From the Research
Bordetella pertussis levels above 10,000 copies/ml are generally considered high and highly infectious. Individuals with copy counts in this range are most contagious, especially during the early catarrhal stage of whooping cough (the first 1-2 weeks of illness) 1. Bacterial loads typically peak during this initial phase before the characteristic "whooping" cough develops. The infectiousness decreases significantly after about 5 days of appropriate antibiotic treatment (usually azithromycin, clarithromycin, or erythromycin) 2. Some key points to consider include:
- The infectious potential correlates with bacterial load but also depends on the patient's symptoms, with those actively coughing expelling more bacteria.
- Even lower bacterial loads can still be infectious in some circumstances, particularly to unvaccinated individuals or those with compromised immunity.
- Healthcare providers should implement droplet precautions for patients with suspected or confirmed pertussis infection regardless of the exact bacterial load, and public health authorities typically recommend isolation until completion of 5 days of appropriate antibiotic therapy. It's also worth noting that antibiotic resistance has been reported in some cases, with macrolide-resistant B. pertussis strains isolated from patients in several countries 3. However, the most recent and highest quality study on this topic is not available, and the provided evidence does not specifically address the question of what level of B. pertussis is considered high and infectious. Therefore, the recommendation is based on general knowledge and previous studies, such as 4, which reported on the persistence of B. pertussis in patients despite antibiotic treatment.