From the Guidelines
A high and potentially infectious bacterial load of Bordetella pertussis is generally considered to be 10^7 to 10^9 colony-forming units (CFU) per milliliter of respiratory secretions. Individuals with this level of bacterial load are highly contagious, particularly during the early catarrhal stage of whooping cough when symptoms resemble a common cold. The infectiousness typically decreases significantly after about two weeks of antibiotic treatment, though untreated individuals may remain contagious for up to three weeks after the onset of the characteristic paroxysmal cough. This high bacterial load is particularly concerning in healthcare and childcare settings where transmission to vulnerable populations can occur. The bacteria attach to ciliated epithelial cells in the respiratory tract using adhesins like filamentous hemagglutinin and pertactin, allowing them to multiply rapidly and produce toxins that damage the respiratory epithelium.
Key Points
- The bacterial load of 10^7 to 10^9 CFU/mL is considered high and potentially infectious 1
- Individuals with this level of bacterial load are highly contagious, especially during the early catarrhal stage of whooping cough
- Antibiotic treatment can decrease infectiousness significantly after about two weeks, but untreated individuals may remain contagious for up to three weeks
- PCR testing is the preferred diagnostic method for early detection of potentially infectious cases, as it can detect high bacterial loads before culture becomes positive
Considerations
- The high bacterial load is particularly concerning in healthcare and childcare settings where transmission to vulnerable populations can occur
- The bacteria's ability to multiply rapidly and produce toxins that damage the respiratory epithelium contributes to the high infectiousness of individuals with this level of bacterial load
- The use of antibiotic treatment, such as erythromycin or azithromycin, can help reduce the duration and severity of symptoms, as well as lessen the period of communicability 1
From the Research
Bacterial Load of B. pertussis
- The bacterial load of B. pertussis considered high and potentially infectious can vary depending on several factors, including the patient's age, vaccination status, and disease severity 2.
- A study found that the median bacterial load was significantly higher in inpatients versus outpatients (4.91 vs. 2.55 log10 CFU/mL) and in patients with complications versus those without (6.05 vs. 2.82 log10 CFU/mL) 2.
- Another study reported that a patient with a high bacterial load of 7.02 (log GEq/mL) at the onset of treatment remained positive with 2.64 and 2.69 during and after a second 7-day course of antibiotic treatment, respectively 3.
- A bacterial load of 4.20 vs. 2.76 log10 CFU/mL was observed in unvaccinated-partially vaccinated patients versus vaccinated patients, respectively 2.
- The study also found that a higher bacterial load was associated with a higher risk of hospitalization and complications 2.
Factors Influencing Bacterial Load
- Age: The bacterial load was inversely correlated with age, with younger patients tend to have higher bacterial loads 2.
- Vaccination status: Unvaccinated or partially vaccinated patients tend to have higher bacterial loads than vaccinated patients 2.
- Disease severity: Patients with more severe disease tend to have higher bacterial loads 2.
- Time to diagnosis: The bacterial load was inversely correlated with time to diagnosis, with patients diagnosed earlier tend to have higher bacterial loads 2.
Antibiotic Treatment and Bacterial Load
- Antibiotic treatment can reduce the bacterial load of B. pertussis, but the effectiveness of treatment may vary depending on the patient's age, vaccination status, and disease severity 3, 2.
- A study found that azithromycin was effective in eradicating colonization in 88% of colonized individuals within 48 hours 4.