Twelve previously uninoculated and 15 WEE-immune English sparrows were inoculated with EEE virus. The average level and duration of viremia did not differ significantly between groups. The control group suffered 50 per cent mortality due to EEE infection, while no deaths occurred in the WEE-immune group.

Nine previously uninoculated and 8 EEE-immune English sparrows were inoculated with WEE virus. WEE viremia was of shorter duration and reached lower maximum levels in EEE-immune birds than in the control birds.

In three horses previously infected with WEE, infection with EEE did not produce central nervous system symptoms nor deaths.

The possible influence of these factors on the epidemiology and geographic distribution of clinical disease produced by these viruses is discussed.

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