Risk factors for infection with Foot-and-Mouth Disease virus in a cattle population vaccinated with a non-purified vaccine in Iran

Abstract

In this study, we estimated the level of Foot-and-Mouth (FMD) virus infection in a cattle-dense north-western province of Islamic Republic of Iran and analyzed putative risk factors for FMD infection. Calves (6-24 months of age) from all 17 districts of West Azerbaijan were tested for antibodies against non-structural proteins (NSP-Ab) of FMD virus. A proportional stratification with a minimum of 30 epi-units was applied for 3 different husbandry systems: villages, dairy and mixed farms. Within an epi-unit, 30 calves were sampled. For the interpretation of ELISA test results, we used the 50% inhibition (50PI) cut-off as per producer's instructions and created one at 75% inhibition (75PI) based on the lowest point of the histogram of PI results. This approach resulted in three categories of outcomes; negative (N), low-positive (LP) and high-positive (HP). A generalized mixed-effect model for binary outcomes was used for analysing putative risk factors and was run for both cut-off values. A total of 8378 calves from 202 villages, 51 dairy farms and 28 mixed farms were eligible for analysis. The percentage of calves testing positive (LP+HP) was 53.7% (95% Confidence interval (CI): 52.6%-54.8%), with 39.6% (95% CI: 38.6-40.7%) testing HP (n=3309) while 14.1% (95% CI: 13.5-15.0%) of calves tested LP (n=1188). Of 281 epi-units sampled, all calves sampled tested negative in only 2 epi-units (0.7% (95% CI: 0.1-2.5%)) and more than 25 calves tested positive in 29 epi-units (10.3% (95% CI: 7.0-14.5%)). Outcomes of regression modelling using the 50 PI cut-off indicated that, for each month increase in age, the odds of testing positive increased 1.01 times (95% CI: 1.00-1.03). The odds of calves testing NSP-positive increased 1.46 times (95% CI: 1.22-1.77) for calves residing in epi-units that had experienced clinical FMD in the 12 months preceding this study. The odds of calves owned by livestock owners who traded livestock testing positive were 1.4-1.6 times higher than those owned by persons not engaged in trading while the odds for calves testing positive in dairy herds was 1.62 (95% CI: 1.10-2.35) times higher compared with calves in villages. The results of the model using the 75 PI cut-off value resulted in comparable estimates, with the age-effect becoming more evident. These results have confirmed widespread FMD infection and were used in developing a risk-based control strategy on FMD, in line with Stage 1 of the Progressive Control Pathway for FMD (PCP-FMD).

Keywords: Foot-and-Mouth Disease; Multi-level logistic regression analysis; Non-structural antibodies; Risk factors.