These modeling analyses demonstrate that the high reported rates of community TB transmission to children in Southern Africa 9-13
can be explained by the interplay between the prevalence of adult infectious TB, social mixing between adults and children and the prevailing domestic ventilation characteristics.
The model in this study was based on the Wells-Riley equation which has been used to examine airborne TB disease transmission since the 1970s 14
in a wide variety of medical and non-medical settings and thus has been useful in examining the relative importance of transmission factors in real-life situations 15-19
. This study used a novel approach of incorporating population data from a specific South African township to populate the Well-Riley equation in order to explore household and non-household TB transmission to pre-school children.
Of particular interest our model indicated that the potential of the existing clinic-based TB control program to reduce transmission to children is somewhat limited. Even reductions in Δ (period of infectiousness) to 30 days by active case finding and rapid TB diagnosis would only have significant impact on transmission to children when ventilation rates in households exceed 6 ACH. However, such high ventilation rates of informal dwellings during cold Cape Town winters might be difficult to achieve throughout the year. As similar reductions in TB transmission could be achieved by separating child and adult sleeping areas, this might be a more practicable stratagem.
Another major finding of our study was that a maximum of 75% of the total annual risk of infection could possibly be explained by the interaction between a child and the limited number of adults resident in a primary household. Pre-school children are susceptible to TB infection predominantly due to exposure to infectious adults 6, 7
and therefore the main determinant of maximal transmission risk in either setting was the number of adults to whom a child was exposed.
Our model also indicated that at least 25% of the risk of infection resulted from exposure to non-resident adults. In well ventilated settings transmission was related to Δ rather than number of households visited. In contrast to transmission risks from adult household residents, transmission from non-resident adults can be markedly influenced by the TB control program's ability to decrease Δ by active case finding. In poorly ventilated non-residence settings transmission risks increased markedly with the numbers of households visited. These analyses indicate that as children become more socially mobile, the potential for transmission in poorly ventilated non-household settings might become the largest contributor to total transmission risk. Indeed, we have reported increasing TB infection rates throughout childhood in this community, which peak at approximately 8% at 15 years of age 11
The strength of this study was the availability of accurate information specific for this community including the ARTI, number of adults and children per household and smear-positive TB notification rates. A caveat is that some important parameters such as Δ and numbers of infective quanta produced by TB diseased adults are difficult to measure directly and estimates were derived from published data. Indeed, Δ may not be identical to the period of diagnostic delay in published studies and smear-positive TB incidence may only approximate to the smear-positive notification rate. The model used the epidemiologic assumption that the TB epidemic was generalized with equal mixing of infectivity and contact risks. However, stochastic transmission events such as close non-household contact with highly infectious individuals are not captured in this model. Despite these limitations, the outputs from the model were robust and were compatible with the previously observed ARTI in this and similar communities 9-11
Our findings may give insight to why TB rates of transmission in South Africa have remained very high, despite apparent improvement in case management by the TB control program 1
. The conditions within crowded African townships with high unemployment rates may have much in common with those conditions present during the industrial revolution of 18th
centuries when TB burdens were also extremely high. Children lived and worked side by side with adults 25
but successive factory acts in UK and USA reduced the childhood exposure to adults in the workplace 26, 27
. Improvement in housing and schooling further reduced close exposures between children and adults. The crèche movement further limited the frequency of contacts between young children and potentially infectious adults 28
and may have potential to decrease non-household transmission in crowded townships. Reduction in household TB transmission in poor informal housing will be difficult to achieve. However, improvement of housing stock should particularly focus on improved ventilation and separation of child from adult sleeping quarters.
These modeled analyses have identified social and environmental factors which contribute to high rates of TB transmission in this community. Social mixing patterns of pre-school children result in TB transmission within the extended rather than nuclear family. Where TB is highly endemic, interruption of community TB transmission requires prompt treatment of source cases. TB control will therefore necessitate an increased focus on active case finding and reduction of diagnostic delays.