Volume 18, Issue 4
  • ISSN: 0002-9637
  • E-ISSN: 1476-1645



The neutralization of toxin observed in preliminary studies with normal human serum prompted further investigations on the prevalence and nature of the neutralizing activity. As shown by toxin neutralization (TN) tests in mice with toxic suspensions of rickettsiae, serum of 50% of 207 normal persons had titers of 1:32 or greater against toxin, regardless of age or residence in the United States. Within this group were 35 persons less than 20 years old who had never received typhus vaccine, and who resided in a typhus-free area (Montana). Serum from all of 25 normal monkeys had titers of 1:512 or greater. With three exceptions, serum specimens positive in murine TN tests did not neutralize toxin of epidemic typhus or react in complement-fixation or microagglutination tests with murine or epidemic typhus antigens. Serum of normal burros, horses, sheep, goats, rabbits, and chickens did not neutralize toxins of either murine or epidemic typhus. The neutralizing factor in serum from normal monkeys was not affected by treatment with potassium periodate or trypsin, but could be completely precipitated by 50% saturation with ammonium sulfate or by acetone. These findings suggest that the murine TN substance detected in the primate serum was of nonspecific origin, not the result of exposure to or , and must be considered in evaluating the significance of the TN test as a diagnostic procedure.


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