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A fatal disease resembling Argentine haemorrhagic fever of man has been produced in guinea-pigs and mice by inoculation with Junin virus. Infected guinea-pigs show macroscopic and microscopic haemorrhagic lesions, marked bone marrow changes, decreased leukocytes and platelets in the peripheral blood, and impairment of immunological response. This response permits differentiation between pathogenic (XJ) and attenuated (XJ Cl(3)) strains. Guinea-pigs inoculated with the XJ Cl(3) strain develop an inapparent infection accompanied by slight haematological changes, the appearance of antibody, and protection against challenge with the pathogenic strain. The attenuated strain has been used successfully as an immunizing antigen in 636 human volunteers. Guinea-pigs infected with Tacaribe virus show cross-protection against Junin virus, with the presence of heterologous neutralizing antibodies. Suckling mice infected with Junin virus develop a typical viral encephalitis; the pathogenicity of the virus decreases with increasing age of the mice. Experiments with thymectomized mice and with mice treated with antithymocyte serum suggest that the pathogenicity of Junin virus in this host is related to the integrity of the thymus-dependent immune system. There is evidence that humoral antibodies do not play any role in the development of the encephalitic lesions but rather protect mice against Junin virus infection. A recent serological survey among laboratory workers and inhabitants of the endemic area has demonstrated the presence of inapparent infection with Junin virus.

Experimental Machupo virus infection of rhesus and cynomolgus monkeys produced a severe illness consisting of an initial clinical phase and a later neurological phase. Cumulative mortality during the two phases was 80% and 95% respectively. Attempts to alter the pathogenesis with decomplementation or immunosuppression resulted in earlier deaths of the monkeys.

Tamiami virus produces a lethal encephalitis in suckling mice, and the illness is mediated, at least in part, by cellular immunity. Infection of extraneural organs, including lymphoid organs, is limited. The same virus produces a widespread infection in its natural host, the cotton rat, but neither symptomatic illness nor cytopathology results. Since antibody is produced, as in the murine infection, suppression of cellular immunity to the virus may be responsible for the non-cytolytic infection. Lymphoid tissue is extensively infected in the cotton rat and a relationship between this lymphotropism and immunosuppression is suggested.

Natural infection with Machupo and Latino viruses occurs only in the cricetine rodent Calomys callosus. Machupo virus induces fatal infection in suckling mice and hamsters, and in adult guinea-pigs, marmosets, and rhesus monkeys. Latino virus kills only suckling hamsters; it produces chronic but non-viraemic infection in Calomys rodents.Machupo virus, in contrast, induces a viraemic immunotolerant infection in suckling Calomys, and a split response in animals more than 9 days of age. Tolerant infection is associated with haemolytic anaemia and splenomegaly, lesions not observed in animals able to clear viraemia and produce circulating neutralizing antibodies. Experimental increase in the fraction of tolerant response was obtained by decreasing the virus dose or by phenotypic inbreeding of rodents. Long-term effects of tolerant infection included mild runting, decreased survival time, and almost total sterility among females, largely caused by fatal virus infection of embryos.

Arenaviruses are known to show antigenic relationships in the complement-fixation (CF) and fluorescent antibody tests but not in the neutralization test. The humoral response to some of the arenaviruses is characterized by a dissociation between the antibodies determined by the CF test and those determined by the neutralization test and also by its late appearance following natural infection. Investigations are reported showing that, as regards the CF test, Junin, Machupo, Amapari, and Tacaribe viruses are closely related, while LCM and Lassa viruses are distantly related to each other and to the remaining viruses in the group. Studies by agar gel diffusion and precipitation have shown complete specificity except among viruses that are very closely related in the CF test. Observations with sera from persons infected with Lassa fever virus show that CF antibodies rarely appear before the 18th day after onset; they can persist, with diminishing titres, for up to 6 years.

Bolivian haemorrhagic fever immunoglobulin of human origin, given either prior to or shortly after experimental infection with Machupo virus, protected rhesus and cynomolgus monkeys against initial clinical illness. Some survivors developed severe neurological signs 30-47 days after virus inoculation and died 4-6 days later. Results from one of the experiments suggested that the development of neurological signs was associated more frequently with high doses of immunoglobulin than with intermediate or low doses.

The indirect haemagglutination test has been developed for use with LCM and Tacaribe viruses employing antibody-sensitized erythrocytes. Both in this test and in the indirect haemagglutination inhibition test cross-reactions between LCM and Tacaribe viruses were revealed. This opens new perspectives for group and species identification of arenaviruses.

A limited but definite need exists for vaccines against Lassa, Junin, and Machupo viruses. Medical and laboratory personnel, as well as defined high-risk population groups, require protection from these highly virulent agents. To date little work has been done on inactivated vaccines for these viruses. A live attenuated Junin vaccine has been tested successfully in more than 600 persons, and a high-passage Machupo virus strain has protected rhesus monkeys against lethal infection produced by a homologous field strain.Work has been initiated on possible heterologous protection induced by infection or antigenic stimulation with arenaviruses not pathogenic for man. Crucial for the eventual development of effective vaccines are the construction of more maximum security laboratories and the further elucidation of the experimental and natural biology of the agents in lower animals and man.