Abstract
Bluetongue virus (BTV) is a double-stranded RNA virus of the Reoviridae family within the genus Orbivirus. Bluetongue virus causes an infectious non-contagious disease in ruminants, which is transmitted by a vector, specific species of Culicoides. In August 2006, BTV-8 was discovered in the Netherlands for the first time and it re-emerged in 2007. BTV-8 seemed to spread fast, and infected a large number of cattle herds. However, objectively quantified field information about the effect of BTV-8 infection was not available. Therefore, the aim of this thesis was to quantify the transmission and impact of BTV-8 in dairy cattle. Transmission In a fully susceptible herd, one BTV-8 infectious cow can infect on average 3.8 (median 2.9; 5th percentile=2.2 and 95th percentile=9.0) other cattle through bites of Culicoides. However, the transmission ratio will vary between herds and months, depending on outside temperatures and herd management. Furthermore, in 2007, there were some factors relating to stable design and grazing management that reduced BTV-8 transmission. Keeping cattle indoors and stables with large horizontal openings in combination with windbreak curtain and open stable doors decreased BTV-8 transmission. It seemed that Culicoides were more likely to feed on grazing cows than on stabled cows and that well-ventilated stables further prevented Culicoides from entering the stable and/or biting the cows. Impact After the BTV-8 epidemic in 2007, BTV-8 infections during gestation appeared to result in the birth of healthy looking, BTV-8 PCR-positive calves. These virus-positive calves mainly originated from cows that became infected during the second half of gestation, and these calves became virus-negative again within five months after birth. The virus-positive calves might play a role in the overwintering of BTV-8. However, whether virus-positive calves can actually start a new infection remains unclear. Vertical transmission of the virus can be prevented by vaccination of the cattle. Furthermore, in the BTV-8 epidemic in north-western Europe, BTV-8 infections were associated with increased mortality, reduced fertility and reduced milk production. The economic consequences of a BTV-8 outbreak depended on the course of infection, clinical or subclinical, and the proportion of susceptible cattle. Whether or not it is economically beneficial for a farmer to vaccinate against BTV-8 depends on the risk of these two factors. The BTV-8 outbreak in north-western Europe was not comparable to other BTV-outbreaks in which clinical signs in cattle were rarely observed. Therefore the epidemic showed that it is uncertain that a new emerging disease will behave itself in the same way as it did in other countries in the past.
Original language | English |
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Qualification | Doctor of Philosophy |
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Award date | 18 Mar 2011 |
Print ISBNs | 978-90-393-5504-6 |
Publication status | Published - 18 Mar 2011 |