Efficacy of Antimicrobial Treatments and Vaccination Regimens for Control of Porcine Reproductive and Respiratory Syndrome Virus and Streptococcus suis Coinfection of Nursery Pigs

Seventy-six, crossbred, porcine reproductive and respiratory syndrome virus (PRRSV)-free pigs were weaned at 12 days of age and randomly assigned to seven groups of 10 to 11 pigs each. Pigs in group 1 served as unchallenged controls. Pigs in groups 2 to 7 were challenged intranasally with 2 ml of high-virulence PRRSV isolate VR-2385 (10(4.47) 50% tissue culture infective doses per 2 ml) on day 0 of the study (30 days of age). Seven days after PRRSV challenge, pigs in groups 2 to 7 were challenged intranasally with 2 ml of Streptococcus suis serotype 2 (10(8.30) CFU/2 ml). Group 2 pigs served as untreated positive controls. Antimicrobial treatments included daily intramuscular injection with 66,000 IU of procaine penicillin G per kg of body weight on days 8 to 10 (group 3), drinking water medication with 23.1 mg of tiamulin per kg during days 8 to 10 (group 4), and daily intramuscular injection of 5.0 mg of ceftiofur hydrochloride per kg on days 8 to 10 (group 5). Vaccination regimens included two intramuscular doses of an autogenous killed S. suis vaccine (group 6) prior to S. suis challenge or a single 2-ml intramuscular dose of an attenuated live PRRSV vaccine (group 7) 2 weeks prior to PRRSV challenge. Mortality was 0, 63, 45, 54, 9, 40, and 81% in groups 1 to 7, respectively. Ceftiofur treatment was the only regimen that significantly (P < 0.05) reduced mortality associated with PRRSV and S. suis coinfection. The other treatments and vaccinations were less effective. We conclude that ceftiofur administered by injection for three consecutive days following S. suis challenge was the most effective regimen for minimizing disease associated with PRRSV and S. suis coinfection.

76头无猪繁殖与呼吸综合征病毒（Porcine Reproductive and Respiratory Syndrome Virus, PRRSV）的杂交仔猪于12日龄断奶，随机分为7组，每组10~11头。第1组作为未攻毒空白对照组。第2至7组于试验第0天（仔猪30日龄）经鼻内接种2ml高毒力PRRSV分离株VR-2385（每2ml含10^4.47半数组织培养感染剂量）。PRRSV攻毒7天后，第2至7组经鼻内接种2ml猪链球菌2型（Streptococcus suis serotype 2）菌株（每2ml含10^8.30菌落形成单位（CFU））。第2组为未治疗阳性对照组。抗菌治疗方案包括：第3组于第8至10天每日肌内注射66000 IU/kg体重的普鲁卡因青霉素G；第4组于第8至10天经饮水给药23.1 mg/kg的泰妙菌素；第5组于第8至10天每日肌内注射5.0 mg/kg的盐酸头孢噻呋。疫苗免疫方案为：第6组在猪链球菌攻毒前接种2剂肌内注射的自体灭活猪链球菌疫苗；第7组在PRRSV攻毒前2周肌内注射1剂2ml的弱毒活PRRSV疫苗。第1至7组的死亡率依次为0%、63%、45%、54%、9%、40%和81%。头孢噻呋治疗是唯一能显著（P<0.05）降低PRRSV与猪链球菌混合感染相关死亡率的方案，其余治疗与免疫方案效果均较差。本研究结论为：在猪链球菌攻毒后连续3天经肌内注射给予头孢噻呋，是降低PRRSV与猪链球菌混合感染相关疾病的最优方案。