Case Herds Research Articles

Milk yield, age at first calving, and the risk of BSE: An analysis at the farm level in France

The epidemic of bovine spongiform encephalopathy (BSE) in France, as in the UK, has affected dairy cattle much more than beef cattle. However, the intensification of dairy herd management as a risk factor for BSE has not to date been analyzed. For this purpose, two databases were merged: the French Milk Records database, and the French BSE database, which can be considered as being devoid of notification bias since July 2001, when systematic tests were implemented. Only pure Holstein herds were considered, which represent the vast majority of total and BSE-affected dairy herds in France. A case-control study was designed so that 20 control herds were matched to each case herd according to the location of the farm and the year of birth of the index case. Three thousand and forty five farms were included, among which 145 with a BSE case notified between July 2001 and July 2003, and 2900 controls. With respect to the risk of BSE, odds ratios for each class of milk yield and age at first calving were estimated by using conditional logistic regression models with appropriate adjustments to herd size. The two main results were the following: firstly, whereas most Holstein herds, with average production between 7000 and 10,000 kg, had nearly the same BSE risk, a small category of very intensive herds, with annual milk yields above 10,000 kg, were significantly more at risk than the other herds. Secondly, a very early first calving (under 26 months of age) was found to be at risk for BSE as compared to other categories, independently of the milk yield. These results are discussed in the light of the known age-dependent susceptibility to BSE.

Quantifying badger exposure and the risk of bovine tuberculosis for cattle herds in county Kilkenny, Ireland

The objectives of the study were to quantify the levels of badger exposure for cattle and to test the hypothesis that increased badger exposure does not increase the risk of bovine tuberculosis (BTB) in a herd. Information that became available from the targeted removal of badgers over the study period, and from a badger-removal project in county Kilkenny, during 1996–1999 was used. The specific location of cattle within each farm, and the length of time that cattle spent in each farm field during the grazing season, and in the barnyard during winter, was used to build an exposure coefficient to quantify the amount of badger exposure that cattle encountered either on pasture or in the barn. The study design was a matched case-control study in which the control herds were selected using incidence density sampling. During the 4-year study period, 543 badgers were removed and of these 96 badgers were classified as tuberculosis positive; 96 BTB herd breakdowns occurred. There was a significant association between case herds and having a higher badger sett exposure coefficient during 1996–1998. No significant association between case herds and having a higher exposure coefficient based on the number of badgers, or the number of tuberculous badgers, during September 1997–December 1999 was found.

Geographical and temporal aspects of an outbreak of off-flavours in bulk-tank milk in Prince Edward Island, Canada

The geographical and temporal dynamics of the outbreak of off-flavours in bulk-tank milk that occurred between September 2000 and June 2002 in Prince Edward Island (PEI) dairy herds were described using spatial, temporal and space–time scan statistics. Results suggested clustering both in time and space. We found one primary cluster (six case herds, feed off-flavour, October 2000 to January 2001) and two secondary clusters formed of feed (5 case herds, October 2000 to January 2001)) and rancid (five case herds, December 2000 to March 2001) off-flavours, respectively. The relative risk (RR) for producing off-flavoured milk was similar for these three clusters (RR = 6.4).

Risk factors associated with Neospora caninum abortion in Ontario Holstein dairy herds

The objective of this epidemiological study was to identify risk factors for Neospora caninum-related abortions in Ontario Holstein dairy herds. A total of 88 herds, consisting of 5080 cattle, and utilizing Dairy Herd Improvement (DHI) services, were divided into three groups. Case ( n = 30) and first control ( n = 31) herds were selected from 1998 and 1999 fetal abortion submissions to the Animal Health Laboratory, University of Guelph, that were histopathologically positive or negative, respectively, for N. caninum. A second control group ( n = 27) was selected from multiple sources of herds sampled within the previous 4 years that had a low seroprevalence (<7%) to N. caninum. Between May and December 1999, all available cows on all farms, in parity one or greater, were blood sampled. The sera were then analyzed for antibody to N. caninum using a kinetic ELISA. A survey administered at the time of sampling recorded information on housing, animal species present, manure management, reproduction, biosecurity practices, wildlife observations, peri-parturient cow management, herd disease history and nutrition. Production and other herd parameters were obtained from DHI records. Logistic regression indicated that the following parameters were positively associated with a N. caninum abortion in a herd: the N. caninum herd seroprevalence (OR = 1.1) , the total number of dogs on a farm (OR = 2.8), the frequency that dogs were observed defecating in mangers (OR = 2.8), the number of horses on a farm (OR = 3.1), the observed annual rate of retained fetal membranes (OR = 1.2) and the observed annual rate of cows returning to estrus after pregnancy confirmation (OR = 1.2). Factors negatively associated were the frequency of stray cats and wild canids observed on a farm (OR = 0.4 and OR = 0.7, respectively) and the housing of heifers on loafing packs (a housing pen divided into feed manger, scrape alley and bedded pack areas, OR = 0.1).

Prevalence of exposure and infection of Lawsonia intracellularis among slaughter-age pigs

The extent of clinical or subclinical infection associated with Lawsonia intracellularis within Dutch pig herds was uncertain. A case-control study of slaughter age pigs was used to study natural infection within Dutch herds and to compare diagnostic methods. From six case herds where clinical disease had been identified recently, and six disease-free herds, 40 pigs of slaughter-age were examined postmortem. The diagnostic methods used were: serology, gross examination, Haematoxylin and Eosin stain (HE), Warthin-Starry silver stain, Lawsonia-specific indirect immunoperoxidase of the ileum, and PCR of ileum mucosa and colon contents. There were 59% seropositive pigs in case herds and 26% seropositive pigs in control herds. Using immunohistochemistry, 57% of case herds and 46% of control herds were bacteria positive in the ileum mucosa. It was concluded that a majority of Dutch herds contain L. intracellularis infected finisher pigs. In some herds this is associated with clinical outbreaks of acute haemorrhagic enteropathy but in other herds no clinical disease is apparent. Many seropositive pigs in herds without clinical disease had evidence of Lawsonia antigen in sites other than the apical cytoplasm of proliferating epithelial cells, particularly the supranuclear region. It was uncertain whether to classify these pigs as having “recovered” from an infection or whether they have a sub-clinical or chronic form of the disease. We concluded that PCR examination of faeces and serology probably provide more specific results than gross examinations at slaughter, and that a monoclonal antibody-based examination of ileum mucosa should be the accepted screening method for this infection.

Factors affecting the herd level of antibodies against Mycobacterium avium subspecies paratuberculosis in dairy cattle

A case-control study was made of Norwegian dairy herds with high and low herd levels of antibodies against Mycobacterium avium subspecies paratuberculosis. A high proportion of the herds had a...

A case-control study investigating risk factors associated with postweaning Escherichia coli diarrhea in southern Ontario

Objectives: To investigate management, housing, and nutrition factors and potential viral infections that might be associated with postweaning Escherichia coli diarrhea in pigs in southern Ontario. Methods: A case-control study involving 50 herds was conducted. Information on hygiene, management, facilities, management of diarrhea, porcine reproductive and respiratory syndrome virus (PRRSV) status, and nursery feed management was collected through a questionnaire. Rectal swabs and sera collected from ten weaned pigs in each herd were tested for F4+ (K88+) E coli and antibodies to two viral pathogens. Hemolytic E coli-like colonies were tested for agglutination in specific anti-F4 antiserum. Results: Twenty-six herds were designated as cases and 22 as controls. Escherichia coli positive for F4 were isolated from pigs in 15 case herds and 3 control herds. Postweaning diarrhea did not occur in the control herds during the study. In case herds, there were more feeder spaces per pen, the first rations offered in the nursery were more likely to be pelleted feed, and nursery pigs and gestating sows were more likely to be vaccinated against PRRSV than in control farms. Implications: Weaning age, weaning weight, and commingling of pigs, which are commonly considered risk factors for postweaning diarrhea, did not appear to be contributing factors in this study. Vaccination for PRRSV was associated with an increased risk of postweaning diarrhea, which occurred in a variety of herd types and sizes.

Risk factors for infection of sow herds with porcine reproductive and respiratory syndrome (PRRS) virus

In 1992, the porcine reproductive and respiratory syndrome virus (PRRSV) of European type (PRRSV-EU) was introduced in Denmark. By 1996, the virus had spread to approximately 25% of the Danish herds. In January 1996, a modified-live vaccine based on the American type of the virus (PRRSV-US) was used in replacement boars for Danish artificial insemination (AI) centres and from July 1996, the vaccine was used in PRRSV-EU infected herds for prevention of disease. Soon after vaccine introduction, PRRSV non-infected herds experienced outbreaks of disease due to infection with PRRSV-US. In this study, we investigated the risk factors (biosecurity level, animals, exposure from PRRSV-US-infected neighbour herds, semen, herd size, pig density and herd density) for infection with PRRSV-US in a cohort of 1071 sow herds; we used a nested case–control study. The retrospective observation period lasted from June 1996 (when they all were non-infected) to October 1997. Seventy-three non-vaccinated, closed sow herds became infected with the vaccine strain during this period. Each case herd was matched with two control herds from the cohort (controls had not been infected at the time of infection in the case herds). The data were analysed using a Cox-regression model. The hazard of infection increased significantly with exposure from PRRSV-US-infected neighbouring herds, purchase of animals from herds incubating PRRSV-US infection, increasing herd size and purchase of semen from boars at PRRSV-US-infected AI centres. The results are consistent with the modified-live vaccine strain spread to other herds by trade with animals and semen and by neighbour (area) transmission. We suggest that virus spread by aerosols was a frequent mode of transmission.

Evaluation of two enzyme-linked immunosorbent assays for detecting Salmonella enterica subsp. enterica Serovar Dublin antibodies in bulk milk.

Two enzyme-linked immunosorbent assays (ELISAs) for the detecting Salmonella enterica subsp. enterica serovar Dublin antibodies in bulk milk were developed and evaluated for potential use in control programs. The ELISAs were based on either lipopolysacharide (LPS ELISA) or flagellar antigen (GP ELISA). Sensitivity was determined with 79 case herds with a wide range of clinical signs. Specificity was determined with 125 Dutch and 200 Swedish control herds. The relation between antibodies in bulk milk, antibodies in serum, and the level of milk production of individual cows was studied with 61 case herds. The optimal optical density (OD) values of the LPS ELISA and the GP ELISA were determined to be 0.2 and 0.5, respectively. The sensitivities of the LPS ELISA and the GP ELISA were 54 and 63%, respectively, with a specificity of 98% for both ELISAs with samples from the Dutch control herds. The specificities for samples from the Swedish herds were 100% for the LPS ELISA and 95% for the GP ELISA. The sensitivity of the combination of tests was 65% when samples were run in parallel, and the specificity was 100% when samples were run in series, irrespective of whether the samples came from Dutch or Swedish control herds. The variance (R(2)) in the OD value for bulk milk samples could be explained by the percentage of seropositive lactating cows in a herd with the LPS ELISA for 51% of the samples and with the GP ELISA for 72%. The variance in the OD value was best explained by the combination of the percentage of seropositive lactating cows in the herd and the mean log(10) serum antibody titer for that herd (R(2) = 62% for the LPS ELISA and R(2) = 75% for the GP ELISA). Case herds more often tested negative by the ELISA with bulk milk when the percentage of seropositive lactating cows was less than 5%. It is concluded that both ELISAs with bulk milk can be used in control programs to distinguish between infected and noninfected herds. Specificity can be increased by using the two tests in combination. Sensitivity was relatively low for both single tests and both tests combined.

Time to first calving and calving interval in bovine virus diarrhoea virus (BVDV) sero-converted dairy herds in Norway

Dairy herds in Møre and Romsdal County, Norway (regarded as initially free from the bovine virus diarrhoea virus (BVDV) infection) were studied retrospectively from 1992 to 1996. The herd reproductive performance (time to first calving, calving interval, and number of breeding services) was investigated for a potential effect of BVDV sero-conversion. The herd culling pattern — possibly affecting the above measurements — was included for investigation. Two different statistical models were used: the generalised estimating equation (GEE) method and multilevel modelling using Gibbs sampling. Though slightly different estimates resulted, both models agreed on an effect of BVDV in the second year after sero-conversion on the herd average time to first calving by — on an average — 14–16 days. In subsets of case herds testing positive for BVDV antibodies among young stock, the impact on time to first calving tended to be more pronounced by an additional increase of 18 days. No effect on the number of breeding services for heifers or cows was observed (indicating a need to search for other determinants than reduced conception risk). There appeared to be no effect of BVDV on the herd average calving interval. There was a tendency for a higher risk for reporting animals lost/died in sero-converted herds, which we believe might be related to the occurrence of mucosal disease.

Outbreaks of porcine reproductive failure: Report on a collaborative field investigation

During the latter half of 1996, a series of abortion outbreaks occurred, primarily in southeast Iowa, with a clinical picture similar to Porcine Reproductive and Respiratory Syndrome (PRRS) in a naïve herd. After initial foreign animal disease investigations proved negative, a collaborative case-control study was designed. Its objectives were to describe these cases of intense reproductive failure; attempt isolation of viral agent(s), particularly postulated variant strains of PRRS virus; and identify potentialmanagement risk factors. Eligible case herds were those experiencing a current outbreak of abortion and submitting samples to one of the seven participating diagnostic laboratories. Control herds were randomly selected by the investigating veterinary medical officer from a list generated by the referring practitioner. For both case and control herds, sow blood samples and fetal tissues were collected for PRRSV serologyand multiple virus isolations, and management data were statistically analyzed to identify potential risk factors. Seventeen case and 34 control herds were enrolled between June 25, 1997 and November 10, 1998. The PRRS virus was isolated from seven cases and one control operation.Sow sera were PRRS positive on 87% of case premises and 55.7% of control premises. These results suggest that the swine abortion outbreaks beginning in 1996 were more likely due to PRRS virus than to an emerging disease. The US pork industry is vulnerable to emerging diseases, and needs to develop rapid capabilities for recognizing new infections and novel pathogens and strategies for detecting and controlling emerging diseases.

Risk factors for clinical salmonellosis in Virginia, USA cattle herds

Veterinarians working in several Virginia, USA counties noticed an increase in clinical salmonellosis in cattle herds during 1994. We conducted a case–control study to identify potential risk factors for salmonellosis in cattle herds in this region. The potential impact of exposure to poultry-related factors was of particular interest because of the close proximity of poultry and cattle operations in this part of the state. Information was collected from 23 case farms and 23 control farms matched by herd veterinarian, type of enterprise (beef vs. dairy), and location. Faecal and environmental samples were collected during herd visits for bacteriologic culture. Salmonella was isolated from 4.7% of 531 faecal, feed, water, and environmental samples collected. Salmonella Typhimurium was the serotype found most frequently in clinical cases in participating herds and from samples collected during study visits. Case herds had a higher percentage of study samples positive for Salmonella. Potential risk factors were screened using methods for pair-matched data and then analyzed in individual conditional logistic-regression models to estimate odds ratios with 95% profile likelihood confidence intervals. Significant factors included: the number of mature cows in the herd (OR=1.01; CI 1.00, 1.03), percent change in the number of mature cows during 1994 (OR=0.68; CI 0.11, 0.96), having calves usually born in a building rather than outdoors (OR=0.17; CI 0.01, 0.98), poultry manure spread on bordering property (OR=4.00; CI 1.00, 26.50), signs of rodents in cattle-housing or feed-storage areas (OR=2.75; CI 0.94, 9.92), and contact of wild geese with cattle or feed (OR=4.5; 1.16, 29.51).

A space–time cluster investigation of an outbreak of acute respiratory disease in Norwegian cattle herds

Outbreaks of acute respiratory disease occurred in several cattle herds in the south-east regions of Norway during the winter and spring of 1995. The present study was conducted to investigate the temporal and spatial dynamics of the occurrence of acute respiratory disease between January and April 1995 in the affected cattle herds, in two neighbouring counties in Norway. This was done to assess the possible role of an infectious agent in these outbreaks, as well as to investigate the possibility of transmission between neighbouring herds. During the study period, a total of 190 (19.8%) outbreaks occurred in 955 cattle herds located in the defined study area. Descriptive maps of the spatial and spatio-temporal patterns of the outbreak were generated, and two different statistical tests of space–time interaction (the Knox test and k-nearest neighbour test) and a cluster-detection test (the space–time scan statistic) were used. The results of all the space–time interaction tests strongly suggested that the case herds were clustered in time as well as in space. In addition, the space–time scan statistic defined the most-likely cluster to be located within the second veterinary district (Toten where the disease occurred) during 9–15 weeks in 1995. The relative risk of a herd being infected inside the most-likely cluster compared with the surrounding study area was estimated to be 3.59 ( p=0.001). The transmission rate between neighbouring farms therefore appeared to have been higher within this cluster than in the surrounding study area. This study supports the hypothesis that one common source of infection was involved in the outbreaks during the study period.

Characteristics of neospora caninum-associated abortion storms in dairy herds in The Netherlands (1995 to1997)

Abortion storms in 50 dairy herds in The Netherlands were reported in which there was a strong association with Neospora caninum-infection. The duration of the abortion storms ranged from 6 to 65 d (mean 41.5 d). The cumulative proportion of aborting cows ranged from 0.11 to 0.57 (mean 0.26) of the animals at risk. An apparent seasonal influence was noted as most abortion storms occurred during the summer and early autumn. The prevalence of antibodies to N. caninum in 50 herds which had had an abortion storm was compared with that of 100 control herds which had no history of an abortion storm. Seroprevalence was estimated by testing a 20% cross sectional herd sample using a tachyzoite lysate-based ELISA method. Seroprevalence in case herds (range 17 to 87%, mean 51.5%) was significantly higher than that in control herds (range 0 to 53%, mean 13.9%). For most herds the Seroprevalence levels were equal across all age groups, which suggests that the infection had been perpetuated by vertical transmission. In these herds, the abortion storms appeared to be induced by factors causing recrudescence of a N. caninum- infection in chronically infected animals rather than being the result of a recent introduction. In 6 case herds the Seroprevalence in the dairy cows was significantly higher than in the young stock, which may have been attributable to superimposed postnatal infection.

Bovine tuberculosis in Northern Ireland: a case-control study of herd risk factors

This paper presents the results of the first formal epidemiological study in Northern Ireland of risk factors for bovine tuberculosis (TB) associated with farm boundaries, neighbours and wildlife. The study...

Seroepidemiologic study of Neospora caninum in dairy herds

Objective To identify risk factors associated with Neospora caninum infection in dairy herds in Québec and to estimate seroprevalence in case and control herds. Design Herd-based case-control and seroprevalence study. Animals 3,059 cows from 24 case and 22 control dairy herds in Québec. Procedure Blood samples were obtained from each cow, and sera were tested, using an ELISA, for antibodies to N caninum. Owners of herds answered questionnaires requesting information on demographics and herd management practices. Seroprevalence was compared between case and control herds, using the Mann-Whitney test. Risk factors were compared between case and control herds, using logistic regression. Results All case herds and 73% of control herds had at least one seropositive cow. Median seroprevalence was significantly greater among case herds (22.5%) than among control herds (7.5%). Dogs were found most often and in greatest numbers on farms housing case herds compared with control herds during the past 3 years. Clinical Implications Although the exact role that dogs have in transmission of N caninum in dairy herds needs to be elucidated, dogs should have limited access to barns and cattle.(J Am Vet Med Assoc 1998;213:1595–1598)

Risk factors for brucellosis seropositivity of goat herds in the Mexicali Valley of Baja California, Mexico

A case-control study was conducted in the Mexicali Valley to identify risk factors for goat-herd seropositivity for Brucella melitensis. Nineteen case herds (≥2 positive results with the 8% rose bengal plate test (RBT)) and 55 control herds (zero positive results in RBT), matched for herdsize and geographic location, were enrolled. Conditional logistic regression was used to construct a multivariable model of the odds of seropositivity using variables assessed in a questionnaire administered to goat ranchers. The final model for herd seropositivity included increased risk from importation of goats from other Mexican states, the presence of La Mancha breed does, and the presence of does born outside the herd. Increasing herdsize was also highly significant ( p<0.01). In addition, a significant ( p<0.05) positive association was found between the presence of seropositive dogs (as assessed by RBT) and seropositive goats on the same ranch.

Epidemiologic herd-level assessment of causative agents and risk factors for winter dysentery in dairy cattle

Abstract Objective To test the association between exposure to bovine coronavirus (BCV) and outbreaks of winter dysentery (WD) in dairy herds and to examine other risk factors for outbreaks of WD in dairy herds. Animals 12 dairy herds in Ohio affected with WD (case herds). For each case herd, 2 unaffected herds from the same area were concurrently used as control herds. Procedure A case-control study was conducted, using herds as the unit of investigation. Multivariate logistic regression modeling was used to identify risk factors for contracting disease. Results 4 factors appeared to increase a herd's risk for WD: increase in herd prevalence of adult cows that had a fourfold or more increase in BCV serum IgG antibody titer; increase in herd prevalence of adult cows that had a fourfold or more increase in bovine viral diarrhea virus (BVDV) titer; housing cattle in tie-stall or stanchion barns rather than free-stall facilities; and use of equipment to handle manure and subsequently handle feed. The adjusted population-attributable risk for these variables was 71, 43, 53, and 31%, respectively, and 99% overall, indicating that these variables had considerable impact on WD outbreaks for the study population. Conclusions and Clinical Relevance In dairies in Ohio, recent herd exposure to BCV appeared to increase the risk for WD outbreaks. Some WD outbreaks might have been associated with acute BVDV infection. Certain housing and management practices may have increased the risk of an outbreak of WD. (Am J Vet Res 1998;59:994–1001)

Detection of Lawsonia intracellularis, Serpulina hyodysenteriae, weakly beta-haemolytic intestinal spirochaetes, Salmonella enterica, and haemolytic Escherichia coli from swine herds with and without diarrhoea among growing pigs

A polymerase chain reaction (PCR) was optimized to detect Lawsonia intracellularis in faeces from naturally infected pigs. By combining a boiling procedure to extract DNA and a nested PCR procedure, a detection limit at 2×10 2 bacterial cells per gram of faeces was achieved. The optimized PCR was used together with conventional culture techniques to detect Serpulina hyodysenteriae, weakly beta-haemolytic intestinal spirochaetes (WBHIS), Salmonella enterica, and haemolytic Escherichia coli, in a case control study to examine selected risk factors for the development of diarrhoea in growing pigs. Herds with diarrhoea were selected as cases and randomly chosen herds without diarrhoea were chosen as controls. Infection with L. intracellularis significantly enhanced the chance of diarrhoea. S. hyodysenteriae, WBHIS group IV ( Serpulina pilosicoli), and S. enterica were isolated only from case herds which indicate that these species may influence the development of diarrhoea. In addition, herd-type had a significant impact, that is specific pathogen-free herds showed an odds ratio at 0.2 relative to conventional herds for the development of diarrhoea.

Epidemiology of Neospora caninum Infection in Québec Dairy Herds

A case-control study was undertaken on 46 dairy herds located throughout the province of Quebec. Case herds (N=24) were herds that had a confirmed Neospora caninum abortion between 1993 and 1995. A questionnaire was administered on site to the dairyman to describe the management, feed storage, abortion history, herd health practices, calf health and presence of wild and domestic animals on the premises.