Furthering our understanding of pathogen transmission in cystic fibrosis
- Correspondence to Dr L Saiman, Division of Infectious Diseases, Department of Paediatrics College of Physicians and Surgeons of Columbia University, 633 West 168th Street, New York, NY10032, USA;
The source of pathogens for patients with cystic fibrosis (CF) is often unknown. Potential sources include the natural environment, the healthcare environment and other patients with CF.1 The relative contribution of these different sources is unknown, but the routes of transmission include direct and indirect contact with infected respiratory tract secretions and infectious respiratory droplets.1 2 Notably, airborne transmission—defined as dissemination of either airborne nuclei or small particles of respirable size—containing infectious agents (eg, Mycobacterium tuberculosis) and remaining infectious over time and distance has not been proved to be a route of transmission between patients with CF.2
Infection control strategies to minimise the spread of potential pathogens are tailored to different modes of transmission.2 When caring for patients harbouring pathogens transmitted by contact with infectious secretions, staff perform hand hygiene before and after touching patients, patient care equipment or horizontal surfaces in patients’ rooms and don gowns and gloves to prevent contamination of their clothing and hands. When caring for patients harbouring pathogens transmitted by infectious droplets, staff perform hand hygiene and don surgical masks. In contrast, when caring for patients infected by pathogens spread by airborne transmission, staff don an N95 respirator and patients are cared for in rooms with specific parameters for negative pressure, air exchanges and ventilation.
Infection control strategies to minimise transmission between patients with CF have been implemented in CF centres around the world. In addition to strategies outlined previously for staff, patients and their families are also taught to implement infection control practices including hand hygiene and respiratory hygiene which includes containment of respiratory tract secretions and maintaining a distance of at least 1 m from others with CF.1
Wainwright and colleagues3 have developed an experimental system to study the potential for airborne transmission of pathogens from patients with CF (see page 926). To do so, these investigators used a cough aerosol sampling method which included 5 min of voluntary coughing. Subjects coughed into a chamber and a vacuum pump pulled air—which included the particles generated during coughing—from the chamber through the stage impactors which contained 400 holes of decreasing diameter through which particles of relevant size penetrated and landed on agar plates for culture. Subjects included 28 adults and children with CF, 26 of whom had chronic infection with Pseudomonas aeruginosa, 13 of whom were studied during a pulmonary exacerbation and 18 of whom harboured the same strain, P aeruginosa AES2.
P aeruginosa was recovered from the subjects, the room air, on settle plates within the chamber and from the cough aerosols. However, as the authors are careful to state, their data do not prove the role of airborne transmission in CF. Their data demonstrate—via an experimental system with questionable relevance for clinical scenarios—that viable P aeruginosa were collected on cough aerosol sampling plates and that 70% of generated particles were within the respirable range (⩽3.3 μm). However, their observations were potentially skewed by a dominant clone of transmissible P aeruginosa whose behaviour may not be generalisable. Furthermore, the applicability of these findings to Gram-positive pathogens such as Staphylococcus aureus is unknown as these organisms were not studied.
Our understanding of droplet transmission is evolving thanks to improved studies of the epidemiology of viral pathogens such as rhinovirus, influenza and SARS.2 Historically, droplets have been defined as >5 μm in diameter with a defined risk distance of ⩽1 m from the infected patient. However, we have learned that these parameters can vary depending on droplet characteristics and environmental conditions. Infectious droplet nuclei ⩽5 μm can arise from desiccation of a droplet, droplets 30 μm in diameter can remain suspended in the air within a defined space such as a patient’s room, and droplets can travel as far as 2–3 m.2 Thus, Roy and Milton have proposed a new paradigm with which to understand the potential for airborne transmission.4 They proposed three categories for pathogens capable of airborne transmission: obligate (only transmitted by airborne transmission); preferential (can be transmitted by other routes such as droplet, but can also be transmitted by airborne transmission); and opportunistic (generally transmitted by other routes, but under special circumstances airborne transmission may occur).
While the relevance of this paradigm for CF is uncertain, it is important to keep an open mind regarding the potential for transmission of CF pathogens via the airborne route. Some CF centres have implemented the universal use of surgical masks by patients with CF to further reduce the risk of transmission and acquisition of potential pathogens. Use of masks has the further advantage of identifying CF patients to one another. Given the increasing importance of viral pathogens in CF and the influenza A H1N1 (SO) pandemic, use of masks by patients with CF is prudent. However, mask use cannot replace other infection control practices such as hand hygiene and respiratory hygiene.
In summary, the current scientific understanding of the increased complexity of transmission of pathogens by the droplet and airborne route and the study presented by Wainwright and colleagues challenge our assumptions that we fully understand patient to patient transmission in CF. Additional studies are needed to expand the relevance of the observations by Wainwright and to demonstrate that actual transmission occurs via the airborne route in CF. Such studies would then allow us to modify our evidence-based infection control recommendations for CF.
Competing interests None.
Provenance and Peer review Commissioned; not externally peer reviewed.