Herpesvirus saimiri was isolated from 22 squirrel monkeys by cocultivation of peripheral lymphocytes with permissive owl monkey kidney monolayer cells. Comparison of virion DNA fragments produced from restriction endonuclease digestion was used as a sensitive measure of strain variability. Although all isolates contained similarities and common features, 19 of the 22 were readily distinguished. Three of the isolates, however, were indistinguishable and possibly were related epidemiologically.

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... stinct subtypes of H. saimiri were not evident by these criteria; Peruvian, Colombian, Guyanan, and Bolivian squirrel monkeys yielded isolates without characteristic features peculiar to the geographic region. Three of three colony-born squirrel monkeys that were tested yielded a strain of virus distinct from that obtained from the mother. In separate experiments, two of three animals chosen at random yielded a strain of virus different from that originally obtained 16 and 22 months previously; only one of the three animals examined yielded the same strain of virus 22 months after the original isolation. The degree of restriction endonuclease fragment variability among H. saimiri strains appeared to be greater than previously observed for other herpesviruses. Herpesvirus saimiri naturally infects most squirrel monkeys (Saimiri sciureus) in which it causes no apparent disease. H. saimiri infection of other species of New World primates frequently results in lymphoma or leukemia. Squirrel monkeys, the natural hosts, are commonly found in the rain forests of South America, and animals obtained from different South American regions have previously been shown to have distinct physical appearances and karyotypes (15). Previous analyses of H. saimiri virion DNA revealed a high degree of intramolecular heterogeneity in guanine-plus-cytosine (G+C) content (for review see reference 11). Unsheared DNA purified from virions is composed of about 90% infectious DNA that bands at a density of 1.705 g/cm3 (45% G+C) in CsCl and about 10% defective, noninfectious DNA that bands at a density of 1.730 g/cm3 (71% G+C) in CsCl. The defective DNA consists of identical tandem repeat units of 1.3 kilobase pairs (kb). Because of its high G+C content and density in CsCl, repetitive DNA is called H-DNA. Infectious virion DNA that bands at a density of 1.705 g/cm3 is composed of 115 kb of unique DNA with an average G+C content of 36% (density in CsCl, 1.695 g/cm3) to which H-DNA repeat units are covalently attached in the same orientation at each end. Since the number of H-DNA repeat units at each end varies, the size of infectious H. saimiri DNA is somewhat variable; the average size of intact infectious DNA is around 160 kb. Infectious virion DNA is called M-DNA, and the central unique 115 kb is called L-DNA. Analysis of size and number of herpesvirus DNA fragments generated by restriction endonuclease digestion has been used by others to distinguish different strains of herpes simplex virus (5, 18), Epstein-Barr virus (2, 12, 22), cytomegalovirus (13), varicella-zoster virus (20), and equine herpesvirus (23), to trace the spread of herpes simplex virus (3, 5, 17) , and to establish equine herpesvirus subtypes 1 and 4 (23). The results have indicated that epidemiologically unrelated herpesvirus isolates can be readily distinguished by the number and distribution of restriction endonuclease cleavage sites in their DNAs. This report presents some unique as-