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In the Netherlands, the mortality rate of ear-tagged calves <1 yr is one of the indicators that is continuously monitored in census data and is defined as the number of deceased calves relative to the number of calf-days-at-risk. In 2017, yearly calf mortality rates were published in the lay press and resulted in discussions about the calculation of this parameter among stakeholders because the same parameter appeared to be calculated in many different ways by different organizations. These diverse definitions of calf mortality answered different aims such as early detection of deviations, monitoring trends, or providing insight into herd-specific results, but were difficult to understand by stakeholders. The aim of this study was to evaluate several definitions of calf mortality for scientific validity, usefulness for policymakers, and comprehensibility by farmers. Based on expert consultations, 10 definitions for calf mortality were evaluated that assessed different age categories, time periods, and denominators. Differences in definitions appeared to have a large effect on the magnitude of mortality. For example, with the original mortality parameter, the mortality rate was 16.5% per year. When the first year of life was subdivided into 3 age categories, the mortality rate was 3.3, 4.5, and 3.1% for postnatal calves (≤14 d), preweaned calves (15-55 d), and weaned calves (56 d-1 yr), respectively. Although it was logical that these mortality rates were lower than the original, the sum of the 3 separate mortality rates was also lower than the original mortality rate. The reason was that the number of calves present in a herd and the risk of mortality are not randomly distributed over a calf's first year of life and the conditional nature of mortality rates when calculated for different age categories. Ultimately, 4 parameters to monitor calf mortality in Dutch dairy herds were chosen based on scientific value, usefulness for monitoring of trends, and comprehensibility by farmers: perinatal calf mortality risk (i.e., mortality before, during, or shortly after the moment of birth up to the moment of ear-tagging), postnatal calf mortality risk (≤14 d), preweaned calf mortality rate (15-55 d), and weaned calf mortality rate (56 d-1 yr). Slight differences in definitions of parameters can have a major effect on results, and many factors have to be taken into account when defining an important health indicator such as mortality. Our evaluation resulted in a more thorough understanding of the definitions of the selected parameters and agreement by the stakeholders to use these key indicators to monitor calf mortality., (The Authors. Published by FASS Inc. and Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).)

Associations between clinical mastitis (CM) and the proportional distribution of patterns in somatic cell count (SCC) on a herd level were determined in this study. Data on CM and SCC over a 12-month period from 274 Dutch herds were used. The dataset contained parts of 29,719 lactations from 22,955 cows of different parities. In total, 207,079 SCC test-days were recorded with 5719 cases of CM; 1561 cases were associated with environmental pathogens (ENV&#95;CM), and 2681 with contagious pathogens (CONT&#95;CM). Definitions of patterns in SCC were based on 3, 4, or 5 consecutive test-day recordings of SCC that differentiated between short or longer periods of increased SCC, and also between lactations with and without recovery. The distribution of those patterns (relative to their maximum) varied among herds. The distribution of SCC patterns was correlated with the incidence rate of CM. Herds with a relatively frequent quick recovery pattern had a 2.5 times more chance of being classified in the upper quartile for CM. These herds also had 2.1 times more chance of being classified in the upper quartile for ENV&#95;CM but only 0.4 times for CONT&#95;CM. Herds with a relatively frequent no recovery pattern had less chance (odds ratio=0.5) of being classified in the lower quartile for CONT&#95;CM. Since the distributions of SCC patterns were indicative for overall, environmental and contagious CM, the necessity to introduce pathogen-specific mastitis control programs in a herd could be determined based on the mean incidences of SCC patterns in that herd.

The ELISA tests that are available to detect an infection with Mycobacterium avium subsp. paratuberculosis (MAP) have a limited validity expressed as the sensitivity (Se) and specificity (Sp). In many studies, the Se and Sp of the tests are treated as constants and this will result in an underestimation of the variability of the true prevalence (TP). Bayesian inference provided a natural framework for using information on the test variability (i.e., the uncertainty) in the estimates of test Se and Sp when estimating the TP. Data from two prevalence studies for MAP using an ELISA in several regions in two locations were available for the analyses. In location 1, all cattle of at least 3 years of age were sampled in approximately 90 randomly sampled herds in each of the four regions of the country. In location 2, in 30 randomly sampled herds in each of three regions, approximately 30 randomly selected cows were sampled. Information about the unknown test Se and Sp and MAP prevalence was incorporated into a Bayesian model by joint prior probability distributions. Posterior estimates were obtained by combining the actual likelihood with the prior distributions using Bayes' formula. The corrected cow-level TP (proportion of infected cows in a herd) was low, 5.8 and 3.6% in locations 1 and 2, respectively. Certain regions within a location differed significantly in herd-level TP (proportion of infected herds). The herd-level TP was 54.3% in location 1 (95% credible interval (CI) 46.1, 63.3%) and 32.9% in location 2 (95% CI: 14.4, 73.3%). The variation in the herd-level TP estimate for location 2 was more than three times as large as the variation in location 1 mainly because of the relatively small number of investigated herds in location 2. In future prevalence studies for MAP, sample size calculations should be based on a very low cow-level prevalence. Approximately 50 and 90% of the herds in the current study had an estimated cow-level TP below 4 and 10%, respectively.

Transmission of bovine herpesvirus 1 (BHV1) within and between herds was studied on the island of Ameland, The Netherlands. There were 50 herds with 3300 head of cattle on the island. Herds were divided into three groups: (1) only containing seronegative cattle, (2) containing seronegative cattle and vaccinated seropositive cattle, and (3) containing only vaccinated cattle. All 23 herds in groups 1 and 2 were monitored. Three major outbreaks of BHV1 infections were observed due to the introduction of infectious cattle. Another major outbreak was most likely induced by reactivation of latent BHV1 in seropositive cattle. The basic reproduction ratio within these herds was estimated at least 4. Only one of these outbreaks led to three secondary outbreaks in susceptible herds in which all cattle were seronegative. These outbreaks were most likely due to respectively, direct animal contact, human transmission, and aerogenic transmission. The basic reproduction ratio between herds in this study was estimated to be 0.6.

An ordinary differential equation model was developed to simulate dynamics of Staphylococcus aureus mastitis. Data to estimate model parameters were obtained from an 18-month observational study in three commercial dairy herds. A deterministic simulation model was constructed to estimate values of the basic (R0) and effective (Rt) reproductive number in each herd, and to examine the effect of management on mastitis control. In all herds R0 was below the threshold value 1, indicating control of contagious transmission. Rt was higher than R0 because recovered individuals were more susceptible to infection than individuals without prior infection history. Disease dynamics in two herds were well described by the model. Treatment of subclinical mastitis and prevention of influx of infected individuals contributed to decrease of S. aureus prevalence. For one herd, the model failed to mimic field observations. Explanations for the discrepancy are given in a discussion of current knowledge and model assumptions.

A 2-year cohort study was conducted to investigate the probability of disease introduction into Dutch dairy farms. The farms were tested regularly for diseases and were visited biannually to collect management data. Ninety-five specific pathogen-free (SPF) dairy farms were selected from a database of bovine herpesvirus type 1 (BHV1)-free farms to study the probability of, and risk factors for, introduction of BHV1, bovine viral diarrhoea virus (BVDV), Salmonella enterica subsp. enterica serotype Dublin (S. dublin), and Leptospira interrogans serovar hardjo (L. hardjo). Although most of the 95 SPF farms had a low risk on introduction of infectious diseases, one disease was introduced into 12 farms and two diseases were introduced into one farm. Three farms experienced an outbreak of BHV1, one farm an outbreak of L. hardjo, two farms BVDV, six farms S. dublin, and one farm both BHV1 and S. dublin. The total incidence rate was 0.09 (0.06-0.12) per herd-year at risk. The results suggest that the "non-outbreak" farms were significantly more closed than the "outbreak" farms. Direct animal contacts with other cattle should be avoided and professional visitors should be instructed to wear protective clothing before handling cattle.

In May 1998, a compulsory eradication programme for BHV1 started in the Netherlands. In December 1999 approximately 24% of Dutch dairy farms were certified BHV1-free (Animal Health Service (AHS)). Ninety-three certified BHV1-free dairy farms participated in a cohort study that investigated the probability of introduction of infectious diseases. The probability of introduction of BHV1 was determined from March 1997 until April 1999. Ninety of these farms remained BHV1-free and could be used as control farms. From January 1997 until March 1998, BHV1 was introduced into 41 BHV1-free dairy farms in the Netherlands (case farms). Management data were collected for both cases and controls and were complete for 37 case farms and 82 control farms. For small data sets and for data in which both low and high frequencies were expected in the contingency tables, the asymptotic methods were unreliable. Our data set clearly resembled such a data set; the risk factors were rare events because the BHV1-free farms were closed farms on which few direct animal contacts occurred. Therefore, an exact stratified modelling approach was most suitable for the data. The study showed that dairy farms should prevent cattle from escaping or mingling with other cattle and that professional visitors should always wear protective farm clothing.

An outbreak of Streptococcus uberis mastitis was described to gain insight into the dynamics of Strep. uberis infections at a herd level. Data were obtained from a longitudinal observational study on a commercial Dutch dairy farm with good udder health management. Quarter milk samples for bacteriological culture were routinely collected at 3-wk intervals from all lactating animals (n = 95 +/- 5). Additional samples were collected at calving, clinical mastitis, dry-off, and culling. During the 78-wk observation period, 54 Strep. uberis infections were observed. The majority of infections occurred during a 21-wk period that constituted the disease outbreak. The incidence rate was higher in quarters that had recovered from prior Strep. uberis infection than in quarters that had not experienced Strep. uberis infection before. The incidence rate of Strep. uberis infection did not differ between quarters that were infected with other pathogens compared with quarters that were not infected with other pathogens. The expected number of new Strep. uberis infections per 3-wk interval was described by means of a Poisson logistic regression model. Significant predictor variables in the model were the number of existing Strep. uberis infections in the preceding time interval (shedders), phase of the study (early phase vs. postoutbreak phase), and prior infection status of quarters with respect to Strep. uberis, but not infection status with respect to other pathogens. Results suggest that contagious transmission may have played a role in this outbreak of Strep. uberis mastitis.

Organised disease control started in the Netherlands in the 18th century with governmental attempts to eradicate cattle plague. At the beginning of the 20th century, the dairy industry and cattle breeding organisations initiated a programme to control infectious diseases by means of a complex system of rewards and penalties. This was also the reason for establishing the Animal Health Service in Friesland in 1919. The history of programmes to control paratuberculosis in sheep, goats, and cattle in various countries is described. The vaccination of young animals seems to be an effective measure in the prevention of clinical paratuberculosis, although changes in management and hygiene practices are also important. A control programme for infectious cattle diseases has a number of phases (a lifecycle) and different components. Two components are essential for success, namely: open and regular communication with farmers, veterinary practitioners, and other people involved and a good registration and identification system for cattle, herds, and veterinary practitioners. The Dutch paratuberculosis programme has 10 herd status levels: 5-10 for non-suspect herds and 1-4 for infected herds or herds of unknown status. The higher the status, the greater the chance that a herd is free of paratuberculosis. An outline is given of the Dutch paratuberculosis programme including its objectives, basic principles for eradication, communication plan, legal action, logistic considerations, and complementary research programme.

In the Netherlands a survey was conducted to estimate the prevalence of paratuberculosis in dairy herds. In total 15822 cows of at least 3 years of age, belonging to 378 herds were tested using an absorbed ELISA. Of these herds, 55% (n=207) had one or more serologically positive cows. Of the positive non-vaccinated herds, most had one (n=98) or two (n=49) positive cows. The percentage positive cows per herd was 2.5+/-3.2%.The true prevalences on cow and herd levels, based on a test sensitivity that ranged from 0.3 to 0.4 and a specificity that ranged from 0.985 and 0.995, were estimated at 2. 7-6.9% and 31-71%. Seven herds had been vaccinated against paratuberculosis and these herds had a significantly higher percentage of serologically positive cows (23%) than the non-vaccinated herds (2.5%). In conclusion, a small percentage of the dairy cows and a high percentage of the dairy herds in the Netherlands is serologically positive. The percentages true infected cows and herds are difficult to estimate precisely due to uncertainties in test sensitivity and specificity.

Paratuberculosis is an infectious disease that is not easily amenable to classical control methods such as treatment and vaccination. Experimental animal models suggest that there could be genetic factors responsible for susceptibility or resistance to infection with the causative agent, Mycobacterium avium subsp. paratuberculosis. The aim of this study was to estimate genetic variation in susceptibility to paratuberculosis in Dutch dairy cattle. Data collected during a vaccination trial, conducted from 1984 to 1994, was used. A total of 3020 cows, with complete pedigree records and infection status at slaughter, were available for analysis. A standard polygenic statistical probit model was used to estimate heritabilities. The estimated heritability of susceptibility to M. avium. subsp. paratuberculosis infection was 0.06 for the overall population. In the subpopulation of vaccinated animals the estimated heritability was 0.09. Other calculations based on the model used in this study argue against a prominent role for vertical transmission. Because the establishment of genetic variation is one of the first steps towards the exploration of the possible use of selection for genetic improvement, the present study provides evidence for the presence of genetic variation in the susceptibility of cattle to paratuberculosis. Because the economic impact of the disease is substantial, the development and application of genetic tools, along with other control methods, could be instrumental in the eradication of paratuberculosis.

Thirty-eight bovine mammary Staphylococcus aureus isolates from diverse clinical, temporal, and geographical origins were genotyped by pulsed-field gel electrophoresis (PFGE) after SmaI digestion of prokaryotic DNA and by means of binary typing using 15 strain-specific DNA probes. Seven pulsed-field types and four subtypes were identified, as were 16 binary types. Concordant delineation of genetic relatedness was documented by both techniques, yet based on practical and epidemiological considerations, binary typing was the preferable method. Genotypes of bovine isolates were compared to 55 previously characterized human S. aureus isolates through cluster analysis of binary types. Genetic clusters containing strains of both human and bovine origin were found, but bacterial genotypes were predominantly associated with a single host species. Binary typing proved an excellent tool for comparison of S. aureus strains, including methicillin-resistant S. aureus, derived from different host species and from different databases. For 28 bovine S. aureus isolates, detailed clinical observations in vivo were compared to strain typing results in vitro. Associations were found between distinct genotypes and severity of disease, suggesting strain-specific bacterial virulence. Circumstantial evidence furthermore supports strain-specific routes of bacterial dissemination. We conclude that PFGE and binary typing can be successfully applied for genetic analysis of S. aureus isolates from bovine mammary secretions. Binary typing in particular is a robust and simple method and promises to become a powerful tool for strain characterization, for resolution of clonal relationships of bacteria within and between host species, and for identification of sources and transmission routes of bovine S. aureus.

This article discusses risk analysis of infectious diseases in a cattle population. Heck cattle living in nature reserve 'De Oostvaardersplassen' were studied as an example of the risk analysis approach. Twenty-five adult cattle were tested for every infectious disease agent that the risk analysis indicated might be prevalent. All sampled cattle tested positive for Bovine Herpesvirus 1, whereas the prevalence of antibodies against other infectious disease agents was below the level that was assumed to be the threshold for spread of the disease to cattle in the surrounding areas. Risk management of infectious diseases was expressed in a so-called 'low-risk profile'. The risk of introduction or spread of infectious diseases was estimated to be very low as long as the population was kept strictly enclosed.

Objective: To determine relative impact of genetic, common-litter, and within-litter factors on puppy mortality., Animals: 2,622 Boxer puppies of 413 litters born during a 14-month period., Procedure: For each puppy, pedigree was determined, and litter in which it was born was registered. Overall mortality and mortality per specific cause of death were analyzed by use of a model that included an additive genetic effect, common-litter effect, within-litter effect, and regression of mortality on inbreeding coefficient. Relative importance of the effects was determined from estimates of the variance in mortality explained by each factor., Results: 22% of the puppies died before reaching 7 weeks old. Stillbirth was the most frequent cause of death, followed by infection. Most observed differences were attributable to within-litter factors, which explained 67% of the variance in death attributable to infection and < or = 96% of the variance in death attributable to asphyxia. Common-litter factors were more important than additive genetic factors. Variance attributed to common-litter factors ranged from 2% for cheiloschisis, palatoschisis, or cheilopalatoschisis to 30% for death attributable to infection, and variance attributed to additive genetic factors ranged from 0% for asphyxia to 14% for euthanatized because of white color. Inbreeding coefficient had a significant effect on death attributable to infection, which increased 0.26% for each percentage increase of inbreeding., Conclusions and Clinical Relevance: Additive genetic factors have less impact on preweaning mortality than common-litter factors, which in turn have less impact than within-litter factors. Mortality attributable to infection increases significantly with increases in inbreeding.

A 2 to 1 matched case control study design was used to analyze herd level risk factors for Neospora caninum-associated abortion storms in 47 dairy herds. Data were obtained using a questionnaire regarding the state of affairs at the farms over the 2 years prior to the abortion storm. The questionnaire included 120 variables considered to be potential risk factors for either introduction of infection or recrudescence of chronic infection. The relationship between risk factors and case control pairs was analyzed by conditional logistic regression using a three-steps procedure. In addition, cross sectional serology was used to assess the possible role of concomitant infections. The main factors that were significant in the analysis and that were considered to have potential biological relevance were the presence of dogs, the presence of poultry, and the feeding of moldy maize-silage during summer. For both the presence of dogs and the presence of poultry on the farms, a linear relationship was found between the number of animals and the assessed risk for an abortion storm. These findings suggest a possible role of these species in the transmission of N. caninum. Further evidence for such a role of dogs was the significant association between the presence of dogs and the presence of seropositive cattle in the control herds. The feeding of moldy fodder is considered to be a factor which may induce recrudescence of a latent N. caninum-infection by mycotoxins causing immune suppression. We also found some evidence for a possible influence of management practices around calving and a high prevalence of retained afterbirths. No significant association was found for herd level prevalence of antibodies to bovine viral diarrhea virus, bovine herpesvirus 1, Leptospira hardjo or Salmonella dublin.

One of the impacts of disease is its effect on milk production. In the present study the effect of an outbreak of bovine herpesvirus type 1 on milk production at the herd level of certified bovine herpesvirus type 1-free dairy farms was modeled. The objective was to study several linear models to quantify the effects of a bovine herpesvirus type 1 outbreak on milk production accounting for the repeated measurements and incorporating our assumptions about the most likely duration of effects of this virus. Because milk production is measured at regular intervals, the data consisted of repeated measurements at the herd and cow levels. A marginal model, a subject-specific random-effect model, and a transition model were developed. The effect of a bovine herpesvirus type 1 outbreak was statistically significant in the random-effect model, and this model fitted the investigated farms best. However, a transition model might be a better model for generalizing the results to the whole population of Dutch dairy farms. The effect of a bovine herpesvirus type 1 outbreak on milk production derived from the random-effect model amounted to, on average, a loss of 0.92 kg of milk per cow per day during a period of 9 wk. The milk production loss varied from almost none to 2 kg of milk per cow per day. This reduction resulted in an average loss of Dfl 372 (Dfl1 = $US $0.50) with lower and upper confidence limits of, respectively, Dfl 12 and Dfl 730 per bovine herpesvirus type 1 outbreak.

This paper presents puppy mortality and postmortem findings for a birth cohort of boxer puppies born in the Netherlands between January 1994 and March 1995. In all, 457 litters were registered, of which 414 (90.6 per cent) were involved in the study. The 414 litters contained 2629 puppies, a mean litter size of 6.4 puppies. Of the 2629 puppies 571 (21.7 per cent) died or were euthanased before they were weaned at 50 days of age; there were 147 (25.7 per cent) stillbirths; 102 (17.9 per cent) were euthanased because they were white; 269 (47.1 per cent) of the puppies died during the first 21 days of life and 53 (9.3 per cent) puppies died between days 22 and 50. The cause of death or the reason for euthanasia was assessed by either the breeder or the veterinarian in 176 of these 269 puppies but was not determined in the other 93 puppies. Three hundred and two puppies were examined postmortem; the most important cause of death or reasons for euthanasia were inflammatory disorders (102; 33.8 per cent), non-inflammatory disorders such as asphyxia and malnutrition (66; 21.9 per cent), euthanasia because they were white (51; 16.9 per cent), and congenital abnormalities (45; 14.9 per cent). No cause of death or reason for euthanasia could be found for 38 puppies (12.6 percent)

A study was done of the descendants of cows from 4 dairy herds in which there had been N. caninum abortion outbreaks. Precolostral antibodies to N. caninum were demonstrated in 34 of 50 (68%) F1 calves and in 14 of 17 (82%) F2 calves from cows that aborted during the outbreaks. In 214 F1 progeny, N. caninum seroprevalence was nearly 50%, and there was a significant association between serostatus of the offspring and serostatus of dams. These observations indicated that congenital infection was an important mode of transmission after abortion outbreaks in these herds. A total of 52 abortions was recorded in 293 pregnancies of F1 progeny cows (1 to 3 pregnancies per animal). It was found that seropositive F1 cows had a three-fold increased abortion risk compared with seronegative F1 cows. In 2 of 10 abortions in seronegative cows evidence for N. caninum infection was found, suggesting that a low level of postnatal infection may also have occurred. It is concluded that N. caninum-infected calves should not be used as replacement stock, to decrease the future risk of abortion in dairy herds.

The incidence of clinical mastitis in dairy cows was estimated in 171 randomly selected dairy herds from the southern part of The Netherlands. A total of 1103 quarter cases was reported. The mean annual incidence rate was 12.7 quarter cases/yr per 100 cows. The modeling incidence rate of clinical mastitis at the herd level indicated that a number of risk factors were associated with a higher rate of clinical mastitis: one or more cows that were leaking milk, one or more cows with trampled teats, no disinfection of the maternity area after calving, consistent use of post-milking teat disinfection, Red and White cattle (Meuse-Rhine-Yssel) as the predominant breed, and an annual bulk milk somatic cell count < 150,000 cells/ml. The following risk factors were associated with a higher rate of clinical mastitis caused by Escherichia coli: cows with trampled teats, no disinfection of the maternity area after calving, consistent use of post-milking teat disinfection, use of a thick layer of bedding in the stall, and the stripping of foremilk before cluster attachment. The following risk factors were associated with a higher rate of clinical mastitis caused by Staphylococcus aureus: Red and White cattle (Meuse-Rhine-Yssel) as the predominant breed, cows with trampled teats, the stripping of foremilk before cluster attachment, no regular disinfection of the stall, no regular replacement of stall bedding, and an annual bulk milk somatic cell count < 150,000 cells/ml.

Results of a 20 month split-udder trial on the effect of discontinuation of postmilking teat disinfection on intramammary infections (IMI) with major and minor pathogens in seven dairy herds with a low somatic cell count are described. The incidence of Escherichia coli IMI was found to be significantly lower, whereas the incidence of IMI with Staphylococcus aureus and minor pathogens was significantly higher in quarters for which postmilking teat disinfection was discontinued than in disinfected quarters. It was concluded that discontinuation of postmilking teat disinfection decreased the incidence of E. coli IMI, accompanied by a, from a practical point of view, acceptable rise in somatic cell count. However, the possible increase in the incidence of S. aureus IMI calls for careful monitoring of the dynamics of IMI with contagious pathogens, when postmilking teat disinfection is discontinued in an attempt to reduce E. coli mastitis.

Results are described of a split-udder trial on the effect of discontinuation of postmilking teat disinfection on the incidence of clinical mastitis in seven dairy herds with a low bulk milk somatic cell count and a high incidence of clinical mastitis. Overall incidence of clinical mastitis was non-significantly lower (18%), whereas the incidence of the most prevalent pathogen associated with clinical mastitis, Escherichia coli, was significantly lower in quarters for which postmilking teat disinfection was discontinued. We concluded that discontinuation of postmilking teat disinfection may decrease the incidence of clinical Escherichia coli mastitis in herds for which standard mastitis prevention measures are executed adequately, bulk milk somatic cell count is low, and incidence of clinical mastitis is high. However, because an increase in intramammary infections with contagious pathogens may occur, care is recommended when advising discontinuation of postmilking teat disinfection.

The effect of cobalt supplementation on serum vitamin B12, growth rate and survival rate was measured in controlled field experiments with Texel twin lambs of the same sex, grazing cobalt-deficient pastures. The non-supplemented lambs had lower serum vitamin B12 concentrations than their supplemented brothers or sisters. During the experiments more lambs died in the non-supplemented than in the supplemented group. At the end of the experiments supplemented lambs weighed (mean live weight) 7.2, 9.5, and 11.0 kg more than non-supplemented lambs in 1991, 1992, and 1993, respectively. Sex-related differences in weight gain and survival rate were observed.

The incidence of clinical mastitis and distribution of pathogens in dairy cows was estimated in 171 randomly selected dairy herds in the southern Netherlands. A total of 1103 quarter cases were reported. The average annual incidence rate was 12.7 quarter cases per 100 cows per year. The most frequent isolates from clinical cases were Escherichia coli (16.9 per cent), Staphylococcus aureus (14.4 per cent), Streptococcus uberis (11.9 per cent) and Streptococcus dysgalactiae (8.9 per cent). Most cases were reported in early lactation: 25.4 per cent in the first month of lactation for all cows, and 39.1 per cent in the first month for first lactation cows. The rear quarters had a significantly higher incidence rate than the front quarters. Cows with an E coli infection showed more general clinical signs than cows infected with S aureus, S uberis and S dysgalactiae. A significantly higher incidence was observed in herds with a low (< 150,000 cells/ml) bulk milk somatic cell count than in herds with a count above 250,000 cells/ml.

In this review a number of the main epidemiological developments in the field of udder health in the Netherlands are summarized. The changes considering the bulk milk somatic cell count are described, as are the population dynamics of subclinical mastitis. Additionally, clinical mastitis caused by Escherichia coli and Staphylococcus aureus and their risk factors are reviewed. Finally, some future developments and the possible applications of these developments are discussed.

In this paper the developments in the field of bovine herd health and production management are illustrated. The current use of herd health and production management methods in veterinary practices are illustrated, and a uniform concept for execution of herd health and production management programs is given. The expected future developments and the critical points in these developments are discussed. The changing role of the veterinarian in bovine practice with the application of herd health is outlined.

The purpose of this study was to find the economically optimal period of first conception in gilts, addressing the issues of lifetime reproductive performance and expected herd life. A profit equation was used to combine the effects into one economic parameter. The data were from 14,910 gilts on 54 farms throughout The Netherlands. The average number of pigs born alive in the first litter increased with older age at conception. In the second litter a similar, but much smaller, effect was observed. Age at first conception had no effect on number of pigs born alive in the third or greater litter. Gilts bred at an older age had a shorter expected herd life than gilts bred at a younger age. Evaluating the reasons for culling revealed that infertility became a more important reason with increasing age at first conception. The proportion culled for infertility increased linearly from 18% at conception on d 200 to 24.5% at conception on d 300. Combining the effect of litter size and herd life led to the conclusion that the profit per gilt (sow) was not significantly affected by her age at first conception. It is concluded that the optimal economic age at first conception was considered to be approximately 200 to 220 d of age when the cost of housing and feed of the gilt from entry to first conception were taken into account.

To determine the additional costs and returns of twin calvings in dairy cattle, an economic model was developed on the personal computer. Data used in the model were recorded on 33 farms over 6.5 yr and included 381 twin calvings. For missing information, assumptions were made from the literature. Additional calf returns turned out to be $2.96. Total additional cost were $71.47, consisting of $100.92 for milk reduction, $39.51 for increased premature culling, $19.25 for increased occurrence of abortion, $5.69 for increased therapy, and $6.09 for increased calving interval. Total losses were on average $171.47 - $62.69 = $108.51 per twin birth. Realistic changes in input variables could not change this negative outcome to a positive result. Therefore, it was concluded that it is not profitable to select to increase the number of twins in dairy cattle.

The diagnostic aspects, incidence, risk factors and prevention of coliform mastitis are reviewed in the present paper. It is concluded that it is not possible to establish an accurate diagnosis of coliform mastitis, which is based on a specific clinical symptom differentiating it from other forms of mastitis. Not any single symptom or combination of symptoms is specific for coliform mastitis. The importance of coliform mastitis in dairy cattle showed a marked increase during the last few decades. Successful programmes for the prevention of mastitis reduced the importance of contagious organisms, but the overall incidence of mastitis has remained unchanged; therefore, the proportion of cases of coliform mastitis increased. The observation that coliform micro-organisms are common bacteria causing mastitis in herds with low bulk milk somatic cell counts implies that the future of the dairy industry will be associated with particular risks. Factors involving the risk of coliform mastitis are divided into cow level factors and herd level factors. The most important cow level factors were: leaking milk in between milkings, lesions of the teats and a reduced capacity of the immune system. Risk factors on the herd level include: the bedding material, the design of cubicles, exposure to daylight in summer-time, exposure to dirt and the use of dry-cow therapy. The possible role of antibiotic dry-cow treatment and teat dipping in colonisation resistance is discussed. Preventive programmes designed to reduce the incidence of coliform mastitis have not been very successful so far.

Ten herds with low somatic cell counts in bulk milk had an incidence of clinical mastitis of only 2.2 per 100 cows whereas 10 other herds with similarly low cell counts had an incidence of 53.6 per 100 cows. The major pathogens in the herds with a high incidence were Escherichia coli, Streptococcus uberis, Staphylococcus aureus and the coagulase-negative staphylococci. The percentage of uninfected quarters in the herds with a high incidence of clinical mastitis was 21.4 per cent compared with 12.2 per cent in the herds with a low incidence of clinical mastitis. The prevalence of coagulase-negative staphylococci, Corynebacterium bovis and Micrococcus species was higher in the herds with a low incidence of clinical mastitis. There was a significant linear relationship between the percentage of uninfected quarters and the incidence of clinical mastitis in the herds with a high incidence of clinical mastitis. In herds with a low incidence of clinical mastitis significantly less teat disinfection after milking was practised. The results suggest that infections with minor pathogens tend to protect cows against mastitis, and that teat disinfection after milking may increase the percentage of uninfected quarters and lead to an increased risk of clinical mastitis in herds with low somatic cell counts in bulk milk.