Survey of infectious agents involved in acute respiratory disease in finishing pigs.

Outbreaks of respiratory disease constitute a major health problem in herds of finishing pigs and their aetiology often remains unclear. In this study, 16 outbreaks of respiratory disease with acute clinical signs in finishing pigs were investigated to determine which infectious agents were involved. From each herd four diseased and two clinically healthy pigs were examined pathologically and for the presence of viruses, bacteria and mycoplasmas. In addition, paired blood samples from 10 groupmates of the diseased pigs were tested for antibodies against commonly known causal agents of respiratory disease. A clear diagnosis was possible in 12 of the 16 outbreaks. Seven were due to an infection with influenza virus and five were due to an infection with Actinobacillus pleuropneumoniae. A combination of influenza virus and A pleuropneumoniae may have caused one other outbreak, but no clear cause could be established for the other three outbreaks.     

Serum haptoglobin concentration as a marker of clinical signs in finishing pigs

A case-control study of 340 finishing pigs aged 10 to 25 weeks in 15 commercial Danish pig herds was carried out to investigate serum haptoglobin concentration as an objective marker of clinical signs of disease. Pigs with different clinical signs were matched to control pigs without clinical signs with respect to herd, pen, estimated weight and gender, and each pig was subjected to a standard clinical examination. In 86 of the case-control pairs, the rectal temperature was also recorded. There was a significantly higher mean haptoglobin concentration in the serum of lame pigs (P<0.0001), pigs with respiratory disease (P=0.0004), pigs with tail or ear bites (P=0.0004) and pigs with diarrhoea (P=0.02). Similarly, a higher mean rectal temperature was recorded in lame pigs (P<0.0001), pigs with respiratory disease (P=0.002) and pigs with tail or ear bites (P=0.0003). There was a significant but low correlation between rectal temperature and haptoglobin concentration in serum (P=0.003, r=0.20). The area under the receiver-operating-characteristic curve was between 0.67 and 0.78 for the different clinical signs. The maximum simultaneous levels of sensitivity (0.61 to 0.71) and specificity (0.61 to 0.77) of serum haptoglobin for the different clinical signs were obtained at a cut-off value of 1.1 mg/ml. At a cut-off value of 1.8 mg/ml, the sensitivity decreased to 0.31 to 0.60, and the specificity increased to 0.82 to 0.86. It was not possible to define a cut-off value which classified individual pigs according to their clinical signs.     

Herd factors associated with the seroprevalences of four major respiratory pathogens in slaughter pigs from farrow-to-finish pig herds

The objective of this study was to investigate sero-epidemiological aspects of Mycoplasma hyopneumoniae (Mh), influenza H1N1 and H3N2 viruses and Aujeszky disease virus (ADV) in fattening pigs from 150 randomly selected farrow-to-finish pig herds. Different herd factors were examined as potential risk indicators for the percentage of pigs with antibodies against the 4 pathogens. The median within-herd seroprevalences of the pathogens were: Mh 76%, H1N1 100%, H3N2 40% and ADV 53%. There was a positive association between the seroprevalences of both influenza viruses, and a negative association between the seroprevalences of ADV and H1N1. The percentage of pigs seropositive for Mh increased with the purchase of gilts and with the season (slaughter date in March-April). The within-herd seroprevalences of both influenza viruses were higher in the case of a higher density of pig herds in the municipality. A higher number of fattening pigs per pen additionally increased the risk of being seropositive for H3N2. The percentage of pigs with anti-gE-antibodies against the wild type ADV increased with higher airspace stocking density in the finishing unit, increasing herd size, increasing number of pig herds in the municipality and slaughter date in March-April. Increased seroprevalences for these 4 respiratory pathogens were mostly associated with pig density in the herd and its vicinity, the winter period, and with the purchase of gilts. Purchase of gilts, number of fattening pigs per pen and airspace stocking density are risk factors that can be managed directly by farmers striving to attain a high respiratory health status of pigs.     

Associations between genetics, farm characteristics and clinical disease in field outbreaks of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) is a disease of domestic swine characterized by exceptionally high clinical variability. This study addresses the question of whether clinical variability in PRRS results from (a) genetic variation among viral isolates and/or (b) variation in management practices among farms on which isolates are found. Genetic data (open reading frame 5 gene sequences) and data on farm characteristics and associated clinical disease signs were collected for 62 PRRS virus (PRRSV) field isolates, representing 52 farms. Clinical disease signs were interrelated — confirming that a true reproductive syndrome exists (involving abortions, infertility in sows, deaths of sows and preweaning mortality).

Pairs of farms experiencing deaths in their sow populations also tended to share viral isolates which were more similar to one another than expected by chance alone. This implies that sow death (one of the more-severe manifestations of PRRS) is under genetic influence. Large herd size was a significant risk factor for the death of sows and for respiratory disease in nursery pigs. All-in–all-out management practices in the nursery were protective against reproductive signs in the sow herd. All-in–all-out management practices in the finishing stages of production were protective against respiratory disease in nursery pigs — but were paradoxically associated with an increased risk of infertility in sows. These results suggest that farm-management practices can also influence which PRRS clinical signs are manifested during an outbreak. In general, signs associated with PRRS appear to result from a combination of genetic factors and herd-management characteristics. The relative contributions of these two influences differ depending on the specific clinical sign in question.     

Factors associated with circulation of pseudorabies virus within swine herds

Data were collected from 104 Minnesota swine farms quarantined for pseudorabies virus (PRV) infection. Each herd was serologically evaluated for the presence of antibodies to PRV in finishing pigs. Herd management practices, swine housing design, and disease profiles were described for each farm. Multiple logistic regression analysis was used to determine which factors were associated with circulation of PRV in the finishing pigs of farrow-to-finish farms. Sixty-seven (64%) of the herds had no serologic evidence of PRV circulation in the finishing section, whereas 37 herds (36%) contained at least one PRV seropositive finishing pig. The odds of a given finishing herd being seropositive for PRV were 2.85 times higher if the finishing pigs were housed in confinement (P = 0.01), 2 times higher if Actinobacillus (Haemophilus) pleuropneumoniae was a clinical problem in the herd (P = 0.03), 1.36 times less for each year that passed since the herd quarantine was issued (P = 0.01), 1.74 times higher if clinical signs of PRV were reported (P = 0.04), and 1.52 times higher if animal protein was included in at least one of the rations (P = 0.08).     

豫南地区规模化猪场猪繁殖与呼吸综合征病毒和猪圆环病毒2型抗体检测与分析

为掌握豫南地区规模化猪场中猪繁殖与呼吸综合征病毒（PRRSV）和猪圆环病毒2型（PCV2）疫苗免疫场猪群抗体水平和非疫苗免疫场猪群病原感染情况,试验利用ELISA方法对该地区猪群进行了PRRSV和PCV2抗体水平检测。结果表明,在疫苗免疫猪群中,PRRSV和PCV2抗体合格率分别为68.8%和58.7%,规模越大的猪场合格率越高,200~500头母猪规模场抗体阳性率最高（分别为76.8%和77.4%）,50头母猪以下场最低（分别为48.3%和44.2%）,种猪的合格率均最高,而育肥猪群合格率均最低。在非疫苗免疫猪群中,PRRSV和PCV2抗体阳性率分别为55.6%和65.3%,PRRSV抗体阳性率最低的是100~200头母猪规模场（46.5%）,最高的是50头母猪以下规模场（68.8%）,断奶仔猪和育肥猪群抗体阳性率均在71%以上;随猪场规模越大,PCV2抗体阳性率越低,200~500头母猪规模场抗体阳性率为41.4%,50~100头规模场和50头以下场的阳性率分别为78.9%和78.6%,种公、母猪的PCV2抗体阳性率最低（分别为37.5%和40.4%）,断奶仔猪和育肥猪抗体阳性率较高（分别为82.2%和79.5%）。本研究反映了豫南地区猪繁殖与呼吸综合征（porcine reproductive and respiratory syndrome,PRRS）和猪圆环病毒病（porcine circovirus disease,PCVD）疫苗免疫猪场的疫苗免疫效果和非疫苗免疫猪场病原感染的实际情况和规律,为该地区PRRS和PCVD防制工作提供了理论依据和指导。     

Relationship between serum acute phase protein concentrations and lesions in finishing pigs

To establish the relationship between serum levels of three acute phase proteins, haptoglobin (Hp), C-reactive protein (CRP) and serum amyloid A (SAA), and the occurrence and severity of lesions at slaughter, a study was carried out using 70 fattening pigs from a finishing unit. Pigs were divided into three groups: Group 1 (25 pigs with clinical signs of disease), Group 2 (25 apparently healthy pigs with lesions at slaughter) and Group 3 (20 apparently healthy pigs with no lesions at slaughter). Serum levels of CRP, SAA and Hp were significantly higher in pigs with clinical signs of disease than in apparently healthy animals. Additionally, in apparently healthy pigs, serum levels of Hp and CRP were significantly higher in animals with lesions than those without lesions. The extent and severity of lung lesions were related to serum levels of Hp.     

Variation in seropositivity for some respiratory disease agents in finishing pigs: epidemiological studies on some health parameters and farm and management conditions in the herds.

The relationship between the extent of seropositivity for Aujeszky's disease virus (ADV), Actinobacillus pleuropneumoniae (App.) serotype 2 and porcine influenza (PI) viruses serotype H1N1 and H3N2 in pigs on the one hand and the health status of the pigs and some farm and management conditions in the herds on the other hand was studied in 45 pig finishing herds, all members of one integration group. The health status was assessed by the extent of clinical signs, the use of veterinary drugs and the prevalence of pathological lesions in pigs at slaughter. There was no relationship between the extent of seropositivity on the one hand and clinical signs and use of veterinary drugs on the other hand. However, there was a positive relationship between the extent of seropositivity and the percentage of pigs with lesions of the respiratory tract at slaughter. Furthermore, the study indicates that the variation in seropositivity between pigs herds is associated with management related factors that particularly influence the possibility of the spreading of viruses. A sero-epidemiological investigation in 14 pig herds with recurrent pneumonia problems revealed a high percentage of seropositive pigs per herd. Furthermore, in a large number of herds, pigs were simultaneously seropositive for ADV and App. serotype 2, for ADV and PI serotype H1N1 or for ADV and PI serotype H3N2.     

Polymicrobial respiratory disease in pigs

Respiratory disease in pigs is common in modern pork production worldwide and is often referred to as porcine respiratory disease complex (PRDC). PRDC is polymicrobial in nature, and results from infection with various combinations of primary and secondary respiratory pathogens. As a true multifactorial disease, environmental conditions, population size, management strategies and pig-specific factors such as age and genetics also play critical roles in the outcome of PRDC. While non-infectious factors are important in the initiation and outcome of cases of PRDC, the focus of this review is on infectious factors only. There are a variety of viral and bacterial pathogens commonly associated with PRDC including porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus (SIV), porcine circovirus type 2 (PCV2), Mycoplasma hyopneumoniae (MHYO) and Pasteurella multocida (PMULT). The pathogenesis of viral respiratory disease is typically associated with destruction of the mucocilliary apparatus and with interference and decrease of the function of pulmonary alveolar and intravascular macrophages. Bacterial pathogens often contribute to PRDC by activation of inflammation via enhanced cytokine responses. With recent advancements in pathogen detection methods, the importance of polymicrobial disease has become more evident, and identification of interactions of pathogens and their mechanisms of disease potentiation has become a topic of great interest. For example, combined infection of pigs with typically low pathogenic organisms like PCV2 and MHYO results in severe respiratory disease. Although the body of knowledge has advanced substantially in the last 15 years, much more needs to be learned about the pathogenesis and best practices for control of swine respiratory disease outbreaks caused by concurrent infection of two or more pathogens. This review discusses the latest findings on polymicrobial respiratory disease in pigs.     

Efficacy of Tilmicosin Phosphate (Pulmotil® Premix) in Feed for the Treatment of a Clinical Outbreak of Actinobacilluspleuropneumoniae Infection in Growing–Finishing Pigs

A double‐blind randomized clinical trial was carried out to investigate the efficacy of tilmicosin (Pulmotil^® premix) for the treatment of a clinical outbreak of Actinobacillus pleuropneumoniae infection in growing–finishing pigs. The effects of tilmicosin administration in the feed at 400 mg/kg and an injection therapy of clinically diseased pigs with long‐acting oxytetracycline (Terramycine^® LA) at 20 mg/kg bodyweight were compared. Both groups, totalling 147 pigs, were compared during a medication period of 15 days and a post‐medication period of 11 days by means of different clinical and performance parameters. During the medication period, the tilmicosin group showed a significant advantage with respect to the number of new disease cases (P < 0.01), and a non‐significant advantage regarding the number of removed pigs (P=0.16), the number of sick pigs that recovered (P=0.27) and the time to recovery (P=0.42). During the post‐medication period, the pigs of the tilmicosin group showed numerical non‐significant benefits (P > 0.05) with respect to the clinical parameters. During the overall study period (26 days), the average daily gain and the feed conversion ratio were both significantly (P < 0.01) better in pigs from the tilmicosin group compared with pigs from the oxytetracycline group. This study demonstrated that in‐feed medication of tilmicosin at a dosage of 400 mg/kg is efficacious for the treatment of a clinical respiratory disease outbreak of A. pleuropneumoniae infection in growing–finishing pigs. Compared with oxytetracycline injection of clinically diseased pigs, the tilmicosin treatment is particularly beneficial in the prevention of new disease cases while increasing or maintaining the performance of the pigs.     

Clinical signs and their association with herd demographics and porcine reproductive and respiratory syndrome (PRRS) control strategies in PRRS PCR-positive swine herds in Ontario

The purposes of this study were to describe the clinical signs observed in PRRS positive herds during a porcine reproductive and respiratory syndrome (PRRS) outbreak in Ontario and to determine associations between these clinical signs and herd demographics and PRRS control strategies. All PRRS polymerase chain reaction-(PCR)-positive submissions to a diagnostic laboratory between September 1, 2004 and August 31, 2007 were identified (n = 1864). After meeting eligibility requirements and agreeing to voluntary study participation, producers from 455 of these submissions were surveyed for information on clinical signs observed in their herds, herd demographics, and PRRS control strategies used in their herds at the time that the PCR-positive samples were taken. Larger herd size was associated with an increased risk of reporting abortion, weakborn piglets, off-feed sows, and sow mortality in sow herds, and with an increased risk of reporting mortality in finishing herds. When disease control strategies were examined, use of a commercial PRRS vaccine in sows and gilts was associated with a decreased risk of reporting weakborn pigs and high pre-weaning mortality, while the use of serum inoculation in breeding animals was associated with an increased risk of reporting off-feed sows and sow mortality. Providing biofeedback of stillborn/mummified piglets, placenta or feces to gilts was associated with an increased risk of reporting respiratory disease and mortality in finishing pigs while all-in/all-out flow in farrowing rooms was associated with an increased risk of reporting sow mortality and weakborn piglets.     

Detection of Actinobacillus pleuropneumoniae Infection in Pigs

It is difficult to control the spread of porcine haemophilus pleuropneumonia caused by Actinobacillus pleuropneumoniae because there is no sensitive and specific way to accurately determine whether or not a pig herd is infected. This paper reports bacteriological and serological techniques used to detect A. pleuropneumoniae infection in pigs from a herd with endemic disease.

The bacteria were isolated from the anterior nasal mucosa of grower pigs, but not from younger or older pigs. Bacteriological culture of several tissues from the respiratory tract showed that nine of ten young finishing pigs were infected, but culture of lung tissue from slaughtered hogs detected infection in only 39 of 288 (13.5%). Both cooler storage temperature and use of selective medium prolonged the time that lung tissue could be stored and the organism still recovered. An enzyme-linked immunosorbent assay detected serotype-specific antibodies in serum of infected pigs.

     

Pleuropneumonia caused by Actinobacillus pleuropneumoniae biotype 2 in growing and finishing pigs.

Actinobacillus pleuropneumoniae biotype 2 was isolated in pure culture or as the predominant isolate from the lungs of 9 growing and finishing pigs with pleuropneumonia. Gross and microscopic lesions resembled those caused by A. pleuropneumoniae biotype 1 serotypes (Nos. 1, 5, and 7) traditionally seen in the United States. The overall mortality rate for growing and finishing pigs on this 1,200-sow farrow-to-finish farm ranged from 0.37% to 0.84% per month from July 1990 to February 1991, and mortality due to respiratory disease ranged from 0.17% to 0.52% per month for the same period. This Actinobacillus species did not require V factor (no satellitism on blood agar with a Staphylococcus streak), was strongly beta-hemolytic, and demonstrated restriction fragment length polymorphisms in hybridization studies with A. suis, A. lignieresii, and A. equuli. Biochemically, the isolate most closely resembled A. pleuropneumoniae, and a DNA fragment considered specific for A. pleuropneumoniae biotypes 1 and 2 was demonstrated using polymerase chain reaction. Necrohemorrhagic pleuropneumonia similar to that caused by A. pleuropneumoniae biotype 1 was reproduced experimentally in 2 4-week-old pigs inoculated intratracheally with broth cultures of the A. pleuropneumoniae biotype 2. This study demonstrated the presence of A. pleuropneumoniae biotype 2 in the United States.     

Simultaneous serological evidence of Actinobacillus pleuropneumoniae, PRRS, Aujeszky's disease and influenza viruses in Spanish finishing pigs

A total of 198 pigs with tachypnoea and temperature >/= 40 degrees C were selected on a Spanish finishing unit, and their sera were examined for antibodies to Actinobacillus pleuropneumoniae (App), porcine reproductive and respiratory syndrome virus (PRRSV), Aujeszky' disease virus (ADV), and swine influenza virus (SIV). Eighty-nine point nine per cent of the pigs were seropositive to App, 88.6 per cent to PRRS, 73.0 per cent to ADV, and 30.6 per cent to SIV. Thirty-one pigs (15.6 per cent) were seropositive for App, PRRSV, ADV and SIV, and only one (0.5 per cent) was seronegative for all. Statistical association was assessed for dual infections but it was not found in any case (P > 0.05). Other parameters (dyspnoea, nasal discharge and coughing) were also recorded, and no significant associations between them and the presence of antibodies against any of the four infections was found.     

育肥猪呼吸道疾病综合征病理学观察和病原学检测

猪呼吸道疾病综合征是一种由病毒、细菌、环境应激和猪体免疫力低下等多因素相互作用引起的呼吸道疾病。2009年2～4月,上海农场某猪场一批育肥猪出现疑似病例,表现为食欲下降,咳嗽,呼吸困难,生长缓慢和料肉比降低。本研究在流行病学调查的基础上,剖检8头病猪观察其病理变化,主要表现为肺脏弥慢性实变,弹性降低,间质增宽,有的水肿、气肿,或纤维素性胸膜肺炎。病原检测结果显示,该发病猪群主要病原为猪繁殖与呼吸综合征病毒、猪圆环病毒2型和副猪嗜血杆菌。本研究为猪呼吸道疾病综合征的防治提供了依据。     

Transmission of pathogenic respiratory bacteria to specific pathogen free pigs at slaughter

The purpose of this study was to evaluate the transmission of pathogenic respiratory bacteria to thirteen 5-month-old specific pathogen free (SPF) pigs, during the slaughtering process in a commercial slaughterhouse. Before transportation, the SPF pigs and the lorry were checked to confirm the absence of pathogenic respiratory bacteria.

Nine SPF pigs (group 1) were in contact in a conventional slaughterhouse with finishing pigs, during 4 h before slaughtering. Four SPF pigs (group 2) were slaughtered immediately at arrival in the slaughterhouse.

Five bacterial pathogens (Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae, Pasteurella multocida, Haemophilus parasuis and Streptococcus suis) were detected by PCR, after slaughtering, from nasal cavities, tonsils and trachea in the two groups of pigs. Lung samples were PCR negative. Three and four bacterial species were isolated from the pigs of group 2 and group 1, respectively. Cultures were negative from the lungs.

All the bacterial species present in the SPF pigs were detected by PCR. P. multocida was isolated, from three samples of scalding water before the onset of slaughtering.

Our results suggest that the SPF pigs became contaminated mainly by the slaughterhouse environment and the scalding water. Histological examinations revealed that during scalding, contaminated water could reach the trachea and the lungs of pigs. Checks conducted at slaughter for respiratory disorders have to be carried on, but nasal cavities and tonsils are not appropriate for bacteriological investigations. Moreover, bacteriological results obtained from the lungs of slaughtered pigs have to be used with carefulness.     

Dunging gutters filled with fresh water in finishing barns had no effect on the prevalence of Salmonella enterica on Brazilian swine farms

Our aim was to assess the importance of dunging gutters filled with water in finishing barns for the prevalence of pigs shedding Salmonella enterica. Some finishing barns in Brazil are provided with a dunging-gutter system which consists of a continuous water flow at the back of solid-floored adjacent pens. Because there is transfer of faecal material between adjacent pens by water in this system and the faecal-oral route of transmission is so important for enteric pathogens, we tested the hypothesis that the presence of this kind of dunging-gutter system in finishing barns affects the prevalence of slaughter-age pigs shedding salmonella organisms in their faeces. The cross-sectional study was conducted on six farms each having barns with and barns without a dunging-gutter system. Breeding, management, nutritional and seasonal factors were similar in both barns on each farm. The two systems did not differ in prevalence of pigs shedding salmonella organisms. Five S. enterica serotypes were isolated: S. Agona, S. Javiana, S. Rissen, S. Sandiego and S. Senftenberg.     

Prevalence and risk factors associated with Cryptosporidium oocysts shedding in pigs in Central Vietnam

We investigated the prevalence and risk factors associated with Cryptosporidium oocysts shedding in pigs in Central Vietnam. A total of 740 single fecal samples collected from diarrheic and non-diarrheic pigs on 89 farms were screened by the modified Ziehl-Neelsen staining method. Prevalence at the animal and the farm levels were 18.1% (134/740) and 71.9% (64/89), respectively. Risk factors for the infection were identified using univariate and multivariate logistic regression analysis. The results revealed that age, sanitary condition and topography were significantly associated with oocyst shedding (P<0.05). Pre-weaned piglets were at the highest risk for infection, followed by post-weaners, sows and finishing pigs. Good sanitary conditions showed positive effects in decreasing oocysts shedding. Topographically, Cryptosporidium was more common in mountainous zone than that in coastal delta zone. There was an association between the occurrence of diarrhea and the level of Cryptosporidium oocyst excretion within infected pigs. This is the first epidemiological investigation of prevalence and risk factors of Cryptosporidium in pigs in Vietnam.     

包膜半胱胺对肥育猪生长性能、免疫功能、抗氧化能力及肠内容物消化酶活性的影响

本试验旨在研究日粮中添加不同水平包膜半胱胺(CCS)对肥育猪生长性能、免疫和抗氧化能力及十二指肠消化酶活性的影响。选择体重(62.12±1.43)kg的健康杜×长×大肥育猪160头,随机分为4个处理,每个处理组4个重复,每个重复10头(公、母各半)。对照组饲喂基础日粮,试验组在基础日粮的基础上分别添加100、200、400 mg/kg CCS。试验猪自由采食饮水,预试期5 d,正试期40 d。结果表明:CCS对肥育猪的平均日增重(ADG)、平均日采食量(ADFI)和耗料增重比(F/G)无显著影响,但100 mg/kg CCS组生长性能指标相对最佳;100、200 mg/kg CCS组血清IgM和补体C3水平均上升(P<0.05),C3水平随着CCS添加剂量的提高呈线性上升(P<0.05);CCS有提高血清Ig G水平的趋势(P=0.07),并呈二次升高(P<0.05);CCS提高了血清中总抗氧化能力(T-AOC)并呈二次变化(P<0.05);血清谷胱甘肽过氧化物酶(GSH-Px)水平和过氧化氢酶(CAT)活性随着CCS水平的提高呈线性及二次变化(P<0.05);肝脏GSH-Px和总超氧化物歧化酶(T-SOD)活性随着CCS水平的提高呈线性及二次增高(P<0.05);CCS提高了十二指肠总蛋白酶和总脂肪酶活性(P<0.05),使总淀粉酶活性呈二次上升(P<0.05)。可见,日粮中添加低剂量的CCS对肥育猪生产性能有一定改善趋势,CCS可提高肥育猪的免疫和抗氧化性能,促进肠道消化酶的分泌。在本试验中,CCS在肥育猪饲粮中的适宜添加水平为100 mg/kg。     

Proinflammatory cytokines and viral respiratory disease in pigs

Swine influenza virus (SIV), porcine respiratory coronavirus (PRCV) and porcine reproductive and respiratory syndrome virus (PRRSV) are enzootic viruses causing pulmonary infections in pigs. The first part of this review concentrates on known clinical and pathogenetic features of these infections. SIV is a primary respiratory pathogen; PRCV and PRRSV, on the contrary, tend to cause subclinical infections if uncomplicated but they appear to be important contributors to multifactorial respiratory diseases. The exact mechanisms whereby these viruses cause symptoms and pathology, however, remain unresolved. Classical studies of pathogenesis have revealed different lung cell tropisms and replication kinetics for each of these viruses and they suggest the involvement of different lung inflammatory responses or mediators. The proinflammatory cytokines interferon-alpha (IFN-alpha), tumour necrosis factor-alpha (TNF-alpha) and interleukin-1 (IL-1) have been shown to play key roles in several respiratory disease conditions. The biological effects of these cytokines and their involvement in human viral respiratory disease are discussed in the second part of this review. The third part summarises studies that were recently undertaken in the authors' laboratory to investigate the relationship between respiratory disease in pigs and bioactive lung lavage levels of IFN-alpha, TNF-alpha and IL-1 during single and combined infections with the above viruses. In single SIV infections, typical signs of swine "flu" were tightly correlated with an excessive and coordinate production of the 3 cytokines examined. PRCV or PRRSV infections, in contrast, were subclinical and did not induce production of all 3 cytokines. Combined infections with these 2 subclinical respiratory viruses failed to potentiate disease or cytokine production. After combined inoculation with PRCV followed by bacterial lipopolysaccharide, both clinical respiratory disease and TNF-alpha/IL-1 production were markedly more severe than those associated with the respective single inoculations. Taken together, these data are the first to demonstrate that proinflammatory cytokines can be important mediators of viral respiratory diseases in pigs.