Incidence, clinical presentation, and antimicrobial resistance trends in Salmonella and Shigella infections from children in Yucatan, Mexico

Abstract

Background: Salmonella and Shigella cause significant morbidity and mortality among children worldwide. Increased antimicrobial resistance results in greater burden of disease.

Materials and methods: From 2005 to 2011, Salmonella and Shigella isolates collected from ill children at a major hospital in Yucatan, Mexico, were subjected to serotyping and antimicrobial susceptibility testing by disk diffusion and agar dilution. The identification of bla CTX, bla CMY, bla SHV, bla TEM, and bla OXA and qnr resistance genes was conducted by PCR and sequencing.

Results: Among 2344 children with acute gastroenteritis, salmonellosis decreased from 17.7% in 2005 to 11.2% in 2011 (p < 0.001). In contrast, shigellosis increased from 8.3% in 2010 to 12.1% in 2011. Compared to children with Salmonella, those with Shigella had significantly more bloody stools (59 vs 36%, p < 0.001), dehydration (27 vs 15%, p = 0.031), and seizures (11 vs 3%, p = 0.03). In Salmonella (n = 365), there was a significant decrease in resistance to ampicillin (43 to 16%, p < 0.001), trimethoprim-sulfamethoxazole (44 to 26%, p = 0.014), and extended-spectrum cephalosporins (27 to 10%, p = 0.009). Reduced susceptibility to ciprofloxacin in Salmonella rose from 30 to 41% (p < 0.001). All ceftriaxone-resistant isolates harbored the bla CMY-2 gene. qnr genes were found in 42 (36%) of the 117 Salmonella isolates with a ciprofloxacin MIC ≥ 0.125 μg/ml. Four were qnrA1 and 38 were qnrB19. Resistance to ampicillin (40%) and trimethoprim-sulfamethoxazole (58%) was common in Shigella (n = 218), but isolates remained fully susceptible to ceftriaxone and ciprofloxacin.

Conclusion: Illness from Salmonella has decreased while severe Shigella infections have increased among children with gastroenteritis in the Yucatan Peninsula. While Shigella resistance to clinically important antibiotics remained unchanged, resistance to most of these, except ciprofloxacin, declined in Salmonella. bla CMY-2 and qnr genes are common in Salmonella isolates.

Keywords: Mexico; Salmonella; Shigella; antimicrobial resistance; beta-lactamase genes; blaCMY-2; incidence; qnr genes.

FIGURE 1

Frequency and clinical presentation of Salmonella and Shigella infections in ill children at a state referral hospital inYucatan, Mexico, 2005–2011. (A) There was a statistically significant trend toward decreasing Salmonella gastroenteritis, from 17.7% in 2005 to 11.2% in 2011 (p < 0.001). In contrast, there was a precipitous increase in shigellosis from 8.3% in 2010 to 12.1% in 2011. (B) Children with Shigella had twice as much fever ≥38.5°C, bloody stools, dehydration, and almost four times as many seizures, compared to those with Salmonella. Children with salmonellosis, however, were more likely to be admitted with hypovolemic shock.

FIGURE 2

Minimum inhibitory concentrations to ciprofloxacin, ceftriaxone, and azithromycin in Salmonella isolates from ill children at a state referral hospital in Yucatan, Mexico, 2005–2011. Thick black lines indicate breakpoints for resistance. Number of isolates for each period was as follows: 2005–2006, n = 147; 2007–2009, n = 138, and 2010–2011, n = 80. (A) Ciprofloxacin MICs. Reduced susceptibility to ciprofloxacin in Salmonella rose from 28% in 2005–2006 to 33% in 2010–2011. Non-susceptibility (MIC = 2) first emerged in 2007 in a S. Anatum isolate and later appeared in S. Typhimurium multidrug-resistant, cephalosporin-resistant isolates. Black line is resistance breakpoint for isolates from stools; red line is breakpoint for systemic infections. CLSI breakpoints for susceptibility and resistance in isolates from stools are 1 and 4 μg/ml, respectively. Breakpoints for susceptibility and resistance in isolates from systemic infections are 0.06 and 1 μg/ml, respectively. (B) Ceftriaxone MICs. Resistance to ceftriaxone and other extended-spectrum cephalosporins significantly decreased from 19.7% during 2005–2006 to 8.8% in 2010–2011. Salmonella isolates were distributed into two distinct populations, one which was susceptible and the other resistant to ceftriaxone. CLSI breakpoints for susceptibility and resistance are 1 and 4 μg/ml, respectively. (C) Azithromycin MICs. No significant changes in azithromycin MICs were noted. Suggested breakpoints for susceptibility and resistance are 16 and 64 μg/ml, respectively (Sjolund-Karlsson et al., 2011).

FIGURE 3

Minimum inhibitory concentrations to ciprofloxacin, ceftriaxone, and azithromycin in Shigella isolates from children with gastroenteritis at a state referral hospital inYucatan, Mexico, 2005–2011. Thick black lines indicate breakpoints for resistance. Number of isolates for each period was as follows: 2005–2006, n = 73; 2007–2009, n = 79 and 2010–2011, n = 66. (A) Ciprofloxacin MICs. Isolates were fully susceptible to ciprofloxacin throughout the study. Two isolates had ciprofloxacin MICs ≥ 0.12 μg/ml, only one of which was resistant to nalidixic acid. CLSI breakpoints for susceptibility and resistance are 1 and 4 μg/ml, respectively. (B) Ceftriaxone MICs. All isolates remained fully susceptible to ceftriaxone with no changes over the study period. CLSI breakpoints for susceptibility and resistance are 1 and 4 μg/ml, respectively. (C) Azithromycin MICs. Suggested breakpoints are the same as those proposed for Salmonella. Although isolates were apparently susceptible to azithromycin throughout the study period, 7–17% of these had an MIC = 16.