Emergence of rifampin-resistant Haemophilus influenzae after prophylaxis

Clindamycin therapy of experimental meningitis caused by penicillin- and cephalosporin-resistant Streptococcus pneumoniae

Although penicillin resistance among Streptococcus pneumoniae strains is increasing in many areas, resistance to clindamycin remains low. In our well-characterized rabbit meningitis model, we conducted experiments to evaluate the bacteriologic efficacy of clindamycin after a penicillin- and cephalosporin-resistant S. pneumoniae strain was intracisternally inoculated. Animals received a loading intravenous dose of 30 mg of clindamycin per kg of body weight and then two doses of 20 mg/kg given 5 h apart. In addition to clindamycin, some animals received dexamethasone (DXM) with or without ceftriaxone. The concentrations of clindamycin in cerebrospinal fluid were from 8.9 to 12.8% of the concomitant concentrations in serum and were unaffected by DXM administration. Mean changes in CFU (log10 per milliliter) at 10 and 24 h were -3.7 and -6.1, respectively, for clindamycin-treated rabbits, -3.6 and -6.3 for clindamycin-DXM-treated rabbits, -3.9 and -5.8, respectively, for clindamycin-ceftriaxone-treated rabbits, and -5.0 and -6.7, respectively, for clindamycin-ceftriaxone-DXM-treated rabbits. By 24 h all but one of the cultures of cerebrospinal fluid (that from a clindamycin-DXM-treated rabbit) were sterile. Because of the potential risk for clindamycin-treated rabbits to develop macrolide-lincosamide resistance, we attempted, unsuccessfully, to induce clindamycin resistance in vitro in two S. pneumoniae strains. Although clindamycin therapy might be effective in selected patients with multiple-drug-resistant pneumococcal meningitis who have failed conventional treatments, clinical experience is necessary before it can be recommended.

Rifampicin prophylaxis for throat carriage of Haemophilus influenzae type b in patients with invasive disease and their contacts

OBJECTIVE

To determine rates of colonisation with Haemophilus influenzae type b among household contacts of children with invasive H influenzae type b disease; compliance among medical staff with recommendations for use of rifampicin prophylaxis; and effects of rifampicin treatment in H influenzae type b colonisation of patients and contacts.

DESIGN

Prospective study of patients and their household contacts.

SETTING

Royal Children's Hospital (the major paediatric hospital) in Victoria, Australia, with catchment population of 4.2 million, including 300,000 children aged under 5 years.

SUBJECTS

234 patients (age range 6 weeks to 8 years) with 235 episodes of all types of invasive H influenzae type b disease admitted during 1988-9 and their contacts.

MAIN OUTCOME MEASURES

Positive cultures of H influenzae type b from throat swabs taken at admission and six weeks subsequently; recording of rifampicin prophylaxis.

RESULTS

The percentage of patients with positive throat cultures fell from 69% (33/48) on day 1 of admission to 9% (4/47) after three days' treatment; at six weeks' follow up 32% (32/99) of patients who had not received rifampicin and 8% (5/61) who had, had positive throat cultures. Among household contacts, 33% (62/190) of children and 7% (25/359) of adults were colonised, and the colonisation rates were similar in contacts of patients with all types of H influenzae type b disease. Rifampicin prophylaxis was indicated in 85 families; in 34% it was not prescribed at all for contacts and in 41% not as recommended. The colonisation rates at follow up were significantly less in siblings given rifampicin (12%, 9/78), particularly in families in which all members were treated (3%), than in those in which they were not (36%) (odds ratio 21.5; 95% confidence interval 3.0 to 103.4).

CONCLUSIONS

The rate of throat colonisation with H influenzae type b was similar among siblings of children with all types of invasive H influenzae type b disease. Colonisation in contacts can be reduced by rifampicin prophylaxis, but some contacts remained colonised or were recolonised by the time of follow up. Medical staff complied poorly with current recommendations for rifampicin prophylaxis, which reduces its intrinsically limited value in preventing H influenzae type b disease.

Rifampin resistance of Legionella pneumophila is not increased during therapy for experimental Legionnaires disease: study of rifampin resistance using a guinea pig model of Legionnaires disease

Isolates of Legionella pneumophila serogroup 1, obtained from guinea pigs with experimentally induced Legionnaires disease, were tested for rifampin resistance. Thirteen isolates were from animals treated with rifampin alone, four isolates were from animals treated with saline, and three isolates each were from animals treated with erythromycin or erythromycin plus rifampin; all of these isolates were derived from the same parent strain, F889. Most of the isolates were obtained from rifampin-treated animals that survived infection but had persistence of bacteria in their lungs at necropsy. No differences in rifampin agar dilution MICs were detected for the 23 isolates and parent strain that were tested. None of the 13 isolates from animals treated with rifampin alone had a high number of resistant organisms detected by using a rifampin gradient plate assay. Thirteen isolates plus the parent strain were tested by using a quantitative method of determining resistance frequency. Considerable heterogeneity among isolates was observed, but there was no evidence of increased resistance for any treatment group. The range of rifampin resistance frequencies was 10(-7) to 10(-8). No evidence for rifampin-induced resistance of L. pneumophila was found in this study.

Epidemiology and prevention of invasive Haemophilus influenzae type b infection

The use of rifampicin prophylaxis is recommended in close contacts of individuals with invasive Haemophilus influenzae type b infection if they include a child less than 4 years old in whom the risk of secondary infection is relatively high. In practice, delays in administration of rifampicin, contra-indications to its use and the difficulty of identifying all contacts at risk can reduce significantly its efficacy. Only 1-2% of cases of H. influenzae type b diseases are attributable to known contact and, at best, rifampicin prophylaxis can have little impact on the incidence. In the USA, one in 200 children less than 5 years old is affected. The incidence is probably similar in Australia but there are local differences which could affect the value of preventative measures. The vaccine recently licensed in the USA is not effective in children less than 18-24 months of age in whom the incidence of invasive H. influenzae type b infection, other than epiglottitis, is highest. Nevertheless, it could prevent more than 30% of cases if given to children at the age of 24 months. Vaccines effective in younger infants should become available soon. The best chance of prevention is by the optimal use of both rifampicin prophylaxis and immunization.

Pathogenicity of a rifampin-resistant cerebrospinal fluid isolate of Haemophilus influenzae type b

Two children in a day care facility developed Haemophilus influenzae type b meningitis. The second child was enrolled in the facility after rifampin had been administered to the other attendees. The isolate from the first child was susceptible to rifampin, but the isolate from the second was resistant. Both isolates had identical outer membrane protein PAGE profiles. To investigate the virulence of these isolates, we inoculated infant rats intranasally with either the rifampin-resistant or rifampin-susceptible CSF isolate. The rates of nasal colonization (14 of 20 and eight of eight animals inoculated with the rifampin-resistant and rifampin-susceptible isolates, respectively) did not differ significantly. However, bacteremia occurred less frequently in pups inoculated with the rifampin-resistant strain than in animals inoculated with the susceptible strain (four of 20 vs eight of eight, P less than 0.0001). Nasal washings, blood, and CSF obtained from animals inoculated with the rifampin-resistant isolate were divided and plated on media containing rifampin (1 microgram/ml) or without rifampin. Except for those from one animal, organisms isolated from blood and CSF grew only on medium lacking rifampin, whereas H. influenzae type b growing from nasal washings was frequently found on both media. We conclude that mutation of H. influenzae to rifampin resistance is a hazard of rifampin chemoprophylaxis. Rifampin-resistant isolates have the potential to cause disease in patients and experimental animals, although they may be relatively less pathogenic than the parent, susceptible organism.

Prophylaxis for bacterial meningitis

Close contact of patients with bacterial meningitis that is caused by either Haemophilus influenzae type b or Neisseria meningitidis are at an increased risk of developing invasive infections with these bacteria. Chemoprophylaxis with rifampin and immunoprophylaxis with vaccines may prevent some secondary infections.

Haemophilus influenzae infections. The impact of resistance on the use of aminopenicillins and other antimicrobials in outpatient therapy

The clinical manifestations of Haemophilus influenzae infections and the growing problem of ampicillin-resistant strains are reviewed. Despite the fact that approximately 25 percent of H. influenzae are resistant to penicillin and ampicillin, oral aminopenicillins (ampicillin, amoxicillin, bacampicillin, and cyclacillin) are commonly used for the outpatient management of these infections. The characteristics of these drugs and the rationale for their use are discussed and compared with alternative approaches to therapy. This subject requires periodic review in consideration of the increasing prevalence of ampicillin and multiple antibiotic resistance among H. influenzae isolates.

Chemoprophylaxis for the prevention of bacterial meningitis

Close contacts of index patients with Neisseria meningitidis or Haemophilus influenzae type b meningitis show higher nasopharyngeal carriage rates and are at an increased risk of contracting the disease. The key issue in the management of such contacts remains close and careful surveillance, because prophylaxis will never result in the absolute prevention of secondary cases. Adequate chemoprophylaxis with rifampin eradicates nasopharyngeal colonization in more than 90% and may prevent both co-primary and secondary diseases. Routine chemoprophylaxis for close contacts of systemic meningococcal diseases is recommended. However, there are several factors inherent in rifampin prophylaxis for the prevention of H. influenzae type b meningitis which exclude its use as a routine measure.

Trimethoprim and rifampin in combination for chemoprophylaxis of household contacts of patients with invasive infections due to Haemophilus influenzae type b

We compared the effectiveness of rifampin-trimethoprim in fixed combination (3.75:1) to rifampin alone in the eradication of Haemophilus influenzae type b carriage among contacts of patients with invasive infection caused by this organism. The study population was composed of 127 index patients and 620 contacts. Twenty-six percent of contacts were colonized. Rifampin-trimethoprim eradicated carriage in 77.6% of contacts (71.1% in contacts less than 5 years, 84.2% in contacts greater than or equal to 5 years) whereas rifampin eradicated carriage in 69.9% of contacts (56.4% in contacts less than 5 years, 81.8% in contacts greater than or equal to 5 years). A single isolate resistant to rifampin and rifampin-trimethoprim was encountered. The eradication rate achieved with this regimen of rifampin-trimethoprim was too low to recommend its routine use. However, a higher dose or longer course might merit clinical trial.

Mutation frequency of Haemophilus influenzae to rifampin resistance

Twenty-two susceptible strains of Haemophilus influenzae were examined for mutation to rifampin resistance (minimal inhibitory concentration, greater than or equal to 10 micrograms/ml). All strains had detectable apparent mutation frequencies with a median minimal inhibitory concentration 2,000-fold greater than that of their wild-type parents. Of the type b strain mutants, 80% (8 of 10) expressed high-level resistance (minimal inhibitory concentration, 750 micrograms/ml) that was 75-fold greater than readily achievable serum concentrations.

Emergence of rifampin-resistant Haemophilus influenzae

Rifampin susceptibility of nasopharyngeal Haemophilus influenzae strains was determined before and after community-wide chemoprophylaxis with rifampin (children) and sequential minocycline-rifampin (adults). All strains were rifampin susceptible before prophylaxis, but 10.3% of the strains at 1 week and 7.5% of the strains at 9 weeks after prophylaxis were rifampin resistant.

In vitro development of rifampin resistance in clinical isolates of Haemophilus influenzae type b

Although all of 14 clinical isolates of Haemophilus influenzae type b strains demonstrated rifampin susceptibility in vitro (minimal inhibitory concentration less than or equal to 0.4 microgram/ml) when an inoculum of 10(4) colony-forming units (CFU) was used, 10 of the 14 strains manifested resistance to this agent when an inoculum of 10(8) CFU was tested. The mutation rate for rifampin resistance ranged from 1 resistant colony per 3.5 x 10(6) CFU to 1 per 4 x 10(7) CFU. The emergence of rifampin-resistant mutants was prevented when trimethoprim was combined with rifampin. This finding suggests that when used alone for prophylaxis of H. influenzae type b nasopharyngeal carriers, rifampin is likely to lead to the emergence of resistant strains.