Streptococcal lymphadenitis is a contagious disease characterized by abscessation of the cervical, mandibular, and cephalic lymph nodes. Affected pigs are generally thrifty and exhibit satisfactory growth rates. The condition can negatively affect work efficiency in slaughter facilities due to condemnation of affected heads, increased processing time required for cleanup of facilities when an abscess is ruptured, and examination of affected carcasses by inspectors for a final disposition on suitability for consumption.
Although it was once a major concern to the swine industry, the significance of Streptococcus porcinus and resulting disease is negligible today. More recently, interest in S porcinus has grown because of connections to genitourinary infections in people. The actual significance of S porcinus to human health is unclear; it is said to be commonly misidentified in laboratories due to reported cross-reactions with S agalactiae, which is recognized as a cause of genitourinary infections in people. Up to 8 serotypes have been described, and anti-phagocytic factors and streptokinase have been identified as potential virulence factors.
Transmission, Epidemiology, and Pathogenesis
Streptococcal lympha-denitis is endemic, and once it occurs on a farm, successive groups of pigs develop abscesses during the growing and finishing period. Pigs may become infected by ingesting S porcinus from draining abscesses and contaminated feed and water; however, recovered carrier pigs are the most common and important source of infection. Recovered pigs harbor S porcinus in the tonsillar tissue and readily transmit the organisms to susceptible pigs via direct contact and by contamination of feed and water. Pigs are considered to be resistant to infection for the first 3–4 wk of life, which is probably related to the influence of passive immunity.
Scattered miliary abscesses develop in the mandibular, parotid, or retropharyngeal lymph nodes within 7 days after infection. By 21 days, abscesses measuring 5–8 cm in diameter are common; they destroy the internal structure of affected nodes and may extend into adjacent tissues. Developing abscesses may reach the skin, rupture, and drain in 7–10 wk. The drained lesions heal by granulation, leaving a dense, fibrous, subcutaneous tract that resolves after several weeks. Deep-seated abscesses may remain undetected until slaughter and tend not to drain into the pharynx. Prevalence of the condition varies and may range from >50% up to 100% in a given lot of market hogs.
Clinical Findings, Lesions, and Diagnosis
Generally, abscesses are the only sign of infection. Abscesses are most common in the mandibular and retropharyngeal lymph nodes and rare in other nodes. Rarely, the disease may cause meningitis, polyarthritis, or septicemia. Diagnosis is by culture and isolation of S porcinus from abscess exudates.
Treatment and Control
In affected herds, piglets should be weaned at 21 days and reared in an environment where continuous flow of facilities is not practiced to reduce bacterial transfer from older pigs to younger pigs. Historically, treatment with penicillin successfully resolved infections that were detected before abscesses became established. Medication of feed with tetracycline at 400 g/ton has also been described as an effective way to reduce the number of abscesses. However, treatment after infection and abscessation is established is not effective in eliminating the bacteria. Vaccination (autogenous) is possible but has not been widely used because cervical abscesses are not viewed as a widespread problem.
Last full review/revision July 2011 by Darryl Ragland