Abstract
We read with interest the article by Gould and her colleagues on the prevalence of BoHV-1 abortions in the United States, as detected by 5 diagnostic laboratories over the past 11 years (2000–2011). 2 The authors found no evidence of a link between a farm history of vaccination and BoHV-1–positive abortion submissions. Our impression is that use of live BoHV-1 vaccines during pregnancy is associated with increased risk of reproductive loss, even when products are used in accordance with label directions.4,5 Several lines of evidence suggest this is possible.3,4,6,7 We wonder whether a laboratory survey such as this, even with a decade’s worth of data from 5 busy diagnostic laboratories, is sufficiently sensitive to confirm or exclude such an association. First, BoHV-1 vaccination can be done at any time during pregnancy, yet abortion submissions to veterinary diagnostic laboratories are weighted toward the latter half of pregnancy. 8 Second, it is our experience that abortion episodes most likely to prompt owners to submit fetuses to diagnostic laboratories involve multiple abortions over a short period. BoHV-1 abortions typically occur 1–2 months after dams are exposed, and tend to be spread out over several weeks; an association with vaccination (if it exists) may be missed,3,7 and such sporadic abortions are likely to be under-investigated. Third, the abortion rate following vaccination of pregnant cattle with live BoHV-1, in animals appropriately vaccinated previously, tends to be lower than the high rates (up to 50%) historically seen in the 1960–1980s in naïve herds exposed to field strains of the virus.
If diagnosticians wish to investigate whether a positive association exists between vaccination with live BoHV-1 during pregnancy and reproductive loss, other approaches are needed to resolve the issue. One is to take the extra step, once a diagnosis of BoHV-1 abortion is made, of testing fetal tissues to establish whether field or vaccine strain BoHV-1 is present. 1 Another is to examine what happens in large herds to the return-to-service rate when use of live BoHV-1 vaccine during pregnancy is discontinued. While not a controlled study, one of us (RC) recently discontinued use of live BoHV-1 vaccination in a >5,000-head dairy herd. Figure 1 shows the number of abortions before and after (arrow) vaccination of pregnant cattle using live BoHV-1 vaccine was stopped. The rate of abortions decreased after live BoHV-1 vaccinated discontinued. We assume but did not prove that this was due to reduced exposure of fetuses to vaccinal BoHV-1.
Pattern of abortions in >5,000-head dairy herd before and after vaccination with a live BoHV-1 product was discontinued. There is abrupt decrease in the number of abortions by week (numbers in square boxes) shortly after use of the vaccine was stopped in September 2009 (vertical arrow). The product was used in accordance with label directions, including prior vaccination 12 months earlier with the same or similar product. Note the decrease in weekly number of abortions that occurred in spite of a rise in total number of milking cows (trend line).
We encourage colleagues interested in the safety of live BoHV-1 products in pregnant cattle to investigate whether the observation is repeatable.