From control to surveillance – the Swiss Bovine Viral Diarhoea (BVD) Eradication Programme

For BVD eradication, it is of high importance to consider the weaknesses and strengths of the testing scheme and the eradication phase. During calf testing, prevention of within herd transmission is of major importance as is the true sensitivity of the whole sampling and testing chain. During serological testing, reliable data of animal movements and herd connections are of higher importance, as the time interval from infection to detection of affected herds is longer. In contrast, determining a BVD negative status of a herd from calf testing is not possible. Virustyping is of importance to investigate infection chains between herds.

Objectives

We inform about the experiences from the Swiss Bovine viral diarrhoea (BVD) eradication programme. In the programme the shift from control to surveillance continued in the last few years. We show the relative importance of case measurements and measurements on farms known to have a higher risk for BVD persistence and re-infections compared to large scale testing. We discuss possibilities to keep the time period with low frequency cases (epidemic tail), following the main clearance, short.

Materials and Methods

In animal disease eradication programmes, determining the time point to shift from control to surveillance is of crucial importance for the effectiveness and the economy of the programme. As control comprises usually large scale testing, it should be as short as possible. Therefore, the shift to surveillance should take place as early as possible. On the other hand, shifting too early to surveillance increases the risk of leaving new infections unrecognized. Traditionally, the shift is done following an extended time of control testing without revealing any cases. In BVD programmes time intervals are long. Thus, there is often a need to change to surveillance even in the presence of new infections.

The Swiss BVD control programme was based on the detection and elimination of persistently infected animals. This was done by testing for antigen or by PCR. After virological testing of all cattle, all newborn calves have been tested for BVD virus for 4 years until the end of 2012.

In 2012, serological tests twice a year of all dairy herds and of a third of the remaining farms was done additionally to calf testing. In 2016 the testing programme will be reduced to once per year for dairy herds whereas it will not change for other herds. However, as case numbers are low, but do not decrease further, measures to control case herds more efficiently are applied since autumn 2015.

We analyze the data from the BVD control programme descriptively, showing the rationale for lowering surveillance while increasing case control measures. We present examples for infection chains during control.

Results

In October 2015 only 30 herds out of a population of 40,089 cattle herds are closed due to BVD. About 99.9% of the herds are free from BVD. Only about 80 herds still have single cows blocked because of known or likely exposure to BVD cases during the susceptible pregnancy time. The prevalence of persistently infected animals (PI) in newborn calves has decreased from 1.4% in 2008 to 0.03% in autumn 2015. In spring 2015 96% of the dairy herds showed no or only few antibodies in bulk milk samples. Due to the surveillance scheme, the figures for non-dairy herds cannot be derived easily but indicate the same trend.

Varying causes were responsible for the prolonged persistence of BVD on a small scale. The causes depended on the testing scheme and the eradication phase. First, during the calf testing, some of the herds could not be cleared from BVD. This was caused by poor herd management and faults in sample taking. Second, when changing from calf testing to serological testing, bovines tested false negative for BVD virus caused some damage, although the absolute number of such animals was small. Third, in the phase of serological testing, problems arising from data inconsistencies in the animal movement database hindering effective tracing forward and backward and ambiguous herd identifying are the main problems. In 2015 we observed about 30 re-infections of herds previously free from BVD. Interestingly, about 95% of the cattle herds have been free from BVD throughout the whole programme.

Herds with a history of BVD were at higher risk to get re-infected.