology of PRDC varies between and within production systems and over time within the same system. In most farms with PRDC problems, one or two viruses, Mycoplasma hyopneumoniae and several opportunistic bacteria work in combination to induce losses associated with respiratory disease. Here are 10 steps you and your veterinarian can take toward finding PRDC solutions.

Step 1: Get an accurate diagnosis.

It is difficult to correctly diagnose the specific causes of PRDC based on clinical signs and gross lesions alone. Because of this, you need access to the more sophisticated and confirmatory diagnostic tools available today.

Diagnostic laboratories have greatly improved conventional diagnostic tests such as virus isolation procedures and have adopted new technologies such as immunohistochemistry, in situ hybridization, and polymerase chain reaction.

Frequent post-mortem animal examinations and serological profiling of the herd are wise investments. They play an important role in determining the most cost-effective PRDC control strategies.

Step 2: Sort the opportunists from the primary pathogens.

From a clinical and diagnostic perspective, swine viral respiratory pathogens can be divided into the following three groups:

1. Well-established primary pathogens capable of inducing clinical disease and lesions. These include:

Porcine reproductive and respiratory syndrome virus.
Swine influenza virus.

Pseudorabies virus.

Probably porcine circovirus.

2. Opportunist pathogens. These viruses usually induce subclinical disease unless the pigs are immunocompromised or if bacteria or other viruses complicate the infection. This group includes porcine cytomegalovirus and porcine respiratory coronavirus.

3. Viral pathogens that are rarely associated with respiratory disease. This group includes paramyxovirus, encephalomyocarditis virus, hemagglutinating encephalomyelitis virus and adenovirus.

Bacterial pneumonia should be thought of in terms of management plus environment plus pathogens. Most of the bacteria that cause pneumonia are commensal of the upper respiratory tract or tonsil. Primary swine bacterial respiratory pathogens include:

Mycoplasma hyopneumoniae.

Actinobacillus pleuropneumoniae.

Bordetella bronchiseptica.

Some strains of Pasteurella multocida.

Septicemic causes of bacterial pneumonia include Salmonella choleraesuis, Actinobacillus suis and Arcanobacterium pyogenes.

Opportunistic bacterial respiratory pathogens include Streptococcus suis, Haemophilus parasuis, Mycoplasma hyorhinus and most strains of Pasteurella multocida.

The airway's branch design causes bacteria to impact the airway mucous layer. The bacteria's virulence factors and inoculum dose determine whether bacteria will clear effectively.

Bacteria are propelled up the mucociliary apparatus and expelled or are phagocytized in the lung or upper respiratory tract by macrophages or neutrophils. Damage to the mucociliary apparatus by viruses (SIV, PRV, circovirus, PRCV) or Mycoplasmas will enhance bacteria-induced respiratory disease.

Viruses such as PRRS may also destroy or decrease the function of pulmonary alveolar macrophages and pulmonary intravascular macrophages making pigs more susceptible to bacterial pneumonia and septicemia.

Step 3: Treat sick pigs early and aggressively.

This can minimize losses. The bacteria involved in PRDC are opportunists; however, those opportunists often are responsible for the animal's death or failure to recover from viral infections. The bacteria are what respond to antimicrobial treatments. These treatments can be injectable antibiotics, water medication or feed medication.

Selection of the drugs is based on antimicrobial susceptibility patterns from farm-specific isolates, pharmaceutical industry data, scientific literature, records from prior response to treatment, product cost, facilities, equipment and labor.

Step 4: Understand and monitor the herd's immune response.

This can be done by:

Monitoring sow-herd immunity and stability.

Emphasizing the importance of good husbandry and piglet colostrum intake.

Understanding passive antibody decay curves for specific diseases

Understanding the quality and proper use of vaccines.
Sow herds should be monitored serologically on a regular basis for PRRS, H1N1 and H3N2 SIV, PRV, Mycoplasma hyopneumoniae and Actinobacillus pleuropneumoniae.

Serology is pretty straightforward for PRV, both subtypes of SIV, Mycoplasma hyopneumoniae and Actinobacillus pleuropneumoniae. Assessing the PRRS status and predicting the future clinical course based on current serology results is much more controversial. Look for extremes in PRRS serology.

Husbandry in the neonatal period is key to realizing rewards for investments in acclimatization and vaccination protocols. If the newborn pig has adequate colostrum intake, the chances of suffering from PRDC are greatly reduced. Sound husbandry practices in the first few days of life will result in higher colostrum intake and less PRDC in the nursery and finishers.

Understanding passive antibody decay curves for the major swine viral respiratory diseases is essential to designing successful segregated early weaning programs to reduce the severity or eliminate respiratory disease problems. The challenge is to decrease the variability in the height and length of these curves in the population. This variability in passive antibody levels is a challenge when it comes to vaccination timing in growing pigs.

Sequential serologic profiles will let you characterize the passive antibody decay curves in herds with PRDC problems. You can then use that information to measure the efficacy of breeding herd immunization and neonatal husbandry, and more appropriately time vaccinations.

Step 5: Understand the epidemiology of the primary pathogens.

Through diagnostic surveys you can monitor over time where each infectious agent persists in the herd and when transmission occurs. You can do this for viruses and some bacteria by regular serological surveys of the breeding herd and growing pigs.

Cross-sectional surveys of the breeding herd are appropriate and are useful to understand disease transmission in growing pigs. The preferred serological surveying method in the breeding herd and growing pigs is sequential bleeding of the same animals over time. For growing pigs this begins at weaning and is repeated every three to five weeks until market.

This demonstrates the specific types and timing of viral, Mycoplasmal and selected bacterial infections. The time required for animals to seroconvert to various pathogens is well established in the literature. Practitioners can then evaluate the serological surveys and appropriately select and schedule the vaccination and treatment protocols.

Swine influenza is an example of disease that primarily manifested itself in grow/finish pigs but which is now less predictable. Most sows and sow herds were solidly immune to SIV, resulting in passive antibody protection for pigs that lasted for 10 or more weeks.

Now, epidemics and endemic infections with SIV are common in suckling and nursery pigs. This is likely due to increasing sow herd size, high gilt replacement rates, multi-sourcing of gilts, earlier weaning ages, wide weaning age ranges and multi-sourcing of nurseries.

Larger sow herds and high gilt replacement rates result in variable exposure to infectious agents in the breeding barn. This translates into variable levels of passive antibody passed on to pigs via colostrum.

The end result is subpopulations of young pigs that are susceptible to diseases, which in the past were not observed until much later in the production system. To address this, SIV vaccines are now commonly used in sow herds to build passive antibody levels that protect pigs until they are 12 to 18 weeks old.

Mycoplasmal pneumonia in the past also was uncommon before 12 to 14 weeks. Now it is common to diagnose Mycoplasma hyopneumoniae and opportunistic bacteria resulting in enzootic pneumonia in 5- to 8-week-old nursery pigs. Control of early-onset Mycoplasmal pneumonia also has moved to sow herd immunization, particularly incoming gilts.

One thing that has not changed – the PRRS virus continues to be a major problem in nursery pigs. That's because passive antibody protection acquired from the sow wanes by 2 to 5 weeks of age. Pigs become susceptible to PRRS during weaning, mixing and moving to the nurseries.

There is a downside to serological profiling when you use it as the only diagnostic tool and monitor only selected pathogens. Serology measures the development of an antibody response in the serum. Serology results demonstrate whether an animal has been exposed to certain pathogens at certain times. It does not determine if certain pathogens are responsible for disease at a certain point in time.

The best way to definitively determine the cause of specific clinical signs at various production stages is to euthanize and perform necropsies on pigs at different ages and stages of disease. These are known as herd cross-sectional necropsies or pathology investigations. By performing necropsies on several pigs on the same day you get a snapshot of how the infectious pathogens flow through the production system.

Because it is only a snapshot it needs to be repeated quarterly or semiannually. This is more effective than the standard approach of making decisions based on information from necropsies of pigs that have died from respiratory disease following unsuccessful treatment.

Evaluation of pathological lesions and recovery of the viruses or bacteria from the affected tissues will confirm the diagnosis.
The combination of regular serological surveys with cross-sectional necropsy exams provides a powerful diagnostic database that will provide a better understanding of respiratory disease epidemiology.

Step 6: Tap information generated by PRDC experimental models and SEW literature.

PRDC associated with PRRS/Mycoplasma hyopneumoniae co-infection is the most common respiratory problem in grow/finish pigs today. Iowa State University veterinary researchers have developed and studied a PRRS/Mycoplasma hyopneumoniae model. Somewhat to the surprise of many, it has been documented that Mycoplasma hyopneumoniae potentiates the severity and duration of PRRS-induced pneumonia. This model has provided an improved understanding of the pathogenesis of PRDC and the ability to test intervention strategies.

Step 7: Use proper biosecurity procedures.

The procedures include:

Carefully select the location of production units.

Know the health status of incoming, purchased breeding stock and pigs.

Obey strict all-in/all-out production rules.

Limit the number of pig sources.

Continually focus on biosecurity.

Incoming breeding stock should always be isolated. Test at least a subset, if not all, animals upon arrival and two to three weeks later.

Long-term PRDC control in growing pigs depends on sound biosecurity in the breeding herd and the verified high-health status of incoming breeding stock.

Step 8: Utilize the reliable vaccines available.

Good quality vaccines are available for SIV H1N1, SIV H3N2, PRV, Mycoplasma hyopneumoniae, Bordetella bronchiseptica and Pasteurella multocida.

If your herd suffers losses associated with these pathogens, consider using vaccines to minimize pathogen impact. PRRS continues to be the leading cause of respiratory disease in growing pigs despite widespread use of several PRRS vaccines. PRRS-associated pneumonia cases have increased nine-fold in the last six years based on submissions to the Iowa State University Veterinary Diagnostic Laboratory.

In some situations, such as PRRS and Strep. suis co-infection, it is more effective to identify and control the concurrent opportunistic infections than it is to vaccinate for PRRS.

Step 9: Assess ventilation, sanitation and other environmental stressors.

The animal's environment is just as important as the medical protocols. The presence of many respiratory diseases can be made subclinical if you provide proper ventilation and sanitation, and pigs are kept comfortable.

Step 10: Understand the restraints of each production system.

After appropriate diagnostics, and you have characterized the pathogens and infection sequence, compliance with proposed control strategies often depends on the restraints of a particular production system.

Examples of common restraints include:
Pig flow pressures.

Production contract stipulations.

Genetic sources.

Building design.

Availability of acclimatization facilities.

Personnel issues.

Getting an accurate diagnosis for porcine respiratory disease complex within your herd isn't impossible, but it's a long road with many twists and turns along the way.

Editor's note: Paul Halbur is a veterinary pathologist at Iowa State University in Ames, Iowa.

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