Eleven-Year Surveillance of Antibiotic Resistance in Streptococcus pneumoniae in Casablanca (Morocco)

Abstract

Objective: To analyze trends of antibiotic resistance rates in Streptococcus pneumoniae from 1998 to 2008 in Casablanca (Morocco). Methods: The antibiotic resistance levels of 955 consecutive nonduplicate isolates were studied using E test and disc diffusion methods. Results were interpreted following Clinical and Laboratory Standards Institute guidelines (2005). Analysis was done according to three periods (1998–2001; 2002–2005; 2006–2008), age, and site of infection. Results: Penicillin nonsusceptibility (PNS) increased significantly over time (15.6%, 17.8%, and 24.8%; p=0.003). Levels of PNS have changed as well: in 2006–2008, 9.1% of the isolates had an MIC ≥2 μg/ml versus 7.7% in 2002–2005 and 3.4% in 1998–2001. The PNS increase was particularly marked in pediatric isolates (21.4%, 25.5%, and 43.3%; p=0.001). There was no significant difference between the rates of PNS in invasive and noninvasive isolates from children, whereas in adults noninvasive isolates were more penicillin nonsusceptible. Amoxicillin and ceftriaxone nonsusceptible isolates were very rare. An increase of resistance rates was also noticed for erythromycin (9.4%, 12.2%, and 14.4%), tetracycline (20%, 18.6%, and 30.5%), and chloramphenicol (5.6%, 5.6%, and 8.1%). Trimethoprim-sulfamethoxazole resistance rates remained stable (22.8%, 20%, and 23.8%). Proportions of dual nonsusceptibility to penicillin and erythromycin, increased from 5.6% to 8.9%. Multiple drug resistance (resistance to 3 or more antibiotic classes) was found in 0%, 2.4%, and 7.7% of all isolates, respectively. Conclusion: The results reported here maybe useful for guiding update of treatment recommendations and suggest the need for continuous surveillance. Increase of antibiotic resistance correlated with antibiotic consumption, stressing the need for elaboration of antibiotic policy in Morocco.

Introduction

S treptococcus pneumoniae is responsible for high morbidity and mortality due to community acquired infections—mainly otitis media—community acquired pneumonia, and meningitis (9,10).

Antibiotic resistance in S. pneumoniae has been increasing regularly over the last three decades, with marked geographic variations and very high levels in some areas (4). This situation stresses the need for local data to guide treatment recommendations.

Data from North Africa, and especially Morocco, are scarce, and the rare published studies are sporadic and include relatively small numbers of isolates (3,11).

In Casablanca (Morocco), the surveillance of antibiotic resistance in S. pneumoniae started in 1994 (1,2) and showed relatively low levels of penicillin nonsusceptibility (PNS), 12.5% and 9.2%, respectively.

We report on an 11-year study of S. pneumoniae antibiotic resistance conducted at the Ibn Rochd University Hospital of Casablanca (1998–2008).

Materials and Methods

The Ibn Rochd University Hospital of Casablanca (Morocco) is a tertiary care hospital comprising of 1,600 beds of which 240 are pediatric. This bed capacity represents 57.9% of public health acute care beds of the Grand Casablanca.

From January 1998 to December 2008, all the isolates of S. pneumoniae recovered at the microbiology laboratory were included. Duplicate isolates were excluded from this study.

Isolates recovered from patients' ≤14 years of age were considered pediatric isolates. Isolates recovered from normally sterile sites (cerebrospinal fluid, blood, pleural fluids, and articular fluids) were considered invasive. Resistance rates have been analyzed according to three periods: 1998–2001, 2002–2005, and 2006–2008.

Isolation and identification of S. pneumoniae have been realized by standard methods (α hemolysis, optochin susceptibility, and bile solubility). Antibiotic susceptibility testing was done following Clinical Laboratory Standard Institute guidelines (6). Oxacillin (1 μg), erythromycin, tetracycline, and trimethoprim-sulfamethoxazole were tested by disk diffusion with antibiotic disks from Biorad on Mueller Hinton Agar (BioMerieux) supplemented with 5% sheep blood. Minimal inhibitory concentrations for penicillin G, amoxicillin, and ceftriaxone have been determined on 5% sheep blood Mueller Hinton Agar with E tests from AB Biodisk.

The breakpoints used for interpretation were those recommended by the Clinical and Laboratory Standards Institute in 2005: ≤0.06 and ≥2 μg/ml for penicillin; for amoxicillin ≤2 and ≥8 μg/ml for nonmeningeal isolates; for ceftriaxone ≤0.5 and ≥2 μg/ml for meningeal isolates and ≤1 and ≥4 μg/ml for nonmeningeal isolates.

Quality control was done with S. pneumoniae ATCC 49619.

Data were analyzed with WHONET5 and EpiInfo 6.4 software. Statistical comparisons were done by the Chi-square test and p≤0.05 was considered significant.

Results

During the survey period, a total of 995 nonrepetitive isolates were studied: 33% (n=315) were recovered from children, and 55.6% (n=531) originated from invasive infections (Table 1). The numbers of isolates from each period were very similar (Table 2).

Table 1.

Origin of 955 Streptococcus pneumoniae Isolates from 1998 to 2008 in Casablanca (Morocco)

	Children (n)	Adults (n)	Total (n)
Invasive samples	252	279	531
CSF	144	127	271
Blood	77	95	172
Biological fluids	31	57	88

Noninvasive samples	63	361	424
Respiratory tract samples	46	297	343
Pus	15	58	73
Otitis media samples	2	6	8
Total	315	640	955

Table 2.

Penicillin Nonsusceptibility Rates in Streptococcus pneumoniae Isolates According to Age

	1998–2001	2002–2005	2006–2008
All ages
n	311	331	313
Total % PNSSP (I+R)	15.6 (12.2+3.4)	17.8 (10.1+7.7)	24.8 (15.7+9.1)
Children
n	89	117	109
Total % PNSSP (I+R)	21.4 (14.3+7.1)	25.5 (13.2+12.3)	43.3 (28.9+14.4)
Adults
n	222	214	204
Total % PNSSP (I+R)	13 (12+1)	14 (9.3+4.7)	14.9 (8.7+6.2)

PNSSP, penicillin nonsusceptible Streptococcus pneumoniae.

PNS rates, all ages included, increased over time, slightly in the beginning from 15.6% during the first period (1998–2001) to 17.8% during the years 2002–2005 and more significantly (p=0.003) in the third period (2006–2008) with 24.8%.

Furthermore, the level of PNS has changed as well: 3.4% of the isolates had an MIC ≥2 μg/ml in 1998–2001 versus 7.7% in 2002–2005 and 9.1% in 2006–2008.

PNS rates in adults remained stable during the three study periods whereas the rates of PNS were significantly higher in isolates recovered from children (21.4% vs. 25.5% and 43.3%; p=0.001).

There was no significant difference between the rates of PNS in invasive and noninvasive isolates when all age groups or children were considered (Table 3). In adults, noninvasive isolates were more penicillin nonsusceptible (p=0.03).

Table 3.

Penicillin Nonsusceptibility Rates (%) in Streptococcus pneumoniae According to Type of Isolates

	Invasive (n=531)	Noninvasive (n=424)	p
All ages	19.8	19.1	0.79
Adults	10.7	16.6	0.03
Children	29.8	33.3	0.58

Nonsusceptibility to amoxicillin and ceftriaxone was very rare, accounting for less than 1% each (Table 4). During the whole study period, only five isolates were amoxicillin resistant (two high level and three low level). Eight isolates were ceftriaxone resistant (three high level and five low level).

Table 4.

Antibiotic Resistance Trends (%) of Streptococcus pneumoniae Isolates in Casablanca (Morocco)

Antibiotics	1998–2001	2002–2005	2006–2008	p
Amoxicillin	0	0.6	1	NS
Ceftriaxone	0.6	0.8	1	NS
Chloramphenicol	5.6	5.6	8.1	NS
Erythromycin	9.4	12.2	14.4	0.053
Tetracycline	20	18.6	30.5	0.002
TSU	22.8	20	23.8	NS

TSU, trimethoprim-sulfamethoxazole; NS, nonsignificant.

Trends in resistance to antibiotics other than penicillin (Table 4) showed that over time, resistance to trimethoprim-sulfamethoxazole remained relatively stable, around 20%, whereas the rates of resistance to chloramphenicol slightly increased (5.6% vs. 5.6% and 8.1%). Resistance to erythromycin was more marked (9.4% vs. 12.2% and 14.4%), whereas resistance to tetracycline increased significantly (20% vs. 18.6% and 30.5%).

Proportions of dual nonsusceptibility, defined as penicillin intermediate or resistant together with erythromycin resistance, increased from 5.6% to 8.9%. Multiple drug resistance (resistance to three or more antibiotic classes) was found in 0% versus 2.4% and 7.7% of all isolates, respectively.

Penicillin nonsusceptible S. pneumoniae isolates were more resistant to other antibiotic classes than penicillin susceptible ones for erythromycin, tetracycline, trimethoprim-sulfamethoxazole, and less markedly for chloramphenicol (Table 5). Among penicillin susceptible isolates, no one showed a decreased susceptibility to amoxicillin or ceftriaxone.

Table 5.

Antibiotic Resistance (%) of Streptococcus pneumoniae According to Penicillin Susceptibility Status

Antibiotics	Penicillin susceptible	Penicillin nonsusceptible
Erythromycin	3.9	40.6
Tetracycline	22.4	53.8
TSU	9	60.6
Chloramphenicol	7.6	11.6

Discussion

The 11-year survey of antibiotic resistance in S. pneumoniae conducted at the Ibn Rochd University Hospital of Casablanca (Morocco) showed an increase of resistance to penicillin, erythromycin, tetracycline, and chloramphenicol. Resistance to amoxicillin and ceftriaxone was marginal. Penicillin nonsusceptible isolates are more resistant to other antibiotics than susceptible isolates, making therapeutic options more problematic. Increases in dual and multidrug resistance were noticed as well, making antibiotic choice even more difficult.

The increase of antibiotic resistance in S. pneumoniae has been attributed to several factors, including sociocultural and economic factors and differences in regulatory practices (8). Among these factors, the role of antibiotic consumption has been amply demonstrated (9,12). A recent European study showed that variations of antibiotic consumption are well correlated to S. pneumoniae PNS rates at country level (12). Antibiotic consumption should be considered according to the volume and the pattern use (12): the much lower rates of PNS in S. pneumoniae in Germany compared with France have been linked to the lower volume of overall antibiotic use and to the relative higher use of narrow spectrum penicillins and much lower use of broad spectrum penicillins in Germany. In Morocco, antibiotic consumption has sharply increased: from 7.01 DDD/1,000 inhabitants (DID) in 1999 to 9.96 DID in 2004 (7). In Casablanca, the economic capital of Morocco, in 2004 the antibiotic consumption was 2.4 times higher than the mean for the whole country and the volume of large spectrum penicillins (J01CA of the WHO ATC classification) almost tripled (5.43 DDI vs. 14.23 DID) between 1994 and 2004. The correlation between antibiotic use and antibiotic resistance in S. pneumoniae showed in this survey was suggested as well for amoxicillin resistance in Escherichia coli responsible for community acquired urinary tract infections in Casablanca (7).

Antibiotic resistance rates vary considerably according to geographic location. In Morocco, other data are available only from the capital Rabat: a study conducted between 1997 and 2001 on 90 isolates (11) reported slightly lower rates (7.8% of PNSP) than to those observed in Casablanca in 1994–1997 (1). A more recent study (2006–2007) of 85 respiratory isolates showed 40%, 14.1%, and 13% for penicillin, amoxicillin, and cefotaxime nonsusceptibility (3). The reasons for such higher rates and such sharp trends are not clear, apart from the respiratory origin of the isolates. On the other part, multicentric surveys in the Mediterranean region showed a very heterogeneous situation (4). Other studies showed that remarkably different rates can be found in regions from the same country (5).

Young age is one of the multiple risk factors linked to antibiotic resistance in S. pneumoniae (9). Results from our series are concordant, since PNS rates were much more higher in pediatric isolates (43.3% vs. 14.9%).

Resistance is more frequent in noninvasive isolates, especially from respiratory tract (9). In our series, similar results were noticed only in adults. The discrepant result found in children may be due to the fact that only 20% of the isolates tested in this age group were noninvasive and that the percentage of isolates from otitis media was very low.

The data reported here maybe useful for guiding the elaboration of local treatment recommendations, and for monitoring the antibiotic resistance trends after the introduction of the pneumococcal vaccine in the Moroccan national immunization program.

Data on antibiotic resistance of S. pneumoniae in Morocco are now available only from the administrative and economic capitals and need to be completed with results from other regions, which suggests the establishment of a national network of laboratories using the same protocols to allow for comparison.

Footnotes

Author Disclosure Statement

This study was funded in part by Glaxo Smith Kline Beecham, Morocco.

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