Levels of SAT serotype foot and mouth disease viruses in tissues of experimentally infected cattle carcasses and those from the wildlife-livestock interface in the Zambezi Region of Namibia

Abstract

A value chain approach was employed as the basis for sanitary management of both food safety and animal disease risks. This was achieved through the integration of hazard analysis critical control point (HACCP) and commodity-based trade (CBT) methodologies. That approach enabled identification of and management of critical control points (CCPs) to enable an ‘appropriate level of protection (ALOP)’ to be achieved, i.e. attainment of negligible risk. This work was done as part of a project led by the Meat Board of Namibia and largely funded by the Millennium Challenge Account – Namibia, aimed at improving market access for beef producers the Zambezi Region (ZR) of Namibia, a foot and mouth disease (FMD)-infected zone. In order to validate the technical basis for the approach, it was necessary to confirm that matured (pH < 6.0), deboned beef from which the lymph nodes and fat had been removed, did not contain detectable quantities of southern African Territories (SAT) serotype FMD viruses even when derived from cattle in the acute stage of infection. This was done by experimental infection of cattle in a bio-secure facility and testing of tissues derived from those cattle immediately after slaughter and exsanguination. Tissues derived from healthy cattle after slaughter at the official abattoir in the ZR were also subsequently tested for viral content. No evidence of infection was found in those animals. Experimental infection involved intradermolingual inoculation of six cattle with three different FMD SAT virus isolates obtained from outbreaks in cattle in the ZR. Forty-eight hours after infection the animals were euthanized and exsanguinated (i.e. acute phase of infection) and selected tissues collected and processed to determine viral genomic content. For the abattoir survey, 148 cattle were sampled for identification SAT serotype viruses in specific lymph nodes of carcasses derived from cattle slaughtered at the abattoir in the ZR. Sub-mandibular, pre-scapular and popliteal lymph nodes from these cattle were examined based on the results of the prior experimental infection study that showed those lymph nodes to consistently contain detectable viral RNA levels. Conventional and real time PCR techniques were used for detection of FMD virus RNA. It was shown that immediately after slaughter and exsanguination, but before maturation, SAT serotype virus RNA was undetectable in striated muscle or fat of acutely infected cattle using the methods employed in this study. A weakness of the method employed in this study was the use of a non-optimized protocol for isolating total RNA from fat tissue. However, high levels of virus RNA were present in some lymph nodes; consistently in the three that were sampled routinely at the abattoir in the ZR. Sampling of the 148 cattle derived mainly from high risk areas and slaughtered at the abattoir failed to reveal presence of FMD virus RNA in any animal. The sample size was however too small to attain statistical significance due to cessation of sampling caused by a FMD outbreak in cattle in the ZR. These results confirm and extend previously published findings on the safety of matured beef from which the bones and lymph nodes have been removed even if such beef is derived from locations that are not recognised as free from FMD.