Evaluation of the Voluntary Johne's Disease Herd Status Program as a source of replacement cattle

The objectives of this study were to evaluate the perceived value that enrolled producers gained from participation in the Voluntary Johne's Disease Herd Status Program (VJDHSP), and to evaluate the risk of infection with Mycobacterium avium ssp. paratuberculosis (MAP) of cattle reared in a presumed Johne's free environment vs. cattle raised in an environment of unknown Johne's status. Producers enrolled in level 3 or 4 of the VJDHSP (98 and 99% probability, respectively, of being free from MAP infection) were interviewed via telephone. Producers were asked questions pertaining to their participation in the VJDHSP, and asked to identify herds to which they had sold replacement heifers. These cattle were presumed to be uninfected before sale. Fifty-nine cattle (identified as having been purchased into infected herds as heifers) were identified as having been raised in uninfected herds and sold to producers with herds of unknown Johne's disease status. On the purchasing farm, fecal and blood samples were collected from each cow of VJDHSP origin and 3 randomly selected home-reared cows per VJDHSP cow matched by lactation as controls. Samples were tested using commercial ELISA (serum) and bacterial culture (feces). Results indicated that enrolled producers saw value in VJDHSP participation, and that cattle reared in VJDHSP herds were less likely to be infected with MAP than herdmates as measured by serum ELISA MAP antibody and by fecal culture. This study provides evidence of the value of the VJDHSP in providing economic value to participants and supports the promotion of VJDHSP herds as a source of replacement cattle of low infection risk for MAP.     

Comparison of individual, pooled, and composite fecal sampling methods for detection of salmonella on u.s. Dairy operations

The objectives of this study were to estimate the prevalence of Salmonella for individual, pooled, and composite fecal samples and to compare culture results from each sample type for determining herd Salmonella infection status and identifying Salmonella serovar(s). During the U.S. Department of Agriculture National Animal Health Monitoring System Dairy 2007 study, data and samples were collected from dairy operations in 17 major dairy states. As part of the study, composite fecal samples (six per operation) were collected from cow areas, such as holding pens, alleyways, and lagoons, where manure accumulates. Fecal samples also were collected from individual cows (35 per operation), and fecal sample pools were created by combining samples from 5 cows (7 per operation). A total of 1,541 composite fecal samples were collected from 260 operations in 17 states, and 406 (26.3%) of these samples were culture positive for Salmonella. Among the 116 operations for which all three sample types were obtained, 41.4% (48 operations) were Salmonella culture positive based on individual samples, 39.7% (46 operations) were positive based on pooled samples, and 49.1% (57 operations) were positive based on composite fecal samples. Relative to individual samples, the sensitivity of composite fecal samples for determining herd infection status was 85.4% and the sensitivity of pooled fecal samples was 91.7%. On 33.6% of operations (39 of 116), Salmonella was cultured from all three fecal sample types (individual, pooled, and composite), and 20 (51.3%) of these operations had exactly the same serovar in all three sample types. Use of composite fecal samples is less costly and time-consuming than use of individual or pooled samples and provides similar results for detecting the presence and identifying serovars of Salmonella in dairy herds. Therefore, composite sampling may be an appropriate alternative to culture of individual samples when assessing Salmonella status in dairy herds.     

Factors associated with fecal shedding of verotoxin-producing Escherichia coli O157 on dairy farms.

Fecal samples were collected from 4,361 dairy cows on 91 dairy operations between 26 February and 8 July 1996. Fecal samples were cultured for Escherichia coli O157, and positive isolates were probed for verotoxin-producing genes. A total of 52 (1.2%) fecal samples on 22 (24.2%) operations were positive for verotoxin-producing E. coli O157. Herds in which samples were collected on or after 1 May 1996 were significantly more likely to test positive than herds sampled before that date (odds ratio = 7.7). Herds maintained on farms on which alleyways were flushed with water to remove manure were 8.0 times more likely to have samples test positive for verotoxin-producing E. coli O157 than were herds maintained on farms cleaned by use of other methods of manure removal.     

Environmental sample characteristics and herd size associated with decreased herd-level prevalence of Mycobacterium avium ssp. paratuberculosis

Environmental sampling is an effective method for estimating regional dairy herd-level prevalence of infection with Mycobacterium avium ssp. paratuberculosis (MAP). However, factors affecting prevalence estimates based on environmental samples are not known. The objective was to determine whether odds of environmental samples collected on farm changed culture status over 2 sampling times and if changes were specific for location and type of housing (freestall, tiestall, or loose housing), the sample collected (i.e., manure of lactating, dry, or sick cows; namely, cow group), and effects of herd size. In 2012–2013 [sampling 1 (S1)] and 2015–2017 [sampling 2 (S2)], 6 environmental samples were collected and cultured for MAP from all 167 (99%) and 160 (95%) farms, respectively, in the province of Saskatchewan, Canada. Only the 148 dairy farms sampled at both sampling periods were included in the analysis. A mixed effects logistic regression was used to determine whether differences between sampling periods were associated with herd size and sample characteristics (cow group contributing to environmental sample, type of housing, and location). In S1 and S2, 55 and 34%, respectively, of farms had at least 1 MAP-positive environmental sample. Correcting for sensitivity of environmental sampling, estimated true prevalence in S1 and S2 was 79 and 48%, respectively. Herds with >200 cows were more often MAP-positive than herds with <51 cows in both S1 and S2. The percentage of positive samples was lower in S2 compared with S1 for all sampled areas, cow groups contributing to samples, types of housing where samples were collected, and herd size categories. However, samples collected from dry cow areas had the largest decrease in MAP-positive samples in S2 compared with all other cow group samples. Herds that were MAP-negative in S1 with a herd size 51 to 100 or 101 to 150 were more likely to stay MAP-negative, whereas MAP-positive herds with >200 cows more frequently stayed MAP-positive. No difference was observed in the odds of a sample being MAP-positive among housing types or location of sample collection in both sample periods. Of all farms sampled, 104 (70%) did not change status from S1 to S2. In conclusion, when herd-level MAP prevalence decreased over the 3-yr interval, the change in prevalence differed among herd size categories and was larger in samples from dry cow areas. It was, however, not specific to other characteristics of environmental samples collected.     

Comparison of fecal pooling strategies for detection of Mycobacterium avium ssp. paratuberculosis in cattle

In herds with typical moderate to low within-herd prevalence, testing for Mycobacterium avium ssp. paratuberculosis (MAP), the infectious agent of Johne's disease, will be more cost-effective if individual fecal samples are cultured in composite pools. However, sensitivity to classify a pool containing 1 or more positive individual samples as positive may depend on pool size and number of individual positive samples within a pool. Fecal samples collected from 994 dairy cows sampled at slaughter were cultured to detect MAP. Culturing was done both individually and as composite pooled samples using the TREK ESP Culture System II broth medium (Thermo Fisher Scientific, Trek Diagnostic Systems Inc., Cleveland, OH). Composite samples consisted of pools containing feces from 3, 5, 8, 10, or 15 cows. The number of individual fecal culture-positive cows within each pool ranged from 0 to 4. Culture of individual fecal samples detected MAP in 36 (3.6%) of the 994 cows. Individual samples that were detected within the first 50 d by TREK ESP Culture System II were more likely to lead to a positive pool result. In total, 840 pooled fecal samples were examined for presence of MAP, and of those, 272 pools actually contained feces from fecal culture-positive cows. The crude sensitivity (proportion of pools that contained at least 1 fecal-positive cow that tested positive) for pools of 3, 5, 8, 10, and 15 was 47, 67, 44, 59, and 39%, respectively. Across pools, an increase of the number of fecal culture-positive samples from 1 to 2 enhanced overall crude sensitivity from 44 to 71%. However, sensitivity did not further increase for pools with 3 or 4 fecal culture-positive samples (63 and 60%, respectively). Additionally, a simulation analysis assessing probability of pooled fecal samples being positive in herds of 50 and 100 cows was conducted. The simulation assumed that 1, 2, or 5 cows per herd were MAP fecal culture-positive and that pools of 5 and 10 were used. This low-prevalence herd simulation indicated that weighted mean herd probabilities of detecting a positive herd ranged between 52 and 99.3%, with the lowest probability for pools of 10 with 1 positive cow in the herd and the highest probability for pools of 5 with 5 positive cows in the herd. However, overall, pools of 5 and 10 had similar diagnostic capabilities, enabling cost savings by utilizing pools of 10.     

Environmental sampling for detection of Mycobacterium avium ssp. paratuberculosis on large California dairies

Environmental samples collected from each of 3 locations on 23 large California dairies were cultured to evaluate the utility of this approach for identifying herds infected with Mycobacterium avium ssp. paratuberculosis. Results were compared with concurrent ELISA testing of ≥ 60 animals in each herd, and with previously performed individual and pooled fecal cultures of 60 animals. The estimated proportions of infected herds did not differ significantly among the testing methods (environmental sampling, 74%; previous fecal culture, 70%; and concurrent ELISA testing, 65%). Measures of agreement between environmental sampling and the results of previous fecal cultures were 70% (observed agreement), 85% (positive agreement), 62% (negative agreement), and 0.47 (kappa), whereas agreement between environmental sampling and concurrent ELISA testing was 65, 75, and 43%, and 0.19, for the same measures, respectively. The proportion of positive environmental samples on each farm was significantly correlated with the proportion of seropositive animals (r = 0.53), suggesting that environmental sampling may also provide a qualitative estimate of within-herd prevalence. Of the sampling locations that were evaluated, samples of lagoon water (15/23; 65%) were significantly more likely to yield a positive result than were composite manure samples (8/22; 36%) collected from the sick/fresh cow pen or from the alleyway (9/23; 39%) where cows exited from the milking parlor. Environmental sampling was an effective and inexpensive method of identifying herds infected with Mycobacterium avium ssp. paratuberculosis.     

Prevalence of Mycobacterium avium ssp. paratuberculosis infections in Canadian dairy herds

Johne's disease is a progressive, chronic disease with inflammation of the small intestine of ruminants caused by Mycobacterium avium ssp. paratuberculosis (MAP). Accurately estimating prevalence of MAP infections is important when controlling spread of infection or monitoring effectiveness of control programs. In the absence of a consistent test method used in prevalence studies across Canada, prevalence estimates among regions and programs cannot be compared. The aim of the current study was to estimate and compare prevalence of MAP infection in Western Canada, Ontario, Québec, and the Atlantic provinces, as well as among varying herd sizes and housing types. On 362 dairy farms located in all 10 provinces of Canada, environmental samples were collected and cultured for detection of MAP. For each herd, 1 sample was collected from the lactating cow area and manure storage. An additional environmental sample was collected from the area where breeding-age heifers were housed. Using prior distributions from previous research, diagnostic sensitivity and specificity were calculated to assess the ability of only 2 environmental samples (manure storage and lactating cow area) to identify MAP-positive farms, resulting in a sensitivity and specificity of 38 and 100%, respectively. We found no difference in sensitivity and specificity when including breeding-age heifers environmental samples. Test characteristics were applied to environmental culture results from the 362 participating farms in all 4 regions, resulting in true prevalence estimates of 66% for farms in Western Canada, 54% in Ontario, 24% in Québec, and 47% in Atlantic Canada. Herds housed in tiestalls had lower prevalence than freestall-housed herds, and herds with 101–150 and >151 cows had higher prevalence than herds with ≤100 cows. This was the first time MAP prevalence was determined using 1 detection method, performed in 1 laboratory, and within a single year across Canada, enabling direct comparisons of prevalence among regions, housing types, and herd sizes.     

Evaluation of milk ELISA fordetection of Mycobacterium avium subspecies paratuberculosis indairy herds and association with within-herd prevalence

Cow-level milk ELISA results can be used to determine herd Mycobacterium avium ssp. paratuberculosis (MAP) status. Milk sample collection is minimally invasive and ELISA results can be obtained quickly and economically. The objectives were to evaluate the herd-level test characteristics of 3 commercial milk ELISA, and to determine the impact of within-herd MAP prevalence on the performance of the milk ELISA herd test. A total of 32 purposively selected herds with a median herd size of 66 milking cows were used in this 2-yr project. Fecal and milk samples were collected from all milking cows at 6-mo intervals. Fecal samples were pooled by cow age, with 5 cow samples per pool; individual fecal culture was completed on cow samples from positive pools. Herd MAP status was defined as MAP positive if, at any point during the longitudinal study, a pooled fecal culture from the herd was positive. Milk samples were analyzed using each of 3 commercial milk ELISA kits; a cow-level result from each ELISA was classified as positive following the respective manufacturer’s recommended threshold for a positive result. Herd-level milk ELISA test characteristics were estimated using generalized estimating equations logistic models, which accounted for repeated measurements. Using a cutoff of 2% milk ELISA-positive cows, milk ELISA herd sensitivity relative to a herd MAP status based on all pooled fecal culture results collected during the study was as follows: ELISA A: 59% [95% confidence interval (CI): 36–78%), ELISA B: 56% (95% CI: 32–77%), and ELISA C: 63% (95% CI: 41–81%). Herd specificity for ELISA A, B, and C was 80% (95% CI: 71–88%), 96% (95% CI: 89–98%), and 92% (95% CI: 86–96%), respectively. The remainder of the analyses focused on results from ELISA B. Herd sensitivity of ELISA B increased as MAP prevalence increased. In herds with a mean MAP prevalence ≤5%, the herd sensitivity of the milk ELISA was low, ranging from 11% when MAP prevalence was 1%, to 62% when MAP prevalence was 5%. Categorical likelihood ratios based on milk ELISA within-herd prevalence predicted that herds with milk ELISA prevalence above 0 but <2% had a similar likelihood to be MAP positive or MAP negative, whereas herds with a milk ELISA prevalence between 2 and 4% were 3.7 times more likely to be MAP positive than MAP negative. All herds with a milk ELISA prevalence >4% were MAP positive. Although milk ELISA B worked well to establish herd MAP status in high-prevalence herds, interpretation was unreliable in MAP-negative and low-prevalence herds.     

Reduction in incidence of Johne's disease associated with implementation of a disease control program in Minnesota demonstration herds

This prospective longitudinal observational study was conducted to evaluate the effect of a standardized control program on the incidence of Johne's disease in 8 dairy herds in Minnesota. Depending on recruitment year, herds were followed for between 5 and 10 yr. Program compliance was evaluated using a cohort risk assessment score by birth cohort. Fecal samples from cows in study herds were tested annually using bacterial culture to detect Mycobacterium avium ssp. paratuberculosis (MAP), and serum samples from study cows were tested using an ELISA to detect antibodies to MAP. Clinical Johne's disease was also recorded. Cohort risk assessment score decreased along birth cohorts. Depending on the follow-up period in each herd, 5 to 8 birth cohorts were followed to describe changes in time to MAP bacterial culture positivity, serum ELISA positivity, MAP heavy shedding status, and clinical Johne's disease. The analysis of time to bacterial culture positivity, serum ELISA positivity, heavy fecal shedding status, and clinical Johne's disease using a time-dependent Cox regression indicated a reduction of the instantaneous hazard ratio by birth cohorts and by cohort risk score; however, the strength of association between the cohort risk score and each of the 4 disease outcomes decreased over time. The age at which the cows first tested positive for bacterial culture, serum ELISA, and heavy fecal shedding, and the age of the cows at onset of clinical Johne's disease signs remained constant for all birth cohorts. Based on herd risk scores, overall herds complied with the recommended management practices in the program. Results were consistent with a within-herd reduction of Johne's disease transmission, and that reduction was associated with herd-level management practices implemented as part of the control program.     

Salmonella in dairy operations in the United States: prevalence and antimicrobial drug susceptibility

Salmonella serotypes are important foodborne pathogens of humans that can be acquired through consumption of contaminated meat and dairy products. Salmonella infection also can be a significant animal health issue. As part of a national study of U.S. dairy operations conducted between March and September 2002, fecal samples were collected from representative cows in 97 dairy herds in 21 states and were cultured to determine the prevalence of Salmonella shedding. Salmonella was recovered from the feces of at least one cow in 30.9% of the herds. Overall, 7.3% of fecal samples were culture positive for Salmonella. The three most frequently recovered serotypes were Salmonella Meleagridis (24.1%), Salmonella Montevideo (11.9%), and Salmonella Typhimurium (9.9%). The susceptibilities of Salmonella isolates recovered were determined using a panel of 16 antimicrobial drugs. Salmonella isolates recovered from dairy cows had relatively little resistance to these antimicrobial agents; 83.0% of the isolates were susceptible to all antimicrobials tested. This study provides updated information on the prevalence and susceptibility patterns of Salmonella in dairy herds and on cow and herd characteristics. These data contribute to our understanding of the ecology of Salmonella in the dairy farm environment.     

Evaluation of environmental sampling and culture to determine Mycobacterium avium subspecies paratuberculosis distribution and herd infection status on US dairy operations

The objectives of this study were to determine the distribution of Mycobacterium avium subspecies paratuberculosis (MAP) in the environment and assess the relationship between the culture status of MAP in the farm environment and herd infection status. The National Animal Health Monitoring System's Dairy 2002 study surveyed dairy operations in 21 states. One component of the study involved collection and culturing of environmental samples for MAP from areas on farms where manure accumulated from a majority of a herd's cows. Operations were selected for inclusion based on perceived risk factors for MAP infection identified in a previously administered questionnaire. Individual animal and environmental samples were collected and used to determine the efficiency of environmental sampling for determination of herd infection status. Individual animal fecal, serum, and milk samples were used to classify herds as infected or not infected based on the presence of at least one test-positive animal in the herd. A total of 483 environmental samples (approximately 5 per farm) were collected, and 218 (45.1%) were culture-positive for MAP. A similar percentage of environmental cultures collected from all designated areas were positive [parlor exits (52.3%), floors of holding pens (49.1%), common alleyways (48.8%), lagoons (47.4%), manure spreaders (42.3%), and manure pits (41.5%)]. Of the 98 operations tested with the environmental sample culture, 97 had individual serum ELISA results, 60 had individual fecal culture results, and 34 had individual milk ELISA results. Sixty-nine of the 98 operations (70.4%) had at least one environmental sample that was culture-positive. Of the 50 herds classified as infected by fecal culture, 38 (76.0%) were identified by environmental culture. Two of the 10 operations classified as not infected based on individual animal fecal culture were environmental culture-positive. Of the 80 operations classified as infected based on serum ELISA-positive results, 61 (76.3%) were identified as environmental-positive, whereas 20 of the 28 (71.4%) operations identified as infected based on milk ELISA were detected by environmental sampling. Environmental sample culturing is less costly than individual animal sampling, does not require animal restraint, and identified more than 70% of infected operations. Environmental sampling is another diagnostic tool that veterinarians and dairy producers can use to determine herd infection status for MAP.     

Detection of Mycobacterium avium subspecies paratuberculosis in naturally exposed dairy heifers and associated risk factors

An observational prospective study was conducted to identify risk factors associated with fecal shedding of Mycobacterium avium ssp. paratuberculosis (MAP) in naturally exposed dairy heifers. The study population consisted of heifers from 8 dairy herds in Michigan participating in a MAP control demonstration project. Ten heifers from 4 age groups (0 to 3, 4 to 6, 7 to 14, and 15 to 24 mo) were selected from each herd every 4 mo for 28 mo and tested for the presence of MAP by fecal culture (FC). Heifers from dams testing positive for MAP by serum ELISA or FC were preferentially selected, with the remainder of the age cohort filled with randomly selected heifers. Logistic regression using generalized estimating equations to account for clustering of data within herds and repeated measures across heifers was used to evaluate the relationship between MAP FC status of heifers and herd risk factors. In total, 1,842 fecal samples were collected from 1,202 heifers. Thirty-six (2%) fecal samples, representing 27 individual heifers, cultured positive for MAP. Heifers shedding MAP were more likely to occur in herds with adult-cow MAP ELISA prevalence >10% (odds ratio = 4.7; 95% confidence interval: 2.0-11.1) and herds milking >300 cows (odds ratio = 5.7; 95% confidence interval: 2.4-13.4). Mycobacterium avium ssp. paratuberculosis can be cultured from the feces of naturally infected dairy heifers. The future performance of these MAP FC-positive heifers is unknown and needs to be explored.     

Comparisons of cows and herds in two progeny testing programs and two corresponding states

Data were USDA genetic evaluations of cows and DHI herd profiles from 4154 Holstein progeny-test herds from two artificial insemination organizations. 21st Century Genetics (Shawano, WI) and Genex (Ithaca, NY), and from 6361 additional herds from Minnesota and New York. We grouped herds into four categories: 21st Century Genetics herds, other Minnesota herds, Genex herds, and other New York herds. Herds were eliminated if they contributed fewer than 10 cows with genetic evaluations and birth dates from January 1989 to March 1995. Data included 83 and 74%, respectively, of first-crop daughters of 21st Century Genetics and Genex progeny-test bulls with genetic evaluations from January 1995 to February 1997. Herds were characterized by DHI profile and cow evaluation data. Daughters of progeny-test bulls with extreme production records (outside of 3 SD) relative to herd mean and variance did not appear in disproportionate numbers among the progeny of bulls likely culled or considered for further use. The two organizations appear to have selected larger, genetically superior, and better managed herds from within their respective regions for progeny-testing purposes. We were not able to predict whether a bull in the progeny-testing programs of these two organizations was going to exceed or fail to meet the pedigree prediction from characteristics of herds in which his daughters performed. Differences between parent average and daughter yield deviations for typical young sires appear to result from Mendelian segregation of genes.     

High herd-level prevalence of Mycobacterium avium subspecies paratuberculosis in Western Canadian dairy farms,based on environmental sampling

Mycobacterium avium subspecies paratuberculosis (MAP) causes chronic progressive enteritis in ruminants. The pathogen is present in most countries with modern dairy production, causing substantial economic losses for the industry. The objectives of this study were to estimate dairy herd prevalence of MAP in the Western Canadian provinces of Alberta and Saskatchewan, and to determine whether herd size and housing system (tie-stall versus freestall or loose housing) affected the risk of a herd testing positive for MAP. Six environmental samples were collected on 360 Alberta farms (60% of registered producers) and on 166 Saskatchewan dairy farms (99%). In total, 47% of the sampled farms in Alberta and 53% of the sampled farms in Saskatchewan had at least one environmental sample that was MAP culture positive and were, therefore, defined as infected. Sensitivity of environmental sampling was estimated using 3 subsequent annual tests performed on 82 farms. Because laboratory protocols were continuously improved throughout the project, the sensitivity increased over time. Therefore, a mean of the sensitivity estimates weighted on sampling year was constructed; this resulted in sensitivities of 68 and 69% for Alberta and Saskatchewan, respectively. Implementing those estimates in an approximate Bayesian computation model resulted in a true herd prevalence of 68% (95% probability interval: 60–80%) for Alberta and 76% (95% probability interval: 70–85%) for Saskatchewan. Herds with >200 cows had 3.54 times higher odds of being environmental sample positive and had more positive samples than herds with <50 cows (neither province nor housing system affected those results). In conclusion, the majority of Alberta and Saskatchewan dairy farms were infected with MAP and larger herds were more often MAP positive than smaller herds.     

Detection of Mycobacterium avium subspecies paratuberculosis: comparing fecal culture versus serum enzyme-linked immunosorbent assay and direct fecal polymerase chain reaction

Mycobacterium avium ssp. paratuberculosis (MAP) is the etiologic agent of Johne's disease in cattle. The disease causes diarrhea, reduced milk production, poor reproductivity, emaciation, and eventually death. Culture on Herrold's egg yolk agar is considered to be the definitive test for diagnosis of Johne's in cattle. This method has moderate sensitivity (30 to 50%) and is 100% specific; however, it can take up to 16 wk due to the slow growth of MAP. Currently, serum ELISA is used to screen herds for Johne's disease, but positive tests must be confirmed culturally or by PCR. The current research sought to evaluate an in-house direct fecal PCR procedure and directly compare it to ELISA using culture as the gold standard. Serum and fecal samples were collected from cows (n = 250) with unknown Johne's status. Fecal samples were processed for culture on Herrold's egg yolk agar and direct PCR. Serum samples were tested using the Parachek serum ELISA. Overall, 67/250 [26.8%, 95% confidence interval (CI) 21.4 to 32.8] animals were culturally confirmed to be shedding MAP. The PCR and ELISA detected 74/250 (29.6%, 95% CI 24 to 35.7) and 25/250 (10%, 95% CI 6.6 to 14.4), respectively. Culture and PCR were able to detect more positive animals than ELISA. Overall, direct fecal PCR was 70.2% sensitive and 85.3% specific when using culture as the gold standard. The ELISA method was 31.3% sensitive and 97.8% specific. When culture reported <10 cfu, the sensitivity and specificity of PCR and ELISA were 57.1 and 85.3%, and 4.8 and 97.8%, respectively. When culture reported 10 to <40 cfu, the sensitivity of PCR and ELISA were 75 and 50%, respectively. When culture reported ≥40 cfu, the sensitivity of PCR and ELISA were 100 and 88.2%, respectively. Specificity could not be calculated at these levels because there were no negative samples. The direct PCR outperformed the ELISA in detecting animals potentially infected with MAP and was not significantly different when compared with culture. The direct fecal PCR method described here provides faster results than traditional culture and is more sensitive than ELISA at detecting animals suspected of Johne's disease. These data support the use of PCR as an alternative method for screening herds for prevalence and diagnosis of Johne's disease.     

Relationships between fecal culture, ELISA, and bulk tank milk test results for Johne's disease in US dairy herds

Objectives were to estimate percentages of seropositive herds with cows shedding Mycobacterium paratuberculosis in feces and milk, and to estimate sensitivity, specificity, and predictive value of an ELISA relative to fecal culture. Dairy cows (n = 712) were randomly selected from 61 herds previously identified by ELISA as positive for Johne's disease. Fecal and bulk tank milk samples (n = 52 of 61 herds) were obtained from 10 states in the United States. Fecal samples were processed by a double centrifugation, double decontamination culture procedure. Milk samples were processed for both culture and DNA analysis by using polymerase chain reaction (PCR). Of 24 herds with at least three cows that had tested ELISA-positive, 79% were also culture-positive, compared with 18 of 37 herds with one or two ELISA-positive cows. Both fecal-culture and ELISA results were available on 651 cows; only 25% of cows that were fecal-culture positive also tested positive by ELISA and over 6% of cows that were fecal-culture negative tested ELISA-positive. Milk samples all cultured negative, but analysis of milk samples by PCR resulted in 68% of herds positive for M. paratuberculosis DNA including 24 of 31 herds with positive fecal cultures and 11 of 21 herds with negative fecal cultures. Sensitivity and specificity of the ELISA compared with fecal culture is lower than previously reported and perhaps best used in screening herds because of limited efficacy to predict infection in individual cows. In addition, contamination of bulk tank milk samples with M. paratuberculosis does occur in seropositive herds, even in some with negative fecal cultures.     

Evaluation of national surveillance methods for detection of Irish dairy herds infected with Mycobacterium avium ssp. paratuberculosis

The aim of this study was to evaluate the utility and cost-effectiveness of a range of national surveillance methods for paratuberculosis in Irish dairy herds. We simulated alternative surveillance strategies applied to dairy cattle herds for the detection of Mycobacterium avium ssp. paratuberculosis (MAP)-infected herds (case-detection) or for estimation of confidence of herd freedom from infection (assurance testing). Strategies simulated included whole-herd milk or serum serology, serology on cull cows at slaughter, bulk milk tank serology, environmental testing, and pooled fecal testing. None of the strategies evaluated were ideal for widespread national case-detection surveillance. Herd testing with milk or serum ELISA or pooled fecal testing were the most effective methods currently available for detection of MAP-infected herds, with median herd sensitivity >60% and 100% herd specificity, although they are relatively expensive for widespread use. Environmental sampling shows promise as an alternative, with median herd sensitivity of 69%, but is also expensive unless samples can be pooled and requires further validation under Irish conditions. Bulk tank milk testing is the lowest cost option and may be useful for detecting high-prevalence herds but had median herd sensitivity <10% and positive predictive value of 85%. Cull cow sampling strategies were also lower cost but had median herd sensitivity <40% and herd positive predictive values of <50%, resulting in an increased number of test-positive herds, each of which requires follow-up herd testing to clarify status. Possible false-positive herd testing results associated with prior tuberculosis testing also presented logistical issues for both cull cow and bulk milk testing. Whole-herd milk or serum ELISA testing are currently the preferred testing strategies to estimate confidence of herd freedom from MAP in dairy herds due to the good technical performance and moderate cost of these strategies for individual herd testing. Cull cow serology and bulk tank milk sampling provide only minimal assurance value, with confidence of herd freedom increasing only minimally above the prior estimate. Different testing strategies should be considered when deciding on cost-effective approaches for case-detection compared with those used for building confidence of herd freedom (assurance testing) as part of a national program.     

Effect of Johne's disease status on reproduction and culling in dairy cattle

Among the costs attributed to Mycobacterium avium ssp. paratuberculosis (MAP) infection in dairy cattle, the effects on reproduction and culling are the least documented. To estimate the cost of MAP infections and Johne's disease in a dairy herd, the rates of calving and culling were calculated for cows in each stage of MAP infection relative to uninfected cows. Data from 6 commercial dairy herds, consisting of 2,818 cows with 2,754 calvings and 1,483 cullings, were used for analysis. Every cow in each study herd was tested regularly for MAP, and herds were followed for between 4 and 7 yr. An ordinal categorical variable for Johne's disease status [test-negative, low-positive (low-shedding or ELISA-positive only), or high-shedding] was defined as a time-dependent variable for all cows with at least 1 positive test result or 2 negative test results. A Cox regression model, stratified on herd and controlling for the time-dependent infection variable, was used to analyze time to culling. Nonshedding animals were significantly less likely to be culled in comparison with animals in the low-shedding or ELISA-positive category, and high-shedding animals had nonsignificantly higher culling rates than low-shedding or ELISA-positive animals. Time to calving was analyzed using a proportional rates model, an analog to the Andersen-Gill regression model suitable for recurrent event data, stratifying on herd and weighted to adjust for the dependent censoring caused by the culling effects described above. High-shedding animals had lower calving rates in comparison with low-shedding or ELISA-positive animals, which tended to have higher calving rates than test-negative animals.     

Within-herd prevalence threshold for the detection of Mycobacterium avium ssp. paratuberculosis antibody–positive dairy herds using pooled milk samples: A field study

Herd-level diagnosis of paratuberculosis using a pool-milk ELISA (pool size: n ≤ 50) is a novel, economical, and convenient method to identify blood serological Mycobacterium avium ssp. paratuberculosis (MAP) antibody–positive herds. To date, the diagnostic performance of the pool-milk ELISA has been described only under laboratory conditions where herd prevalence was simulated by the preparation of milk pools consisting of milk samples of cows with a known MAP status determined by fecal culture. In our observational study, test performance under field conditions was studied using pooled milk and individual blood samples. A total of 486 herds within the MAP prevalence reduction program of Lower Saxony, from which pooled milk and individual blood ELISA results were available, were assigned to this study. Data were analyzed for the period between January 1 and December 31, 2018, the first year after herd testing became obligatory in this federal state of Germany. To evaluate whether pooled milk samples reliably distinguish between herds with a MAP-apparent blood serological within-herd prevalence (MAP-Ab-WHP_app) ≥5% and herds with a MAP-Ab-WHP_app <5%, the distribution of the MAP-Ab-WHP_app was compared between pool-positive and pool-negative herds. The MAP-Ab-WHP_app was 3.4% (median; 95% confidence interval = 0–11.4%) in pool-positive herds and 1.2% (median; 95% confidence interval = 0–6.4%) in pool-negative herds. Only 10.8% (n = 12) of the pool sample–negative herds had a MAP-Ab-WHP_app ≥5% and were therefore false negatives, given the aims of the MAP prevalence reduction program. Hence, the pool-milk sampling strategy seems well suited to distinguish between herds with a MAP-Ab-WHP_app ≥ 5% and herds with a MAP-Ab-WHP_app <5% since only 10% of serum MAP-ELISA positive herds were missed. Employing a logistic regression model, we estimated that the minimum blood serological MAP-Ab-WHP_app to detect a pool-positive herd with a probability of 95% was 8%, which fits well with the aim of the MAP prevalence reduction program to focus on herds with a MAP-Ab-WHP_app of ≥5%. Despite the limitations of the control approach, which include milk pool sample collection and a low sensitivity of the ELISA used in milk pools and serum samples, the aims of the MAP prevalence reduction program can be achieved. The results of these field data support that pool-milk sample ELISA is a useful, economical, and low labor–intensive tool to identify herds seropositive for MAP in a MAP prevalence reduction program.     

Phosphorus reduction through precision feeding of dairy cattle

A study was conducted on 4 dairy farms in the Cannonsville Reservoir Basin (Delaware County, NY) to identify feeding strategies in commercial dairy herds that will reduce manure phosphorus and mass farm phosphorus balance. Lactating cow diets on all 4 farms were evaluated monthly for 28 mo using the Cornell Net Carbohydrate and Protein System. Milk production and herd reproductive performance were measured monthly. Manure phosphorus content was measured every 6 mo. Reduced phosphorus diets (precision feeding) were implemented in 2 of the herds. Mean herd phosphorus intakes in the 4 herds ranged from 107 to 165% of requirement. Dietary phosphorus intakes in the 2 herds where diets were modified were reduced from 153% of requirement to 111%, an average reduction of 25%. Predicted phosphorus intakes and manure excretions were reduced 11.8 kg/yr per cow. After dietary adjustments in the 2 herds, fecal phosphorus concentrations decreased 33%. Milk production was not adversely affected by reduced phosphorus diets. Whole farm mass phosphorus balances (amount of phosphorus remaining on the farm) on the 2 farms were reduced 49%, with the percentage of imported phosphorus remaining on the farm reduced to less than 45%. Achieving feed phosphorus reductions similar to those of this study on all of the estimated 7000 to 8000 mature dairy cattle in the Cannonsville Basin could reduce feed phosphorus imports and manure phosphorus excretions more than 64,000 kg/yr. This would slow the rate of phosphorus accumulation in agricultural soils in the Cannonsville Basin, which over time could reduce the 50,000 kg/yr average total phosphorus loading of the Cannonsville Reservoir.