Surveillance of Infectious Bovine Rhinotracheitis in marker-vaccinated dairy herds: Application of a recombinant gE ELISA on bulk milk samples.

Infectious Bovine Rhinotracheitis (IBR) occurs worldwide, requiring significant resources for eradication programs or surveillance purposes. The status of infection is usually detected by serological methods using the virus neutralization test (VNT) or enzyme-linked immunosorbent assay (ELISA) on individual sera. The gE DIVA (Differentiating Infected from Vaccinated Animals) vaccines approach, adopted in order to reduce the virus circulation and prevent clinical signs, have tightened the range of available methods for the serological diagnosis. Different gE blocking ELISA could be performed to detect specific antibodies in sera of infected or whole virus-vaccinated animals but with less sensitivity if applied to bulk milk samples, especially in marker-vaccinated herds. A new rec-gE ELISA was recently developed in Italy and applied with good performances on blood serum samples. The present paper focuses on the application of a rapid protocol for purification/concentration of immunoglobulin G (IgG) from bulk milk and on the use of the new rec-gE indirect ELISA. The study involved three different partners and 225 herds (12,800 lactating cows) with different official IBR diagnostic statuses. The diagnostic specificity of the method was demonstrated closed to 100% while the diagnostic sensitivity was strictly related to the herd-seroprevalence. Considering 2.5% as the limit of detection of within-herd seropositivity prevalence, the diagnostic sensitivity showed by the proposed method was equal to 100%. A single reactivation of a whole strain vaccine in an old cow was detected inside a group of 67 lactating cows, showing the field applicability of the method.

Dynamics of infection and immunity in a dairy cattle population undergoing an eradication programme for Infectious Bovine Rhinotracheitis (IBR).

Several countries within the European Union (EU) have successfully eradicated Infectious Bovine Rhinotracheitis (IBR), while others (e.g. Germany) are making efforts to achieve IBR-free status. EU member states IBR eradication programmes must meet Community legislation requirements that ban breeding farms from purchasing positive animals, from using whole-virus IBR vaccines, and from inseminating cows with semen from positive bulls. A follow-up study from 2002 to 2005 was carried out in the province of Trento (Italy), where a compulsory programme for IBR eradication was started in 1998. IBR outbreaks (identified on the basis of seroconversion of sentinel animals) were concentrated in larger positive herds. A higher incidence was recorded between 2003 and 2004. An association between markedly high temperatures in the summer of 2003 and virus reactivation has been suggested but is yet to be confirmed. The practice of driving cattle to common alpine pastures for the summer season did not play a significant epidemiological role in IBR transmission. Premising that only seronegative animals are allowed to enter dairy farms, animal movement increases the infection risk to a moderate extent. The long-term persistence of IBR antibodies was more pronounced in animals positive for antibodies to the glycoprotein E (gE). Scattered seroconversions, occurring mostly in positive herds, require careful interpretation in order to avoid overestimating the incidence of the infection at herd level.