Spatial and stochastic simulation to evaluate the impact of events and control measures on the 1997-1998 classical swine fever epidemic in The Netherlands. II. Comparison of control strategies

Using the spatial, temporal and stochastic simulation model InterCSF, several alternative pre-emptive slaughter strategies that could have been applied in the Dutch Classical Swine Fever (CSF) epidemic of 1997–1998 were evaluated. Furthermore, effects of changes in some disease-spread and disease-control parameters were studied. InterCSF simulates the spread of CSF between farms through local spread and contacts (animals, transport and persons). Disease spread is affected by control measures implemented through different mechanisms (e.g. depopulation of infected farms, pre-emptive slaughter, movement control). The starting point for the evaluation of strategies was a simulated basic scenario, which mimicked the real epidemic. Strategies were compared using epidemiological as well as economic results. Economic results were generated by a separate model (EpiLoss) that calculated the direct losses and consequential losses for farmers and related industries. The comparison of the different alternatives to the basic scenario led to some general conclusions on the Dutch CSF-epidemic. Pre-emptive slaughter seemed to be an effective strategy to reduce the size of an epidemic, if started at an early stage. Economically, pre-emptive slaughter was not as expensive as expected; the resulting smaller size of the epidemic, combined with less welfare slaughter, led to much lower overall losses. Furthermore, although large movement control areas seemed effective in reducing the size of the epidemic, the total losses were relatively high because of subsequent welfare slaughter. If infection probabilities could be reduced, for example by improved biosecurity, the resulting epidemics would be much smaller.     

Repeatability in ranking of dairy farms on technical and economic performance over years

Repeatability of farm average 305-day milk production and gross margin per 100 kg of milk was evaluated for 39 farms. Ranking of gross margin, its underlying factors (i.e. milk price, returns from cull cows and calves, costs of concentrates, and costs of roughage purchases per 100 kg of milk), and 305-day milk production was not completely random over the four years of the study. The coefficient of concordance ranged between 0.55 and 0.82. The costs of roughage purchased had the lowest concordance over time, and 305-day milk production had the highest concordance. For each year and each farm, the difference between average gross margin and farm-specific gross margin was calculated. The standard deviations (SD) of these values was calculated for each farm, and showed differences between farms in variability in gross margin over years (the farm-year-specific SD varied between farms from 0.56 to 5.73). All the underlying factors showed a deviation over years. So, variability of gross margin can be due to changes in all underlying factors. The impact on gross margin of purchased roughage was not of major importance because its absolute impact on the gross margin is small. We concluded that milk-production data over one year is a reliable indicator for the typical farm milk production. Because gross margin fluctuates considerably over time, however it is preferred to base economic research on data from more than one year.     

Probability of and risk factors for introduction of infectious diseases into Dutch SPF dairy farms: a cohort study

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.     

Adaptive conjoint analysis to determine perceived risk factors of farmers, veterinarians and AI technicians for introduction of BHV1 to dairy farms

A study was carried out to determine the possibility of a more-closed farming system for (Dutch) dairy farms. The objective of the study was to provide effective and economically profitable management advice for improving the animal-health status of farms. Management measures will only be successfully applied if supported by farmers and their advisors (such as veterinarians). Therefore, the perception of farmers and advisors of the importance of various risk factors for the introduction of diseases to a farm was determined by using bovine herpes virus type 1 (BHV1) as an example.

As part of the study, an evening-long workshop was organized and run thrice. In total, 49 farmers, veterinarians and AI technicians participated in these workshops. The computerized questionnaire technique was based on adaptive conjoint analysis (ACA). ACA has the advantage that participants can work with a large number of risk factors in a relatively short period of time. Another advantage of ACA (compared with standard questionnaires) is that the answers from each participant can be checked with regard to consistency with respect to the importance assigned to them. Data from participants with inconsistent responses can be excluded from further analyses. The results of the ACA interview were compared with the risk factors reported in the literature as being associated with BHV1 status (e.g. purchase of cattle, participation in cattle shows) and with farmers' actual management to prevent the introduction of diseases.

The workshop participants were all operating in the dairy sector and they seemed well aware of the risk of direct animal contacts for the introduction of BHV1. Farmers thought visitors to be more risky than did AI technicians and (especially) veterinarians. Farmers who purchased cattle or participated in cattle shows were of the opinion that the risks of direct animal contacts were more important than did farmers who were not involved in those practices. Farmers whose farms were BHV1-positive (and participated in cattle shows more often) thought the risk of participation smaller than did farmers with BHV1-negative farms.     

Economic aspects of reproductive failure in dairy cattle. II. The decision to replace animals

Until now, economic research to support the dairy farmer's policy with respect to the replacement decision has mainly been concerned with culling for production. However, in most cases the replacement decision for cows suffering from ill health or the after-effects thereof is also an economic one. In this paper the question approached is: how long is it profitable to continue inseminating dairy cows with poor fertility, and differing in age and productive capacity, before the decision to cull them must be made? At each heat, the criterion for the decision is that a cow should be inseminated with the aim of retention if the sum of expected differences in profits during her remaining expected life in the case of pregnancy, compared with replacement at the optimal stage of the current lactation, still exceeds zero.This criterion has been expressed in an economic replacement model for dairy cows. The essence of the model is a comparison of expected incomes of a cow present in the herd with an average herd life of 4 lactations, an average calving interval of 365 days, age and stage of lactation) below which it is not profitable to inseminate empty cows has been calculated. This was done at 10 stages in each lactation, from 65 to 245 days after calving at 20-day intervals. In the first instance, these calculations were made within a herd with an average herd life of 4 lactations, an average claving interval of 365 days, an average milk production of slightly more than 5300 kg per cow per year and a genetic increase in milk production of 1% per year. Moreover, a time preference was assumed for present over future income (discounting).In this basic situation, it appeared to be profitable to continue inseminating cows with poor fertility for a long time: even up to 8–9 months after calving in young cows with an average production level or higher. It was concluded from a sensitivity analysis that the calculated critical production levels were practically independent of several factors, especially those (e.g., milk price) that affect the expected income of both the cow present in the herd and the replacement cow. Factors which did have a considerable influence were persistence of milk production during lactation, and repeatability of a longer calving interval of the cow concerned.Finally, the possible use of the present model for the replacement decision with respect to other diseases is discussed.     

Economic aspects of reproductive failure in dairy cattle. I. Financial loss at farm level

Some years ago, systematic research was initiated to improve quantitative insight into the economic impact of diseases and disease control in livestock. This paper deals with the financial loss at farm level caused by reproductive failure in dairy cattle. Economic calculations concerning the calving interval of cows which differ in age, relative production level and persistence of milk production during lactation were made. On average, an optimal interval of 1 year or less was established, while the loss per day lengthening of the interval (norm-loss) amounted to 1–2 Dutch guilders (Dfl.). Also, 676 norm-losses due to forced replacement because of reproductive failure were determined, dependent on age and relative production level of the culled cow and average herd life of the farm. On average, this norm-loss was determined to be almost Dfl. 500 per culled cow. Based on 71 farms in an investigation in the province of Overijssel, total loss per farm was determined, using the calving interval data, forced replacement data and norm-loss rate described above. On average, the calculated loss amounted to Dfl. 63 per cow per year, of which Dfl. 35.50 resulted from sub-optimal calving interval and Dfl. 27.50 from forced replacement due to reproductive failure. Costs for veterinary treatment and drugs were not included in these elements. Total loss due to reproductive failure was estimated to average about Dfl. 80 per cow per year, which equals about 2% of the gross production value or 10% of an average farmer's income.Finally, considerable differences in loss between farms have been determined. The difference between the 20% of farms with the highest and the 20% of farms with the lowest calculated loss is even more than the average loss. Thus improvment may be achieved on many farms, e.g., with the help of a herd health programme.     

Economic losses due to paratuberculosis in dairy cattle

The results of a study of the economic losses caused by paratuberculosis in dairy cattle are reported. The losses in production and the determination of lost future income due to premature disposal are emphasised. A decrease in milk production of 19.5 per cent compared with the lactation two years before culling was recorded in animals showing clinical signs of paratuberculosis. The decrease in production in the last lactation but one compared with the previous lactation was 5 per cent. In animals with non-clinical forms of paratuberculosis these decreases in production were 16 per cent and 6 per cent, respectively.     

A three year herd health and management program on thirty Dutch dairy farms. III. Economic evaluation of fertility control

Fertility control was part of a herd health and management program on thirty dairy farms in the province of Overijssel. A comparable control group of thirty-one farms was available. After two years of application of the program the improvement in income feedcosts margin per cow on the program farms exceeded that on control farms by Dfl. 176. Within the large variation in income between the farms it was not possible to accurately distinguish the effects of the different aspects of the program, despite the use of detailed techniques such as Factor Analysis: thus another approach to determine the income effect of fertility control was necessary. The total loss per farm due to sub-optimal fertility was determined, using calving interval data, forced replacement data and norm-loss rates. From computation of this loss on program and control farms, the income effect of fertility control was deduced. Excluding the cost of the program, a significant (P less than 0.01) positive effect of fertility control on the total calculated loss due to sub-optimal fertility was determined (average Dfl. 25.50 per cow, or about Dfl. 1,700 per farm, after two years of program application). This income effect differed considerably between farms with an initial good or poor situation regarding fertility. The program contributed more to the improvement of insufficient or moderate fertility than to the prevention of a deterioration in herds with excellent fertility, although on the latter farms the total program had a considerable positive effect.     

The embryonic vertebrate heart tube is a dynamic suction pump

The embryonic vertebrate heart begins pumping blood long before the development of discernable chambers and valves. At these early stages, the heart tube has been described as a peristaltic pump. Recent advances in confocal laser scanning microscopy and four-dimensional visualization have warranted another look at early cardiac structure and function. We examined the movement of cells in the embryonic zebrafish heart tube and the flow of blood through the heart and obtained results that contradict peristalsis as a pumping mechanism in the embryonic heart. We propose a more likely explanation of early cardiac dynamics in which the pumping action results from suction due to elastic wave propagation in the heart tube.