30 October 2010

5 minute read

In this study, the prevalence of BVDV persistent infection in US calves was low (0.12 per cent of calves) and the herd prevalence (those with at least one BVDV persistently infected animal identified) was modest (8.8 per cent). The single-sample testing for most of the positive animals in this study did not allow confirmation of persistent infection with BVDV, so some transient infections may be included.

Previous studies on weaned calves entering feedlot or stocker operations have detected a BVDV persistent infection prevalence of 0.3 per cent (6/2,000, 95-per cent confidence interval 0.14–0.65 per cent) [Loneragan et al., 2005], 0.32 per cent (3/938, 95-per cent confidence interval 0.1–0.94 per cent) [Larson et al., 2005b], and 0.4 per cent (86/21,743, 95 per cent confidence interval 0.32–0.49 per cent) [Fulton et al., 2006]. In each study, sampling and testing designed to identify persistently infected animals may have also allowed inclusion of transiently infected animals. The reasons for the difference in prevalence estimates between this survey and previous studies are not clear.

This study sampled younger calves, which would be expected to have a higher prevalence of persistent infection than older animals, as persistently infected calves have an increased mortality rate (Houe, 1995). In this study, no BVDV persistently infected calves were found on operations that had tested for BVDV during the previous 3 years. History of previous testing in herds was not reported in the previous studies.

The proportion of operations that had tested for BVDV persistent infection during the previous 3 years may have been higher in this study than in previous studies and might account for some of the difference in prevalence. Estimates in table a., p 21 show that only 4.2 per cent of all cow-calf operations tested calves for persistent infection with BVDV during the previous 3 years, but among operations that participated in BVDV sampling for this study, 16.8 per cent had tested calves during the previous 3 years.

The operations that submitted ear-notch samples in this survey were larger and more likely to implement testing, vaccination, and reproductive technologies than the nonsubmitters and may represent a population less likely to have BVDV persistently infected animals compared with populations in previous studies. Animal prevalence in this study for operations that had not tested any cattle for BVDV during the previous 3 years was 0.16 per cent.

Previous studies have detected a herd prevalence of persistent BVDV infection of 9 per cent (6/66 herds, 95-per cent confidence interval 3.4–18.7 per cent) [Bolin et al., 1985], and 10.2 per cent (13/128 herds, 95-per cent confidence interval 5.5–16.7 per cent) [Wittum et al., 2001], which is similar to the prevalence found in this study (8.8 per cent). Wittum et al. (2001) found a 19-per cent BVDV persistently infected herd prevalence in herds in which the attending veterinarian suspected BVDV persistent infection (10/52 herds, 95-per cent confidence interval 10–33 per cent) and a 2.7-per cent herd prevalence in randomly selected herds (2/75, 95-per cent confidence interval 0.5–10 per cent).

In this study, herd prevalence on operations that had not tested for BVDV persistent infection during the previous 3 years was 10.7 per cent (18/168 herds). No BVDV persistently infected positive operations were found among operations that had tested for BVDV persistent infection during the previous 3 years.

The prevalence of BVDV persistent infection was similar across all calf age groups in this study. As stated previously, persistently infected calves are expected to have increased mortality rates, which would produce a decreased prevalence with age. This, however, was not observed, and the per centages of calves that were born dead or died prior to weaning were similar between BVDV persistently infected positive and negative operations. The morbidity and mortality associated with different strains of BVDV persistent infection is variable and, along with the low prevalence of herd infection detected in this study, may account for the lack of observed effect.

The low prevalence of BVDV persistent infection detected in this study limits the ability to detect associations between management practices and BVDV persistent infection risk. One general biosecurity practice was found to be associated with BVDV persistent infection: no positive animals were found on operations that reported testing during the previous 3 years. Although this does not suggest testing performs perfectly in eliminating BVDV persistent infection, it is supportive of the effectiveness of BVDV testing. Effects of BVDV infection on positive operations were not clear, as no differences could be detected in the proportion of calves that were born dead or died prior to weaning.

Generally, producers knew at least the basics about BVDV persistent infection but were uncertain about the value of testing. Vaccination is the most commonly practiced intervention against BVDV persistent infection. While breeding females are the most important group to vaccinate to prevent the production of additional persistently infected calves, calves aged 22 days to weaning were just as likely to be vaccinated as other classes of cattle. Fewer than one of four operations (23.8 per cent) gave a BVDV booster to cows. Most operations do not vaccinate breeding females for BVDV, potentially leaving them more vulnerable to BVDV exposure.

Management practices such as quarantine and testing of cattle brought onto the operation are necessary to prevent disease from BVDV. Most operations quarantined none of the new cattle brought onto the operation during the previous 12 months, and very few tested any calves for BVDV persistent infection during the previous 3 years. Vaccination can clearly provide protection against introducing BVDV infection into the herd; however, with low levels of testing, no quarantine of incoming cattle, and modest vaccination levels it appears many operations have no biosecurity and biocontainment plan for BVDV.

Additional efforts are needed to increase producer education, to quantify the economic cost of BVDV infection and the value of testing programs, and to better understand the indications for herd testing and targeted surveillance, quarantine, and vaccination in cow-calf herds. There are likely several reasons for the low level of testing and quarantine. For some producers, lack of knowledge of effective BVDV control methods is likely important.

Other producers may not believe the value of quarantine and testing is sufficient to warrant the effort and expense. Some producers may understand the importance of BVDV in cow-calf herds but do not believe that it is present in their herd or that they are at risk for bringing it onto the operation and, thus, do not test.