Acute presentation: flu-like illness with cough;
Subacute: recurrent "pneumonia";
Chronic: exertional dyspnea, productive cough, and weight loss;
Most patients have abnormal imaging studies (chest x-ray or high-resolution CT). Crepitant rales are heard in some cases. Pulmonary function testing shows a restrictive defect in early disease and a restrictive, obstructive or mixed defect in late disease. Precipitating antibodies are neither sensitive nor specific, and their presence is no longer considered a hallmark of HP. Some patients have decreased diffusion capacity and arterial hypoxemia. If the diagnosis is in doubt, bronchoalveolar lavage (BAL) typically shows lymphocytosis. Surgical lung biopsy may be indicated if bronchoscopy is nondiagnostic. The disease latency varies from a few weeks to years after first exposure. Symptoms appear or worsen within a few hours of antigen exposure to bioaerosols of microbial or animal antigens or rarely to a few reactive chemicals. Complete recovery usually occurs if exposure is terminated early. Otherwise, the disease may progress to interstitial fibrosis. [Murray, p. 1783-1799] From 1980 -2002 in the US, 814 deaths from HP were identified. Of these 814 cases, 56% were unspecified in type, 37% were farmer's lung, 4% bird fancier lung, 1% bagassosis, 1% humidifier lung, and 1 case each for mushroom worker lung and maple bark stripper lung. [Hyperlink] "In the diagnostic evaluation of patients with ILD [interstitial lung disease], it can be challenging for clinicians to distinguish cHP [chronic hypersensitivity pneumonitis] from idiopathic pulmonary fibrosis. . . . Accordingly, an identified exposure combined with HCRT features of cHP and BAL lymphocytosis greater than 40% was the only scenario experts believed sufficient to establish a diagnosis of cHP without the need for a lung biopsy." [
PMID 29172641] "During the period studied, the commonest suspected causes of OHP [occupational HP] in the UK were exposures to metalworking fluids (MWFs, 35%), farming (17%) and birds (11%)." [
PMID 27919062] In a very large series of 710 patients with ILD, a BAL lymphocyte count of < 30% was associated with a radical (90%) reduction in the likelihood of HP." [
PMID 28943282]