Managing respiratory disease is one of the most common and most vexing problems that producers face. In swine, respiratory disease is almost always a multifactorial problem, with multiple infectious agents contributing to the clinical presentation. In the biological milieu of the upper respiratory tract and airways, environmental stressors and novel infectious agents change commensal or endemic organisms into pathogens.

It is this complicated interplay that we do not fully understand that creates the greatest challenge in the management of swine respiratory disease (SRD). The primary challenge in evidence-based management of SRD under commercial conditions is proper sick pig identification to allow intervention in a timely manner. Once this is achieved, applying the appropriate therapy for their state in the progression of the disease event is a straightforward task.

In the United States, the most common presentation of SRD involves a viral respiratory infection, typically porcine reproductive and respiratory syndrome (PRRS) or Influenza A virus, followed by a bacterial pneumonia due to commensal organisms of the respiratory tract such as Actinobacillus suis, Pasteurella multocidia or Haemophilus parasuis 3-4 days after the initial viral infection. Depending on the age of the pig, Mycoplasma pneumonia may also be involved in SRD. Figure 1 (on page 14) demonstrates the complex interplay among infectious agents, management and the host in SRD. As summarized in Figure 2, the clinical presentation often begins with a subtle change in attitude (depression), which is easily masked by the animal when they encounter humans in the pen due to a “flight-or-fight” response. This depression is followed 24-48 hours later by signs of inappetence (sunken sides, lack of “gut” fill) and a febrile response. All of this typically precedes any overt signs of respiratory distress. Often the fever starts to regress after the acute viral insult, and then rises again as the secondary bacterial pneumonia becomes established and more pronounced respiratory signs appear.

 

Figure 1: Swine respiratory disease in the pig is often a multiple-infection event with a viral infection and subsequent increase in macrophages as part of the innate immune response, followed by a wave of bacteria that grow in the damaged lung.

Typically, five to seven days after the initial onset of the viral infection, it is easy to observe weight loss (easy-to-observe backbone), and a mild to moderate, mixed broncho-interstitial bacterial pneumonia is present.

Interestingly, the amount of depression declines over the progression of the disease. All of this is summarized in Figure 3 (see page 18).

Early Disease Recognition Key

While we tend to think about “barns” of pigs, our ability to understand and teach this progression of SRD in individual pigs to our caregivers is critical to the success of our control strategies. Improving the ability of our caregivers to identify pigs with SRD early in the course of disease is the foundation for all control programs in growing pigs. It allows us to provide appropriate supportive and anti-infective care that minimizes the damage from the secondary bacterial infection.

Without this ability, our caregivers will delay not only intervening in individual animals with SRD and miss the opportunity to decrease suffering in the affected animal, but they will delay notifying the veterinarian of the change in health status and limit our ability to apply the most effective therapy for the population.

As an example of the rapid progression of disease, we used a computerized sound analysis program to “count” the number of coughs by pigs in a barn. During an outbreak of influenza, our computerized counting system detected an increase in coughs 24 hours prior to the caregivers reporting coughing and three days before there was a decrease in water consumption. 

To fully implement evidence-based approaches to antibiotic therapy in SRD, we need to do a better job of measuring outcomes of our interventions. In the U.S. cattle feeding industry, it is common to track case fatality rates for all cattle that are treated. This information has helped shift and mold decisions on how antibiotics are used.

More importantly, it has led to the development of objective measures of the progression of respiratory disease in the individual that is being treated. The use of rectal temperature and a standardized lung score can predict with a high degree of accuracy the case fatality rate in feedlot cattle. Considerable work is underway to validate the most appropriate antibiotics to use in each scenario.

Adapting these measurement techniques to swine medicine is the next step in implementing effective evidence-based medicine for SRD. 

Applying antibiotic therapy in an evidence-based manner is a matter of objectively understanding where the individual pig and the population of pigs are in the progression of disease. In general, all of the modern antibiotics achieve an acceptable level of in vitro (lab-based) sensitivity against the bacteria of concern in SRD. The challenge in applying antibiotics in an evidence-based manner is to use an antibiotic that meets the needs of the animal to have acceptable rates of clinical cure. This difference is due to the poor correlation between in vitro sensitivity and in vivo (in the animal) responses. When matching products that are easy to administer (single-dose or oral mass treatment) with the progression of disease, careful consideration is needed. 

In every case, early treatment produces better clinical cure rates than late treatment. While the selection of antibiotics can be important in determining clinical cure rates, the timing of the application of those antibiotics is more important. Once consolidation occurs in the lung tissue, the ability of antibiotics to penetrate the tissue and reach the “bug” are greatly reduced. For the antibiotic to be effective we must get the “drug to the bug.” The progression from mild airway disease that has caused little damage to the lung (bronchitis) to severe pneumonia is rapid: often over a period of 24-48 hours. Finding pigs prior to progression allows for higher clinical cure rates regardless of antibiotic, as drug-bug contact is easier to achieve and healing takes less time. 

Early in the clinical course of disease, antibiotics that have a long duration of effect and are excreted into the airway are of great benefit, as any bacterial infection is limited to the airway and the primary disease is viral.  Duration of therapy is important as the secondary bacterial infection has not peaked in intensity yet, and the antibiotic needs to be available when this happens (Figure 2, page 16). The use of the macrolide and triamilide antibiotics is indicated in this period of disease as they persist in the airways for a long period (>5 days) of time.

Figure 2: Swine respiratory disease in the pig is often a multiple-infection event with a viral infection and subsequent increase in macrophages as part of the innate immune response, followed by a wave of bacteria that grow in the damaged lung.

Conversely, later in the progression of disease, when the bacterial component of SRD is more established and the disease now involves lung tissue consolidation and the airways, antibiotics which have the highest concentration in the airways (macrolides) will produce a less satisfactory clinical outcome. In this case, antibiotics that have high tissue concentrations (β lactams, floroquinolones, floranfenicols, pleuromutilins, tetracyclines) will be more effective than the macrolides due to their tissue distribution. 

Duration of therapy is another challenge in the more progressed cases of SRD. While concentration-dependent antibiotics (fluoroquinolones) can be very effective in individual isolated animals, they present a clinical challenge in animals that are re-exposed to the agents of SRD before they have had a chance to develop a complete immune response and the lung has healed. In large populations where re-exposure to the agents of SRD is common, better clinical outcomes are often achieved when antibiotics with long duration of persistence in the animal, multiple-dose therapies or combinations of concentration-dependent ones followed by oral or multidose therapies are used. It is perfectly rational to use short-duration, injectable antibiotics in individuals and oral, long-duration therapy in the population to match antibiotic availability in the target tissue with the course of disease.

Managing sick pigs with respiratory disease requires careful observation of individuals, outstanding daily care and proactive use of interventions. Caregivers are the linchpin in making this process successful.

Leaders in production organizations need to appreciate the demands on caregivers and help them understand how disease progresses in individual pigs. Working as a care team, the caregivers, leaders and veterinarians can improve the well-being and performance of pigs with respiratory disease by addressing the multiple factors that lead to disease in pigs and outbreaks in populations.    

Figure 3: The host response to the viral and bacterial infection leads to behavioral changes that subsequently lead to weight loss. These changes are mediated by the innate immune response. Most importantly, the physical changes (weight loss) occur days after the behavioral changes, and often after the primary infection has started to resolve.