The pathological effects of prolonged asbestos ingestion in rats

Our plain-English summary of The pathological effects of prolonged asbestos ingestion in rats, by R.E. Bolton, J.M. Davis, D. Lamb (Environmental Research, 1982).

The study

• Authors: R.E. Bolton, J.M. Davis, D. Lamb
• Journal: Environmental Research
• Published: 1982;29(1):134-50
• Cited by: 18 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The effects of prolonged ingestion of amosite, crocidolite, and chrysotile UICC standard reference asbestos samples were examined in groups of laboratory rats. Animals were given over 250 mg per week for periods up to 25 months and were monitored for the remainder of their life span. Animals were examined for evidence of pathological effects and gastrointestinal mucosal cytokinetic disturbances and for signs of penetration and dissemination of fiber. There was no excess of malignant tumors in the experimental groups when compared with control animals, and no gastrointestinal mucosal abnormalities were found. An examination of the cytokinetic status of a subgroup of animals exposed to asbestos similarly showed no evidence of any adverse effects of prolonged ingestion of amosite asbestos fiber. Detailed electron microscopic examination of various tissue residues for the presence of asbestos fibers was also undertaken. No penetration and/or damage to any of the gut tissues was found. Although occasional fibers were found in a variety of tissue residues, there was no evidence of preferential retention of fibers within any specific tissue and no sign of higher fiber burdens in those animals with tumors. It was concluded that there were no significant adverse effects of prolonged asbestos ingestion in healthy laboratory rats. The implications of these findings are discussed in the light of other published work.

PubMed has no abstract for this study. This is the authors' abstract, copied from the article.

In plain English

In this animal experiment, 115 male Wistar-derived rats were split into five groups. Three ate margarine mixed with UICC amosite, crocidolite or chrysotile "A" asbestos (about 250 mg per rat weekly) for up to 25 months. One control group ate plain margarine and one ate the normal diet only. Most rats were kept for life and autopsied when moribund; a small subgroup was removed for intestinal cell studies. Malignant tumors numbered 1 (amosite), 1 (crocidolite), 5 (chrysotile), 4 (margarine controls) and 2 (untreated controls). These differences were not statistically significant, and no primary malignant tumors of the gut lining were found. One amosite rat had a malignant stomach-muscle tumor and one chrysotile rat a pleural tumor resembling rat mesothelioma; the authors urged caution. The chrysotile group had 11 benign tumors, a statistically significant excess over controls. The authors attributed this excess mainly to mesenteric hemangiomas in 4 of the 22 chrysotile rats, and judged it unlikely to be treatment-related. Electron microscopy found very few fibers in tissues and no gut penetration. The authors concluded prolonged asbestos ingestion had no significant adverse effects in healthy rats, noting their microscope could miss very fine fibers.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Animal experiment: long-term feeding study in rats with lifetime follow-up of most animals and autopsy
• Who was studied: 115 male HAN spf Wistar-derived rats, aged 10 weeks at the start, in five groups: amosite 24, crocidolite 22, chrysotile 22, margarine (positive) control 24, untreated (negative) control 23. Most were kept for the rest of their lives. A small subgroup was removed for intestinal cell-proliferation studies.
• Exposure: UICC standard reference amosite, crocidolite, or chrysotile "A" asbestos, given in a margarine-based dietary supplement at 5 mg asbestos per gram of margarine. This averaged approximately 250 mg of asbestos per rat per week (0.27% by weight of food consumed), for periods up to 25 months.
• Main results:
  • Malignant tumors: 1 in the amosite group (24 rats), 1 in the crocidolite group (22), 5 in the chrysotile group (22), 4 in the margarine control group (24) and 2 in the untreated control group (23). The differences were not statistically significant.
  • No primary malignant tumors of the gastrointestinal mucosae were found in any group.
  • One amosite-treated rat had a malignant tumor arising in the stomach muscle layers. One chrysotile-treated rat had a pleural histiocytic tumor resembling a conventional rat mesothelioma. The authors said these findings called for caution in interpretation. They also noted that similar tumors occasionally arise spontaneously in this rat strain, although none was seen among the 47 controls.
  • Benign tumors numbered 1 in the amosite group, 5 in the crocidolite group and 11 in the chrysotile group.
  • The authors reported a statistically significant excess of benign tumors in the chrysotile-treated group compared with controls. They attributed the excess mainly to mesenteric hemangiomas in four chrysotile-treated rats (an incidence of 4 in 22 animals).
  • Because fiber searches found no widespread penetration or spread of fiber, the authors judged it unlikely that the higher number of benign tumors in chrysotile-exposed rats was due to treatment.
  • Mean survival times did not differ significantly between the groups. Most rats survived beyond 700 days of age.
  • In a cell-turnover subgroup (8 amosite-treated rats, 8 margarine controls, 6 untreated controls, about 28 months old), cell production in the descending colon was lower in amosite-treated rats than in untreated controls (13 versus 24 cells per crypt per hour). Margarine controls were lower still (9). The authors attributed the depression to the margarine diet, not to asbestos.
  • Scanning electron microscopy of ashed tissues found very few asbestos fibers. No tissue retained fibers preferentially, and fiber presence did not correlate with disease lesions.
  • No penetration of, or damage to, the gut tissues was found.
• The authors' conclusion: The authors concluded that prolonged asbestos ingestion had no significant adverse effects in healthy laboratory rats. They also concluded that the normal healthy gut appeared to maintain an effective barrier against ingested asbestos.
• Limitations the authors state:
  • The resolution of the scanning electron microscope made it difficult to identify fine fibers less than 0.1 micrometers in diameter.
  • Estimates of crypt cell production were subject to large errors because only a small number of animals was available.
  • The study did not examine whether intestinal dysfunction affects fiber penetration or damage.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Amosite, Chrysotile, Crocidolite.

Medical terms in this study, explained in our Lexicon: Amosite, Crocidolite, Chrysotile, UICC, Fiber Burden.

Related studies

• The short-term effects of chronic asbestos ingestion in rats — R.E. Bolton, J.M. Davis (1976)
• Light and electron microscope studies of the rat digestive tract following prolonged and short-term ingestion of chrysotile asbestos — R. Jacobs, J. Humphrys, et al. (1978)
• Enhanced lipid peroxidation and lysosomal enzyme activity in the lungs of rats with prolonged pulmonary deposition of crocidolite asbestos — J. Jajte, I. Lao, et al. (1987)
• The sizes, shapes, and mineralogy of asbestos structures that induce lung tumors or mesothelioma in AF/HAN rats following inhalation — D.W. Berman, K.S. Crump, et al. (1995)
• Nucleic acid metabolism in the rat following short-term and prolonged ingestion of chrysotile asbestos or cigarette-smoke condensate — R. Jacobs, M. Weinzweig, et al. (1978)
• Distribution of 6-6(n)-3[H]sucrose and its radiolabeled degradation products from isolated, perfused rat small intestine loops following prolonged ingestion of chrysotile asbestos — R. Jacobs, R.J. Richards (1980)

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