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Experimental Toxoplasmosis and Vaccine Tests in Aotus Monkeys

We studied Aotus lemurinus, Panamanian night monkeys, for susceptibility to Toxoplasma infection and for their capacity to develop immunity using either sufadiazine prophylaxis or the non-persistent ts-4 vaccine. The animals were highly susceptible to infection with a mouse pathogenic (T265) and a mouse nonpathogenic (T163) Toxoplasma isolate. A calculated single bradyzoite by mouth gave rise to infection which was fatal in nine to 12 days. Chemoprophylaxis with 60–300 of sulfadiazine mg per day for up to 40 days protected the animals; however this was followed by fatal reactivation of infection between 11 and 70 days after treatment was stopped. Vaccination was carried out in two or three doses subcutaneously. Challenge was performed in 26 animals using both Toxoplasma isolates. Five monkeys (19%) survived for over a year, 10 died after a prolonged illness, and 11 died as rapidly as the seven controls. Safety tests showed the vaccine to be nonpathogenic in 111 adults except for slight fever and local inflammation, although one of four juveniles died from disseminated infection. Vaccination of 25 pregnant monkeys was non-pathogenic; however two of 25 fetuses were aborted, one of which was infected and one newborn had microphthalmia, retinitis and a cataract; four of the offspring were not tested. When six lactating monkeys were vaccinated, Toxoplasma was not transmitted to the infants. The high susceptibility to Toxoplasma and the low immunizability was circumstantially attributed to absence of exposure and lack of selection by Toxoplasma of these arboreal monkeys even though about 50% of terrestrial animals from the same area were infected. Therefore, because they are apparently unselected by Toxoplasma, these monkeys proved to be a sensitive primate model for safety tests. They are however, unsuitable for evaluation of vaccine potency which was demonstrated in mice, hamsters and rabbits.