Long term outcome of soybean epidemic asthma after an allergen reduction intervention

Background-Asthma outbreaks due to the inhalation of soybean dust released from handling of soybean in the city harbour occurred in Barcelona, Spain f rom 1981 to 1987. The installation of bag filters in the responsible silo w as followed by a substantial reduction of airborne soybean dust released in to the atmosphere and the disappearance of asthma outbreaks. A study was un dertaken to assess the relevant outcomes in asthma patients affected by soy bean epidemic asthma eight years after this environmental intervention. Methods-A repeat case-control study was performed in 1995 on a population o f subjects with epidemic and non-epidemic asthma previously assessed in 198 9. The same protocol was used in both surveys to collect data from patients via a questionnaire and respiratory function, skin and laboratory tests we re performed under blinded conditions with regard to epidemic and non-epide mic status. Environmental soybean allergen in pollution filters was measure d by means of a RAST inhibition technique. Results-During 1995 and 1996 the 24 hour mean airborne levels of soybean al lergen on a sample of 39 unloading days (range 31-269 U/m(3)) were systemat ically below the lowest level ever detected during an epidemic day (1500 U/ m(3)). Measurable levels of serum IgE antibodies against soybean were still present in 55% of patients with epidemic asthma compared with 6.0% of thos e with non-epidemic asthma (p<0.05). These proportions were almost identica l to those observed in 1989. The proportion of patients with soybean asthma with symptoms in 1989 who reported the absence of symptoms in 1995 was sim ilar to the control subjects, so most of the relative risks (RRs) of improv ement were near to 1. The only statistically significant differences betwee n the two groups were a smaller proportion of patients with epidemic asthma showing improvement in terms of being woken up by attacks of coughing (RR improvement 0.47; 95% CI 0.22 to 0.99) and the need for treatment at the em ergency room (RR improvement 0.63; 95% CI 0.41 to 0.96). Conclusions-Eight years after a large reduction in the levels of airborne s oybean allergen half of the former soybean epidemic asthma patients were st ill sensitised to soybean. These results indicate an initial improvement in soybean epidemic asthma in the two years following the intervention with n o further improvement in subsequent years.