Bulk milk testing for antibody seroprevalences to BVDV and BHV-1 in a rural region of Peru
Introduction
Bulk milk testing for detection of antibodies is a fast, non-invasive and cost-effective method that has been invaluable in the control programmes for bovine viral-diarrhoea virus (BVDV) and bovine herpesvirus type 1 (BHV-1) in Sweden, Norway, Finland and Denmark (Bitsch and Rønsholt, 1995, Lindberg and Alenius, 1999, Nylin et al., 2000). It can be used as a first step in a control strategy to discriminate between possibly infected and non-infected herds (that is, to give a qualitative measurement of the infection status of a herd for a particular infection). Furthermore, for certain endemic infections like BVDV and BHV-1, there is also a correlation between the antibody level in a bulk milk sample and the within-herd prevalence of antibody-positive cows (Niskanen et al., 1991, Niskanen, 1993, Hartman et al., 1997, Nylin et al., 2000, Beaudeau et al., 2001). Bulk milk testing is an efficient tool to detect introduction of infections at an early stage in previously uninfected herds by detecting changes in antibody levels between paired samples (Lindberg and Alenius, 1999). The presence of only one or a few lactating individual cows with high antibody titres to BVDV within a herd of otherwise-seronegative cows will give rise to a positive bulk milk test (Niskanen, 1993).
BVDV is widespread throughout the world. The infection tends to be endemic in most populations, with 60–85% of the cattle showing neutralising antibodies to the virus. Estimations of the economic impact of the infection have been made in high milk-producing countries such as England and Denmark, with annual national losses calculated at between US$ 10 and 40 million per million calvings (Houe, 1999). Infection with BVDV results in a clinical spectrum ranging from subclinical to the highly fatal form known as “mucosal disease” (Brownlie, 1990). Infection early in pregnancy can lead to calves born immunotolerant to (and persistently infected (PI) with) BVDV (Van Oirschot, 1983). PI calves shed large quantities of virus and are the main active vectors of viral transmission within the herd (Lindberg and Alenius, 1999). Trade with PIs or with non-PI dams carrying PI foetuses (PI carriers) constitutes the major route for the transmission of virus between herds (Lindberg et al., 2001). Herds with active infection (i.e. with presence of PI animals) typically show a high prevalence of seropositive cows and thus a strongly positive bulk milk test (typically ODs>0.8 according to experiences gained in the Swedish BVDV programme) unless the PI animals were introduced only recently (Houe and Meyling, 1991, Drew et al., 1999). The presence of lactating antibody-positive PI carriers in the herd also will be reflected as a strongly positive bulk milk test, because these animals have markedly high antibody titres when sampled in late gestation (Brownlie et al., 1998, Lindberg et al., 2001).
BHV-1 is an economically important infection of cattle; it causes disease manifestations including infectious bovine rhinotracheitis (IBR) and infectious pustular vulvovaginitis (IPV)/infectious pustular balanopostitis (IBP). BHV-1 is an enzootic infection on the B list of the Office International des Epizooties (OIE), and has world-wide distribution. As with other herpesvirus, BHV-1 can establish latency (for review, see Gibbs and Rweywmamu, 1977, Kahrs, 1981, Straub, 2001). During latency, the agent is harboured in the sensory ganglia as viral DNA (Ackermann et al., 1982). Reactivation from the latent state and reexcretion of virus can occur (after stimuli such as stress or corticosteroid treatment) with the risk of transmission to susceptible animals (Pastoret and Thiry, 1985, Thiry et al., 1987). Upon introduction of the virus into a susceptible herd, the infection typically spreads rapidly and completely—resulting in clinical or subclinical disease and seroconversion (Hage et al., 1996), given that reactivation of latent infection occur.
The information on epidemiology and impact of BVDV and BHV-1 in developing countries in general, and in low-producing rural regions in particular, is limited. It seems reasonable to assume, however, that these infections have important impact on production and on economic development for the rural communities, and that a reduction of prevalence would be beneficial. Considering the economic situation in the rural regions of developing countries, any control strategy must use the available resources as efficiently as possible. By using bulk milk testing, information on the infection status in the regional population of herds is gained at a low cost. Furthermore, by using a non-invasive sampling method, the risk of iatrogenic infection transmission is reduced. This might be particularly advantageous when working among small holders in rural communities where sufficiently sterile working conditions may be hard to achieve.
Studies of individual-cow antibody prevalence of BVDV and BHV-1 have been carried out in three major dairy areas in Peru (with results ranging between 50 and 80% and between 15 and 30%, respectively; Rivera, 2001), but knowledge on incidence and regional impact is lacking. There are no official control programmes against these diseases in the country, and even though vaccination do occur in some regions, no official vaccination regimen exists.
Our purpose was to introduce the use of bulk milk testing for detection of antibodies to BVDV and BHV-1 and to investigate the current status of these infections in the Mantaro Valley (the major dairy district in the central highlands of Peru). Bulk milk testing offers a simple and inexpensive alternative to blood sampling, and could be a valuable tool for disease control in regions (such as the Mantaro Valley) with limited economic resources.
Section snippets
The region
The Mantaro Valley in the Andean Department of Junin is one of the main agricultural regions in the central highlands. It is a rural region where 30% of the economically active population has livestock production as the main economic activity (Dı́az et al., 1999), and where mixed small-scale animal and crop farming (with emphasis on dairy production) is the dominant system (Fernandez-Baca and Bojorquez, 1995). According to the national census (INEI, 1998), 40–50% of the population in these
Results
The true prevalence of BVDV antibody-positive herds was 96% (range: 89–100%), 27% of the herds had high levels of antibodies (class 3; Table 1). The median OD for the 55 positive herds was 0.42 (range: 0.07–0.97).
Twenty-seven of the 60 herds were antibody-positive to BHV-1 and the true prevalence was 51% (range: 45–57%). The median OD for the 27 positive herds was 0.85 (range: 0.10–1.99), and that for the 33 negative, 0.01 (range: 0–0.04). Interestingly to us, none of the herds had ODs between
Discussion
The selection of herds in this survey was not based on randomisation and we cannot expect the prevalences to generalise from our sample. Nevertheless, it is the largest herd-level serosurvey of BVDV and BHV-1 carried out in rural areas in Peru, and also the first to provide an estimate of the seroprevalence of dairy herds in a selected population that are likely to have had recent active BVDV infection or to contain PI animals.
In the selection, large herds were voluntarily over-represented,
Acknowledgements
The authors would like to thank Professor Stefan Alenius for comments on the manuscript. We would also like to extend our thanks to veterinary colleagues: Caroline Åkesson, Hugo Sanchez, Bety Morales, Carlos Arana de la Cruz and Charlotte Ståhl for co-operation in the Mantaro Valley, Peru, and to Martin Olin for linguistical reading of the manuscript. This work was supported by the Swedish International Development Co-operation Agency, Sida/SAREC.
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