This work was aimed at addressing issues of misconception that hamper the advancement of bovine viral diarrhea virus (BVDV) control in a Belgian context. To demonstrate that recognizing BVDV infection by its clinical presentations is difficult, the detailed pathology and clinical presentations associated with BVDV infections were documented in three case reports. A subclinical form of BVDV infection was highlighted by looking at differences in bulk milk somatic cell counts between BVDV-infected and non-infected herds. Furthermore, it was assessed whether the BVDV management currently in practice in Belgium is efficient. Finally, some recommendations were made for the Belgian plan to control BVD in the future and eventually to eradicate the BVD virus by bringing together current general knowledge and findings originating from this thesis.
Field experience reveals that not everyone is convinced of the proposition that BVD cannot sufficiently be recognized by its clinical symptoms. The cases described here show that the clinical presentations can be rare and various. In a first case, cytopathic BVDV was detected in a 10-days-old calf during an outbreak of hemorrhagic neonatal diarrhea. Since non-cytopathic BVDV had not been detected in the same animal, this was an extraordinary finding, in particular because of the young age of the calf. Moreover, the case pointed to the role of BVDV in co-infections. In the two other cases, attention was not immediately drawn to BVDV as a potential cause. A 2-day-old calf showed spontaneous skin bleeding and thrombocytopenia. By testing blood samples, it was proved that the calf was persistently infected with a BVDV1b strain. The other case reported was of a cow suffering from hemorrhagic proctocolitis. The cow was transiently infected with BVDV1b. Although it is still generally accepted that BVDV2 is almost exclusively responsible for BVDV associated with severe disease, both the calf and the cow described were infected with BVDV1b.
Subsequently, a study on 406 Belgian dairy farms showed an association between the degree of BVDV infection of herds and the bulk milk somatic cell count (BMSCC), a proxy for subclinical mastitis. Herds considered BVDV-free had significantly lower BMSCC than herds where, according to BVDV-antibody optical density ratios in bulk milk, active BVDV infection was still present, or where BVDV had circulated recently.
It is evident that laboratory testing is indispensable when a disease, such as BVD, is not recognizable through clinical diagnosis. Tests are necessary for both the diagnosis of BVD in individual animals and for surveillance. Continuous monitoring is used to detect BVDV infection of herds and to alert in time for re-infection. Also, to examine why, in Belgium, the prevalence of BVDV remains high, the voluntary attempts at BVDV control in Flanders were studied. The results showed that BVDV control is inefficient in most herds. In a majority of herds (63%) the BVDV status was unknown, since continuous monitoring was not implemented in the approach. Moreover, in 71% of herds, cattle were vaccinated against BVDV without previous knowledge of the BVDV status, thus without any previous detection or the removal of persistently infected cattle.
Experience from the past teaches that BVDV control programs can succeed only if they are conducted at a regional or national level. Therefore, the proper method to implement an efficient BVDV control programme for Belgium is to run a national, mandatory program. In the final part of the discussion of this thesis, requirements are adduced for the eradication of the BVD virus over an acceptably short period of time.