| Non-fibrotic HP | High-level intermittent exposure of usually an identifiable antigen | Hours or days following significant exposure | Acute or insidious onset of productive cough, dyspnea, and fatigue often associated with an intermittent flu-like syndrome (fever, chills, malaise, cough, chest tightness, dyspnea, and headache) | Diffuse fine bibasilar crackles, mid-inspiratory squeaks | Symptoms resolve gradually within 12 hours to several days after exposure removal and may recur following re-exposure | In case of exposure’ avoidance may have a favorable prognosis with the possibility of stabilization or complete recovery |
| Fibrotic HP | Low-level continuous exposure to a frequently unknown antigen | Months after exposure | Insidious disease with no apparent acute episodes. Progressive dyspnea, cough, fatigue, malaise, and/or weight loss | Bilateral crackles, cyanosis, cor pulmonale, and finger clubbing (in 50% of patients) | End-stage fibrotic disease. Exacerbation may occur despite antigen avoidance | Poor prognosis |
