Kenneth
D. Rosenman, M.D.
Michigan State University Extension
Respiratory
hazards that farmers and family members may be exposed to
include various grains that can be contaminated with fungi,
bacteria or microbial toxins; pesticides; solvents; gasoline
and diesel fuels; and irritant gases such as oxides of nitrogen
and ammonia. The specific issue to be discussed in this CMASH
newsletter is silo-filler's disease.
Silo-filler's
disease is a chemical pneumonitis from exposure to oxides
of nitrogen. These gases are produced within hours and sometimes
up to two weeks after fresh plant material is placed in silos.
The
typical clinical presentation is that the patient is acutely
ill with symptoms of shortness of breath and cough and may
also have chest pain and tightness in the chest. On physical
exam they are dyspneic and typically have rales. They may
be febrile and have wheezing. Typical laboratory findings
are hypoxia on arterial blood gases, leukocytosis and pulmonary
edema or diffuse reticulonodular infiltrates on the chest
x-ray. Methemoglobineuria has been described in some patients.
On history, the patient will typically relate having entered
a silo or the silo chute within 1-3 days after the silo had
been filled. At the time of exposure, a patient may be overcome
and die from the fumes, or may have symptoms of a cough, light-headedness,
shortness of breath, chest tightness and choking, or may be
entirely symptom free. Many times there is a symptom free
period that ranges from « to 42 hours between the time of
entering the silo and development of the chemical pneumoniti
. If the concentration in the air of oxides of nitrogen are
high enough, the patient may have smelt a bleach-like odor,
otherwise they may have noted a yellowish or reddish vapor
on top of the plant material, or noted dead birds or insects
near the base of the silo (see Figure 1).
Patients
with acute hypersensitivity pneumonitis may present with a
similar onset of acute shortness of breath, fever and lung
infiltrates as silo-filler's disease. However, with hypersensitivity
pneumonitis the patient will have been exposed to dust and
not gas and the time of the year will typically be different.
Silo filling of corn silage generally begins in late August
and continues throughout the fall in Michigan. Some farmers
begin filling silos with grass silage in the spring and continue throughout the growing season. Hypersensitivity pneumonitis
is an immunologic reaction to moldy grain dust and there would
need to be time for the grain to become moldy (see Physician's
newsletter vol. 1, issue 1, Winter 1992).
In addition
to supplemental oxygen and mechanical ventilation, high doses
of corticosteroids are generally given for silo-filler's disease
although no controlled clinical trials have been performed.
Fever has been reported to persist for up to six days and
the pulmonary infiltrates to persist up to five days.
Some
authors recommend continuing oral steroids for three to four
weeks after an acute presentation to prevent bronchiolitis
obliterans which can be a late sequela. Patients with bronchiolitis
obliterans present two to six weeks after exposure with shortness
of breath, cough, and sometimes fever. Chest x-ray may show
a diffuse reticulonodular or miliary pattern on the chest
x-ray. Some patients, however, show only hyperinflation. With
bronchiolitis obliterans, pulmonary function testing ma show
restriction or obstruction, although typically lung volumes
are normal. The diffusing capacity is frequently reduced.
On biopsy there is a patchy fibrosis involving the air spaces
and terminal and respiratory bronchiolus.
Silos
were first built in the United States in 1875. The first deaths
from exposure to gases from silos were reported in the medical
literature in 1914. The initial deaths were attributed to
asphyxiation from build up of carbon dioxide. A report in
1949 mentioned a brownish gas from a silo was the cause of
the patient's respiratory disease. In the mid 1950's a number
of case reports were published which recognized that oxides
of nitrogen were the causal agent for the death and/or respiratory disease associated with silos. In air tight silos, which
are typically unloaded from the bottom, carbon dioxide is
the toxic gas which is normally produced. Carbon dioxide replaces
oxygen and the patient may asphyxiate. Carbon dioxide does
not cause lung disease. However, the more common type of silo
is the non-airtight silo which is unloaded from the top. Oxides
of nitrogen are concentrated in the chamber above the silage.
The
percent of nitrate in the plant material ensiled determines
how much oxides of nitrogen are produced. Weeds have the highest
percentage of free nitrate (7.5%); corn and sorghum (5.5%);
oats, wheat, barley and rye (4.0%), brome, orchard grass,
timothy (3.0%); and alfalfa, red clover (1.0%). Most reports
of silo-filler's disease have been reported with silos filled
with com silage where the entire plant is chopped into small
chunks. Factors which increase the nitrate content of plants
includes drought followed by rain and prolonged cloudy weather,
plants with leaf and root damage, and plants grown in soil
with high levels of nitrogen.
Most
farmers are aware of the hazards of entering a silo soon after
filling stops. No data are available on how many Michigan
farmers have had silo-filler's disease; however, case reports
continue to be reported in the medical literature. The Mayo
Clinic reported seeing 12 patients with silo gas exposure
from 1980 - 1987. There are an estimated 15,000 silos on dairy
farms in Michigan. Approximately 60 percent are the non-airtight
top unloading type of silo where oxides of nitrogen are form
d.
An excellent review of silo-filler's disease patients seen at
the Mayo Clinic has been reported (Douglas WW, Hepper NGG, Colby
TV. Silo-Filler's Disease. Mayo Clin Proc 1989;
64:291-304.).
Author
Abstract. Between 1955 and 1987, 17 patients were examined
at the Mayo Clinic shortly after exposure to silo gas. All
exposures had occurred in conventional top-unloading silos.
Acute lung injury occurred in 11 patients, 1 of whom died;
early diffuse alveolar damage with hyaline membranes and hemorrhagic
pulmonary edema and acute edema of the airways were found
at autopsy. In one patient, hypoxemia and transient obstruction
of the airways developed, but no pulmonary infiltrates were
noted. One patient had symptoms for 5 weeks and diffuse confluent
pulmonary infiltrates; many years later, chronic obstructive
pulmonary disease developed (he had, however, been a heavy
smoker before exposure).
Bronchiolitis
obliterans was not observed in the other patients, probably
because of less severe exposure or early corticosteroid
therapy. Prophylactic corticosteroid therapy is advised
for workers who have been exposed to silo gas.
The
management of patients with established acute lung injury
is reviewed. Previously unreported patterns of exposure
to silo gas in conventional silos are described, and recommendations
for avoiding exposure are suggested.
There continue to be studies reported of increased risks of
cancer among farmers. A recent study from Alberta (Fincham SM,
Hanson J, and Berkel J. Patterns and Risks of Cancer
in Farmers in Alberta. Cancer 1992; 69:1276-1285.) reported
elevated lip and prostate cancer. This study did not show elevated
melanoma, leukemia, lymphoma or brain cancer. All these other
cancers have been reported in other studies to be increased
among farmers.
Author
Abstract. Data on occupations and life styles of patients
with cancer have been collected since 1983. To investigate
cancer patterns and risks in farmers in Alberta, all farmers
were abstracted and compared with nonfarmers in the data base,
using case-control analysis. Controls were patients with cancer
at any site except the index site. Significantly elevated
odds ratio (OR), adjusted for age and smoking, were found
among the farmers for cancers of the lip (OR = .22, 95% confidence
interval (CI) = 2.14 to 4.84) and prostate (OR = 1.31, 95%
CI = 1.11 to 1.55). Crude risk for lung cancer was significantly
lower in farmers, but statistical significance disappeared
when risk was adjusted for smoking (OR = 0.81, 95% CI = 0.65
to 1.02). Farmers were at considerably lower risk of malignant
melanoma of the skin, compared with non-farmers (OR = 0.57,
95% CI 0.36 to 0.91).
Disclaimer
and Reproduction Information: Information in NASD does not represent
NIOSH policy. Information included in NASD appears by permission
of the author and/or copyright holder. More
NASD Review: 04/2002
This document
is Physician's Newsletter, Vol. 1, No. 3, a series of the Center
for Michigan Agricultural Safety and Health, Michigan State
University, East Lansing, Michigan. This newsletter was made
possible in part by a grant from the National Institute for
Occupational Safety and Health, which provided funding for the
Michigan Agricultural Health Promotion System Project. Publication
date: Summer 1992.
Dr.
Kenneth D. Rosenman, M.D., Editor, Howard J. Doss, Agricultural
Safety Specialist, Department of Medicine and Department of
Agricultural Engineering, respectively, Michigan State University,
East Lansing, Michigan 48824-1323. Funded by Cooperative Agreement
UO5-4506052-02 and 03. This information is for educational
purposes only.
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