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1.
Of swine from 104 herds, 2,616 were tested for antibodies against Toxoplasma gondii, using an ELISA. Data were analyzed according to swine type, herd size, facility type, and season. The true prevalence of toxoplasmosis was estimated as 5.4% among finishing swine and 11.4% among sows and gilts. Herds with less than 100 breeding swine were significantly (P less than 0.05) more likely to be infected than were herds with greater than or equal to 100 breeding swine. The rate of seropositivity in breeding swine was approximately the same in infected herds, regardless of herd size. Herds with finishing swine maintained in total confinement were as likely to become infected as were herds maintained in other types of facilities, but infected herds with finishing swine maintained in confinement appeared to have a lower in-herd prevalence than did herds maintained in other types of facilities (P = 0.09). Seasonal effects were not observed, and prevalence remained relatively constant throughout the year.  相似文献   

2.
Swine and free-living nonporcine species from 20 Iowa swine farms were surveyed for antibodies against encephalomyocarditis virus (EMCV). The microtitration serum neutralization test was used, and antibody titers ≥ 1:8 were considered positive. The overall prevalence in 267 sows in various groups was 37.8% (range 20-86%). The prevalence in sows maintained in total confinement was significantly lower than in sows not maintained in total confinement (p = 0.01). Prevalence in sows was not associated with other herd characteristics or with abundance estimates of rats and/or mice.

Free-living animals tested included 74 domestic cats (Felis domestica), 203 house mice (Mus musculus), 15 mice of the genus Peromyscus, nine Norway rats (Rattus norvegicus), 34 opossums (Didelphis virginiana), 14 raccoons (Procyon lotor), and seven striped skunks (Mephitis mephitis). Of these, the only seropositive animals were two domestic cats.

The results of this study failed to implicate the free-living species surveyed as important reservoirs of EMCV for swine, and suggested that swine themselves and/or some species not included in this survey are the main reservoir of EMCV for swine in Iowa.

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3.
The aim of this study was to provide baseline data on the population and herd Salmonella seroprevalence in sows and finishers. For the population estimates in 1996 and 1999 and the herd prevalences for sows and gilts, blood samples from swine vesicular disease (SVD) and pseudorabies monitoring programmes were used and tested in an indirect Salmonella enzyme-linked immunosorbent assay (ELISA). The herd prevalence for finishers was determined using blood samples collected at two slaughterhouses.The population prevalence for finishers in 1996 and 1999 was 23.7 and 24.5%, respectively, and for sows 40.5 and 60.4%, respectively. The prevalence in free range (FR) finishers was significantly higher (44.6%) than in intensively housed finishers in 1999, identifying a hazard group for possible extra pork and pork product contamination. Of 406 finishing herds, 9% were completely seronegative for Salmonella (cut-off OD%>10). Of these 406 finishing herds, 69.7% had Salmonella-status I (low prevalence), 21.7% status II (moderate prevalence) and 8.6% status III (high prevalence) (cut-off OD%>40). In 46 multiplying sow herds, 20 breeding sow herds and 20 matching replacement gilt herds, the average herd prevalences were 54, 44.4 and 19.3%, respectively. Two gilt herds were completely seronegative. The prevalence in the gilt herds was never higher than in the matching breeding sow herds. Agreement on methodology and calibration of ELISA tests would make these results comparable between countries and is a prerequisite for a co-ordinated and integrated program to reduce Salmonella in pork in the European Union.  相似文献   

4.
In Belgium, pseudorabies in swine has been the subject of a mandatory eradication programme since 1993. From December 1995 to February 1996, a survey was conducted in the five provinces of northern Belgium to estimate the provincial pseudorabies virus (PRV) herd seroprevalence. Seven hundred and twenty randomly selected herds were included in this survey. To detect recently infected animals, only young sows were sampled. The results show that 44% of these herds had an important number of PRV-seropositive young sows. The highest herd seroprevalence was observed in West Flanders (68%), followed by Antwerp (60%), East Flanders (43%), Limburg (18%), and Flemish Brabant (8%). Assuming a diagnostic test sensitivity and specificity of 95% and 99%, respectively, and a true PRV within-herd prevalence of 43%, the overall true PRV herd prevalence was estimated to be 35%. A logistic multiple-regression revealed that the presence of finishing pigs was associated with a two-fold increase in odds of a herd being seropositive (odds ratio (OR)=2.07, 95% confidence interval (CI)=1.31–3.26); a breeding herd size ≥70 sows was associated with a four-fold increase in odds of a herd being seropositive (OR=4.09, 95% CI=2.18–7.67); a pig density in the municipality of ≥455 pigs/km2 was associated with a 10-fold increase in odds of a herd being seropositive (OR=9.68, 95% CI=5.17–18.12). No association was detected between the PRV herd seroprevalence and purchase policy of breeding pigs (purchased gilts, or use of homebred gilts only).  相似文献   

5.
The objective of the present study was to investigate culling guidelines for gilts and sows in Japanese commercial herds and to compare differences between culling guidelines and actual culling practices in different herd productivity groups. A questionnaire survey was undertaken to obtain information on culling guidelines in 115 commercial swine herds that participated in the PigCHAMP data-share program. The questionnaire included questions on guideline values for culling intervals and the number of conception failure occurrences that would trigger a culling decision to be made. Ninety-two of the 115 herds (80.0%) returned appropriate data for the study and were included in the present study. In addition to questionnaire data, culling data regarding the actual culling intervals and number of reservices for gilts and sows culled during 2007 to 2008 were also obtained for the same herds from a PigCHAMP database. Culled gilts and sows were divided into 4 female groups on the basis of the stages of their reproductive life when they were culled: unmated gilts, mated gilts, unmated sows, and mated sows. Culling intervals in unmated gilts and sows were defined as the number of days from birth or weaning to culling, respectively, whereas in mated gilts and sows culling intervals were the number of days from last service to culling. Three herd productivity groups were formed on the basis of the upper and lower 25th percentiles of pigs weaned mated female(-1)·yr(-1):high-,intermediate- or low-performing herds. For unmated gilts and sows actual culling intervals were 15 d shorter than the guideline culling intervals in the surveyed data submitted by producers (P < 0.05). This shorter actual culling period for unmated gilts and sows did not vary significantly between herd productivity groups in any parity. However, for mated gilts and sows the actual culling intervals were at least 30 d longer than the guideline culling intervals (P < 0.05). Guideline and actual culling intervals for mated gilts and sows were at least 10 d shorter in high-performing herds than in low-performing herds (P < 0.05). High-performing herds had lower proportions of sows culled after the second reservice than intermediate- or low-performing herds in parity groups 0 to 5 (P < 0.05). In conclusion, culling guidelines for mated sows differed between herd productivity groups, and culling guidelines for mated gilts and sows were not strictly followed in any herd group in the commercial herds.  相似文献   

6.
Samples of serum from 76 gilts, 1440 sows, 1473 piglets and 3093 finishing pigs from 96 farrow-to-finish herds were tested for antibodies to encephalomyocarditis virus (EMCV) in microtitre serum neutralisation tests employing two strains of virus, one associated with myocarditis and the other with reproductive failure. The total seroprevalence of EMCV infection was 2.48 per cent. There was no significant difference between the seroprevalence of the reproductive failure strain (1.6 per cent) and the myocardial strain (1.85 per cent). The seroprevalence was higher in the gilts (6.57 per cent) and sows (5.13 per cent) than in the piglets (1 per cent) and finishing pigs (1.84 per cent), and the highest titres were observed in the sows (1:540) and finishing pigs (1:640). In the gilts, the difference in seroprevalence between the reproductive failure strain (3.95 per cent) and the myocardial strain (5.33 per cent) was wider than in the other groups.  相似文献   

7.
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).  相似文献   

8.
Encephalomyocarditis virus (EMCV) has been found on pig farms worldwide and can cause myocarditis in young pigs and reproduction disorders in sows. So far, clinical signs of EMCV have not been reported in the Netherlands. The aim of this study was to estimate the seroprevalence of EMCV infection in Dutch sow herds. A total of 277 Dutch sow herds were randomly selected, from which 3237 serum samples were collected. These samples were tested for EMCV antibodies using the virus neutralization test (VN test). The apparent prevalence of EMCV antibodies was 9.3% in the total sow population, and the apparent herd prevalence was 58.8%. An exact determination of the prevalence of EMCV infections in the Dutch sow population was not possible because the characteristics of the VN test under field circumstances were not known. However, Dutch sow herds seem to be infected with EMCV because the distribution of positive blood samples in the tested sow population was significantly different from that expected if random false-positive reactions had occurred.  相似文献   

9.
A total of 796 sows and gilts from 30 Danish sow herds were examined three times at intervals of 6 weeks for serum antibodies to Leptospira bratislava by the microscopic agglutination technique (MAT) test. The prevalence of seroreactors with positive titer values, 1:100, at the three successive tests were 2.7%, 2.5% and 2.9%; 4.5% of the animals were positive in at least one of the three tests, and 2.2% showed a greater than two-fold rise in titer between two consecutive samplings. Of the 30 herds, 21 (70%) had ever-positive within-herd prevalences in sows and gilts of 4–13%. The risk of a herd having one or more positive sow was positively associated with a herd size of > 141 sows, and distinct regional differences in the prevalence of positive herds were observed. The reproductive performance of the 21 herds with seroreactions was poorer than the performance of the nine herds without positive reactions concerning the variables: ‘days from weaning to last service’ (2.7 days more, P = 0.07), ‘percentage of sows returning to heat’ (4.0 percentage units more, P = 0.03), ‘services per farrowing’ (0.04 more, P = 0.04), ‘farrowing percentage’ (4.3 percentage units lower, P = 0.06), and ‘stillborn pigs per farrowing’ (0.16 more, P = 0.02). No association between the MAT serological status of the herd and the incidence of medical treatments of sows and gilts could be found. A high prevalence and low cumulative incidence of seroreactors was demonstrated in first-parity gilts, followed by a low prevalence and cumulative incidence from parity 2 to 3, and a high prevalence and cumulative incidence at the fifth parity.  相似文献   

10.
Strategies for the elimination of pseudorabies virus (PRV) from swine herds include test and removal, offspring segregation, and depopulation/repopulation. The prevalence of PRV in a herd is a major factor in selection of the most appropriate strategy. The purpose of the study reported here was to describe the prevalence of PRV in adult swine in PRV quarantined herds in Minnesota, and to determine herd factors associated with the seroprevalence. Questionnaires describing the health history of the herd, management practices, and design of the swine facilities were obtained from the owners of 142 quarantined herds. Blood was collected from 29 finishing pigs over the age of 4 months, up to 29 adult females, and all herd boars. Factors considered to be significant in a bivariate analysis were combined in a stepwise multiple logistic regression analysis. The prevalence of PRV-seropositive adults in each herd was bimodally distributed among the 142 herds. In 42 (30%) of the herds, none of the females tested was seropositive, which represented the lower mode. At least 90% of the adults tested were seropositive in 30 (21%) of the herds and represented the higher mode. The odds of the breeding swine of a given herd having a PRV seroprevalence of greater than or equal to 20% as compared with having a seroprevalence of less than 20% was 1.654 times higher per 50 adults in the herd, 13.550 times higher if the finishing pigs were seropositive, 2.378 times higher if sows were housed inside during gestation, and 1.481 times lower per number of years since the imposition of quarantine.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   

11.
In 2005, methicillin-resistant Staphylococcus aureus was found in pig herds and in humans in contact with pigs. To determine the prevalence of, this now-called livestock-associated (LA) MRSA among pig herds in The Netherlands and to identify and quantify risk factors, an observational study of 202 pig herds was performed between 2007 and 2008. Five environmental wipes and 60 nasal swabs from each herd were collected, and microbiological analysis was performed on single environmental samples and pooled nasal samples. A herd was considered MRSA-positive if ≥1 sample tested positive. The prevalence of MRSA-positive herds was 67% in breeding herds and 71% in finishing herds. Multivariable logistic regression analysis was then performed on data from 171 breeding herds. The number of MRSA-positive herds increased from ～30% at the start to ～75% at the end of the study, most likely due to transmission between herds. The prevalence of MRSA increased with herd size, as ～40% of smaller herds (<250 sows) were MRSA-positive compared to >80% of larger herds (>500 sows). Other risk factors (e.g. antimicrobial use, purchase of gilts and hygiene measures) were not significantly associated with MRSA, though associated with herd size. Herd size appeared to be a compilation of several factors, which made larger herds more often MRSA positive.  相似文献   

12.
In theory, pseudorabies virus (PRV) may be eliminated from any size of breeding herd by phased test and removal if replacement gilts are not infected with PRV, culling decisions are partially based on PRV status, and the cull rate is higher than the incidence rate of PRV. Annual cull rates are commonly at least 50%, but little information exists on the incidence of PRV within enzootically infected swine herds. The purpose of this study was to develop a method by which spread of PRV could be detected among breeding swine within enzootically infected herds and to determine the incidence of PRV infection in these herds. Data were collected from 17 herds that were quarantined for PRV and ranged in size from 120 to 1,100 sows. At each herd, within the first 5 days of introduction, a group of approximately 30 replacement gilts was identified, vaccinated with a glycoprotein X-deleted PRV vaccine, and blood sample was collected. The owner of 1 herd had a nonvaccinated breeding herd and elected to leave incoming gilts nonvaccinated. After vaccination, blood samples were collected every 1 to 2 months for an average of 13.6 months. Serum samples from vaccinated gilts were tested for antiglycoprotein X antibodies by a specific differential ELISA. Samples from nonvaccinated gilts were evaluated by serum neutralization test. Product-limit method was used to estimate the probability of not becoming infected with PRV. Spread was detected in 7 of 8 herds that had more than 400 sows and in 2 of 9 herds that had less than 400 sows.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   

13.
Vaccination programs to eradicate pseudorabies virus (PRV) are being considered in several countries. Knowledge of factors that influence PRV transmission within vaccinated breeding herds may contribute to the success of these programs. A multivariate analysis of variance of the PRV-seroprevalence in sows in 209 herds (average seroprevalence 67.0% per herd) in the southern Netherlands revealed the following risk indicators: (1) presence of finishing pigs; (2) production type (producers of finishing piglets had a higher seroprevalence than producers of breeding stock); (3) vaccination of the sows during nursing (in comparison with vaccinating all sows simultaneously at 5 month intervals, or vaccination during the second half of gestation); (4) pig density in the municipality where the herd was located (seroprevalence increased with higher pig density); (5) herd size less than 100 sows; (6) average within-herd parity (seroprevalence increased with higher withinherd parity); (7) replacement pigs raised on the premises; (8) vaccine strain administered to the sows. Purchase policy (breeding pigs purchased between 10 weeks and 7 months of age, or use of home-bred gilts only) did not significantly contribute to the multivariate model.  相似文献   

14.
Data were collected from 39 Minnesota swine farms quarantined for pseudorabies virus (PRV) infection. Each herd was serologically evaluated for antibodies to PRV in the sows, boars, and finishing pigs. To identify PRV-seropositive swine herds, the Kappa statistic was used to estimate the effectiveness of evaluating the PRV serostatus of boars or of finishing pigs. Using the serostatus of all herd boars, the sensitivity (with 95% confidence interval) of identifying PRV-infected herds was 58 +/- 22%, and the specificity was 100 +/- 0%; Kappa statistic was 0.55. Using the serostatus of a representative sample of finishing pigs, the sensitivity of identifying PRV-infected herds (with 95% confidence interval) was 63 +/- 22%, and specificity was 87 +/- 23%; Kappa statistic was 0.40. The PRV serostatus of herd boars or of a representative sample of finishing pigs did not accurately reflect the PRV serostatus of the herd.  相似文献   

15.
Sera were collected from 6 large farrow-to-finish swine herds infected with pseudorabies virus (PRV) in Illinois. All herds were participating in the Large Herd Cleanup Study, a USDA-initiated project to evaluate the feasibility of eradicating pseudorabies from large farms (greater than 400 sows) by use of a combination of vaccination and management changes. Herd size ranged between 425 and 1,500 breeding females. Between April and July 1990, sera for measurement of PRV antibodies were obtained from 113 to 156 sows and 112 to 162 finishing pigs (body weight greater than 70 kg)/herd. Duplicate sera from 30 sows and 30 market-weight pigs/herd were obtained for measurement of serum antibodies to the following associated organisms: swine influenza virus, transmissible gastroenteritis virus, encephalomyocarditis virus, Actinobacillus pleuropneumoniae, Eperythrozoon suis, and 6 serovars of Leptospira interrogans. Prevalence of PRV antibodies attributable to field virus infection ranged between 53.8 and 100% for sows and between 0.7 and 97.3% for finishing pigs, as determined by the appropriate differential test for the vaccine being used on each farm. In only 1 herd, PRV seroprevalence was increased with higher sow parity. For associated infections, the risk of seropositivity attributable to PRV was not significant (for most infections) on all farms and varied among farms. Thus, pseudorabies did not appear, in general, to increase susceptibility to infection with other disease agents.  相似文献   

16.
Multiparous sows from 19 central Iowa swine farms were tested for antibodies against Toxoplasma gondii by the modified direct agglutination test. Antibody titers of 1:32 or greater were considered positive. Rodents, domestic cats, opossums (Didelphis virginiana), raccoons (Procyon lotor) and striped skunks (Mephitis mephitis) were live-trapped on each farm and similarly tested for antibodies. The overall prevalence of T. gondii antibodies in the species tested was 39/273 (14.3%) swine, 31/74 (41.9%) cats, 2/588 (0.3%) house mice (Mus musculus), 0/21 mice of the genus Peromyscus, 0/9 Norway rats (Rattus norvegicus), 1/34 (2.9%) opossums, 4/14 (28.6%) raccoons and 2/7 (28.6%) striped skunks. The overall prevalence was significantly greater in adult cats versus juvenile cats, adult male cats versus adult female cats, and adult raccoons versus juvenile raccoons. The prevalence of T. gondii antibodies in sows was compared with the prevalence in each non-swine species on a farm basis in order to identify existing associations. The prevalence in sows (and each of the non-swine species) was also analyzed on a farm basis for association with farm characteristics or swine management practices, including the degree of confinement of swine, population size and average parity of breeding female swine, estimated cat population size, and estimated mouse and rat abundance. Average titers of seropositive animals were compared on a species basis. The prevalence in sows which were totally and continuously confined was lower than that in sows which were not totally and continuously confined. The prevalence in sows from farms with an average parity of less than 2.0 was significantly lower than that in sows from farms with an average parity of 2.0 or greater. These results suggested that the prevalence of T. gondii antibodies in swine increased with age and that prevalence in swine could be reduced through total confinement. No associations could be established between prevalence in sows and prevalence in non-swine species or other farm characteristics/swine management practices. However, the high prevalence of T. gondii antibodies in cats suggested that fecal contamination of the environment by cats may be the most significant source of toxoplasmosis for swine. The extremely low prevalence of T. gondii antibodies in house mice suggested that this species was not an important source of T. gondii for swine in Iowa.  相似文献   

17.
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.  相似文献   

18.
Serological surveys were conducted on the gilts and adult sows in 4 herds endemically infected with porcine parvovirus. The study assessed the influence of the type of management of breeders on the spread of virus infection and the influence of endemic parvovirus infection on reproductive parameters of the herd. The practice of holding gilts and sows in groups did not reliably promote infection or maintain a 100% level of active immunity amongst adult sows in 2 of 3 group husbandry herds. In the 4 herds, the prevalence of adult sows (greater than 12 months) with active immune haemagglutination inhibition titres (greater than or equal to 256) ranged between 44% and 100%, while between 0% and 100% of gilts (6 to 12 months of age) had active immune titres. Fully susceptible gilts older than 9 months of age held in groups, failed to become infected by 12 months of age on farms endemically infected with PPV. In 2 herds a continued low infection rate of gilts resulted in increasing the potential of breeding animals becoming susceptible to parvovirus infection as infected sows were replaced by noninfected gilts. In both herds, epidemics of parvovirus infection followed, which were characterised by an increase in reproductive failure. Parvovirus infection during the first 70 days of pregnancy reduced the average number of piglets born alive per litter by 1.6 piglets (p less than 0.05). This was due to the combined effect of more piglets being born dead per litter and an overall reduction in litter size.  相似文献   

19.
Culling patterns in selected Minnesota swine breeding herds.   总被引：2，自引：0，他引：2       下载免费PDF全文
This investigation was conducted to study culling patterns in swine breeding herds. Data were obtained from 89 Minnesota swine breeding herds and included 5918 sows and 1324 gilts for a total of 7242 culled femaled. Each producer was involved for 12 consecutive months. They were asked to record every female that was removed from the herd, the reason for its culling and its parity. The annual culling rate for the sample averaged 50%, but varied considerably between herds ranging from 15% to 85%. Culled females had produced an average of 3.77 litters. Half of the females culled did not produce more than three litters. Reproductive failure accounted for 32% of all removals. The average parity of the females culled in that category was only 2.37: almost 33% were gilts. Failure to conceive represented 75% of all females culled for reproductive failure. Proportionally, culling as a result of anestrus was higher in gilts. It accounted for 33% of all gilts culled for reproductive failure which was twice as much as for sows. Inadequate performance accounted for 17% of all removals. These sows had produced an average of 5.11 litters. These results indicated that few animals were culled on the basis of first litter performance. Old age comprised 14% of all removals and the average parity at culling for this category was 7.11. Death accounted for 12% and the average parity for these females was 3.40. Locomotor problems and peripartum problems were the cause of 28% and 23% of all deaths, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   

20.
The use of an ELISA that can differentiate between swine infected with pseudorabies virus (PRV) and swine vaccinated with a specific PRV vaccine was evaluated on an individual and herd basis, and a system for interpreting ELISA results on a herd basis was developed. In 17 herds, recently introduced replacement gilts, seronegative for PRV, were vaccinated with a thymidine kinase- and glycoprotein X (gpX)-deleted vaccine. After vaccination, blood samples were collected from these gilts approximately every 1 to 2 months for up to 19 months. Serum samples were analyzed for antibodies to gpX antigen, using a commercially available ELISA kit according to the manufacturer's protocol. Herd status was determined as positive, suspect, or negative, according to the serum sample:negative control (S:N) values of the samples collected from the herd. From the 17 herds, 130 evaluations were performed. On 49 (38%) of the 130 herd evaluations, 1 or more gilts had suspect test results. Additional testing was required in 19 (39%) of these 49 herd evaluations to determine the PRV infection status of the herd. Status of herds having gilts with suspect results and no positive results was usually negative after retesting. Herds having gilts with positive results were unlikely to have negative status after retesting.  相似文献   

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