Epidemiologic Notes and Reports Silo-Filler's Disease in Rural New York

On September 18, 1981, at a farm in Mohawk, New York, a 39-year-old farmhand was overcome while climbing up the chute of a recently filled concrete stave silo and later died of presumed silo-filler's disease. The case report follows.

In the preceding 10 days, the farm owner had filled this silo with Sudex grass and chopped corn silage. On September 18, he asked a farmhand to climb up the unloading chute inside the silo and toss out fresh silage. When the farmhand climbed the chute, he became short of breath and confused and had to descend. He made a second attempt but again had to climb down and was noted to be cyanotic, pale, and diaphoretic.

At a local hospital, the examining physician noted cyanosis and respiratory distress; blood pressure of 84/60; pulse, 128; respiration, 32; and temperature, 37.5 C (99.5 F). The patient had wheezes and crackles on auscultation of his chest but no signs of consolidation; after asthma was diagnosed, he was treated with epinephrine, intravenous aminophylline, and steroids. White blood count was 31,000; hematocrit, 57.8%; and hemoglobin, 18.6 gm. Arterial blood gas examination while on 2 liters of nasal oxygen showed pH 7.35; PaCO((2)), 32 mm Hg; PaO 45 mm Hg; and a calculated bicarbonate of 17.6 mEq/L. An electrocardiogram showed a sinus tachycardia, and a plain chest radiograph disclosed extensive fluffy bilateral infiltrates.

The patient was moved to the intensive care unit, where a tentative diagnosis of pneumonia was made. He became agitated, would not wear an oxygen mask, and remained in shock. Five hours after admission, he experienced cardiopulmonary arrest and died despite vigorous efforts to resuscitate him.

Post-mortem examination the next day showed grossly edematous lungs with pleural effusions (200 ml) on both sides; the right lung weighed 900 gm, and the left weighed 1,000 gm. Microscopy of the lungs showed alveoli flooded with proteinaceous material; the alveolar walls were intact. No bacteria, fungi, or evidence of viral disease was found. Early bronchiolitis was present; no evidence of asthma could be seen. There were no granulomas or hyaline membranes.

An investigation at the farm 2 weeks later failed to uncover any problem with the corn silage, which was still being unloaded. The cows were eating normally and producing the usual amounts of milk. The farmer reported that, following his farmhand's illness, he had turned on the silo blower and sent another worker up to toss out corn; no ill effects had occurred.

Several factors support the diagnosis of silo-filler's disease, an illness caused by the inhalation of nitrogen oxides: i.e., rapid onset of symptoms following a recent filling of the silo and histology classic for toxic exposure. Reported by DS Pratt, MD, JJ May, MD, Section of Pulmonary Medicine, Mary Imogene Bassett Hospital, Cooperstown, R Rothenberg, MD, State Epidemiologist, New York State Dept of Health; Immunology Section, Laboratory Investigations Br, NIOSH, CDC.

Editorial Note

Editorial Note: The case outlined above is typical for massive exposure to nitrogen oxides. Silo-filler's disease represents an occupational hazard associated with ensiled crops. Laboratory studies have shown that toxic levels of NO, NO((2)), and N((2))O((4)) are regularly produced in silos (1,2). Because these oxides are dense, they tend to settle in the chute and around the base of the silo, and exposure often occurs without anyone's entering the silo. Although NO((2)) is brown and has an odor, N((2))O((4)) is colorless and odorless, and exposure can occur without warning (3). If undetected by smell or sight, the potent nitrogen oxides may be inhaled deep into the lungs, where contact with the mucosal moisture produces nitric acid, which burns the airways, respiratory bronchioles, and alveoli. In fatal exposures, vascular collapse and the outpouring of serum rapidly produce shock and death. In another clinical course associated with silo-filler's disease, exposure causes cough and chest tightness. Although these conditions clear spontaneously, illness may return in three weeks with severe symptoms of fever, chills, and shortness of breath. Biopsies show a bronchiolitis obliterans with granuloma formation. This second pattern appears to respond to steroids (4).

Fatal and serious exposures to nitrogen oxides are not unique to farming but have been reported in association with arc and acetylene welding (4), burning cellulose nitrate (5), and dynamite blasting (6). Diesel fumes, furnace gases, and chemical processes involving the generation of NO((2)) (3) are also potentially dangerous.

It is possible to prevent this type of exposure in the farm industry if farmers are aware of the following dangers and use the suggested safety measures: 1) Silos begin to produce NO((2)) within 4 hours after filling, and no one should enter or come in close contact with a recently filled silo. 2) Some crops (oats, corn) produce more NO((2)) than others, and heavily fertilized crops, cloudy conditions, and rain raise the risk of NO((2)) production. 3) Although NO((2)) levels are generally low and within a safe range after 2 weeks, dangerous amounts may remain for months if the silo has not been opened (7). 4) If possible, enclosed areas should be ventilated for 20 minutes before anyone enters, and individuals should be equipped with a full-face mask and an air supply.

References

1. Commins, BT, Raveney FJ, Jesson MW. Toxic gases in tower silos. Ann Occup Hyg 1971;14:275-83.

2. Wang LC, Burris RH. Mass spectrometric study of nitrogenous gases produced by silage. Agricultural and Food Chemistry 1960;8:239-42.

3. Zenz C. Oxides of nitrogen (NO((2)) and NO). In: Zenz C, ed. Developments in occupational medicine. Chicago: Year Book Medical Publishers, 1980:378-82.

4. Jones GR, Proudfoot AT, Hall JI. Pulmonary effects of acute exposure to nitrous fumes. Thorax 1973;28:61-5.

5. Nichols BH. Clinical effects of inhalation of nitrogen dioxide. AJR 1930;23:516-20.

6. Becklake MR, Goldman HI, Bosman AR, Freed CC. The long-term effects of exposure to nitrous fumes. Am Rev Tuberc 1957;76:398-409.

7. Parkes WR. Occupational lung disorders. 2nd ed. London: Butterworths, 1982:475.

8. Key MM, Henschel AF, Butler J, Ligo RN, Tabershaw IR. Occupational diseases: a guide to their recognition. Cincinnati: National Institute for Occupational Safety and Health, 1977:427. (DHEW Publication; No. (NIOSH) 77-181).

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