Zika virus (II). Pathogenicity and physical properties

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Summary 1. (1) A description is given of the adaptation to mice of two strains of Zika virus. Zika is the name of a forest area near Entebbe, Uganda, where both strains of virus were isolated. One of the strains was isolated from a pyrexial rhesus monkey which was being employed as a yellow fever sentinel and the other was obtained from a batch of A. africanus. 2. (2) The signs of infection in mice are described. While mice of all ages tested are susceptible to intracerebral inoculations with Zika mouse brain virus, mice of 2 weeks of age and over can rarely be infected by the intraperitoneal route. Mice younger than 2 weeks are highly susceptible to intraperitoneal inoculation of the virus. 3. (3) Zika virus is highly neurotropic in mice and no virus has been recovered from tissues other than the brains of infected mice. 4. (4) Cotton-rats, guineapigs and rabbits show no clinical signs of infection after intracerebral inoculation of late passage mouse brain virus. 5. (5) Monkeys develop an inapparent infection after subcutaneous inoculation with mouse brain virus. After intracerebral inoculation one of five monkeys showed a mild pyrexia, the others showed no signs of infection. Viraemia during the first week after inoculation has been found in all monkeys tested and antibody has been demonstrated by the 14th day after inoculation. 6. (6) Of 99 human sera tested, 6 (6.1 per cent.)-have neutralized more than 100 LD50 of virus. Antibody has also been found in the serum of one of 15 wild monkeys tested. 7. (7) The size of Zika virus is estimated to eb in the region of 30 to 45 mμ in diameter. The virus may be preserved up to 6 months in 50 per cent. glycerol and up to 30 months after drying. It is susceptible to anaesthetic ether and the thermal death point is 58°C. for 30 minutes. 8. (8) Neuronal degeneration, cellular infiltration and areas of softening are present in infected mouse brains. Cowdry type A inclusion bodies have been found, particularly in the brains of young mice showing extensive lesions.
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