Swine Actinobacillus pleuropneumoniae on the Rise
Iowa State University Veterinary Diagnostic Laboratory has seen an increase in Actinobacillus pleuropneumoniae (APP) cases in the past several months, and multiple practitioners in the Upper Midwest have reported increasing clinical outbreaks of the disease. Eight farms in the Oklahoma panhandle have experienced outbreaks since December 10, 2021, involving pigs from multiple sow sources despite the fact that the sow herds weren’t APP positive. Seven farms near Iowa Falls have had lateral APP outbreaks since November 2021, affecting late finishing pigs. These incidents led the Swine Health Information Center and the American Association of Swine Veterinarians to host a free webinar on APP, because the outbreaks have resulted in increased morbidity and mortality in production systems where the disease was previously well controlled. The goal was to provide information about the disease and discuss recommended management strategies.
Background and etiology
APP causes severe pleuropneumonia in pigs, which is highly contagious and typically characterized by sudden onset, a short clinical course, and high morbidity and mortality. The pathogen was first reported in the United States in 1957, and outbreaks increased as swine production was industrialized. Currently, APP is found throughout countries where swine are raised, and the disease is common in the United States, Canada, and Asia. APP can affect pigs in all age brackets, but the disease is most common in 6- to 20-week-old animals. Outbreaks are usually seen from late fall to early spring when temperatures fluctuate greatly or during severe weather conditions.
Actinobacillus pleuropneumoniae is a hemolytic, Gram negative, capsulated coccobacillary rod, which is highly host specific for swine. APP has at least 15 serotypes that are commonly recognized, and the most common ones identified in the United States are 1, 3, 5, and 7. The APP organism secretes exotoxins, called RTX toxins, which are cytotoxic, hemolytic, or both, depending on the serotype. APP can survive in discharge, such as nasal secretions, but will die quickly if dried.
Transmission and pathogenesis
Transmission usually occurs through direct contact with infected nasal secretions, but aerosol transmission can occur over short distances. Pigs who survive APP often remain carriers, and they also can be subclinically infected carriers, having occasional mild disease outbreaks. The organism can spread via fomites, such as boots and clothing, but these items are only infectious for a short period of time. Situations that facilitate transmission or precipitate outbreaks include overcrowding, inadequate ventilation, unusual stress, and infection with other respiratory pathogens.
The incubation period is 12 to 36 hours, and APP quickly colonizes the affected animal by first invading the epithelial cells in the tonsils and then proceeding to the lower respiratory tract. Toxins released by APP damage endothelial cells and pulmonary alveolar macrophages, causing extensive vasculitis throughout the lungs, which is followed by thrombosis and infarctions in the lung tissue.
Clinical signs
Clinical signs vary depending on whether the disease is acute or chronic APP. In acute APP, sudden deaths are common, and early signs include high fever, lethargy, anorexia, stiffness, vomiting, diarrhea, and prostration. As the disease progresses, the pigs exhibit marked dyspnea, open-mouth breathing, and a foamy, bloody discharge from the nose and mouth. Extremities become cyanotic in the early stages, and this progresses to generalized cyanosis as circulation fails. Pigs who survive the acute stage typically will have a chronic cough and slow growth caused by pleural adhesions and lung abscesses.
Diagnosis
APP is suspected when a farm has a sudden onset of an acute, rapidly spreading respiratory disease causing high morbidity and mortality. Other diagnostic measures include:
- Necropsy — The thoracic cavity of pigs infected with APP usually contains blood-tinged fluid. One or both diaphragmatic lobes are typically hemorrhagic and necrotic. Dark red or black infarcted areas are noticeable in the lungs. Bloody froth often fills the larger airways, and interlobular edema may be present. Pleural adhesions and lung abscesses are often found in chronic cases.
- Polymerase chain reaction (PCR) tests — These tests target the RTX toxin genes and can help differentiate from Actinobacillus suis infection.
- Serology tests — Tests, such as the complement fixation, enzyme-linked immunosorbent assay, and hemolysin neutralization serology tests, are useful to diagnose APP on a herd basis.
Treatment
Injectable antibiotics are the best way to treat APP since affected pigs are typically unable to eat or drink enough to receive medication through their food or water. Best results are seen when pigs are treated immediately after clinical signs occur. Clinically ill pigs and all pigs in contact with them should be treated on the first day signs are observed. High morbidity and mortality typically occur regardless of treatment.
Prevention
Proper management strategies can decrease a herd’s risk for APP, and may include:
- Vaccination — Older type vaccines are serotype specific and only protect against the homologous serotypes, but new generation vaccines are cross protective for all serotypes.
- Closed herd — Herds on APP-free farms should be managed as closed herds.
- Quarantine — Incoming pigs should be tested, isolated for a minimum of 30 days, and inspected daily.
- Screening — Herds that supply replacement breeding stock should be screened for APP on the basis of herd history, clinical inspections, and regular lung examinations at slaughter.
- Limit stress — Overcrowding and other situations that could cause the herd stress should be avoided.
- All in/all out — All in/all out production should be practiced, and careful cleaning and disinfection should occur between farrowings.
- Predisposing diseases — Diseases such as porcine reproductive and respiratory syndrome virus should be eliminated or controlled.
- Water — The herd should have a plentiful supply of clean water since temporary water deprivation can trigger disease.
- Ventilation — Good ventilation and a warm air flow should be maintained.
- Antibiotics — Appropriate antibiotics should be kept on hand so treatment can be started promptly if an outbreak occurs.
APP causes significant loss for affected swine producers, but strategies can be employed to reduce a herd’s risk. Hopefully, using these techniques will decrease the occurrence of outbreaks in the Upper Midwest and across the country.
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