Evaluation of stocking density and subtherapeutic chlortetracycline on Salmonella enterica subsp. enterica shedding in growing swine

The objective of this research was to determine the effect of stocking density and inclusion of subtherapeutic chlortetracycline in the diet on Salmonella fecal prevalence and antimicrobial resistance in growing swine. A 2 x 2 factorial design was employed on a privately owned commercial swine farm. Four finisher rooms were included in the study. Two of the rooms received 50 g/tonnes of chlortetracycline in the ration, two rooms received no antimicrobials in the feed. In each room, alternate pens were assigned to either high stocking density (0.60 m2/pig) or low stocking density (0.74 m2/pig). Pigs were placed in the finisher rooms at 10 weeks of age and followed for 6 weeks. Individual fecal samples were collected from the floors of each pen and cultured once weekly. Antimicrobial resistance phenotypes were determined. Data were analyzed using multilevel, multivariable logistic regression. Pigs fed chlortetracycline were at increased odds (OR 6.88, 95% CI 2.77-17.12) to shed Salmonellae. No other associations between treatments (CTC and stocking density) and Salmonella prevalence or reduced susceptibility to antimicrobials were identified. Variance in the odds of a fecal sample to be positive was distributed mostly at the lowest level, the individual fecal sample. The increased risk of shedding associated with inclusion of subtherapeutic chlortetracycline in swine diets is discordant with previous results by our group, suggesting farm or strain specific factors may impact this association. Understanding this risk may provide a potential intervention for controlling Salmonella pre-harvest.     

Seroprevalence of and Risk Factors for Toxoplasma gondii in the US Swine Herd Using Sera Collected During the National Animal Health Monitoring Survey (Swine 2006)

The United States Department of Agriculture (USDA) initiated the National Animal Health Monitoring System (NAHMS) in 1983 to collect, analyse and disseminate data on animal health, management and productivity in US domestic livestock populations, including swine. The programme includes an on‐farm serological sampling component which can be used to monitor seroprevalence of various pathogens, including Toxoplasma gondii. The purpose of this study was to determine the seroprevalence of T. gondii in grower/finisher pigs using sera collected during NAHMS Swine 2006 and to determine farm level factors associated with differences in seroprevalence on farms where sera was collected during the Swine 2006 survey. Sera and data on management practices for this study were collected from 185 grower/finisher swine production sites located in 16 states accounting for >90% of US swine production (Arkansas, Colorado, Iowa, Illinois, Indiana, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Carolina, Ohio, Pennsylvania, South Dakota, Texas and Wisconsin). A total of 6238 sera were tested for T. gondii antibodies using a commercial ELISA assay (Vet. Parasitol. 128 , 2005, 177). Seroprevalence in this study, as determined by ELISA, was 2.6%, with a herd prevalence of 21.6% and a mean within‐herd prevalence of 2.7%. Analysis of swine management practices indicated that rodent control methods and carcass disposal methods were associated with differences in the number of T. gondii positive samples on farm. These results are consistent with current epidemiological knowledge of the transmission of Toxoplasma on the farm (ingestion of organic matter containing oocysts, or ingestion of infected animal tissues). Production practices which eliminate these sources of exposure can reduce the risk of Toxoplasma infection in pigs, and reduce the likelihood of human infection from consumption of infected pork.     

Longitudinal study of Salmonella enterica in growing pigs reared in multiple-site swine production systems

Intensive longitudinal investigations of breeding and growing pig populations in two multiple-site swine production systems were conducted in NC, USA. Five cohorts of sows and individually identified growing pigs from their litters were serially sampled in order to determine the prevalence and serotypes of Salmonella enterica in each stage of production based on fecal culture. In addition to fecal samples, feed and environmental samples were obtained. Fifteen different serotypes were isolated from the two systems, the most frequently isolated serotypes were S. typhimurium var Mbandaka and S. typhimurium var Copenhagen. Pig prevalence estimates ranged from 0 to 48.1%. Environmental contamination was frequently encountered despite cleaning and disinfection. Feed was rarely (2/800, 0.25%) identified as S. enterica positive. We observed highly variable patterns of S. enterica prevalence and serotype profiles within cohorts over time and among cohorts within systems. These observations indicate that point estimates of S. enterica prevalence and serotypes cannot be considered as reliable indicators of the S. enterica status of farms, and that uncontrolled studies of interventions to control S. enterica may yield misleading results. These findings are critical to the design of epidemiological studies of S. enterica on swine farms and may suggest that cohort level, as opposed to farm or company level events or management practices, may be important as potential risk factors for S. enterica fecal shedding in market age pigs.     

Evaluation of environmental and management-related risk factors associated with chronic mastitis in sows

OBJECTIVE: To evaluate environmental and management-related risk factors associated with chronic mastitis in sows. ANIMALS: 1,254 sows from 76 herds. PROCEDURE: Prevalence of chronic mastitis was determined by a veterinarian who performed clinical examinations at the time of weaning and approximately 1 week later in a sample of the sow population on each farm. Information concerning environmental factors and management practices was collected. In addition, the herd veterinarian made an assessment of the farmer's skills in swine production. RESULTS: Use of partly slatted floors in the farrowing pens, use of disinfectants between batches in the farrowing and breeding areas, feeding lactating sows whey, and avoiding cutting or grinding of the piglets' teeth were significantly associated with a decreased risk of chronic mastitis. A high hygienic standard on the farm, as determined by the herd veterinarian, was associated with a significant reduction in the prevalence of mastitis. CONCLUSIONS AND CLINICAL RELEVANCE: Chronic mastitis in sows is a common disease that has a negative influence on productivity. Results indicate that certain management practices and environmental factors influenced the development of mastitis, which may contribute to the development of methods useful for controlling the disease.     

后非洲猪瘟时代之大围产期风险管理

非洲猪瘟背景下,猪场恢复生产过程中面临诸多风险。恢复生产即恢复连续生产的过程,在连续生产过程中,大围产期是复产的高风险期。文章旨在分析大围产期存在的风险因素,并为非洲猪瘟背景下,猪场做好大围产期风险管理提供参考借鉴。     

Mortality attributed to respiratory problems among finisher pigs in the United States

In the 1995 National Swine Study of the United States National Animal Health Monitoring System, producers identified respiratory problems as the leading cause of death in pigs during the grower/finisher phase of production. Over a six-month period, 61.7±4.1% (mean±SEM) of operations reported at least one death attributed to respiratory problems among finisher pigs (based on 388 operations representing operations with ≥300 finisher pigs in 16 states). Mean mortality attributed to respiratory problems was 0.9±0.1% of finisher pigs per operation. Stepwise logistic regression (using SAS) was used to identify factors associated with operations attributing at least one death to respiratory problems, and to identify factors associated with reporting ≥2% mortality attributed to respiratory problems. Final models were run with SUDAAN to account for the sampling strategy. Attributing at least one death to respiratory problems was associated with having ≥3000 pigs enter the grower/finisher unit over a six-month period; diagnosis of Haemophilus (or Actinobacillus) in the past 12 months; and keeping pigs in the grower/finisher unit >120 days (as compared to <100 days). Not having a farrowing facility, mean weaning age <28 days, and <50% of finisher pigs on solid concrete only were associated with reporting ≥2% mortality attributed to respiratory problems.     

Trichinae certification in the United States pork industry

Control of Trichinella infection in U.S. pork has traditionally been accomplished by inspection of individual carcasses at slaughter or by post-slaughter processing to inactivate parasites. We propose that an alternative to individual carcass testing or processing can be used when pigs are raised in production systems where risk of exposure to Trichinella spiralis has been mitigated. Declines in prevalence of this parasite in U.S. domestic swine during the last 30 years, coupled with improvements in pork production systems, now allow Trichinella control to be shifted to the farm through implementation of specific pork production practices. Knowledge of risk factors for exposure of swine to T. spiralis was used to develop an objective audit of risk that can be applied to pork production sites. In a pilot study, 461 production site audits were performed by trained veterinary practitioners. The on-farm audit included aspects of farm management, bio-security, feed and feed storage, rodent control programs and general hygiene. Of the 461 production site audits, 450 audits (97.6%) indicated compliance with the required good production practices. These sites are eligible for certification under the U.S. Trichinae Certification Program and will be audited regularly to maintain that status. The described trichinae certification mechanism will establish a process for ensuring the Trichinella safety of swine, and ultimately food products derived from swine, at the production level.     

Prevalence of Salmonella spp. on Canadian pig farms using liquid or dry-feeding

The objective of this study was to determine whether the shedding and antibody titre to Salmonella was lower for pig herds provided liquid-feed compared to those on traditional dry rations. Twenty liquid-feeding farms and 61 dry-feeding farms were selected. The amount of antibodies to Salmonella in sera from 15 finisher pigs on each of 80 Ontario swine farms was analyzed by means of enzyme-linked immunosorbent assay (ELISA). In addition, the presence of Salmonella on the 20 liquid-feeding farms and 21 of the dry-feeding farms was assessed by culture of 15 fecal samples taken directly from finisher pigs and five pooled pen-fecal samples at each farm.

A cut-off of OD% 10 was used. The Salmonella sero-prevalence differed between the two groups of farms. At least one pig tested sero-positive on 98% of the dry-feeding farms and 84% of the liquid-feeding farms (P < 0.05). A multi-variable mixed linear regression model with the farm as a random variable and farm factors as the fixed effects was fitted. Crude optical density (OD) of the individual pig was considered as the continuous dependent variable. Dry-feeding and antimicrobial daily usage was associated with crude OD (P < 0.05). In addition, crude OD increased with increasing herd size (P < 0.05).

Salmonella was isolated from 25 out of 420 fecal samples (6%) from dry-feeding farms compared to three out of 400 samples (0.8%) from liquid-feeding farms. Eight of the dry-feeding farms (38%) tested positive compared to only three of the liquid-feeding farms (15%). Salmonella was also recovered from the pen environment on five dry-feeding farms but were not isolated from the facilities using liquid-feeding. Salmonella Typhimurium was isolated from four farms in the dry-feed group and on one farm with liquid-feeding. The one S. Typhimurium isolate from the liquid-feeding farm exhibited no antimicrobial resistance, but those from dry-feeding farms were resistant to four or more antimicrobial agents. The results of the logistic regression, with farm as a random effect showed that dry-feeding [OR = 2.7 (1.1–15.1)] and continuous flow system [OR = 2.3 (1.2–12.7)] increased risk of finding Salmonella in the individual pig.

These findings indicate that liquid-feeding and all-in all-out management of the grower–finisher barns can reduce the Salmonella prevalence.     

An analysis of mortality in the grower/finisher phase of swine production in the United States

Over a 6-month period, the mean mortality risk (based on 393 operations participating in the United States National Animal Health Monitoring System 1995 National Swine Study, and representing operations with ≥ 300 market hogs in 16 states), was 2.3 ± 0.2% in the grower/finisher production phase (where figures after the ± represent the standard error of the estimate). Mortality ≥ 4% was experienced by 13.5 2.9% of grower/finisher operations, while 63.6 ± 5.3% had ≤ 2% mortality. To identify factors associated with ≥ 4% mortality, stepwise logistic regression [Statistical Analysis Systems, (1989). SAS/STAT^® User's Guide, Version 6, 4th edn, Vol. 2. SAS Institute, Cary, NC, 794 pp.]was performed twice: once using operations with all mortality rates, and again excluding operations with between 2% and 4% mortality. Final models were run with SUDAAN [Shah, B.V., Barnwell, B.G., Bieler, G.S., (1996). SUDAAN User's Manual, Version 6.40, 2nd edn. Research Triangle Institute, Research Triangle Park, NC, 492 pp.] to take the sample design into account. In addition, SAS and SUDAAN logistic regression models were developed to analyze factors associated with > 2.3% mortality among grower/finisher pigs. Mean weaning age ≤ 28 days entered all models as being associated with increased mortality in the grower/finisher unit. Not obtaining all grower/finisher pigs from farrowing units belonging to the operation was associated with ≥ 4% mortality among grower/finisher swine. Not typically giving grower/finisher pigs antibiotics or other agents as disease-preventives or growth-promotants in the feed or water, and ranking producer organizations as very or extremely important sources of antibiotic information were associated with ≤ 2.3% mortality in the grower/finisher phase.     

Evaluation of farm management practices as risk factors for clinical listeriosis and fecal shedding of Listeria monocytogenes in ruminants

OBJECTIVE: To assess seasonal variation in prevalence of Listeria monocytogenes on ruminant farms and identify management practices associated with ruminant listeriosis and fecal shedding of L. monocytogenes. STUDY DESIGN: Case-control study. SAMPLE POPULATION: 2056 samples of feces, feed, soil, and water from 24 case farms with listeriosis and 28 control farms without listeriosis. PROCEDURE: Samples were collected and evaluated via bacterial culture for L. monocytogenes. Univariate associations between farm management practices and listeriosis and fecal shedding of L. monocytogenes were assessed. Multivariate models were developed to identify farm management practices associated with listeriosis and fecal shedding of L. monocytogenes. RESULTS: The prevalence of L. monocytogenes on cattle, goat, and sheep farms was seasonal, especially in fecal samples, with peak prevalence in winter. Although the prevalence of L. monocytogenes in feedstuffs from small-ruminant farms also peaked during winter, the bacterium was detected at a constant rate in cattle farm feedstuffs throughout the year. Farm management practices, animal health and hygiene, and feedstuff quality and storage were associated with ruminant listeriosis and fecal shedding of L. monocytogenes. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that the prevalence of L. monocytogenes on ruminant farms is seasonal, management practices are associated with ruminant listeriosis and fecal shedding of L. monocytogenes, and the epidemiologic features of listeriosis differ in cattle versus small ruminants. Awareness of risk factors may be used to develop control measures to reduce animal disease and introduction of L. monocytogenes into the human food chain.     

Development of a sanitary risk index for Salmonella seroprevalence in Belgian pig farms

The aim of this study was to develop a scientifically based Sanitary Risk Index (SRI), defined as an objective measure of the Salmonella seroprevalence in a pig herd based on the risk factors being present on the farm. Therefore, an observational epidemiological study was adopted to infer risk factors for the Salmonella seroprevalence of market pigs. A total of 204 Belgian farrow-to-finish pig herds were included in this cross-sectional study. The antibody titre to Salmonella in sera was analysed by means of an enzyme-linked immunosorbent assay (ELISA) for an average of 58 finisher pigs on each farm. A detailed questionnaire, covering an extensive range of potential risk factors was completed by each participating pig producer. Pearson correlation coefficients between the average sample to positive ratio (S/P)-value of a herd and the within-herd proportion of seropositive pigs were high. Significant risk factors associated with the average S/P-value of a herd were identified by a general linear mixed model. Feeding of meal, providing wet feed, having a hygienic-lock facility, using boot baths, applying the strict all in/all out procedure, programming the temperature in the zone of thermal neutrality and disinfecting between batches were all associated with lower average S/P-values. Sampling in summer, using a clean downtime, decreasing floor space per animal as well as increasing herd size were related with higher average S/P-values. The SRI consists of the above-specified risk factors together with their relative weight. Determining the Salmonella risk of a new herd by the SRI is primarily based on the quantification of the farm specific risk factors present and results in an average S/P-value of the herd. The model was validated using a set of conventional farms. In conclusion, the SRI is a useful preliminary screening tool which forms the basis for targeted sampling but cannot replace the serological herd classification with regard to Salmonella prevalence.     

Syndromic survey and molecular analysis of influenza viruses at the human–swine interface in two West African cosmopolitan cities suggest the possibility of bidirectional interspecies transmission

Influenza viruses are frequently transmitted between pigs and their handlers, and among pig handlers. However, reports on socio‐environmental variables as potential risk factors associated with transmission of influenza in West African swine production facilities are very scarce. Syndromic survey for influenza was therefore conducted in Ibadan, Nigeria, and Kumasi, Ghana, in order to identify and elucidate selected socio‐environmental variables that may contribute to the occurrence and distribution of influenza‐like illness (ILI) among swine industry workers. In addition, molecular analyses were conducted to elucidate the nature of influenza viruses circulating at the human–swine interface in these cities and better understand the dynamics of their transmission. Influenza viruses were detected by type‐specific and subtype‐specific RT–PCR. Sequencing and phylogenetic analyses were carried out. Socio‐environmental variables were tested by both univariable and multivariable regression methods for significance at p < 0.05. Three risk factors for ILI were identified in each city. These included “frequency of visit of pig handler to pig pen or lairage” (Ibadan: risk ratio [RR] = 1.54, 95% confidence interval [CI] = 1.36–1.79, p = 0.02; Kumasi: RR = 1.28, 95% CI = 1.11–1.71, p = 0.01) and “pig handler's awareness about biosecurity measures” (Ibadan: RR = 7.09, 95% CI = 2.36–21.32, p < 0.001; Kumasi: RR = 4.84, 95% CI = 1.98–11.80, p < 0.001). Influenza A(H1N1)pdm09 viruses, with M genes closely related to those which circulated among pigs in the two cities during the same period, were detected among Nigerian and Ghanaian pig industry workers. These findings suggest the possibility of bidirectional transmission of influenza at the human–swine interface in these cities and underscore the need for more extensive molecular studies. Risk factors identified may assist in the control of human‐to‐human and human‐to‐swine transmission of influenza in the West African swine industry.     

Associations of antimicrobial uses with antimicrobial resistance of fecal Escherichia coli from pigs on 47 farrow-to-finish farms in Ontario and British Columbia

This study assessed the associations between antimicrobial use and other management practices in pigs and antimicrobial resistance in generic Escherichia coli recovered from feces of weaner and finisher pigs on 39 purposefully selected farrow-to-finish farms in Ontario and 8 in British Columbia. Antimicrobials (n = 13), most frequently penicillins and tetracycline, wer dministered to different age groups of pigs on study farms through various routes of administration. Logistic regression was used to model risk factors to antimicrobial resistance in fecal E. coli of pigs for the following antimicrobials: ampicillin, apramycin, carbadox, cephalothin, chloramphenicol, kanamycin, neomycin, nitrofurantoin, spectinomycin, streptomycin, sulfamethoxazole tetracycline, and cotrimoxazole (trimethoprim and sulfamethoxazole). Use of antimicrobials in weaner pigs compared with use in finisher pigs was associated with resistance in most models. There was phenotypic evidence of different mechanisms of resistance selection, including direct selection [use of carbadox was associated with carbadox resistance (OR = 6.48)]; cross-resistance [use of spectinomycin was associated with streptomycin resistance (OR = 2.29)]; and possible co-selection [ceftiofu use was associated with tetracycline resistance (OR = 6.12)]. These results provide further evidence that use of antimicrobials in pigs selects for resistance among fecal E. coli within and between classes of antimicrobials.     

规模猪场PRRS风险因素分析

猪繁殖与呼吸综合征造成养猪产业巨大经济损失,预防和消除该病已成为世界性难题。本文根据猪繁殖与呼吸综合征流行病学特性,结合规模化猪场生产管理实际,采用风险列举法对规模猪场PRRS传入和场内传播流行的风险因素进行了系统分析。提出规模猪场PRRS风险因素主要包括猪场选址、用品和工具进入、员工和访客进入、活猪运输、饲料运输、死猪处理、粪便管理、猪舍条件、生物传播媒介、引种等外部风险因素和PRRSV状态、管理措施、猪只流动、设施条件、协同病原、猪舍布局等内部风险因素。这为进一步开展规模猪场PRRS风险评估研究,制订科学有效的PRRS风险管理措施奠定了良好基础。     

Associations among antimicrobial drug treatments and antimicrobial resistance of fecal Escherichia coli of swine on 34 farrow-to-finish farms in Ontario, Canada

Logistic regression was used to model associations between antimicrobial treatment and resistance among fecal Escherichia coli of finisher pigs at the farm level. Four sets of potential risk factors representing different levels of refinement of antimicrobial use on farms were modelled on resistance to antimicrobials. Final models for each antimicrobial were constructed from treatment and management variables significant on initial screening, and corrections for overdispersion were made. In general, in-feed antimicrobial treatment of pigs was more consistently associated with an increased risk of resistance than individual-animal treatment. Antimicrobial treatment in starter rations was significant in final models of resistance to ampicillin, carbadox, nitrofurantoin, sulfisoxizole, and tetracycline. Treatment in grower–finisher rations was significantly associated with resistance to ampicillin, spectinomycin, sulfisoxizole, and tetracycline. There was little evidence that in-feed antimicrobials increased the risk of resistance to gentamicin, which is a drug used only for individual-pig treatment in this study population. These results suggest that antimicrobial medication of rations of post-weaning pigs selects for and maintains antimicrobial resistance among E. coli of finisher pigs. Although resistance was common on farms that did not medicate rations of post-weaning pigs, the results indicate that antimicrobial use does increase the risk of resistance to the antimicrobials studied.     

Biosecurity practices on intensive pig production systems in Chile

Chile eradicated classical swine fever (CSF) in April 1998, following a 17-year eradication programme. The authors describe biosecurity levels of pig farms in Chile after the eradication of CSF. A formal survey was administered to 50 large integrated pig farms, which represented almost 60% of the swine population. The main topics on the questionnaire were production, health management, biosecurity, insurance and information about CSF outbreaks in the past. Biosecurity practices were analysed according to the criteria stated by Barcelo and Marco in 1998. A scoring system to measure biosecurity was designed and pig farms were classified according to this score. An adjusted specific measure is discussed as a potential indicator of risk for disease infections. The authors explore associations between biosecurity herd size and insurance policy against CSF.     

Investigation of exposure to swine influenza viruses in Ontario (Canada) finisher herds in 2004 and 2005

The epidemiology of influenza in the North American swine population has changed since the emergence of a triple-reassortant H3N2 influenza virus. Although seen previously in North America, the Ontario swine population had likely been free of viruses of the reassortant H3N2 lineage until 2005. The objective of this study was to investigate the frequency and distribution of exposure to H1N1 and H3N2 subtypes in the Ontario finisher pig population prior to and after the H3N2 outbreak that occurred in 2005. This included investigating prevalence and spatial distribution of positive herds, assessing proportion of random variation at different hierarchical levels, and evaluating selected demographic factors and management procedures as potential risk factors. In total, 919 and 978 sera collected in cross-sectional studies from 46 and 49 finisher herds in 2004 and 2005 were tested by a H1N1 subtype-specific and a H3N2 subtype-specific commercial ELISA. For the H1N1 subtype, the point prevalence of positive herds (>3 reactors) was 19.5% and 30.6% in 2004 and 2005, respectively. For the H3N2 subtype the point prevalence of positive herds (>3 reactors) was 6.5% and 40.8% in 2004 and 2005, respectively. Sera from 2004 that were positive on H3N2 ELISA did not cross-react with any of the H3N2 variants used as antigen on a sequential HI test. Only herds positive for H3N2 subtype in 2005 clustered in space (P<0.01). The H1N1 status in 2005 was associated with the H1N1 status in 2004, and with reported distance to the nearest herd. The H3N2 status in 2005 was associated with reported distance to the nearest herd and a type of replacement gilt source. For H3N2, distance seemed to be important even after controlling for type of gilt source. Most variability in seropositivity was between herds with little variability between pens. This study confirms that in 2005, the epidemic H3N2 subtype co-circulated with endemic H1N1 subtype in the Ontario finisher herds. We concluded that in Ontario, the endemic H1N1 subtype was likely maintained through circulation within herds and sites with common flow. Whereas the transmission of epidemic H3N2 subtype was attributed to local spread, which could include different modes of direct, indirect, and airborne transmission. We emphasize the importance of establishing routine monitoring systems that would allow using molecular tools, and maintaining serum banks as a useful resource for retrospective comparisons.     

Prevalence of Yersinia enterocolitica shedding and bioserotype distribution in Ontario finisher pig herds in 2001, 2002, and 2004

We investigated characteristics of Yersinia enterocolitica infection in Ontario finisher pig herds. Our specific objectives were to estimate or test: prevalence of Y. enterocolitica shedding in finisher pigs, bioserotype distribution, agreement between the herd-level tests based on sampling pig and pooled fecal samples, whether bioserotypes cluster by farms, and whether Y. enterocolitica-positive herds cluster spatially. In total, 3747 fecal samples were collected from 100 farms over the years 2001, 2002, and 2004 (250 total herd visits). Fecal samples were tested by culture and positive isolates were biotyped and serotyped.Apparent pig-level prevalence of Y. enterocolitica was 1.8%, 3.2%, and 12.5% in 2001, 2002, and 2004, respectively. Estimated true pig-level prevalence of Y. enterocolitica was 5.1%, 9.1%, and 35.1% in 2001, 2002, and 2004, respectively. Herd-level prevalence was 16.3%, 17.9%, and 37.5% in 2001, 2002, and 2004, respectively. In all years, the most common bioserotype was 4, O:3, followed by bioserotype 2, O:5,27. Kappa between herd-level status based on pig and pooled samples ranged between 0.51 and 0.68 for biotype 1A and bioserotype 4, O:3, respectively. For 4, O:3, a significant bias in discordant pairs was detected, indicating that pig samples were more sensitive than pooled samples in declaring a herd as positive. Farms tended to be repeatedly positive with the same bioserotype, but positive study farms did not cluster spatially (suggesting lack of between herd transmission and lack of a common geographic risk factor).     

Prevalence of and risk factors for influenza in southern Ontario swine herds in 2001 and 2003

This research included 2 prevalence studies and a risk-factor investigation conducted in 2001 at 93 sites with sows only, finishers only, or both. In 2001, 1300 serum samples from sows in 65 herds and 720 serum samples from finisher pigs in 72 herds were tested for antibodies to swine influenzavirus (SIV) of H1N1 subtype with an enzyme-linked immunosorbent assay (ELISA). In 2003, 1140 serum samples from sows in 76 herds were tested for antibodies to SIV of H3N2 subtype with a hemagglutination-inhibition assay based on A/Swine/Colorado/1/77 and A/Swine/Texas/4199-2/98 isolates. The apparent pig-level H1N1 seroprevalence in 2001 was 61.1% and 24.3% in sows and finishers, respectively. The apparent pig-level seroprevalence in 2003 for H3N2 A/Sw/CO/1/77 and A/Sw/TX/4199-2/98 in sows was 0.6% and 0.7%, respectively. The factors associated with sow-herd H1N1 positivity included pig or farm density at different geographic levels, an external source of breeding pigs, number of animals on site, and decreasing proximity to other barns. Higher-parity sows had higher odds of seropositivity, but there was significant random variability in this association among herds. The odds of finisher-herd SIV positivity were higher with large herd size, high pig farm density, and farrow-to-finish type of farm. Finisher herds were SIV-positive only if source sow herds were positive. Simultaneously, 45% of finisher herds were SIV-negative although sow source herds were positive.