A new diagnostic approach to serological surveillance of IBR marker vaccinated diary herds in Cuneo Province: A field study

OBJECTIVES In Italy, control and eradication measures have been implemented in order to reduce the economic losses caused by Bovine Herpesvirus 1(BoHV1) infection. Since 2004 a voluntary eradication programme is in place in Piedmont region, but current economic situation greatly reduced the progression of eradication. For this reason, once a year are performed individual serological monitoring on gE negative herds and a spot test on young animals in positive farms. In order to contain the costs of IBR surveillance, the use of bulk milk, already collected for monitoring programmes of other infections like brucellosis and leucosis, has to be considered as a possible diagnostic solution. However, the low level of IgG in bulk milk samples as well as the reduced sensitivity of gE blocking ELISA applied to this matrix, seems to discourage this choice. The recent introduction of an indirect gE ELISA using milk IgG sequential precipitation reagents (Eradikit® Bulk Milk Surveillance Kit plus, In3diagnostic, Italy) seems a promising tool for IBR surveillance of vaccinated herds. In this study the method was applied to pooled milk samples in dairy herds and compared with individual serum assay over a 12 months period, allowing the estimation of the diagnostic sensitivity and specificity under field condition. MATERIALS AND METHODS Three hundred dairy farms of Cuneo province, comprising 28000 lactating cows, were enrolled in this study. Individual serum samples were collected and tested by whole virus and gE blocking ELISAs, and sanitary status updated as follows: 158 gE negative and 142 positive herds. Individual milk samples were monthly collected by breeder association according to periodical milk parameter determination and were used to make milk pools (n=1365 pools in two sessions) of up to 40 lactating cows (median 34, IQR 30-37). Therefore, considering the outcomes of the official test in serum samples, the within-pooled milk prevalence was calculated and the field sensitivity of the method was assessed. Pool samples were processed for IgG concentration and indirect ELISA procedure, according to manufacturer protocol. The results of the first two sessions of analyses are reported below. RESULTS All pools from gE negative farms scored negative to Eradikit® BMSK-plus (specificity: 100%, 95%CI: 97.7%-100%). Among positive farms, all but 2 (n=69) having IBR prevalence of lactating cows greater than 2.5% scored positive, while dairy herds having IBR prevalence lower than 2.5% scored negative in all pools. In the latter farms gE positive serum samples were mainly attributable to old vaccinated animals. In one occasion a previously classified gE negative farm turned positive during the serological annual control. Pooled milk test, carried out in two occasions before the annual control, readily detect viral circulation. CONCLUSIONS Field specificity and sensitivity of Eradikit® BMSK-plus seems adequate for surveillance of gE negative herds and for detection of viral circulation at prevalence level greater than 2,5%. However, during the late stage of eradication, the proposed method would require reduced pool size to detect a weak positive (old vaccinated) sample.