1. A virus has been isolated from two patients suffering with epidemic keratoconjunctivitis.

2. At first the virus could be maintained only by the inoculation of tissue cultures (serum ultrafiltrate and embryonic mouse brain) with conjunctival scrapings or with emulsified mouse brains from early passage animals. Later it caused a fatal disease in every mouse into which it was inoculated, and could, then be readily maintained in mice.

3. The virus proved pathogenic for unweaned white Swiss mice by the intranasal, intraperitoneal, and intracerebral routes; for adult mice by the intranasal and intracerebral routes, and for rabbits by only the intracerebral route.

4. Although the titer of tissue cultures rarely exceeded 10–2 or 10–3, the virus, once established in mice, increased in potency until titers of 10–5 and 10–6 were attained. Mice injected with either the emulsion of mouse brain tissue or with the tissue culture material in these dilutions developed symptoms within a definite incubation period; once the disease was initiated, it followed a characteristic course for a period of a few to 24 hours, and consistently terminated in death.

5. The pathological changes in mice were limited to the central nervous system, and were not particularly distinctive. The neurotropic character of the virus is further shown by the fact that only the brain tissue was consistently pathogenic for mice.

6. Serial tissue cultures could be maintained only at room temperature, and when the inoculum from virus-infected cultures into fresh tissue cultures contained ground-up cells.

7. The highest level of potency in cultures occurred on about the 6th day at room temperature.

8. The virus passed without difficulty through an E-K Seitz filter (double pads) and through all grades of Berkefeld filters.

9. The virus passed consistently through graded collodion membranes with an A.P.D. of 75 to 100 millimicrons and to a lesser extent through those with an A.P.D. of 50 to 75 millimicrons. Membranes with an A.P.D. of less than 50 millimicrons retained the virus.

10. The mouse virus was not neutralized by anti-lymphocytic choriomeningitis serum, antiherpes serum, normal human serum, or serum from cases of non-specific conjunctivitis or keratitis.

11. Mice hyperimmunized to Theiler's virus were susceptible to the mouse keratoconjunctivitis virus. The latter virus was also not neutralized by hyperimmune Theiler rabbit serum.

12. The mouse virus could be neutralized by serum from the two patients from whom the virus had been isolated, and also by that from the three patients convalescing from epidemic keratoconjunctivitis in California and the serum of a convalescent in New York. The neutralization data were confirmed by tests on 15 additional convalescent serums (unpublished data).

13. A mild but characteristic picture of epidemic keratoconjunctivitis was reproduced in a human volunteer following inoculation with the mouse virus.

14. The serum of the human volunteer, while not neutralizing the mouse virus before infection, contained neutralizing antibodies 1 month after infection.

15. Development of antibodies was demonstrated in one patient (R.H.) in the present series, and in six other patients of another series (unpublished data).

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