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The detection and elimination of animals persistently infected with bovine viral diarrhoea virus (BVDV) is the key to BVD control. This method has proven to be very efficient in eradicating BVDV in a herd. Several pitfalls in the detection procedure can make that some persistently infected (PI) animals do not get identified or are removed too late, supporting the assumption that circulation of the virus could be possible in absence of PI animals. Furthermore the risk of reintroduction is high since the prevalence of BVD is high in the Netherlands and Belgium. Based on both practical experience and literature, here we review critical control points in order to minimise the risk of a false negative BVDV screening.

As a result of a BVDV infection in a herd consisting of 95 adult cattle 17 cows aborted their calves within a period of 3.5 months, one third got severe diarrhoea, 3 cows died and an increased percentage of the cattle got lochiometra after calving or abortion. The disease was diagnosed by paired serological testing of cattle with diarrhoea or abortion and post mortem examination of several aborted calves. From one foetus BVDV virus was isolated and subsequently subtyped by sequencing. Of aborting cattle, the testing results were influenced by the interval between infection and abortion. These results indicate that a primary infection with BVDV subtype 1b can cause severe clinical symptoms in a dairy herd.

The losses caused by an BVD-outbreak on a Dutch dairy farm with approximately 100 dairy cows were determined. In this particular farm, calves which were born around the same time as a detected carrier calf were not sampled. These carrier calves were kept on as replacement heifers and caused additional loss on top of the losses already caused by the primary outbreak. The total economic loss was more than f 96.000,- or nearly f 1000,-per cow. This case report illustrates the potential for serious economic loss when an BVD free herd is reinfected combined with less adequate control measures.

Since 1976 nineteen naturally occurring and eight experimentally induced incidents of congenital bovine viral diarrhoea virus (BVDV) infections were detected in routine laboratory diagnosis of swine fever. In nearly all naturally occurring cases, the disease was limited to one litter only. Clinical signs of disease and gross pathological lesions resembled those of chronic swine fever. Nearly all piglets born infected with the disease died within four months of birth. One pig remained persistently infected and immunotolerant until slaughtered at the age of 24 months; one of its litter mates seroconverted when six months old. In contrast, postnatal infections of pigs with BVDV strains isolated from swine ran a nearly or completely subclinical course. A serological survey of slaughter sows and boars showed that 20 per cent of the animals had antibodies against BVDV. The percentage of seropositive pigs was significantly higher on farms where cattle were present, and the percentage increased with age regardless of whether cattle were present. Sheep, pigs and humans can also transmit BVDV infections to pigs.

Winter dysentery is a highly contagious, herd-enzootic disease of cattle, most frequently observed during the winter season. The disease is characterized by a sudden onset of severe diarrhoea, which normally disappears spontaneously within two or three days. The aetiology continues to be obscure, though a virus is a probable causative agent.

Recent studies on bovine virus diarrhoea (BVD virus) afforded a deeper understanding of the epizootiology of this virus. It is of vital importance to determine whether BVD virus infection occurs within the uterus prior to the 120th day of gestation, during a later stage of pregnancy or after birth. When infection occurs before the 120th day of gestation, normal or abnormal calves which persistently carry the BVD virus, will be delivered. When infection occurs during a later stage of gestation, the calves which are born, will be immune to BVD virus. Symptoms of the disease (directly or indirectly associated with BVD virus) may appear in infections at an age under six months. As a rule, effects of BVD virus infection are slight in older animals. The very severe clinical picture of Mucosal Disease is confined to persistently infected carriers of BVD virus. The most important source of BVD virus infection consists in persistently infected carriers of BVD virus, which continue to excrete large quantities of BVD virus. The diagnosis of BVD virus infection is usually established at a late stage during the course of the disease so that vaccination is no longer relevant. Moreover, it is not clear to which extent immunity of gestating animals, induced by BVD virus vaccines, will be able subsequently to protect a foetus from infection due to BVD virus.