Progress has been made concerning the causes of neoplasms in fish, but many questions remain. The following conclusions and suggestions for additional study are based on literature reviewed in this chapter.

1. Both oncogenic viruses and chemical carcinogens appear to be common causes of fish neoplasia. However, interactions between viruses of fish and environmental factors, especially chemical pollutants, have not been adequately considered.

2. Differentiation between neoplasia and various non-neoplastic lesions continues to be a problem. Historically, neoplasia in fish has been defined almost exclusively by histological appearance. Although the current state of knowledge concerning oncogene mutation or expression and associated changes in cell function are not adequate to provide defining criteria for neoplasia, rapid advances in this field offer hope for a more objective definition of these diseases. However, it is also possible that the line between hyperplasia and neoplasia is as fuzzy biochemically and physiologically as it is histologically.

3. Regression of fish neoplasms, including some considered malignant, needs additional study. Frequent or rapid regression suggests that there are changes in the immune system or that the growth advantage of neoplastic cells is altered. Another possibility is that some of these lesions are not neoplasms, or at least not malignant neoplasms.

4. Temperature has important effects on development and regression of neoplasms. Although temperature is a major factor in all aspects of poikilothermic physiology, specific mechanisms involved in temperature-related changes in the behaviour of fish neoplasms have not been adequately considered.

5. The importance and usefulness of transplantation of neoplastic tissue from one fish to another need to be clarified. Successful transplantation of tumours between fish, often inbred or syngeneic, has occasionally been used as evidence of the neoplastic nature of the lesion. Basic information is needed about transplant rejection in fish, and factors that affect growth of normal tissue when transplanted to syngeneic fish need to be determined.

6. Fish neoplasms metastasize less often and less aggressively than do similar tumours in mammals. Although several hypotheses for this difference have been proposed, additional research is required to test these possibilities.

7. Most neoplasms of wild fish do not appear to affect the size of fish populations; however, shifts in age distribution can occur. Additional consideration should be given to potential long-term effects if high frequencies of neoplasms occur for several generations.

8. A common theme in many studies of neoplasms occurring in wild fish is the usefulness of certain types of tumours as sentinels for the presence of chemical carcinogens that could have human health implications. These neoplasms can also be useful as indicators of environmental degradation that has serious direct effects on aquatic ecosystems, including fish populations.

9. Fish offer several advantages over other animals in screening for carcinogenicity, but additional refinement is needed for test procedures. Factors that need consideration include the relative advantages of various species and routes of exposure. Approaches for shortening duration of tests should be considered, including use of oncogene alterations or preneoplastic lesions as end-points.

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