Self-clearance from BVDV infections—A frequent finding in dairy herds in an endemically infected region in Peru
Introduction
Bovine viral diarrhoea virus (BVDV), endemic in most cattle-raising countries, is a well-known causative agent for reproductive failure and impaired animal health. BVDV infections have, due to the high prevalence, a significant economic impact on the cattle industry, and annual national losses have been estimated at between 10 and 40 million USD per million calvings in high milk-producing countries such as England and Denmark (Houe, 2003).
Foetal infection in early gestation leading to persistently infected (PI) individuals is the key to the evolutionary success of BVDV (for review see Grooms, 2004). These PI individuals shed large quantities of virus and are the main sources of viral transmission (Lindberg and Alenius, 1999). Prevention of foetal infection is, consequently, the key to BVDV control. The conventional way to control or reduce losses caused by BVDV has been vaccination (Brock, 2004). Vaccination is in general implemented on a herd-to-herd decision basis, and may reduce economic losses caused by clinical disease, but does not appear to result in reduction of the prevalence of BVDV (O’Rourke, 2002). During the last decades eradication programmes against BVDV, without the use of vaccines, have been implemented in some European countries (Lindberg and Alenius, 1999, Greiser-Wilke et al., 2003). These have been based on an initial determination of herd BVDV status and increased herd biosecurity to prevent introduction of BVDV in non-infected herds, followed by systematic identification and elimination of PI animals from infected herds. The national BVDV programmes in the Scandinavian countries, as well as the regional programmes in a few other countries in Europe, have had success with control of BVDV, and are aiming towards eradication (Sandvik, 2004, Hult and Lindberg, 2005, Rossmanith et al., 2005). Experiences from the Swedish BVDV control programme have shown that self-clearance, i.e. the process whereby an infection is eliminated from a population without intervention, is an important phenomenon that works in favour of control. Self-clearance occurs when PI animals do not succeed in establishing additional persistent infections before they are removed from the herd (due to death, trade or culling). The phenomenon is thought to be more frequent in smaller herds, as a result of quickly established herd immunity due to the efficient spread of virus from PI animals to the surrounding group (Lindberg and Houe, 2005).
The importance of the PI animal for within-herd transmission and persistence is well established, even though a few reports suggest that BVDV may persist in a herd in absence of PI animals (Moerman et al., 1993, Moen et al., 2005). Once the PIs are eliminated from a herd the infection will virtually stop, and this is why much attention is given to the process of identification and elimination of PIs (Pillars and Grooms, 2002, Zimmer et al., 2002, Brodersen, 2004, Letellier et al., 2005). This requires whole-herd testing which is a major investment for producers, especially in regions with limited economic resources. Due to self-clearance, however, it is probable that a long-term reduction in prevalence of BVDV can be achieved even without intervention, as long as adequate attention is given to systematic measures aimed at preventing re-introduction of infection.
The purpose of this study was to estimate the prevalence of herds with evidence of BVDV exposure but without indication of current BVDV infection, in a population of dairy herds with endemic BVDV infection, and without systematic BVDV control, as an indirect measure of the probability of self-clearance. Furthermore, we wanted to investigate possible herd and management factors associated with self-clearance.
Section snippets
Study population
The region of Arequipa is the most important milk shed in Peru (Bernet et al., 2001a). The climate is subtropical, sunny and arid with an annual rainfall below 100 mm. Dairy production is concentrated to irrigation zones on the vast desert plateaus at altitudes of between 1000 and 1500 m above sea level, irrigated with water from the higher-altitude Andean region. Small-scale farming dominates, and more than 80% of the farms are less then 5 ha, have a maximum of 10 cows, and produce less than 90
Bulk tank milk testing
Bulk tank milk samples from totally 221 herds were collected in August 2003 (n = 192) and February 2004 (n = 195). The results demonstrated a very high level of exposure to BVDV in the region, and the proportion of herds with high antibody levels in the BTM (i.e. class 3 herds) was above 95% on both occasions. The median CODs in BTM in August 2003 and February 2004 was 1.36 and 1.18, respectively (range: −0.01 to 2.35 and 0.13–2.06, respectively; Fig. 2).
Out of the 221 selected herds 166 were
Discussion
Our results demonstrate a very high level of exposure to BVDV in the surveyed area, in a national as well as an international comparison (Paton et al., 1998, Graham et al., 2001, Mainar-Jaime et al., 2001, Ståhl et al., 2002, Viltrop et al., 2002), partly explained by the high cattle density in the region and the lack of systematic control measures because these have been shown to be important predictors for the prevalence of infection in populations with endemic BVDV infection (Houe, 1995,
Acknowledgements
We thank Professor Stefan Alenius for comments on the manuscript and Mariluz Arainga for technical assistance. We also thank Rafael Alameda, Leonel Diaz and the employees at Gloria S.A. for their assistance with sampling and data collection, and finally the dairy farmers in Arequipa for their participation in this study. This work was supported by the Swedish International Development Cooperation Agency, Sida/SAREC.
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