TERM PAPER FOR PH792D:
CURRENT ISSUES IN ENVIRONMENTAL HEALTH: ADVANCED INDUSTRIAL HYGIENE PRACTICE
EXPOSURE TO MICROORGANISMS AND MICROORGANISM-PRODUCED TOXIC SUBSTANCES IN AGRICULTURE:
A REVIEW ON HEALTH HAZARDS AND
EXPOSURE ASSESSMENT
SUBMITTED TO DR. PREMLATA MENON
WRITTEN BY GUO QING ZHANG
APRIL 24, 1997
INTRODUCTION
Most agricultural workers involved in jobs in either crop cultivation and handling and animal husbandry. Exposure to microorganisms and microorganism-produced toxic substances is highly concerned among agricultural workers in these two main area in recent years (Abramson, 1989; Smith, 1989; Farant, 1989; Warren, 1989; Donham, 1989; May et al., 1989; Morey, et al., 1989; Morring, 1989; Donham, 1995). The hazardous exposure related to crop cultivation and handling is microbial exposure from moldy crops and plant materials. Exposure to microorganisms and microorganism-produced toxic substances may occur during harvest, but occupational exposure occurs also during post harvest treatment, e.g. during storage, refining or preparing of agricultural products. In animal husbandry, the concerns are focus on the animal confinement facilities (Dutkiewicz et al., 1988). Between 0.64 and 2.96 million workers and family members are occupationally exposed to poultry, swine, or beef and daily confinement and processing operations in the U.S. (Merchant and Donham, 1989) This paper reviews some literature on occupational health hazards and exposure assessment on exposure of agricultural workers to microorganisms and microorganism-produced toxic substances.
HEALTH HAZARDS RELATED TO MICROBIAL EXPOSURE
Microbial growth may occur in various crops and food, particularly during damp and humid conditions. Exposure to microorganisms may represent a potential health hazard, with respect to allergic reactions, asthma, toxicosis, and airway inflammation (Rylander, 1994; Schenker et al., 1991; Dockhorn and Neilburger, 1980; Von Essen et al., 1990). Microorganisms produce a variety of compounds, including volatile organic compounds (Gerber, 1968; Kaminski, et al., 1974; Strem, G. Et al., 1993; Bemjesson, 1993), cell-wall toxins such as endotoxins and 1-3-beta-glucans (Goto et al, 1994; Pratt et al., 1994), allergens, and specific mycotoxins. Endotoxins and 1-3-beta-glucans may cause airway inflammation at low exposure levels, and may contribute to the development of bronchial hyperresponsiveness and asthma (Rylander, 1994; Palchak et al., 1988; Jacobs,1989). Clinical and epidemiological evidence suggests that endotoxin plays a role in causing acute manifestations of the Abyssinosis syndrome@ (e.g., chest tightness accompanied by across-shift FEV1 decline, and mill fever) (Castellan et al., 1995). Mycotoxines are potent biological agents, and reports on mycotoxicosis (e.g. ergotism) dates back to 1000 A.D(Anon. 1991). Recently, aflatoxins were classified as human carcinogens by the International Agency for Research on Cancer (IARC, 1993). Most studies on mycotoxines deals with oral exposure through contaminated food, and there are only few publications on occupational airborne exposure to mycotoxines in airborne spores. Toxins from Stachybotrys sp., and aflatoxins from Aspergillus flavus have been shown to cause various types of health impairments in humans, including immunosupression, dermatitis, conjunctivitis, upper and lower airway symptoms, fatigue, and headache (Anon. 1991)
Inhalation fever (Organic dust toxic syndrome)
Inhalation fever, or organic dust toxic syndrome (ODTS), are terms describing a noninfectious, febrile illness associated with chills, malaise, myalgia, a dry cough, dyspnea, headache and nausea that occurs after heavy organic dust exposure (Schenker et al., 1991; Dockhorn and Neilburger, 1980; Von Essen et al., 1990; Rask-Andersen, A. 1995). This syndrome is thought to be an acute febrile reaction to organic dust exposure distinct from allergic alveolitis. ODTS has been reported from a variety of environments in which workers are at risk of mold dust exposure such as moldy hay, moldy straw, and moldy grain. A number of mold species and bacteria can cause ODTS including Thermophilic Actinomycetes, Flavobacteria, Aspergilli, and other fungi, as well as algae (Rask-Andersen, 1995). In the Nordic countries, pulmonary reactions to inhalation of dust from moldy hay occasionally occurs among farmers (Rask-Andersen, 1995). Among farmers, ODTS is reported to be much more common than allergic alveolitis (Rask-Andersen, 1995), the incidence being 10-190 cases / 10000 farmers (Von Essen et al., 1990).
Allergic alveolitis (hypersensitivity pneumonitis)
This disease has been described mainly among farmers that have been exposed to high concentrations of moldy organic dust ("farmers lung") (Schenker et al., 1991; Dockhorn and Neilburger, 1980; Von Essen et al., 1990; Rask-Andersen, A. 1995). It is characterized by acute recurrent pneumonia with fever, cough, chest tightness, and lung infiltrates. There are diagnostic criteria to be fulfilled for the diagnosis of extrinsic allergic alvolitis (Terho, 1986). Allergic alveolitis is a rare disease, among farmers the incidence rate is estimated to be 2-30 cases / 10000 farmers (Von Essen et al., 1990). In Japan, the most prevalent form of allergic alveolitis is reported to be summer-type hypersensitivity pneumonitis, caused by indoor molds during summer (Ando, et al., 1991).
EXPOSURE ASSESSMENT
Dust measurements
Personal exposure measurements of airborne total dust, respirable dust, or organic
dust have been applied, by conventional hygienic methods, to indirectly measure exposure of microorganisms and their toxic metabolites. Total dust can be sampled on cellulose acetate filter, and determined by gravimetric analysis (Gemhe, et al., 1983; Wieslander et al., 1994). If both inorganic dust and organic dust is present in the air, organic dust concentration can be determined as the weight difference before and after low-temperature ashing of the total dust samples collected on the filters (Wieslander et al., 1994). Respirable dust can also be determined by direct reading instruments (e.g. SIBATA PH5), based on the measurement of Rayleight light scattering by a lase diod (Bjemnsson, et al., 1995). Such direct reading instruments may have a lower accuracy than gravimetric methods for dust measurements. They are, however, particularly useful as a tool to identify emission sources, and to evaluate the effects of measures aiming to reduce the dust exposure.
Measurements of microorganisms and microbial exposures
There are various types of exposures related to microbial growth. By tradition, microbiologists have measured only viable spores in air samples, by cultivation and identification of mold species. In many types of occupational environments, however, only a small proportion of the airborne microorganisms are viable. Biological effects of nonpathogenic microorganisms are similar for viable and non-viable organisms. In addition, there are cases when bacteria, and not only molds, should be included in the exposure assessment. One method capable of detecting both viable and non-viable molds and bacteria is the CAMNEA method. This method
utilizes pumped air sampling on polycarbonate filters (Nucleopore, Pleasanton, California, USA), followed by acridine orange staining, and counting of microorganisms by epiflourescence microscopy (Palmgren et al., 1986). The CAMNEA method has been demonstrated to be a simple and accurate mean to measure microbial exposure in highly contaminated occupational environments (Eduard, et al., 1990). There are also chemical and biological methods available to quantify specific compounds related to microbial exposure. Endotoxins are toxic constituents to the cell wall of gram negative bacteria, and (1-3)-beta-glucans are constituents of the cell wall of microfungi. Both endotoxins and (1-3)-beta-glucans can be detected by the Limulus assay, a biological method (Goto et al, 1994; Pratt et al., 1994). Because of possible interaction between these toxins, and the nonspecificity of the Limulus system, there is a need to develop new sensitive and specific analytical methods for these cell-wall toxins, e.g. by means of gas chromatography and mass-spectrometry. Microorganisms may also emit volatile organic compounds (VOC). Some of these VOC's are specific compounds produced exclusively by microorganisms (e.g. geosmin or 1-octen-3-ol) (Gerber, 1968; Kaminski, et al., 1974; Strem, G. Et al., 1993; Bemjesson, 1993). It has recency been shown in Sweden that measurements of MVOC can be used also as a rapid and economical indicator of mold growth in stored cereals (Bemjesson, 1993). The most commonly occurring metabolites from grain-detoriating Aspergillus sp. and Penicillium sp. were found to be 3-methylfuran, 2-methyl-1-propanol, and 3-methyl-1-butanol. It was therefore concluded that these are the best indicators of mold growth in stored cereals. One particular advantage of MVOC analysis, as compared to cultivation of mold spores, is possibility to get a representive air sample from the whole batch of cereals in the store, and the possibility to get the analytic results within one day.
Exposure assessment results
Grain and forage
Mycotoxigenic fungi are found in grain and forage. Species of field and storage fungi in the genera Fusarium, Aspergillus, and Penicillium, in particular, are potential producers of toxic metablities, e.g., trichothecenes, aflatoxins, and ochratoxins (Smith, 1989). Some of the mycotoxins are potent carcinogens (Wyllie and Morehouse, 1977). Studies have shown that long-term inhalation of spores of Stachybotrys atra, a straw saprobe, may result in intoxication (Cockcroft et al., 1983).
Many bacterial species have been recorded as pathogens, epiphytes, or saprobes of grain and forage crops (Smith, 1989). Spores of the yellow pigmented strains of Erwinia herbicola are predominant among the Gram-negative airborne bacteria. Some isolates of these produce potent endotoxins (Cockcroft et al., 1983).
Table 1 shows species of fungus identified in airborne grain dust samples collected in terminal, transfer, and country grain elevators. Table 2 shows airborne spore concentrations found in various types of Canadian grain elevators. Table 3 shows mycotoxins and associated fungi in West Canadian grain and Table 4 shows levels of Aflatoxins in corn and associated dusts during harvesting and handling. These tables give more detailed information on exposure nature of microorganisms and their toxic metabolites.
Table 1 (adapted from Farant, 1989
Table 2 (adapted from Farant, 1989)
Table 3 (adapted from Abramson, 1989)
Table 4 (adapted from Abramson, 1989)
Animal confinement facilities
The relatively warm and humid environment in animal confinement buildings provides a rich culture medium for fungi and bacteria. Common fungi include species of Pencillium, Fusarium, Aspergillus flavus, Scopolariopsis, Verticillium, Phycomycete, and Paecilomyces (Merchant and Donham, 1989; Harries and Cromwell, 1982; Donham, 1986; Donham et al., 1985;). Both Gram-positive and Gram-negative bacteria are common, and relatively high levels of endotoxin occur in both swine and poultry confinement and processing facilities (Thelin et al., 1984; Donham, 1986; Donham et al., 1985; Thedell et al., 1980; Olenchock et al., 1982; Clark et al., 1983; Lenhart, 1984; Jones et al., 1984; Merchant and Donham, 1989). Thermoactinomyces candidus, Micropolyspora faeni, and Saccharomonspora viridis, also occur in swine confinement facilities and silage unloading (Donham, 1986; Donham et al., 1985; Dutkiewicz, 1978; Morey et al., 1989).
Poultry workers are exposed to a variety of airborne microorganisms and endotoxins. Table 5 and 6 shows concentrations of airborne viable microorganisms and endotoxin in different poultry production sectors. For daily farm workers, silage handling is the main operation of microorganism exposure. Microorganisms in silage and silage dust are shown in Table 7,8 and 9. Swine farm worker are exposed to relatively high concentrations of microorganisms and endotoxin as indicated in Table 10.
Table 6 (adapted from Morring, 1989)
Table 5 (adapted from Morring, 1989)
Table 7 (adapted from May et al., 1989)
Table 8 (adapted from Morey et al., 1989)
Table 9 (adapted from Morey et al., 1989)
Table 10 (adapted from Donham et al., 1989)
Table 11 (adapted from Donham, 1995)
Conclusions and recommendations
Exposure to microorganisms could risk a potential health hazard, with respect to allergic reactions, asthma, toxicosis, and airway inflammation. Microorganisms produce a variety of compounds, including volatile organic compounds, cell-wall toxins such as endotoxins and 1-3-beta-glucans, allergens, and specific mycotoxins. These microorganism-produced toxins may cause respiratory disorders and many other health problems. Agricultural workers involved in jobs in either crop cultivation and handling or animal confinement facilities are highly exposed to microorganisms and their toxic metabolites. Grain harvest and handling, poultry, swine, daily and beef producing in confinement facilities, and silage handling are occupations where it is important to minimize the exposure to the workers to reduce occupational health hazards.
The current maximum grain dust exposure threshold in the U.S. (OSHA-PEL: 15 mg M-3; ACGH-TLV 10 mg M-3 ) and Canada (10 mg M-3) should be revaluated and that consideration be given to a lower standard. A TLV of 4 or 5 mg M-3 has been considered as the best estimate of the appropriate TLV for grain dust (doPico et al., 1983; Enarson et al., 1985; Ratney, 1989; Huy et al, 1991). ACGIH has recommended a TLV of 4 mg M-3 for grain handling facilities but need more information to establish a PEL for grain dust. Exposure response studies include assessment of response to endotoxin, bioaerosol, ammonia, and microbes showed dose-response relationships between exposure and pulmonary function test and symptoms among workers in confinement swine buildings (Donham et al., 1989; Heederik et al., 1991). The threshold levels have been suggested by Donham (1995) as shown in Table 11. These exposure thresholds can be used as guideline for exposure assessment and control for agricultural workers involved in jobs in either crop cultivation and handling or animal confinement facilities.
The measurements of airborne total dust, respirable dust, or organic dust can be applied, by conventional hygienic methods, to indirectly measure exposure of microorganisms and their toxic metabolites. The CAMNEA method has been demonstrated to be a simple and accurate mean to measure microbial exposure in highly contaminated occupational environments. The measurement of microbial volatile organic compounds (MVOC) as a rapid and economical indicator of microbial growth in stored crops should be further evaluated. If possible, the exposure to potential occupational biohazards should be quantified by exposure measurements.
Exposure control could be done by improvements in the environment through 1) decrease generation of bioaerosol through improved management practices; 2) removal of contaminants through use of local exhaust ventilation, or electrostatic precipitation; 3) protection of the individual with personal airway protection devices (Donham, 1995). Mold growth in the crop should be avoided, and special precaution is needed when moldy plant material is handled.
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