A linear programming assessment of the profit from strategies to reduce the prevalence of Staphylococcus aureus mastitis

We used a linear programming model to estimate the financial returns to a Staphylococcus aureus testing and control program over a 1-year period for a 100-cow herd, with a 8636-kg rolling-herd average. Six tests, which vary in sensitivity from 0.80 to 0.98 and specificity of 0.99, were examined in simulated herds with 10, 20 and 30% prevalence of S. aureus infection. Sensitivity of these results to a range of assumptions regarding rolling-herd average, milk price, somatic cell-count premium, and cost and cure rate of dry treatment were examined to determine the profits from the program. The profits of a control program are most dependent upon prevalence, cell-count premium, and cost of dry treatment. In our simulation for a 100-cow herd, a testing and control program appears to cost less than US$10 per cow per year, and pays for itself within 1 yr, except under the lowest prevalence and most-adverse conditions (low yield, high cost of dry treatment, or low SCC premium).     

Pooled fecal culture sampling for Mycobacterium avium subsp. paratuberculosis at different herd sizes and prevalence.

A stochastic spreadsheet model was developed to obtain estimates of the costs of whole herd testing on dairy farms for Mycobacterium avium subsp. paratuberculosis (Map) with pooled fecal samples. The optimal pool size was investigated for 2 scenarios, prevalence (a low-prevalence herd [< or = 5%] and a high-prevalence herd [> 5%]) and for different herd sizes (100-, 250-, 500- and 1,000-cow herds). All adult animals in the herd were sampled, and the samples of the individuals were divided into equal sized pools. When a pool tested positive, the manure samples of the animals in the pool were tested individually. The individual samples from a negative pool were assumed negative and not tested individually. Distributions were used to model the uncertainty about the sensitivity of the fecal culture at farm level and Map prevalence. The model randomly allocated a disease status to the cows (not shedding, low Map shedder, moderate Map shedder, and heavy Map shedder) on the basis of the expected prevalence in the herd. Pooling was not efficient in 100-cow and 250-cow herds with low prevalence because the probability to detect a map infection in these herds became poor (53% and 88%) when samples were pooled. When samples were pooled in larger herds, the probability to detect at least 1 (moderate to heavy) shedder was > 90%. The cost reduction as a result of pooling varied from 43% in a 100-cow herd with a high prevalence to 71% in a 1,000-cow herd with a low prevalence. The optimal pool size increased with increasing herd size and varied from 3 for a 500-cow herd with a low prevalence to 5 for a 1,000-cow herd with a high prevalence.     

Use of partial budgeting to determine the economic outcome of Staphylococcus aureus intramammary infection reduction strategies in three Ohio dairy herds

Efforts to reduce the prevalence of Staphylococcus aureus intramammary infection were monitored in 3 Ohio dairy herds. Bacteriologic culturing of milk from all lactating cows in each herd was completed multiple times to identify infected cows and monitor reduction. Partial budgeting techniques were used to determine the economic outcome of the reduction program. Of particular emphasis was the economic impact of culling to maintain or achieve milk quality premium payments on the basis of bulk tank somatic cell counts. The prevalence of S aureusinfected cows was reduced in each herd. Culturing of milk from all lactating cows appeared to be an effective method to identify infected cows. Although numbers were limited, it also appeared that culturing of composite quarter samples was effective as a herd screening test to identify S aureus-infected cows. Bacteriologic culturing had a negative financial impact in all 3 herds. Using partial budgeting to assess the economic impact of the programs, it was determined that 2 herds experienced negative financial impacts as a result of an excess culling rate when compared with a 12-month baseline period prior to the initiation of the project. All herds had increased milk production per cow during the study as measured by the mature-equivalent method. However, when actual production was considered, increased milk production in each herd was not as great as that of other Ohio herds enrolled on Dairy Herd Improvement Association testing programs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Management strategies to decrease the prevalence of mastitis caused by one strain of Staphylococcus aureus in a dairy herd

The dairy herd at Washington State University had an outbreak of mastitis caused by a single strain of Staphylococcus aureus. The outbreak strain, termed novel, could not be controlled with routine contagious mastitis pathogen control procedures (incidence, 3.4 infections/100 cow months; peak prevalence > 22%). Our objective was to implement mastitis control measures that would decrease the incidence and prevalence of intramammary infection (IMI) caused by S aureus in the herd. The following intervention strategies were successfully implemented: strict segregation of cattle with IMI caused by S aureus, intensified culling of cattle with multiple-quarter IMI caused by S aureus, and inducing cessation of lactation of infected quarters in single-mammary-quarter infected cattle. One year after implementation of these control measures, incidence of IMI caused by S aureus was 0.35 infections/100 cow months, and prevalence had decreased from 20 to 8%.     

Subclinical mastitis of beef cows

Two beef cow herds were investigated to determine the prevalence of subclinical mastitis and to determine whether it contributed to decreased weaning weight of calves nursed by affected cows. Quarter samples obtained about 30 days after calving were evaluated by the California mastitis test and by direct microscopic cell count. In 1 herd, a 92-cow confinement operation, 17 quarters of 12 cows were infected with Staphylococcus aureus, Streptococcus sp other than agalactiae, or Klebsiella sp. Mean 205-day adjusted weights were 203.9 kg and 218.2 kg for calves nursed by infected vs noninfected cows. In the other herd, a 75-cow range-pasture operation, 8 quarters of 8 cows were infected, all with S aureus. Mean 205-day adjusted weights were 162.5 kg and 174.6 kg for calves nursed by infected vs noninfected cows.     

Pooled-sample testing as a herd-screening tool for detection of bovine viral diarrhea virus persistently infected cattle.

The study was conducted to develop methodology for least-cost strategies for using polymerase chain reaction (PCR)/probe testing of pooled blood samples to identify animals in a herd persistently infected with bovine viral diarrhea virus (BVDV). Cost was estimated for 5 protocols using Monte Carlo simulations for herd prevalences of BVDV persistent infection (BVDV-PI) ranging from 0.5% to 3%, assuming a cost for a PCR/probe test of $20. The protocol associated with the least cost per cow involved an initial testing of pools followed by repooling and testing of positive pools. For a herd prevalence of 1%, the least cost per cow was $2.64 (95% prediction interval = $1.72, $3.68), where pool sizes for the initial and repooled testing were 20 and 5 blood samples per pool, respectively. Optimization of the least cost for pooled-sample testing depended on how well a presumed prevalence of BVDV-PI approximated the true prevalence of BVDV infection in the herd. As prevalence increased beyond 3%, the least cost increased, thereby diminishing the competitive benefit of pooled testing. The protocols presented for sample pooling have general application to screening or surveillance using a sensitive diagnostic test to detect very low prevalence diseases or pathogens in flocks or herds.     

Theoretical basis and empirical implications of the bovine brucellosis market slaughter testing program in the United States

With reductions in federal funding, the Market Slaughter Testing (MST) surveillance system might once again become the primary bovine brucellosis surveillance system for the beef-cow population in the United States. Thus, understanding the weaknesses and/or strengths of the MST surveillance system will be crucial in determining the most effective program in a limited funding control/eradication program.An estimation of a hypergeometric distribution used in previous studies to describe the effectiveness of the MST system is employed and parameters of this distribution are estimated from secondary data. Results analyzed by herd size, and within-herd infection level yield a distinct herd size bias in the MST surveillance system. For example, the probability of detection in a 9-cow herd is 24% compared to 85.4% for a 645 cow herd after 1 year of infection. This herd size bias implies that secondary testing may be efficiently u used by concentrating testing in the smaller herds when funds for secondary testing are limited.MST detection probabilities can vary 30% between the build-up and liquidation phases of the cattle cycle, i.e., secular upswings and downswings in cattle inventory numbers. First Point of Concentration testing (FPC) of purchased replacements can partially offset the effects of the cattle cycle. A 95% herd vaccination level can reduce the probability of detection as much as 40% when compared to a similar herd that is not vaccinated.     

An economic justification of "blitz" therapy to eradicate Streptococcus agalactiae from a dairy herd

Streptococcus agalactiae was identified as the cause of mastitis in a 240-cow dairy herd. Forty-five per cent of the herd had cell counts over 500,000/ml, and 28 per cent had cell counts over 1,000,000/ml. Dry cow therapy was used regularly but teat dipping had not been used for three years. The procedures at milking were modified, teat dipping was introduced, and the herd was divided into two according to cell count. The 120 cows with higher cell counts were treated with 300 mg erythromycin (Erythrocin intramammary; Sanofi Animal Health) preparation per quarter at two consecutive milkings. Towards the end of lactation, all the 90 lactating cows in the herd were again treated with erythromycin. Milk samples were collected from all the cows in the herd 12 months after the initial treatment, and S agalactiae was isolated from only one replacement heifer which had been purchased after the treatments with erythromycin. The butterfat and protein levels in the milk were compared with those of a similar, but untreated, herd for 12 months before and after therapy. The butterfat levels rose sharply after treatment, and financial assessment showed a 41 per cent return on investment in the 12 months following the treatment.     

Raised herd somatic cell count due to Staphylococcus aureus following the failure of an automatic teat spraying system

This study describes the failure of a single jet exit race automatic teat spray (ATS) system resulting in the spread of Staphylococcus aureus infection in a 135-cow dairy herd, which showed an increased herd somatic cell count from 91,000/ml to 554,000/ml over a nine-month period. S aureus was isolated from 34 of 46 high cell count cows. The milking procedures were modified and manual teat spraying was restarted. Bacteriology was used to identify S aureus positive high cell count cows, and first and second lactation cows were treated during lactation. If their cell counts were not reduced, these were then culled. High cell count S aureus cows in lactation three or above were culled. The three-month geometric mean cell count fell to below 150,000/ml within five months. As all replacements were home-bred, S aureus infection must have spread from within the herd itself. All other causes have been eliminated, and this spread is attributed to the failure of the ATS to carry out effective postmilking teat disinfection. The advantages and disadvantages of ATS systems are discussed, especially in relation to robotic or voluntary milking systems.     

Apparatus for pasteurising teat cup liners between cows in a herringbone parlour.

In a herringbone milking parlour, teat cup liners were deliberately contaminated in turn with Staphylococcus aureus, Escherichia coli, Streptococcus agalactiae and Sterp uberis. Contamination was achieved by filling the liners with milk that contained 10(6) test organisms per ml. After the clusters had been back-flushed with water at 85 degrees C for five seconds, normal swabbing methods failed to recover any contaminating organisms from the teat liners in 56 tests out of 64. After 10 seconds back-flushing no recoveries were made in the same number of tests. The apparatus developed to effect this back-flushing for a particular herringbone parlour is described, with details of its routine use during milking. For a 100-cow herd, the running cost of such equipment using a five-second back-flush is estimated at no more than 4 pounds per week and, in its present form, would not add more than 10 seconds to the total milking time for each cow. Improvements in design of the apparatus, and in milking techniques arising from the routine use of the device, are also considered.     

RESPITE--a computer-aided guide to the prevalence of pneumonia in pig herds

An interactive computer-based guide (termed RESPITE) was developed to demonstrate to pig producers the expected level of pneumonia in pig herds which follow specified management practices known to influence the severity of pneumonia problems. A program was used to evaluate the combined effect of known risk factors present in the particular herd on the prevalence of lung lesions in slaughter pigs. The expected prevalence was estimated using formulae that were derived from observations and data in the published literature. Eleven risk factors considered are the number of pigs in the same room, all-in/all-out versus continuous flow of pigs, closed versus open herd, temperature fluctuation ratio, width of the buildings, space allotted per pig, the type of partitions separating adjacent pens, the presence or absence of diarrhoea as a clinical problem, liquid versus solid manure disposal, ascarid control efficiency, and the presence or absence of active Aujeszky's disease in the herd. Each of the factors can be assessed in the farrowing, nursery and grow-finish stages of production and the appropriate observations are entered. The computer uses the data to calculate the expected prevalence of pneumonia in animals leaving that stage of production. Of the pneumonia developing in the farrowing, nursery and grow-finish stages, 90 per cent, 75 per cent and 25 per cent respectively is considered to resolve before slaughter inspection at 100 kg. Adjusting for this degree of resolution, the program then estimates the expected final prevalence of pneumonic lesions in slaughter pigs. Comparison of predictions with slaughterhouse data from four herds indicated that the guide was realistic in its estimates.     

Single versus double testing of meat-juice samples for Salmonella antibodies,in the Danish pig-herd surveillance programme

In Denmark, a national serological surveillance-and-control programme for Salmonella in pigs has been in operation since 1995. The programme is based on the Danish mix-ELISA and uses double testing (two ELISA-wells used per sample) of meat-juice samples taken in relation to slaughter. All herds are classified monthly into one of the three levels; the classification is based on the percentage of positive serological results in the previous 3 months. In connection with evaluation of the programme in 2001, we investigated whether single testing (testing in one well only) could be expected to be sufficiently precise compared to double testing. Data from the year 2000 were used, and mathematical modelling. Single testing was simulated by randomised selection of one of the two results in the double testing. A slight increase in the prevalence of Salmonella-positive samples (1.02-1.09 times more through the four quarters of the year 2000) was found in the simulated single testing, as compared to the double testing. Around 0.5% of the herds would be allocated to another herd level in single testing-almost equal numbers one level up and one level down. No herd being seronegative in double testing would be allocated to levels 2 or 3 (herds with >40 or >70%, respectively, serological reactors) in single testing. The prevalence of "false-positive" diagnoses (positive in single testing and negative in double testing) and inversely defined "false-negative" diagnoses varied from 4.2 to 8.7% and from 3.2 to 4.5%, respectively, through the four quarters of the year 2000. The probability of allocating a herd to a wrong level due to sampling error was on the average 6.2 (varying from 1.66 to over 100) times higher than the probability of allocating a herd to a wrong level due to the test inaccuracy introduced by going from double to single testing. This is, however, an average; a herd with a true prevalence close to one of the level border cut-offs (40 and 70% weighted seroprevalence, respectively) would have a higher risk of being allocated to a wrong level than a herd with a true prevalence far from the level border cut-offs. The results are based on the current Danish sample sizes in the surveillance scheme, which implies that 60, 75 or 100 samples are taken annually in a herd, depending on its size. Other sample sizes would produce other results.     

Economic implications of bovine leukemia virus infection in mid-Atlantic dairy herds

OBJECTIVE: To determine the baseline costs of bovine leukemia virus (BLV) infection, including costs of clinical disease and subclinical infection, in a dairy herd representative of the mid-Atlantic region and compare these costs with the cost of a test-and-manage BLV control program. DESIGN: Stochastic spreadsheet model. SAMPLE POPULATION: A commercial Holstein dairy herd with 100 milking cows. PROCEDURES: A spreadsheet model was developed. The overall cost of infection included the cost of clinical disease (ie, lymphosarcoma [LS]) and the effects of subclinical infection on milk production and premature culling. Model input values and distributions were designed to reflect economic conditions in the mid-Atlantic region. Relative costs of infection and control were calculated for infection prevalences of 20, 50, and 80%. RESULTS: Estimated mean cost to the producer per case of LS was 412 dollars; for a herd with a 50% prevalence of BLV infection, annual incidence of LS was 0.66. Mean annual cost of subclinical infection at a 50% prevalence of infection was 6,406 dollars. Mean annual cost of a test-and-manage control program was 1,765 dollars. The cost of clinical disease and subclinical infection varied substantially with the prevalence of infection, whereas the cost of control varied with herd size. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggested that a basic BLV control program may be economically beneficial in herds in which the prevalence of BLV infection is > or = 12.5%. Farm-specific considerations may factor prominently when weighing the costs and benefits of an individual BLV control program.     

Controlling highly prevalent Staphylococcus aureus mastitis from the dairy farm

In 57 Holstein cows where the dairy farm uses a milking parlor system, the somatic cell count (SCC) increased persistently in the bulk milk (monthly mean 52.3 x 10(4) cells/ml; range 21 to 94 x 10(4) cells/ml). We detected S. aureus in 24 (41.2%) of the 54 lactating cows and in 29 (12.8%) of 227 quarters of the 57 milking cows in the herd. A control program was implemented in an effort to eradicate S. aureus mastitis from this dairy farm. The control plan established improved handling of the lactating cows, improved milking procedures, dry-cow therapy, and culling of infected cows. The program was monitored for 3.5 years by frequent checkups on the rate of S. aureus infection, the SCC, and the changes in milk composition. Eighteen months after the control program was started, the rate of S. aureus infection in the quarter milk decreased dramatically, and no S. aureus isolates were found in the milk of the remaining cows. The SCC in the bulk milk of the herd dropped to a monthly mean of <20 x 10(4) cells/ml. In conclusion, the control program was effective for controlling persistent S. aureus mastitis in this dairy herd.     

Prevalence and risk factors for Maedi-Visna in sheep farms in Mecklenburg-Western-Pomerania

Despite indications of a considerable spread of Maedi-Visna among sheep flocks in Germany, prevalence studies of this important infection are hardly available. Prior to any health schemes and guidelines, knowledge about regional disease distribution is essential. Depending upon herd size, 70 farms were randomly selected, of which 41 cooperated. A total of 2229 blood samples were taken at random and serologically examined. For assessment of selected farm characteristics a questionnaire exercise was conducted at all farms involved. The average herd prevalence is 51.2%, the within-herd prevalence is 28,8%. In the unvariate analysis of risk factors, small (10-100 sheep) and large (> 250 sheep) farms are more MVV-affected than medium sized farms. The average stable and pasture space per sheep is larger at non-infected- compared to infected farms. Owners judgement on general herd health turns out to be better at non-infected compared to infected farms. Taking infected farms only, the risk of within-herd prevalence above 20% is significant higher in crossbred than in purebred flocks.     

Survey of Mycobacterium avium subspecies paratuberculosis serological status in beef herds on community pastures in Saskatchewan

Johne's disease is a well recognized problem in dairy herds. Relatively little information is available on either the prevalence or the control of Johne's disease in commercial cow-calf operations. In the fall of 1999, blood samples were collected during pregnancy testing from cows on community pastures in Saskatchewan. Sera from these cows were analyzed using a commercial ELISA for antibodies to Mycoplasma avium subspecies paratuberculosis. All cows from each herd examined at the community pastures were sampled. Of the 1799 samples tested, 15 had sample to positive (S/P) ratios greater than 0.25 and were considered positive (apparent sample prevalence, 0.8%; 95% CI, 0.4% to 1.5%). If we assume test sensitivity of 25% and specificity of 98% as recommended by the National Johne's Working Group, the true sample prevalence is not significantly different from 0.0%. The ELISA S/P results for the antibody test-positive animals ranged from 0.27 to 2.5. If a herd was classified as positive based on one test-positive animal, the average herd apparent prevalence was 15.2% (95% CI, 7.1% to 28.6%). If the potential for false-positive results was considered with 2 or more positive animals being required for positive herd status, the herd prevalence was 3.0% (95% CI, 0.4% to 13.4%). Because of the very low prevalence in cow-calf herds, future research to identify risk factors and control points should target problem herds and utilize a case-control study design.     

Benefits and costs of a control program for an epizootic of Staphylococcus aureus mastitis

In response to an epizootic of Staphylococcus aureus mastitis, the owner of a 2,000-cow dairy instituted a treatment, prevention, and control program (hereafter, "intervention program") consisting of teat dipping, bacteriologic culturing of milk from all fresh cows and from cows with clinical signs of infection, and culling cows found to be infected. The program was highly beneficial, yielding $2.40 in direct benefits for every $1 invested.     

An estimated prevalence of Johne's disease in a subpopulation of Alabama beef cattle.

The objective of this study was to estimate the overall prevalence of animals that were infected with Mycobacterium avium ssp. paratuberculosis in a subpopulation of Alabama beef cattle. This was determined using a commercial enzyme-linked immunosorbent assay (ELISA) for the detection of M. avium ssp. paratuberculosis-specific antibodies in serum. Serum was collected from 79 herds that were participating in the Alabama Brucellosis Certification program. A total of 2,073 beef cattle were randomly tested by selecting 30 animals per herd in herds greater than 30 and selecting all animals in herds 30 and less for testing. It has been estimated that the commercial ELISA test used has a 60% sensitivity and a 97% specificity. Of the 79 herds tested, 29 herds were seronegative, 24 herds had 1-2 positive animals, and 26 herds had 3 or more seropositive animals. The average number of infected animals per positive herd was 3.3. In addition, a calculated minimum of 53.5% of the herds were identified as Johne's positive herds with a 95% confidence level. Of the total number of animals tested, 8.0% (166/2,073) of them were positive by the ELISA. After adjustments for test sensitivity and specificity and the proportion of animals sampled per herd, the true prevalence was calculated to be 8.75%. These data suggest that approximately 50% of the herds are infected with M. avium ssp. Paratuberculosis, and the overall prevalence of infection in Alabama beef cattle is approximately 8%, which correlates with other previously published regional estimates.     

Impact of a Mastitis control program in Valdres,Norway, during 1982–1985

The present study examined the effect of the mastitis control program (MCP) on milk production, incidence of mastitis, incidence of teat lesions, culling, log₁₀ of the bulk tank milk somatic cell count (LBMSC) and economic returns in Valdres, Norway, during 1982–1985. The major economic impact of the MCP was due to improved milk quality and to a lesser degree due to increased milk production. This might be explained by the Norwegian policy aimed at reducing the milk surplus through production quotas. The net present value of MCP, using a 15% discount rate, for an average 12-cow herd was 5599-3372 Nkr ($800-482) for production within or above the milk production quota, respectively. Since the program was provided free of charge to the individual farmer, this was also the net benefit to the MCP-participating farmer.     

Economic decision analysis model of a paratuberculosis test and cull program.

A spreadsheet program was written to perform decision tree analysis for control of paratuberculosis (Johne's disease), when testing all adults in a herd and culling all animals with positive test results. The program incorporated diagnostic test sensitivity, specificity, and test cost with the cost or value of each of the 4 possible outcomes; true-positive, true-negative, false-positive, and false-negative test results. The program was designed to repeat the analysis for the independent variable pretest paratuberculosis prevalence (0 to 100%). Model output was graphed as profit or loss in dollars vs pretest prevalence. The threshold was defined as the pretest prevalence at which benefit-cost equaled zero. Reed-Frost disease modeling techniques were used to predict the number of Mycobacterium paratuberculosis-infected replacement heifers resulting from infected cows during a control program. Sensitivity analysis was performed on variables of the decision tree model; test sensitivity, specificity, test cost, and factors affecting the cost of paratuberculosis to a commercial dairy. A test and cull program was profitable when paratuberculosis caused greater than or equal to 6% decrease in milk production if the pretest prevalence was greater than 6%, test sensitivity was 50%, test specificity was 98%, and the testing cost was $4/cow. Test specificities greater than 98% did not markedly affect the threshold for tests with a 50% sensitivity and costing $4/cow. Test sensitivity had minimal effect on the threshold. Using a diagnostic test with a 50% sensitivity and a 98% specificity as an example, test cost was shown to affect the threshold prevalence at which the test and cull program became profitable.(ABSTRACT TRUNCATED AT 250 WORDS)