Porcine respiratory disease complex is a general term for respiratory diseases. Control efforts should comprehensively address four areas: biosecurity, stress reduction, appropriate immunization, and medication protocols.
Porcine respiratory disease complex (PRDC) is a general term for respiratory diseases in nursery and growing-finishing pigs caused by the interaction of multiple pathogenic agents, including various viruses and bacteria, as well as environmental stress and many other factors. Porcine respiratory disease complex continues to affect herd health, leading to increased mortality and culling rates in affected herds, reduced growth efficiency, and higher production costs, causing serious harm and economic losses to the pig industry. To better address the challenges that porcine respiratory disease complex poses for pig farms, this article focuses on its causes, disease characteristics, harmful effects, and main control measures.
1, Causes Associated with Porcine Respiratory Disease Complex
The causes of porcine respiratory disease complex are highly complex. Major contributing factors include mixed infections involving multiple pathogenic agents, production management, environmental stress, and many other factors. The interaction of these factors leads to the occurrence and progression of porcine respiratory disease complex.
1.1 Infectious factors
Respiratory disease complex is usually caused by the combined action of several related pathogens (Table 1) (Hansen et al. 2010; Palzer et al. 2008); primary pathogens are mainly viral pathogens and Mycoplasma hyopneumoniae. Primary pathogens can reduce local or systemic defenses in pigs, resulting in secondary infections by various bacteria. Mixed infection with multiple pathogens is a typical feature of porcine respiratory disease complex.
Table 1 Related pathogens confirmed to cause respiratory disease complex in pig herds
Porcine reproductive and respiratory syndrome virus is an immunosuppressive virus. Infection can cause viremia and damage pulmonary macrophages, resulting in severe immunosuppression that promotes infection by various pathogens, including Mycoplasma hyopneumoniae and Actinobacillus pleuropneumoniae. Coinfection with the latter can cause sudden death in a herd.
Mycoplasma hyopneumoniae is also a primary pathogen of porcine respiratory disease complex. It can impair the respiratory tract’s local ability to clear pathogens, increasing pigs’ susceptibility to other pathogens. According to studies, Mycoplasma hyopneumoniae makes porcine reproductive and respiratory syndrome more complex and increases the severity of porcine circovirus type II infection.
1.2 Production Management
The occurrence of porcine respiratory disease complex is closely related to the quality of herd husbandry and management. Poor health among introduced replacement pigs and inadequate isolation and acclimation can cause respiratory disease complex to persist throughout the herd. High stocking density, stress from moving or mixing pigs, and production systems that do not use all-in/all-out management may all contribute to the occurrence of respiratory disease complex.
1.3 Environmental stress factors
The occurrence of porcine respiratory disease complex is closely related to changes in temperature. Based on experience, the incidence of porcine respiratory disease complex is higher during seasons with lower temperatures and frequent temperature fluctuations. Improper environmental management of pig housing is also one of the important factors that can trigger respiratory disease complex. Improper pig housing design and management of ventilation facilities can result in poor ventilation. Excessively high concentrations of ammonia, dust, and microorganisms in pig house air are harmful to respiratory health. For example, when the ammonia concentration reaches 50~100mg/mL, it can seriously interfere with the normal function of the respiratory mucosa and trigger respiratory disease complex.
In addition to the above causes, factors such as genetics, the herd immunization program, nutritional status, and other diseases (such as intestinal diseases) may reduce pigs’ ability to resist disease and trigger the occurrence and spread of respiratory diseases in the herd.
2 Characteristics of the combined incidence of porcine respiratory diseases
Due to the complexity of the pathogens involved, porcine respiratory disease complex (PRDC) typically presents with a range of clinical signs and pathological changes, but it can generally be associated with specific ages and stages of onset.
2.1 Clinical signs of porcine respiratory disease complex
Clinical signs in affected pigs typically fall into three categories: acute, subacute, and chronic. Acute cases may involve sudden death. In cases with a slightly longer course and in subacute cases, signs include elevated body temperature, depression, and a marked decrease in feed intake. Common signs include difficulty breathing, often with abdominal breathing, rapid and labored breathing, coughing, nasal discharge, and increased eye discharge. After progressing to chronic cases, affected pigs may show coughing and labored breathing, weight loss, slow growth, pale skin, and a gradual decline in uniformity within the herd (Figure 1).
Figure 1 Typical respiratory signs in growing-finishing pigs at 90 days of age
Figure 2 Clinical signs of pleuritis (adhesions between the lungs and chest wall)
2.2 Pathological features of porcine respiratory disease complex
Respiratory lesions caused by porcine respiratory disease complex can be pathologically classified into three types: rhinitis, pneumonia, and pleuritis. The characteristic pathological change in rhinitis is deformity and atrophy of the nasal turbinates. Depending on the pathogens involved, pneumonia can be classified as bronchopneumonia, interstitial pneumonia, and embolic pneumonia. Typical pathological changes in pleuritis often include large amounts of fluid or fibrinous exudate in the thoracic cavity; as the disease progresses, adhesions between the chest wall and lungs are often seen.
2.3 Age of onset and stages of porcine respiratory disease complex
According to the decline patterns of maternal antibodies and antibodies induced by relevant vaccines in pig herds, porcine respiratory disease complex commonly occurs during late nursery and grower-finisher stages, particularly at 13~20 weeks of age, a period colloquially known as the 13-week wall.
3 Measures to prevent and control porcine respiratory disease complex
With the ongoing development of intensive farming, enclosed farming environments and excessively high stocking densities have led to the widespread occurrence of porcine respiratory disease complex. The high mortality rate, low weight gain, increased feed-to-meat ratio, and increased costs of vaccines and medications associated with the complex have caused serious economic losses to the pig industry (Sales et al.2010). Therefore, porcine respiratory disease complex is widely recognized as one of the most serious problems facing modern pig farming. To better reduce the impact of porcine respiratory disease complex on the profitability of pig farming, the following prevention and control measures are proposed for pig industry professionals to consider applying.
3.1 Implement strict biosecurity measures on pig farms
Introduce healthy replacement breeding pigs, while strengthening isolation and acclimation management for replacement breeding pigs; establish sound biosecurity facilities and biosecurity management procedures that are easy to implement, and enforce them strictly.
3.2 Strengthen pig herd production management and reduce stress in the herd
Pig farms must provide the herd with an environment suitable for growth and survival and reduce various forms of stress to lessen the impact of porcine respiratory disease complex. Improve pig-house ventilation and insulation facilities, and balance insulation and ventilation according to the farm’s actual conditions and seasonal changes; ensure appropriate stocking density and reasonable grouping. To reduce the circulation and spread of various pathogens within the farm, all-in/all-out management must be implemented wherever possible, and housing vacated by relocated pigs must be thoroughly cleaned and disinfected before being left empty.
3.3 Implement a reasonable pig herd vaccination program
Based on the disease situation on the farm, select suitable vaccines targeting the viral and bacterial pathogens that cause porcine respiratory disease complex, and develop a reasonable vaccination program. Also monitor vaccine antibody levels regularly and continuously to ensure the effectiveness of vaccination.
3.4Reasonable Medication Control Program
To reduce the impact of porcine respiratory disease complex on susceptible pig populations, it is essential to develop a reasonable program for medication-based prevention or treatment. Appropriate timing of medication and the selection of susceptible antibiotics are the basis for controlling the bacterial factors involved in porcine respiratory disease complex. To understand the impact of porcine respiratory disease complex on pigs raised in the finishing herd, and to assess the effectiveness of vaccination and medication control programs, farm veterinarians are advised to conduct regular slaughter inspections.