Tuberculosis in a South African prison - a transmission modelling analysis

Abstract

Background: Prisons are recognised internationally as institutions with very high tuberculosis (TB) burdens where transmission is predominantly determined by contact between infectious and susceptible prisoners. A recent South African court case described the conditions under which prisoners awaiting trial were kept. With the use of these data, a mathematical model was developed to explore the interactions between incarceration conditions and TB control measures.

Methods: Cell dimensions, cell occupancy, lock-up time, TB incidence and treatment delays were derived from court evidence and judicial reports. Using the Wells-Riley equation and probability analyses of contact between prisoners, we estimated the current TB transmission probability within prison cells, and estimated transmission probabilities of improved levels of case finding in combination with implementation of national and international minimum standards for incarceration.

Results: Levels of overcrowding (230%) in communal cells and poor TB case finding result in annual TB transmission risks of 90% per annum. Implementing current national or international cell occupancy recommendations would reduce TB transmission probabilities by 30% and 50%, respectively. Improved passive case finding, modest ventilation increase or decreased lock-up time would minimally impact on transmission if introduced individually. However, active case finding together with implementation of minimum national and international standards of incarceration could reduce transmission by 50% and 94%, respectively.

Conclusions: Current conditions of detention for awaiting-trial prisoners are highly conducive for spread of drug-sensitive and drug-resistant TB. Combinations of simple well-established scientific control measures should be implemented urgently.

Fig. 1

The effect of prison cell overcrowding on TB transmission probabilities is plotted against time periods of infectiousness up to 180 days. They are shown for 3 levels of overcrowding; 250% approximates the current level cell occupancy; 100% represents implementation of current South African statutory minimum occupancy of 3.44 m² of floor space per inmate,^– and 50% corresponds to international space recommendations.

Fig. 2

The effect of length of period of restriction in prison cell per day on TB transmission probabilities is plotted against time periods of infectiousness up to 180 days, shown for 3 time periods of cell occupancy during a 24-hour period; hours per day, 12 hours per day and 8 hours per day. NB: 23 hours per day is the current period of restriction to cells in Pollsmoor prison.

Fig. 3

The effect of increasing levels of cell ventilation on TB transmission probabilities is plotted against time periods of infectiousness up to 180 days. They are shown for 4 values of ventilation air change per hour (ACH): 1 ACH (current estimated cell ventilation), 3 ACH (minimal international recommendation), 8 ACH (moderately increased ventilation) and 12 ACH (the optimal level of ventilation recommended by WHO for health care settings).

Fig. 4

TB transmission probabilities for 3 scenarios: scenario 1 – approximates the status quo using parameters of 250% overcrowding, 1 air change per hour (ACH) and cell occupied for 23 hours per day; scenario 2 – immediately achievable parameters of 100% occupancy, 3 ACH and cell occupied for 23 hours per day; scenario 3 – optimised implementation of international recommendation of 5.4 m² floor space, 14 hours of cell occupancy per day and 12 ACH.