Infectious bovine rhinotracheitis (IBR) is an important epizootic disease in cattle caused by bovine herpesvirus type 1. Switzerland is free of IBR since 1993 and annual random testing is put in place to control freedom of the disease. Animals that are antibody positive are considered infected and have to be culled. Since 2012 bulk milk testing is the major mean of control. If a bulk milk sample is antibody positive in ELISA, all animals of this holding have to be tested individually using serum samples. Unfortunately, since 2013, an increasing number of bulk milk samples are positive without confirmation on the single animal level, thus causing high costs and obscuring the epidemiological status. Recent studies from Germany point to bovine herpesvirus type 2, the virus causing herpesmammilitis, a relatively harmless skin disorder on the udder, to be the cause of cross reacting antibodies in milk samples resulting in "false" positive IBR testing. In order to test if this may be the source of the problem in Switzerland too, we tested milk and serum from 10 animals of 20 farms with a positive bulk milk result and 20 from farm with a negative bulk milk result for the presence of antibodies against BoHV-1 and BoHV-2 by ELISA and serum neutralisation test (SNT).
Somewhat surprisingly, the seroprevalence for BoHV-2 was higher than estimated. Nearly 30% of all animals tested were antibody positive for BoHV-2 (28.5%). However, comparing the BoHV-1 and BoHV-2 results in bulk milk, we found no statistically significant correlation between BoHV-1 false-positive result and the BoHV-2 antibody level. From the 389 tested serum samples, only 15 were positive in the BoHV-1 antibody ELISA but proofed negative in the subsequent SNT. These numbers represent a test specificity of 97% which is an acceptable level. However, among these 15 fals-positive sera were statstically more BoHV-2 positive (10) than negative (5). In summary, we found a tendency for BoHV-2 to being involved in the false-positive bulk milk results but no statistically sound evidence for a clear correlation. BoHV-2 may be one out of several influencing factors. Further studies on the quality and quantity of the antibody response against BoHV-2 in view of a cross-reaction with the BoHV-2 diagnostics are necessary
Cover Illustration: Analysis of a possible reaction of antibodies against BoHV-2 causing bovine herpes mammillitis in IBR bulk milk surveillance