SUMMARYStudies of the resistance patterns to infection with a murine cytomegalovirus in inbred strains of mice revealed the existence of resistant and susceptible strains. Resistance was found to be associated with possession of the H-2 k allele at the major histocompatibility locus of the mouse. The F 1 hybrid between a resistant strain (C3H/HeJ) and a susceptible strain (BALB/c) was found to have a resistance intermediate between that of both parents, indicating that the gene(s) controlling resistance is partly dominant. Susceptible BALB/c mice could be made resistant to lethal infection by pre-treatment with thioglycollate broth but not by pre-treatment with endotoxin or BCG. Resistant C3H]HeJ mice could be made susceptible to lethal infection by pre-treatment with cyclophosphamide.

Summary This paper summarizes the isolation of arboviruses from mosquitoes collected in the Ord Valley between 1972 and 1976. A total of one hundred and ninety five strains of at least fifteen antigenically distinct viruses have been isolated. Seven of these isolates appear to be ‘new’ antigenic types, and several are undergoing further testing. These are three new rhabdoviruses (Kununurra [OR194], a virus provisionally named Kimberley [OR250] and OR189 [provisionally named Parry's Creek]), three ungrouped, non‐haemagglutinating viruses (OR379, OR512, OR869) and a virus (OR540) which reacts to Poly Anopheles A world grouping fluid. The remaining viruses have been previously identified in Australia. These include Murray Valley encephalitis (MVE), Kunjin, Kokobera, Sindbis, Koongol, Wongal, Wongorr and a virus in the Corriparta serological group. The most important finding of these studies is that MVE displays an annually recurrent pattern of activity with a peak seasonal transmission rate at the end of the wet monsoon. This is the first definition of a probable endemic focus of MVE activity in Australia. The major vector for the majority of the viruses isolated was, by inference, Culex annulirostris. However, Aedeomyia catasticta was implicated as a major vector of the Corriparta group virus.

The resistance of adult mice to acute lethal infection with murine cytomegalovirus is controlled by genes linked to the H-2 complex. The k haplotype is approximately 10 times more resistant than the b or d haplotypes. Susceptibility is inherited as a completely dominant trait. At least two genes within the H-2 complex are involved, one mapping to the K/IA subregion and the other to the D subregion. The data suggest that interactions may occur between these K- and D-end genes which further affect resistance to the virus. The precise mechanism of H-2 gene control of resistance to murine cytomegalovirus remains to be elucidated. Non-H-2 linked genes also affect resistance to the virus, particularly in the C57BL genetic background, which is associated with an increased resistance to murine cytomegalovirus. Newborn mice of all strains are equally susceptible: both the H-2- and the non-H-2-associated resistances develop in the first few weeks of life and are retained up to at least 18 months of age.

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