Trends in resistance to penicillin and erythromycin of invasive pneumococci in Portugal

Abstract

Antimicrobial resistance of pneumococci is influenced by serotypes, antimicrobial consumption and vaccine use. Serotyping of 697 out of 1331 pneumococcal isolates, recovered in Portugal from 1994 to 2004, showed that the theoretical rate of heptavalent conjugate vaccine coverage was 91.7% and 63.6% for penicillin and erythromycin non-susceptible strains, respectively, in children up to 1 year old. The use of amoxicillin and erythromycin decreased in the vaccine period 2001-2004 (P=0.04 and P<0.01, respectively) but azithromycin usage increased in the same period (P<0.01). By using linear regression models, we evaluated the role of antimicrobial and vaccine use in the trends of resistance to penicillin and erythromycin among the isolates. The models suggest that the use of macrolides was the main factor associated with an increase of penicillin and erythromycin non-susceptible isolates from adults (P<0.01) and erythromycin non-susceptible isolates among children (P=0.006). These models also suggest that heptavalent vaccine is failing to reduce antimicrobial resistance as expected, possibly due to the increased consumption of azithromycin (P=0.04). The efficient use of new antibiotics may reverse the present trends of antimicrobial resistance.

Fig. 1

Distribution of non-susceptible strains to penicillin and erythromycin by pneumococci serotypes isolated from invasive disease in Portugal between years 1999 and 2002. (a) Distribution of non-susceptible strains to penicillin by serotypes isolated from children aged <2 years (□) and <18 years (■). (b) Distribution of non-susceptible strains to penicillin by serotypes isolated from adults aged ⩾18 years (■) and ⩾65 years (□). (c) Distribution of non-susceptible strains to erythromycin by serotypes isolated from children aged <2 years (□) and <18 years (■). (d) Distribution of non-susceptible strains to erythromycin by serotypes isolated from adults aged ⩾18 years (■) and ⩾65 years (□).

Fig. 2

Serotype vaccine coverage according to age group for penicillin and erythromycin non-susceptible strains. (a) Percentage of strains with serotypes included in the vaccine formulations (SVF), according to age group for penicillin non-susceptible strains. (b) Percentage of strains with SVF, according to age group for erythromycin non-susceptible strains. (c) Percentage of strains with SVF, according to age group for susceptible and non-susceptible strains.

Fig. 3

Observed annual rates (1994–2004) of antimicrobial non-susceptible pneumococci isolated from invasive disease in Portugal and respective linear regression models. (a) Observed (■) and predicted (–△–) annual rates of penicillin non-susceptible strains for children (PC). (b) Observed (■) and predicted (–△–) annual rates of penicillin non-susceptible strains for adults (PA). (c) Observed (■) and predicted (–△–) annual rates of erythromycin non-susceptible strains for children (EC). (d) Observed (■) and predicted (–△–) annual rates of erythromycin non-susceptible strains for adults (EA). (e) Scenarios for annual rates of penicillin non-susceptible strains for children: predicted (–△–) without vaccine (–×–), without azithromycin use (–◊–), without vaccine and azithromycin use (–+–).