Latent infection in experimental spirochætes

Abstract

The following observations were made while studying immunity to Spirochætal recurrentis in white rats. Since it had been suggested that an immunity might be maintained by persistent latent infection, it was necessary to make careful examinations for spirochætes in all animals exhibiting resistance to infection.

The work was done with strains of spirochætes which usually produced well-marked infection with one or more relapses. During the observation of several hundred animals it was noticed that parasites occasionally reappeared in the peripheral circulation after unusually long absences. It often happened that spirochætes were found in blood films, on one or two days, after they had been absent for from ten to fifteen days; once spirochætes were seen after an absence of twenty-one days. Blood, taken from the tail, was examined for spirochætes in thick dehemoglobinized and stained films. Examinations, throughout the work, were made daily.

It often happens that a normal infection follows when a rat is inoculated with blood from an animal, once infected, although repeated examinations have failed to find spirochætes in it. In many instances, infection resulted when rats were inoculated with blood taken from other rats which were killed from fifteen to twenty days, in one instance twenty-seven days, after spirochætes were last found in them by microscopical examination.

Although the inoculation of blood, and other material, obtained by killing an experimental animal, is sometimes a more efficient method of detecting spirochaetal infection, it is not always so and, since it involves the destruction of the animal supplying the material inoculated, it is a method which cannot be repeated. Inoculation of material in which living spirochætes have been seen occasionally fails to infect one of two rats inoculated at the same time; sometimes, also, the infection produced remains unseen by microscopical examination and is only detected by killing the inoculated animal and by sub-inoculating another rat with blood from it.