Some cow groups contained both heifers and cows; \* Seroprevalence category defined as: Low, <20%; Moderate, 20–80%; and High, >80% seropositive.

The frequency of management groups likely to have experienced an outbreak of BVDV infection (i.e., >30% of AGID test results were ≥3; Table 2) varied between years. In 2009, approximately 3 in 10 and 2 in 10 heifer and cow groups, respectively (Table 2), were likely to have experienced an outbreak of BVDV infection. However, in 2011 only 1 in 10 and 1 in 20 heifer and cow groups, respectively (Table 2), were likely to have experienced an outbreak of BVDV infection. Outbreaks of BVDV infection in groups of heifers are relatively common due to the recognised generally lower age-specific seroprevalence [6]. A serological study of groups of mating age heifers and first-lactation cows in 12 herds in the Northern Territory found that in nearly half of the herds most of the cattle were likely to be susceptible to infection [8]. Further, these estimates of the prevalence of outbreaks of BVDV infection are similar to those derived from collation of laboratory investigations conducted in Queensland and the Northern Territory [9]. Differences between years in the prevalence of outbreaks of infection are likely to reflect the cyclical changes in proportion of naïve and PI cattle in management groups, strongly influenced by herd culling and heifer replacement practices.

Overall, there is no evidence that the prevalence of infection in beef cattle in northern Australia, either at the herd or animal level, has changed significantly between 1967 and 2009 to 2011.


**Table 2.** Distribution of management groups of unvaccinated heifers and cows by prevalence of recent BVDV infection [1].
