Summary The major features of the tests surveyed are shown in Table I. In a tier system of tests for teratogenicity, the Chernoff test is at a different level than the other assays described here. It is not appropriate for screening large numbers of chemicals, but may be useful for studies of smaller groups of agents, for example to confirm data from a prescreen. Although the test is certainly easier, cheaper and uses less than half the animals of a Segment II test, it is still much more expensive and time-consuming than most alternative tests. Of the remaining alternatives, whole embryos or organs in culture encompass the widest range of mammalian developmental events and are invaluable in the study of teratogenic mechanisms. They are, however, also inappropriate for screening large numbers of chemicals. The methods are technically demanding, relatively expensive and use reasonably large numbers of pregnant mammals. To screen a group of, say, 20 chemicals involves a considerable investment of time and, in fact, no study of this size has been reported. In certain specific circumstances, they may be a useful adjunt to testing; for example, if treated human serum samples are freely available, if a drug has a unique action on rodent dams which confounds evaluation of the standard in vivo tests, or if human metabolism is important and can be mimicked in vitro. Sub-mammalian and sub-vertebrate species offer considerable advantages; reduced cost, relative rapidity and no requirement for laboratory animals. FETAX provides some indication of teratogenicity in relation to embryotoxicity, while CHEST and the planarian and Drosophila assays measure only teratogenic potential, or more strictly speaking, embryotoxic potential, although it should be possible to derive some assessment of hazard with each of the latter systems. The Hydra system is cheap, quick and easy and is commercially available. It is the only assay specifically designed to estimate teratogenic hazard and may offer considerable advantages as an alternative screen. The metabolic cooperation assay has not generated sufficient data to enable evaluation. The neural crest cell assay is not well developed as a routine screen, and objective endpoints which are not measures of general cytotoxicity must be devised. The viral morphogenesis and Drosophila embryo cell assays have both produced encouraging validation data. With further assessment, the viral system may be shown to be useful, but it is a relatively complex assay and its relevance to teratogenesis is obscure. The Drosophila system is easier, has been used with more chemicals and is developmentally relevant. However, it has not produced dose-response data to evaluate potency or hazard, and must be improved so that it can more clearly distinguish cytotoxicity. The measurement of endpoints in the neuroblastoma cell line assay requires further refinement, and contributions of growth inhibition or stimulation to effects on differentiation must be examined. In combination, tumour cell attachment and HEPM may prove valuable. Alone, HEPM appears to be an assay for cellular toxicity, not teratogenicity, and the attachment assay suffers from a high rate of false negatives because it measures only one cell phenomenon. Although micromass cultures use mammalian tissue, are not the cheapest assays and require some skill for full evaluation of the results obtained, they show considerable promise. Validation data are encouraging, the assay includes several developmental processes and the use of multiple endpoints permits specific developmental toxicities to be evaluated.
AN INFECTIOUS OPHTHALMIA OF CATTLE
