الفهرس 
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Abstract
This study was conducted on December 2018 to investigate an outbreak of foot-and-mouth disease (FMD) in a vaccinated dairy herd (n = 4,145) and to identify the associated risk factors. Samples from vesicular lesions and cardiac muscles of clinically infected and dead animals were collected on the 14th day of the outbreak and tested by RT-PCR assay. Foot-and-mouth disease virus (FMDV) A Asia Iran-05 and SAT2 Libya 12-like viral RNAs were detected in clinical samples. Disease frequencies were calculated, and epidemic curves were plotted. Statistical analysis was done using Chi-square test method (χ2) with relative risk (RR) estimates for morbidities and mortalities in animals with different ages, reproductive states and number of parities. Our data indicates that the outbreak occurred due to introduction of a field virus into cattle with minimal matching protective immune response. Previous vaccination with a multivalent vaccine did not prevent replication of a field virus that is an antigenic match to one of the vaccine seed viruses; with subsequent development of a mixed infection. The total cumulative incidence for the 31-day follow up period was 49.8% and the total mortality rate was 0.8%. The total incidence rate was 21 cases/1,000 cows/day, with confidence interval (CI) 20.32, 22.15. Analysis of epidemiological data revealed that lactation is the primary factor in disease development and mortalities in dairy herds (p < 0.005), possibly due to increased frequency of exposure and higher virus loads. Within this group, cows with 1 parity are more vulnerable in terms of disease development (relative risk 1.2, 95% CI 1.121, 1.285) but not mortalities (p = 0.359). Correlations between FMD development and age should only be considered in the context of the reproductive state. Our analysis revealed that a reduction of the overall disease impact can be achieved by reduction of virus burdens in farms during outbreaks.