Journal of Virological Methods 166 (2010) 21–27 Contents lists available at ScienceDirect Journal of Virological Methods j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j v i r o m e t Quantitative single serum-dilution liquid phase competitive blocking ELISA for the assessment of herd immunity and expected protection against foot-and-mouth disease virus in vaccinated cattle Blanca Robiolo a , José La Torre a , Sergio Duffy b , Emilio Leon b , Cristina Seki a , Adriana Torres c , Nora Mattion a,∗ a Centro de Virología Animal, Instituto de Ciencia y Tecnología Dr. Cesar Milstein, CONICET, Saladillo 2468, C1440FFX, Ciudad de Buenos Aires, Argentina b Instituto Nacional de Tecnología Agropecuaria, Unidad de Epidemiología, (INTA-CICVyA), CC25, (1712), Castelar, Argentina c Comisión Provincial de Sanidad Animal, COPROSA, Buenos Aires, Argentina Article history: Received 9 October 2009 Received in revised form 4 February 2010 Accepted 9 February 2010 Available online 17 February 2010 Keywords: Foot-and-mouth disease Herd immunity Quantitative slpELISA Single serum dilution a b s t r a c t A single serum-dilution liquid phase ELISA (slpELISA) was standardized to be used for serological evalu- ation of herd immunity against foot-and-mouth disease. The absorbance value at a dilution 1:64 of each serum sample was interpolated in a standard curve by plotting the antibody titers of six control sera determined by end point dilution liquid phase ELISA (lpELISA),against the absorbance values for the same control sera at 1:64 dilutions. A straight line was obtained by linear regression analysis ( r > 0.90) in the titer range of 1.40–2.40. The reliability of the antibody titers was confirmed by the simultaneous titration of 60 cattle sera by slpELISA and lpELISA, which showed an acceptable correlation (R 2 > 0.87) for viral strains A24/Cruzeiro, A/Argentina/01, O1/Campos and C3/Indaial. Titers obtained by both meth- ods were not significantly different (p > 0.05), thus confirming that slpELISA could be used successfully to replace the conventional serial dilution ELISA for the assessment of protection status of cattle in epi- demiological studies. In addition, this quantitative slpELISA provides an adequate method for monitoring the effectiveness of vaccination campaigns and is also suitable for the assessment of seroconversion of naive animals during early stages of infection. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Foot-and-mouth disease (FMD) has been the most impor- tant disease affecting animal production in Argentina since first recognized in 1870.Although the slaughter policy has been con- sidered and applied in specific cases,the main strategy adopted by Argentina against FMD was compulsory massive vaccination of cattle ( Mattion et al., 2004). Presently, the Office International des Epizooties (OIE) rec- ognizes two different regions in Argentina: a FMD free zone (Patagonia),where vaccination is not practised,and an FMD free zone where vaccination is practised. More than 97% of the Argen- tine cattle population is located in the region where vaccination is practised. Cattle are vaccinated systematically twice a year, while other susceptible species are not.All young cattle must be vaccinated twice before they are moved to other premises. The for- mulation of the vaccines in Argentina has been changed according ∗ Corresponding author. Tel.: +54 11 4686 6225; fax: +54 11 4686 6225. E-mail addresses: mattion@fibertel.com.ar, nmattioncevan@centromilstein.org.ar (N. Mattion). to the FMD virus (FMDV) strains circulating in the field. Presently, the tetravalent vaccine is composed of inactivated oil adjuvanted suspensions of viral particles belonging to the strains A24/Cruzeiro, A/Argentina/01, O1/Campos and C3/Indaial. It is known that the effectiveness of the vaccination campaigns depends on many factors, such as vaccine efficacy, vaccination strategy and vaccination coverage, among others.Due to the size and territorial heterogeneity of Argentina and the magnitude of the cattle population, FMD vaccination campaigns have encountered a number of operational challenges. In the province of Buenos Aires, for instance, 22 million bovines are vaccinated twice a year and the campaigns are managed by 105 Sanitary Units, in almost the same number of counties. For these reasons, the evaluations of the effec- tiveness of the vaccination campaigns are of crucial importance. The level of antibodies against different serotypes of FMDV is measured using a liquid phase blocking competitive ELISA (lpELISA) (Hamblin et al., 1986) whose development and validation have been published (Maradei et al., 2008; Periolo et al., 1993; Robiolo et al., 1995). However, for the large number of samples to be tested in epidemiological studies, the use of this lpELISA is labor-intensive and expensive. Instead, herd immunity can be studied more read- ily using a single serum-dilution liquid phase ELISA (slpELISA), 0166-0934/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jviromet.2010.02.011