Health care-acquired aspergillosis and air conditioning systems.

Aspergillus species (Fig. 1) are conidia-forming fungi that are responsible for a wide spectrum of human illnesses. About 19 species of Aspergillus have been documented as agents of human disease, but the majority of infections are caused by Aspergillus fumigates (Fresenius, 1850), Aspergillus flavus (Link,1809), Aspergillus niger (Van Tieghem, 1867), and Aspergillus terreus (Thom, 1918) [1, 2]. Clinical syndromes and diseases associated to Aspergillus spp. can be: “cutaneous” (primary skin and burnwound infections, especially in neonates and children); “hypersensity” (allergic bronchopulmonary aspergillosis); “acute invasive” (pneumonia, ulcerative tracheobronchitis, osteomyelitis,..);“chronic invasive” (chronic pneumonitis) [3-5]. The primary route of acquiring Aspergillus infections is by the inhalation of conidia [6]. The respiratory tract is therefore the most common portal of entry of Aspergillus spp. conidia [7]. Conidia of the most commonly involved pathogenic Aspergillus spp. are relatively small, with sizes ranging from 2 to 5 μm [8, 9]. The small size of conidia permits them to deposit deep into the lung after inhalation and to reach the pulmonary alveolar spaces, where they may germinate to form hyphae. Humans inhale approximately 200 conidia of filamentous fungi per day, 7 of which reach the alveoli. These conidia are phagocytosed by the alveolar macrophages and destroyed [10]. The outcome of infection depends more on host factors than on the virulence of the individual Aspergillus species [2, 4, 11]. Therefore, host immunity plays a major role in determining who may be at risk of developing invasive aspergillosis (IA). The principal host defences against invasive Aspergillus infection are alveolar macrophages, which target inhaled conidia, and neutrophils, which target the hyphal stage [12-14]. Immunocompromised patients are extremely susceptible to local invasion of respiratory tissues by deposited conidia, resulting in invasive aspergillosis [12, 15]. Most cases of health care-acquired invasive aspergillosis present with pneumonia [8]. However, while invasive aspergillosis occurs typically in severely immunocompromised patients, cases of surgical site infection have been reported in immunocompetent individuals [8, 16].