Application of nested-PCR for detection of foot-and-mouth disease viral sequences in tonsil of slaughtered cattle with clinically normal appearance in Iran

Persistent or inapparent infection, the so-called carrier state, is an important feature of foot-and-mouth disease (FMD) in ruminants. This may occur in non-vaccinated as well as in vaccinated ruminants following exposure to infectious FMD virus. Although the amount of infective virus that can be recovered from carriers is small, the virus can be present in some ruminants for months and in cattle for years. FMD is enzootic in Iran. The present study was carried out for determination of FMD viral genome in clinically normal cattle slaughtered at Zyaran Abattoir. A total of 133 tonsil tissue samples was collected and total RNA was extracted from each sample individually. RT-PCR and subsequently nested-PCR were carried out on each sample using FMDV specific primers from the 3D-3A region. A 222 bp DNA fragment was amplified from positive samples. Of the 133 tonsil tissue samples, 46 were found positive. Results indicated that the frequency of FMD carriers among tested animals is 34.59%. Statistical analysis did not show any significant differences (P>0.05) between positive samples in relation to sex, age and breed. The high frequency of carriers could be due to extensive FMDV circulation among susceptible animals. This finding suggests that, at least at present, full vaccination coverage is required for domestic ruminants in Iran to increase their resistance to field virus exposures.