Re: Outbreak of typhoid fever in vaccinated members of the French Armed Forces in the Ivory Coast

Abstract

Investigations on the 2001 outbreak of typhoid fever in vaccinated French troops have been meticulous. Laboratory studies on 24 clinically labeled cases pointed towards 14 troops to be typhoid confirmed [1]. The four Salmonella isolates during the outbreak had similar antibiotype. The isolates would merit their evaluation for any phenotypic expression of Vi antigen. Vi-negative isolates that would not be agglutinated by typing anti-Vi sera have been known. During the 1960s such isolates were recorded during phage typing in different geographical locations globally [2]. Vi-negative strains would cause a clinical picture closely resembling the one caused by Vi-positive strains. Oral administration of 107 colony-forming units of Vi-positive and Vi-negative strains caused disease in 56 and 25% adult volunteers. The diseases pattern was indistinguishable among those receiving isolates with or without Vi antigen [3]. Undoubtedly only the stored rather than the fresh isolates from the 2001 outbreak [1] would be avail able for any phenotypic evaluations for Vi antigen. That would not jeopardize any pseudo-Vi-negative labels attributable to storage. Genetic control of Vi expression is now well understood and it has been possible to confirm truly Vi-negative Salmonella ser ovar typhi. Employing a muliplex PCR method for the genetic locus for Vi expression, one truly Vi-negative isolate has been reported from Pakistan [4]. Furthermore, it has been possible to analyze stored isolates by PCR for ViaB and SPI-7. The precise genetic components linked with Vi negative strains could be detected in the peripheral blood of typhoid fever [5]. To conclude retrospective analysis of 2001 typhoid isolates [1] for Vi antigen would establish whether the existing Vi polysaccharide vaccines would indeed be effective during an exposure to isolates lacking such antigen components. In the event of Vi-negative iso lates, upon their entry to intracellular niche, they continued to be protected from Vi-antibody. Obvi ously, improvised vaccines [1], with excellent cellular and humoral immunity, would be effective to address infections by Vi-negative Salmonella typhi [4, 5].