- • Known repeated exposure to an immunologically
sensitizing organic or chemical antigen.
- • Symptoms and signs frequently include dyspnea on
exertion and inspiratory rales.
- • Pulmonary physiology typically demonstrates a restrictive
or mixed pattern with reduced diffusion capacity of the lung for
carbon monoxide (DlCO).
- • Abnormalities on chest imaging most commonly include
infiltrates on radiograph and centrilobular nodules on high-resolution
computed tomography (HRCT).
- • Lung histopathology demonstrates cellular bronchiolitis
and interstitial granulomas.
Hypersensitivity pneumonitis (HP), also known as extrinsic allergic
alveolitis, refers to a constellation of inflammatory lung diseases
caused by repeated exposure and immunological sensitization to a
variety of organic and chemical antigens. Diagnosis relies on a
combination of findings including a history of repeated antigen
exposure; characteristic signs and symptoms, most commonly including
exertional dyspnea and inspiratory crackles on examination; pulmonary
function abnormalities often including decreased diffusion capacity
and restrictive changes; radiological abnormalities, classically
with infiltrates on chest radiograph or centrilobular nodules on
HRCT; and characteristic histological findings of cellular bronchiolitis
and interstitial granulomas.
The three general categories of antigens that cause HP are microbial
agents, animal proteins, and low-molecular-weight chemicals. Microbial
agents include bacteria and fungi. Multiple species of thermophilic
bacteria frequently contaminate decaying vegetable matter and are
causally associated with farmer’s lung, the prototypical
example of HP. Thermophiles as well as other bacterial species may
also contaminate ventilation systems and humidifiers and cause HP
from exposure to indoor aerosols. Nontuberculous mycobacteria contaminating
hot tubs and metal working fluids are also thought to cause HP.
Many fungi, including Rhizopus, Penicillium,
Aspergillus, and Alternaria, are
causative antigens in various occupational settings such as woodworking,
agriculture, and lumber milling, and as contaminants of air-handling
systems or water-damaged indoor settings. Avian antigens are the
most common animal proteins associated with HP. Environmental and occupational
history taking should always examine the possible presence of birds
in the home and participation in bird hobbies. Cases of HP have
been associated with apparently trivial exposures such as feather
duvets and decorations, down comforters, and dusty laundry from
bird hobbyists. A few low-molecular-weight chemicals are known to
cause HP, including isocyanates, the pesticide pyrethrum, Pauli’s
reagent (sodium diazobenzenesulfate), and copper sulfate.
The epidemiology of HP is problematic due to variable clinical
case definitions and underrecognition of disease. Most epidemiological
studies have focused on farmer’s lung; estimates of incidence
and prevalence vary significantly by region. HP prevalence among
dairy and cattle ranchers in Wyoming was found to be 3%,
whereas prevalence rates in Wisconsin dairy farmers are as high as
12%. Prevalence of HP among bird hobbyists ranges from
0.5% to 21%. Among isocyanate workers, the prevalence
is approximately 1%. Attack rates in outbreaks of HP may
be quite high. HP occurred in 52% of office workers exposed
to a contaminated humidification system. Attack rates of 27% and
65% in sequential HP outbreaks occurred among lifeguards
exposed to microbial-contaminated aerosols at an indoor swimming