Temperature and air pollution levels are
associated with hospitalisations caused by Pneumocystis
pneumonia (PCP) in people with HIV, investigators from San Francisco report in
the online edition of Clinical Infectious
Diseases. Hospitalisations due to PCP were highest in the summer months and
there was a significant association with an increase in temperature and higher
levels of sulphur dioxide (SO2).
"The identification of both climatological
and air pollution constituents associated with the development of PCP is a
novel and important observation", write the authors.
PCP is an AIDS-defining opportunistic
infection. It was a major cause of death in people with HIV in the era
before effective antiretroviral therapy was introduced, and the infection
remains the second most common AIDS-defining illness in the US.
Environmental factors such as temperature
and air pollution have been associated with an increased risk of a number of
lung disorders. However, the role of climate and air pollution in the
development of PCP are poorly understood.
Investigators from San Francisco General
Hospital therefore designed a study involving 457 people with HIV
admitted to their facility between 1997 and 2008 with confirmed PCP.
Information on climate and air pollution
levels one week, one month and two months before each hospitalisation were
obtained by the investigators.
Most of the patients (89%) were men, 48%
were white and the median age at the time of admission to hospital with PCP was
40 years. The patients had advanced HIV disease with a median CD4 cell count of
just 31 cells/mm3. Only 61 (13%) had received PCP
prophylaxis in the three months before hospitalisation.
Rates of admission differed significantly (p < 0.05) according to season of the year, and were highest in summer (129
hospitalisations), followed by spring (125 admissions). There were only 91
admissions in the winter months.
There was a significant association between
increases in temperature and PCP hospitalisations. Each 5oF increase
in temperature increased the risk of admissions by 41% (OR = 1.41; 95% CI,
Air pollutant levels were also associated
with hospitalisations. A one-unit increase of SO2 parts per billion
increased the risk of admissions by 80% (OR = 1.80; 95% CI, 1.15-2.83).
An analysis of environmental conditions two
weeks before hospitalisation showed that increases in temperature (p < 0.01)
and higher SO2 (p < 0.001) both had a significant association with
Higher temperature (p = 0.03) and SO2
(p < 0.01) one month before admission were also significant risk factors.
There was also a significant association with temperature two months before
hospitalisation (p < 0.01).
The authors suggest “SO2 exposure
in the patients described in the present study contributed to an impairment of
pulmonary defence mechanisms, and aggravated pre-existing and evolving PCP
symptoms resulting in these patients seeking medical care.”
An unexpected result was that higher carbon
monoxide levels were found to be protective and reduced the effects of SO2.
“It is difficult to find a good explanation for this association,” comment the
They conclude, “Further multicenter studies
are needed to identify if these factors are also predictors of PCP admissions
in other geographical locations and in other immunocompromised groups. Animal
studies are also needed to better understand the biological mechanisms behind
the impact of climatic air pollution on PCP occurrence.”
The rigour of the study was praised in an
accompanying editorial. “This is an important area of study and hopefully the
authors will continue to pursue the role of the environment on the
colonization, infection and pneumonia from Pneumocystis.”