Salmonellosis in Lagos, Nigeria: Incidence of Plasmodium falciparum-associated Co-infection, Patterns of Antimicrobial Resistance, and Emergence of Reduced Susceptibility to Fluoroquinolones

The present study was undertaken to examine the status of antimicrobial resistance in Salmonella-associated diseases, by verifying possible emergence of reduced susceptibility to fluoroquinolones in Salmonella isolates and determining the incidence of Plasmodium falciparum-associated co-infection with Salmonella serotypes. Antimicrobial resistance in clinical isolates of Salmonellae was examined for a 12-month period. Four hundred and forty-one patients comprising two groups were recruited. Group A comprised 235 patients diagnosed by clinicians of having pyrexia, and group B included stool samples of 206 patients presenting with gastroenteritis. Samples were cultured and isolates identified, and drug susceptibility testing was performed using the standard methods. Of the 235 samples screened in group A, 42 Salmonella isolates and 107 Plasmodium spp. were identified. Of the 42 Salmonella isolates, 19 (45.2%) were Salmonella Typhi, 9 (21.4%) S. Enteritidis, and 7 (16.7%) each of S. Paratyphi and S. Arizonae. Plasmodium spp.-associated co-infection with Salmonellae was observed in 16 patients mostly in complicated typhoidal cases and S. Enteritidis-associated bacteraemia. Fifty-three of the 206 stool samples from group B patients were confirmed positive for bacterial pathogens, made up of 35 Salmonella and 18 Shigella isolates. Of the Salmonella isolates, 18 (51.4%) were S. Enteritidis, 11 (31.4%) S. Arizonae, 4 (11.4%) S. Paratyphi, and 2 (5.7%) S. Typhi. There was no statistically significant difference (p < 0.01) in antimicrobial resistance patterns exhibited among typhoidal Salmonellae isolated in 2000 and 2005. A similar trend in resistance was recorded for non-typhoidal Salmonellae (p < 0.05). For the first time in Lagos, Nigeria, Salmonella isolates (10-18%) with reduced susceptibility to both ciprofloxacin and ofloxacin at MIC50 and MIC90 values of 0.015 and 0.03 microg/mL respectively were found. Despite this development, ciprofloxacin and ofloxacin remain the drug of choice for severe cases of salmonellosis, although caution should be exercised by clinicians in their prescriptions such that fluoroquinolone antibiotic therapy is used only in laboratory-proven cases of typhoid fever and Salmonella-associated bacteraemia to preserve its efficacy.