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Two strains of Venezuelan equine encephalitis (VEE) virus were examined for the ability to replicate in, as well as to produce death among donkeys. One, a low passage strain known as strain P676, was originally isolated from mosquitos in Venezuela. The other, strain V-38, was isolated from a horse brain in 1938 and had undergone an unknown number of laboratory passages; it is used extensively for the preparation of inactivated VEE vaccine. Both strains were found to be approximately equal in their ability to infect donkeys. However, a quantity as small as 50% hamster intraperitoneal infectious units of strain V-38 resulted in fatal infection. On the other hand, as much as 631 million infectious units of strain P676 were nonfatal in one of two donkeys. It appears that strain V-38 is approximately 100 million times more virulent than strain P676 in equine species. One donkey which received strain P676 demonstrated a biphasic pattern of clinical illness and viremia, and there is suggestive evidence that another animal experienced a second and fatal clinical response 3 weeks after virus inoculation.
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