Bulk Milk Testing Research Articles

Spatial distribution of Bluetongue surveillance and cases in Switzerland

Swiss Bluetongue surveillance from mid 2007 to mid 2008 was a combination of monthly bulk milk testing of 200 cattle herds in zones of higher risk for vector presence and intensification of passive clinical surveillance. The spatial scan statistic and Moran's I statistic were used to determine clustering of surveillance data. The results show a high level of surveillance intensity for BT in Switzerland in 2007. In the region encompassing the Cantons of Aargau, Basel-Landschaft, Basel-Stadt and Solothurn, where cases were detected in 2007, the surveillance was significantly higher than in the rest of Switzerland. Six cases of Bluetongue were detected within the surveillance system. The prevalence estimates 9.62% (95% CI = 3.25%-18.85% versus 0.98% (95% CI = 0.2%-2.22%) were also significantly higher in the area with higher surveillance intensity. Spatial variation in surveillance data should to be considered if a disease event is analysed on a national scale. The spatial variation of prevalence estimates should be considered in future Bluetongue surveillance in Switzerland.

Diagnostic performance of the Pourquier ELISA for detection of antibodies against Mycobacterium avium subspecies paratuberculosis in individual milk and bulk milk samples of dairy herds

The objective of the study was to determine the diagnostic performance of the Pourquier ELISA for detection of antibodies against Mycobacterium avium subsp. paratuberculosis (Map) in individual milk samples and in bulk milk samples. For individual milk samples the specificity of the Pourquier ELISA was estimated by testing a panel of individual milk samples from certified Map-free herds. The relative sensitivity of the assay in individual milk samples and agreement of the results with those of serum samples was estimated by testing panels of paired serum-milk samples from seropositive cattle, whole-herd investigations, and moderate or heavy shedders. The specificity of the ELISA for individual milk samples was still 99.8% at a cut-off of 20% sample to positive ( S/ P) value, clearly lower than the cut-off defined by the manufacturer (30% S/ P). The relative sensitivity for individual milk samples as compared with positive serum samples was 87% for a cut-off of 20% S/ P, and 80% for a cut-off of 30% S/ P. The sensitivity of this ELISA for detection of high shedders was >90% both for individual milk and serum samples, also agreement was very good ( κ = 0.91 for all paired samples). The specificity of the Pourquier ELISA in bulk milk samples was investigated by testing bulk milk samples from certified Map-free herds. Feasibility of bulk milk testing was investigated by titrating ELISA positive individual milk samples in negative milk. In addition, 383 bulk milk samples from herds with a known within-herd seroprevalence were tested. The specificity of the ELISA for bulk milk samples was 100% at a cut-off of 12.5% S/ P. At the cut-off recommended by the manufacturer (30% S/ P) performance of the bulk milk ELISA related to herd status (≥2 seropositive cows) was rather poor, corresponding with a sensitivity of 24% and a specificity of 99% relative to serology. However, at the revised cut-off for bulk milk of 12.5% S/ P and a within-herd seroprevalence of ≥3%, sensitivity and specificity relative to serology were 85% and 96%, respectively. Given the current herd-level seroprevalence in The Netherlands, these test characteristics corresponded with positive and negative predictive values for bulk milk of 67% and 94%, respectively. In conclusion, the diagnostic performance of the Pourquier ELISA for individual milk samples creates opportunities for a cheaper and more feasible testing scheme, while the diagnostic performance for bulk milk samples warrants further consideration.

Application of repeated bulk milk testing for identification of infection dynamics of Neospora caninum in Thai dairy herds

In this prospective study, repeated bulk milk tests were evaluated to establish the Neospora caninum antibody status and to describe infection dynamics and changes in prevalence of N. caninum in Thai dairy herds. Bulk milk from 418 herds in northeast Thailand were collected at three consecutive samplings, in December 2002 (sampling 1), April (sampling 2) and December 2003 (sampling 3). All samples were analysed for presence of N. caninum antibodies by iscom ELISA, and the optical density values were dichotomized at three different cut-offs. Herd status at either sampling 1 or sampling 2 was used to predict herd status at sampling 3. Changes in both sensitivity and specificity at the first samplings at all cut-offs were consistent with commonly seen patterns when evaluating performance of diagnostic tests. In addition, the predictive positive values (PPV) of herd status at each of the two samplings were more affected by time of sampling than choice of cut-off, whereas the predictive negative values (PNV) increased with increasing cut-off. Herd status at the first samplings was also interpreted in combination, i.e. herds that were negative at both samplings were considered negative and positive otherwise in Combination A, while herds positive at both samplings were considered positive in Combination B. When using these combinations, Combination A gave higher PNV but lower PPV than Combination B at all cut-offs. Using the optical density value 0.20 as cut-off to establish the herd N. caninum antibody status, 270 herds (65%) kept their status throughout the study period, but 148 herds converted their status at least once. Interestingly, 136 (86%) of the 158 herds that were considered negative at sampling 1 and sampling 2 remained negative at sampling 3. In addition of the 159 herds considered positive at the first two samplings, 134 (84%) were still positive at sampling 3. We concluded that repeated bulk milk testing at regular intervals provided better information about herd N. caninum status than a single test. Further, our results showed that the infection is prevalent in northeast Thailand, but that a herd can keep a negative infection status despite the frequent presence of dogs.

Indirect Enzyme-Linked Immunosorbent Assay for Detection ofBrucella melitensis-Specific Antibodies in Goat Milk

Brucella melitensis is the cause of brucellosis in sheep and goats, which often results in abortion. Few cases of B. melitensis infection in goats have occurred in the United States over the last 25 years. However, vigilance must be maintained, as it is for the bovine milk industry, to ensure that brucellosis is not introduced into the U.S. goat population. The objective of this study was to develop a sensitive and specific indirect enzyme-linked immunosorbent assay (iELISA) for the detection of B. melitensis-specific antibodies in goat milk. Brucella salt-extractable protein extract was employed as an antigen, and a horseradish peroxidase-labeled polyclonal anti-goat antibody was used as an anti-species conjugate. Thirteen of 13 (100%) individual infected goat milk samples tested positive and 134 of 134 (100%) uninfected bulk milk samples tested negative by the developed iELISA. Three positive milk samples with high, medium, and low absorbance values were used to simulate one positive animal in an otherwise negative herd. By this estimation, one high-titer animal could be detected in a herd of >1,600 animals. Detection estimates for medium- and low-titer animals were one positive animal per herd of <200 and 50 animals, respectively. Based on this estimation, it is recommended that herds be sampled in groups of 50 animals or less for bulk milk testing. The iELISA developed for this study was found to be sensitive and specific and shows potential for use as a bulk milk test for the detection of B. melitensis-specific antibodies in goat milk.

BVDV and BHV-1 Infections in Dairy Herds in Northern and Northeastern Thailand

Bulk milk samples from 220 dairy herds were collected at 9 public milk collection centres in the northeastern and northern Thailand, and a subset of 11 herds was selected for individual testing. The samples were tested for presence of antibodies to BVDV and BHV-1 using an indirect ELISA. The results from the bulk milk testing demonstrated a moderate level of exposure to BVDV and BHV-1 (73% and 67%, respectively). However, the low proportion of herds with high BVDV antibody-levels (13%) and the low within-herd seroprevalence of BVDV and BHV-1 in the 11 herds (24% and 5%, respectively), particularly among the young stock (15% and 0%, respectively), demonstrated a low prevalence of active BVDV infection and a low rate of reactivation of latent BHV-1. The presence of a self-clearance process was also indicated by the results from the individual testing. Moreover, a surprisingly low prevalence of BVDV and BHV-1 antibody-positive herds at one of the milk centres was found. This centre was established 5–10 years before the others. Our impression is that this reflects the self-clearance process, where consecutive replacement of imported infected animals without further spread has resulted in a nearly total elimination of the infections.Based on our experiences and on these results we are convinced that this process can continue if there is awareness of herd biosecurity. This is especially important in the context of a future intensification of the dairy production.

Bulk milk testing for antibody seroprevalences to BVDV and BHV-1 in a rural region of Peru

Bulk milk from 60 herds of dairy cattle in a rural region in the central highlands of Peru was tested for antibodies to bovine viral-diarrhoea virus (BVDV) and bovine herpesvirus type 1 (BHV-1). None of the herds had been vaccinated against BVDV or BHV-1. Commercially available indirect ELISA-kits were used for antibody detection. True prevalences of BVDV and BHV-1 antibody-positive herds were 96 and 51%, respectively. A relatively low proportion of strongly positive herds suggests, however, a low prevalence of active BVDV infection. BVDV optical densities (ODs) in bulk milk increased with herd size—indicating a higher within-herd prevalence in the larger herds (probably, in part a consequence of a higher rate of animal movement into these herds). For BHV-1, this pattern was not found; a relatively high proportion of the herds was free from BHV-1 infection in each size category. This could indicate a low rate of reactivation of latent BHV-1 infection.

Predictive values of serum and bulk milk sampling for the presence of persistently infected BVDV carriers in dairy herds

The aim of this study was to examine whether it is possible to predict the presence of persistently infected (pi) animals with bovine viral diarrhoea virus on dairy farms in The Netherlands, based on a few blood samples of the herd, possibly in combination with a bulk milk test for antibodies. In 25 herds with, and 24 herds without, pi animal(s) the probabilities of obtaining at least x antibody positive animals out of a sample of n animals were calculated, with n varying from 3 to 7 and values for x that were considered were n, n−1 to n−x. This probability, among animals 9–24 months old, ranged from 0·70 to 0·96 for herds withpi animals and from 0·13 to 0·37 for herds without. Using the result of bulk milk testing in addition did not add to the prediction. It was concluded that, due to the high percentage and large variation of antibody positive animals in herds without pi animals, it is not possible to predict the presence of pi animal(s) in dairy herds in The Netherlands using these methods.

Modelling the effect of surveillance programmes on spread of bovine herpesvirus 1 between certified cattle herds

For the eradication of an infectious agent, like bovine herpesvirus 1 (BHV-1), surveillance and certification can be used to reduce the transmission between herds. The goal of surveillance is that a certified herd that becomes infected is detected timely so that infection of several other certified herds is prevented. What counts is whether the reproduction ratio R, i.e. the average number of certified herds infected by one infected certified herd can be kept below 1. To support policy makers in making decisions about the minimal demands for a surveillance programme in an eradication campaign of BHV-1 in cattle, two mathematical models were investigated. With these models, the basic reproduction ratio between herds was calculated. The surveillance programmes were characterised with sample size, sampling frequency, test sensitivity, herd size, vaccination status, and contacts between herds. When R between herds is below 1, then the surveillance programme is sufficiently good to prevent spread of infection, provided that R is estimated well. In the model based on bulk milk testing sample size was replaced by a threshold at which bulk milk can be found positive. The R between herds was mainly influenced by the vaccination status, sampling frequency, and contacts between herds. Herd size moderately affected the outcome. Test sensitivity and sample size, however, were of minor importance. If herds of 50 cows became free of BHV-1 without vaccination, then spread of infection between herds might be prevented when animals within herds are sampled once a year (milk or blood samples). This frequency needs to be intensified, being twice a year, for larger herds and/or herds with extensive contacts with other herds. When bulk milk is sampled instead, sampling should be done at least every 5 months and more intensively, being each month, with larger herd sizes and more contacts between herds.

Evaluating control strategies for outbreaks in BHV1-free areas using stochastic and spatial simulation

Several countries within the EU have successfully eradicated bovine herpesvirus type I (BHV1), while others are still making efforts to eradicate the virus. Reintroduction of the virus into BHV1-free areas can lead to major outbreaks — thereby causing severe economic losses. To give decision-makers more insight into the risk and economic consequences of BHV1 reintroduction and into the effectiveness of various control strategies, we developed the simulation model InterIBR. InterIBR is a dynamic model that takes into account risk and uncertainty and the geographic location of individual farms. Simulation of a BHV1-outbreak in the Netherlands starts with introduction of the virus on a predefined farm type, after which both within-farm and between-farm transmission are simulated. Monitoring and control measures are implemented to simulate detection of the infection and subsequent control. Economic consequences included in this study are related to losses due to infection and costs of control. In the simulated basic control strategy, dairy farms are monitored by monthly bulk-milk tests and miscellaneous farms are monitored by half-yearly serological tests. After detection, movement-control measures apply, animal contacts are traced and neighbour farms are put on surveillance. Given current assumptions on transmission dynamics, we conclude that a strategy with either rapid removal or vaccination of infected cattle does not reduce the number of infected farms compared to this basic strategy — but will cost more to control. Farm type with first introduction of BHV1 has a considerable impact on the expected number of secondarily infected farms and total costs. To limit the number of infected farms and total costs due to outbreaks, we suggest intensifying the monitoring program on farms with a high frequency of cattle trade, and monthly bulk-milk testing on dairy farms.

Epidemiological characteristics of bovine herpesvirus 1 infections determined by bulk milk testing of all Dutch dairy herds

Samples of bulk milk were taken from all 33,636 Dutch dairy herds in November 1994 and tested for the presence of bovine herpesvirus 1 (BHV-1) antibodies with a gB-blocking ELISA....