Calculating the risk of infection of mycobacterium tuberculosis in endemic settings

Johnstone-Robertson, Simon Peter (2012-03)

Thesis (MSc)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: The annual risk of infection (ARI), a measure of recent transmission, has been described as the most important parameter in tuberculosis (TB) epidemics. Nevertheless, mounting evidence suggests all factors contributing to TB transmission are not yet completely understood. This research was performed to investigate the role various parameters, e.g. overcrowding, period of infectivity, ventilation, and infectivity of source cases, play in TB transmission. An established airborne transmission risk model, the Wells-Riley equation (WRE), was modified to account for scenarios where unknown numbers of infectious individuals may be present. Subsequently, the ARI for three indoor locations conducive to TB transmission were calculated. Two locations (households and minibus taxis) were identified in a social mixing survey conducted within a South African community where TB is endemic as a part of this research. The third location (prison) was identified in an earlier independent study in the same community. The impact various interventions could have in reducing the ARI associated with each location was explored. Poor ventilation, severe overcrowding, extended exposure periods, and high incidence rates contributed to high TB transmission risks in each location. The household-associated ARI was related to the number of resident adults. Current TB control programs will only reduce the ARI if household ventilation levels are improved simultaneously. Similar reductions in the ARI could be achieved by trebling current ventilation levels or by separating child and adult sleeping areas. Neighbouring households can also contribute substantially to the ARI. The minibus taxi-associated ARI for drivers and commuters was considerable but readily reduced by opening windows or keeping the fresh-air fan on. Reducing TB case prevalence through active or passive case-finding would reduce the ARI substantially. The prison-associated ARI was proportional to levels of overcrowding. No single intervention, such as improved ventilation, decreased lock-up time, or improved case-finding, would decrease the ARI substantially, but concurrent implementation of all of them to meet national or international standards would. This research shows TB is not only transmitted in epidemics by highly infectious TB cases, but that any TB case, no matter how infectious, has the potential to infect susceptible people under the right conditions.

AFRIKAANSE OPSOMMING:Die jaarlikse infeksierisiko (ARI) – maatstaf van onlangse siekteoordrag – word as die belangrikste parameter in tuberkulose- (TB-)epidemies bestempel. Nietemin dui toenemende bewyse daarop dat nie álle faktore wat tot TB-oordrag bydra, volledig verstaan word nie. Hierdie navorsing is onderneem om ondersoek in te stel na die rol van verskillende parameters – byvoorbeeld oorbevolking, tydperk van aansteeklikheid, ventilasie en die aansteeklikheid van brongevalle – in TB-verspreiding. Gevestigde model vir die raming van siekteverspreiding deur die lug, die Wells-Riley-vergelyking (WRE), is aangepas vir scenario’s waar onbekende aantal aansteeklike individue moontlik aanwesig is. Daarna is die ARI bereken vir drie ingeslote ruimtes wat TB-oordrag bevorder. Twee van die ruimtes (huishoudings en minibustaxi’s) is ten tyde van die navorsing uitgewys in sosialevermengingsopname in Suid-Afrikaanse gemeenskap waar TB endemies is. Die derde ruimte (gevangenisse) is uitgewys in vroeëre onafhanklike studie in dieselfde gemeenskap. Die navorser het gevolglik bepaal watter moontlike impak verskillende intervensies op die verlaging van die ARI in elke ruimte het. Swak ventilasie, ernstige oorbevolking, verlengde blootstellingstydperke en hoë voorkomsyfers het in elke ruimte tot hoë TB-oordragrisiko bygedra. Die huishoudingsverwante ARI het verband gehou met die aantal volwassenes wat in die huis woon. Huidige TB-beheerprogramme sal slegs die ARI kan verlaag indien huishoudelike ventilasievlakke terselfdertyd verbeter word. Drie keer beter ventilasievlakke of die skeiding van kinders en volwassenes se slaapareas kan soortgelyke verlagings in die ARI teweegbring. Buurhuishoudings kan ook aansienlik tot die ARI bydra. Die minibustaxi-verwante ARI vir bestuurders en pendelaars was beduidend, maar kan betreklik maklik verlaag word deur vensters oop te maak of die varslugwaaier aan te hou. Die vermindering van die voorkoms van TBgevalle deur aktiewe óf passiewe gevalle-opsporing kan die ARI ook beduidend verlaag. Die gevangenisverwante ARI het met vlakke van oorbevolking verband gehou. Geen enkele intervensie soos beter ventilasie, korter toesluittye of beter gevalle-opsporing sal die ARI aansienlik verlaag nie, maar die gelyktydige inwerkingstelling van ál hierdie intervensies in pas met nasionale of internasionale standaarde kan wél. Hierdie navorsing toon dat TB in epidemies nie net deur hoogs aansteeklike TB-gevalle oorgedra word nie, maar dat enige TB-geval, ongeag hoe aansteeklik, die siekte in die regte omstandighede na vatbare mense kan oordra.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/20195
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