Sunday 10 August 2008
TB associated with household crowding in NZ
Research using census data and notified tuberculosis (TB) cases over a five year period shows that the risk of this
disease is significantly associated with household crowding in New Zealand.
The research by He Kainga Oranga/Housing and Health Research Programme at the University of Otago, Wellington found that
TB is still an important infectious disease in this country and the risk of infection is increased by household
crowding. The study, which was published in the latest issue of the Journal of Epidemiology and Community Health,
examined 1898 cases of TB over the five year period 2000-2004.
“While the focus of TB control is on prompt case detection and effective treatment, better housing conditions and
reduced levels of crowding also have a role in controlling the spread of this disease,” says lead researcher Associate
Professor Michael Baker.
This is the first published study of its type in New Zealand or Australia to look at the effects of household crowding
on TB, while controlling for other important risk factors for this disease.
“Unlike some previous studies, we were able to control for the effects of known risk factors, notably poverty and
migration from high incidence countries. Linking to census data also allowed us to include a relatively large number of
cases.” says Associate Professor Michael Baker.
The study particularly focussed on the young (under 40 years) New Zealand-born population where TB is most likely to
have been caused by recent infection. In this group, the study found that for every 1% increase in the average household
crowding level of a neighbourhood there would be an 8% increase in the expected TB count, assuming the other variables
were held constant. People living in the most crowded quintile (20%) of neighbourhoods would expect to have a TB risk
approximately 70% higher than those living in the least crowded quintile.
“The finding that TB rates are associated with crowded living conditions fits with what we know about the transmission
of this bacteria via small airborne droplets,“ says Associate Professor Michael Baker. “Household crowding increases
opportunities for active cases to infect other household members, particularly children.”
The study was not able to determine if ethnicity had an independent effect on the risk of TB in NZ, separate from
overcrowding and low income. Dr Baker says this question will require further research and ideally an individual-based
study (e.g. case–control) in recently infected cases.
The second important finding from the study was that there was little evidence of TB transmission from migrants to the
NZ born population. TB incidence among NZ born people under 40 years was not significantly associated with the
proportion of the neighbourhood population who were migrants from high incidence countries.
This study builds on previous work by researchers at He Kainga Oranga/Housing and Health Research Programme on the
epidemiology of TB. This research has shown that, contrary to common belief, New Zealand’s TB rate is not increasing and
has stayed at or below 10 cases per 100,000 since the mid-1980s. Although the majority of TB cases are in recent
immigrants from developing countries there is no indication that migrants are spreading the disease locally to any
significant degree.
At a global level TB remains one of the most important infectious disease in terms of infection rates and as a cause of
premature death. At least one third of the world’s population is infected with the bacterium and 10% of these people
will progress to active disease. The impact of TB is further worsened by the HIV epidemic and rising levels of drug
resistance.
“Even though New Zealand is managing TB well within our own borders, there is no cause for complacency in the face of
this global epidemic”. says Associate Professor Michael Baker.
Funding for this study was through the Health Research Council and the NZ Population Health Charitable Trust.
ends