Tuberculosis in public ground transport – is there enough evidence to justify contact tracing?

Abstract

Travel in public transport, especially in confined spaces, provides an opportunity for the transmission of tuberculosis (TB). In air travel, tracing of passengers to control the further spread of a pathogen is a more or less established approach in infectious disease control in many industrialized countries; however, literature reviews on the risk of TB transmission during public ground travel do not show robust evidence. On short trips, contact tracing (CT) of passengers may be unfeasible since passenger data are generally not collected. In long-distance ground travel, passenger data may not be collected routinely; incomplete data and limited or delayed access to passenger details may be further obstacles. The logistic hurdles and limited evidence suggest that CT in public ground transportation should not be a priority of TB control. However, under specific circumstances, CT in public ground transport might be considered on a case-by-case basis after an evidence-based risk assessment.

Keywords:

• contact tracing
• evidence
• infectious disease control
• public transport
• tuberculosis

Europe is well connected by public transport networks that allow high mobility within and between countries. Road, rail, air and water transportation are widely available across the continent. Nearly half a billion citizens in 27 countries of the EU enjoy access to various means of transport; travel by public transport has increased substantially over the past decades [1].

Passengers using means of public transport may be at risk of infection and disease when they are exposed to infectious passengers. Travel in public transport, mostly in confined spaces, provides opportunities for transmission of airborne infectious diseases such as tuberculosis (TB), meningococcal disease, influenza, and severe acute respiratory syndrome. In air travel, it is a more or less established approach to trace passengers for infectious disease control with the objectives of preventing further spread of a pathogen, offering post-exposure prophylaxis, or providing information and treatment to passengers who had contact with a fellow passenger or personnel shedding an infectious agent/disease aboard an aircraft [2]. In general, contact tracing (CT) is defined as the identification of persons who may have been exposed to an infectious disease by an infectious person and ensuring that they are aware of their exposure [3,4].

The potential transmission of TB during air travel has received considerable attention by public health professionals. This led to the development of guidelines and international recommendations on CT of passengers exposed to people with pulmonary TB who sat in adjacent rows for longer than 8 h (including ground delays) with further criteria if the index case has multidrug-resistant TB or extensively drug-resistant TB [5]. In addition to the WHO guideline [5], the European Centre for Disease Prevention and Control published a systematic review [6,7] that supports the WHO approach.

Although no generally accepted guidelines for CT after exposure to TB or other infectious diseases in public ground transport such as railways, buses/coaches or underground railway/metro have been published, the risk of TB transmission during ground travel may be higher than in air transport since most aircraft used for long-distance flights are equipped with high efficiency particulate air filtration systems that filter bacteria above 0.3 µm, hence removing Mycobacterium tuberculosis [8]. An additional factor that reduces the transmission risk of TB and of other airborne infectious diseases in modern aircraft is the use of laminar air flow in ventilation systems [9].

However recent literature reviews on the risk of TB transmission during air travel [9] as well as during public ground travel [10,11] do not show robust evidence; transmission of TB during travel in railways or buses/coaches may occur, yet the risk of transmission and the public health impact remain largely unknown. The risk of TB transmission is still poorly understood and hard to predict for public ground transport where crowding, the duration of exposure including cumulative exposure (repeated exposures by regular commuting, e.g. on a school bus), ventilation and other environmental characteristics are important and diverse factors [10,11].

It is well known that follow-up of contacts after potential transmission of TB and other airborne infectious diseases in public ground transport is often difficult; contact investigations of passengers may be limited or unfeasible since passenger data (name, telephone number, email address, etc.) are generally not collected on short trips when traveling by local buses and trains, metros, trams, etc. In places with debit card travel systems (Travelcard, etc.) that are often used by commuters and could theoretically permit the identification of passengers, the volume of passengers tends to be too high to pinpoint to specific trams and trains. In long-distance railway or bus travel with the longest exposure times, passenger data may not be collected routinely. If details are collected, incompleteness of data, limited or delayed (retrospective) access to passenger details, and company policies protecting business interests may be further obstacles to timely and effective CT [10,11].

Evidence that potential transmission of TB in public ground transport is a prominent public health issue has not been generated yet. Taking into account the scarce evidence and the multiple logistic bottlenecks, only few events would justify contact investigations [10]. A comprehensive and evidence-based risk assessment is required to support and facilitate the decision whether or not to conduct CT in public ground travel; recently developed CT risk assessment profiles for TB (and other airborne infectious diseases) in ground transportation may support rational and, if available, evidence-based decision making with respect to CT [12]. Compared to WHO and European Centre for Disease Prevention and Control guidelines, the CT risk assessment profiles take into account the drug resistance pattern of the index case in the decision-making process for or against CT. In cases of multidrug-resistant or extensively drug-resistant TB, a decision for CT becomes more likely. The selected publications in literature reviews on potential transmission of TB in mass transportation describe mostly contact investigations after possible TB transmission during ground transport in industrialized countries. The majority of those publications report on investigations in school busses with cumulated exposure during repeated trips between home and school, where CT was a viable intervention. No studies on contact investigations in low-income countries with high prevalence of TB were found [10,11], very few reports describe the risk of TB transmission during public ground travels in a setting with endemic TB [13,14]. In a cohort study on the association between public commuter transport in Lima, Peru, and pulmonary TB in workers, it was shown that the use of minibuses increased the risk of pulmonary TB [13].

It seems likely that the risk of TB transmission during ground travel in poorer regions with a high burden of TB is higher than in developed countries with lower TB prevalence. Nevertheless, the magnitude of the public health impact of TB transmission arising from public ground travel remains unidentified. Since tracing of contact persons in mostly anonymous public ground transport tends to be inefficient and requires substantial financial and human resources, we suppose that CT in public ground transport is rarely feasible in low-income countries. The scarce available evidence suggests that contact investigations in public ground transportation should not be a priority of TB control. Because contact investigations in the setting of air travel seem questionable as well, we suggest a reevaluation of international recommendations on CT in aircrafts. Resources should be directed to efficient means of TB control in order to achieve the United Nations’ Millennium Development Goal to reverse the spread of TB by the year 2015. However, in the frame of specific circumstances, such as confirmed pulmonary TB at organized trips (group travel, school trips, etc.), CT in public ground transport might be considered on a case-by-case basis after a careful evidence-based risk assessment.

Financial & competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.