Strategies aimed at conferring rapid and efficient protection against FMDV have to face one main challenging factor: the rapid, acute infection caused by this virus, which makes the absence of sufficient amounts of antibodies or other interfering factors essential for protection. The traditional emergency vaccines based on virus inactivation could be effective in preventing disease within 4 to 5 days post-vaccination, due to a critical role for innate immune defenses [2, 39] . However, signs of disease can appear as early as 2 days post-exposure. Thus, it is necessary to develop antiviral strategies capable of inducing early protection in the face of an outbreak that threatens to spread widely. Recently, we and several other groups demonstrated that RNAi can be potentially used as a therapeutic Table I or prophylactic mechanism against FMDV [6, 24, 33] . However, to be effective against RNA viruses such as FMDV, which are highly antigenically variable, the siRNA must be targeted to conserved regions. Here, we provide evidence that siRNA targeted to conserved regions of the FMDV genome could effectively inhibit the virus replication of multiple FMDV serotypes. In this study, three genes, namely 3D, VP4 and 2B, were selected as targets for three reasons. First, it is well known that different siR-NA have different efficiencies in inducing RNAi [21] [22] [23] . Second, the proteins encoded by these genes have essential functions in different phases of the viral replication cycle. The RNA-dependent RNA polymerase (3D) is a key enzyme in viral RNA replication [34, 44] ; VP4 is a structural protein, located inside the mature virus particle, and is involved in conversion of provirions to mature virions [1] ; and 2B is involved in membrane rearrangements required for viral RNA replication and capsid assembly [19] . Third, we compared 14 reported FMDV isolates of serotype O and serotype Asia I. The analysis showed that the sequences in these regions are relatively conserved between different FMDV serotypes. Efficient delivery of siRNA into cells or organs in vivo remains a major bottleneck in antiviral therapy. A number of methods for delivering siRNA in vivo have been tested [3, 15, 27, 49] . Liposome formulations are generally used [3, 15, 49] , but a simple intranasal administration of naked siRNA has been shown to be effective against respiratory viruses [3] . Chen et al. [6] used adenoviruses as the vectors for the delivery of duplex RNA to the target tissue, and total RNA were extracted from five tissues (oropharynx, lung, liver, muscle, and epidermis of the foot) of guinea pigs treated with rAd5 and assayed for rAd5 GFP mRNA. The vast majority of rAd5 was found in the liver. In cloven-hoofed animals, the oropharynx has been identified as the major site of FMDV replication during acute and persistent infection [37] . Brown et al. [4] indicated that the FMDV RNA was localized in the tonsil, pharynx and the tracheo-bronchial lymph node. The different tissue distribution of rAd5 and FMDV is probably related to antiviral activity of rAd5 in vivo. Our overall goal was to find a siRNA vector that can spread through the similar infection and transmission tract as FMDV. Attenuated Salmonella has become popular for expressing foreign antigens as a vaccine strain or as a delivery vector of DNA vaccines [10, 12] . Gray et al. [18] found that S. choleraesuis was most often recovered from the lymph node, tonsil, lung and alimentary tract of swine inoculated with S. choleraesuis. These studies aroused our interest in evaluating the potential of Salmonella as a vector for siRNA delivery. We found that 80% of guinea pigs were completely protected when inoculated with 10 9 CFU of p3D-NT56/rS. cho, and that rS. cho also conferred marked antiviral activity in swine. Surprisingly, the antiviral potential of RNAi was evidently impaired in animals treated with a high dose of rS. cho. Srinivasan et al. [43] demonstrated that variation in the initial dose of infection with Salmonella had a profound effect on the response of Salmonella flagellin-specific CD4 T cells in vivo, and that low-dose infection could evade Salmonella flagellin-specific T cell activation completely. Mercado et al. [32] reported that a 25-fold dilution in bacterial challenge dose did not significantly alter the size or kinetics of the Listeria LLO 91-99 -specific CD8 T cell response. The low-dose effect may represent a general mechanism to increase virulence. The immune evasion could probably make the lower dose salmonella more efficient as a vector to carry siRNA.