The oral and parenteral infections of chimpanzees receiving echo Types 6 and 4 viruses successively are described. The two infections, spaced 2½ months apart, and given by the same route in each animal, failed to induce overt disease. The inapparent infections were demonstrated by virus excretion in the throat and the stools and the development of neutralizing antibodies. Complement-fixing antibodies also appeared after Type 6 infection, but fell more rapidly than the neutralizing antibodies.
After oral infection, echo-6 virus was found for equal periods in both the throat and feces, but echo-4 persisted in the throat for much longer periods than in the lower bowel. Almost no virus carriage occurred after parenteral inoculation. No true viremia was exhibited in any of the animals.
One of the chimpanzees had neutralizing antibodies against Type 6 virus in its pre-inoculation serum. It responded extraordinarily to the Type 6 exposure, developing antibody levels of 1:50,000 to echo-6, of 1:1024 against the echo-6' variant, and of 1:64 against the echo 6'' variant.
Although Type 4 antibodies developed after the exposure, they proved difficult to measure by ordinary methods. However, they could be satisfactorily assayed by the plaque reduction method.
Three other chimpanzees fed echo-2, echo-3, and an untypable echo virus, respectively, yielded results confirming those established with Types 4 and 6.