Characterization of the New Fecal Form of Toxoplasma gondii

Abstract

Fecal excretion of toxoplasma was found in 28 of 29 dye test-negative adult cats, 7 of 12 dye test-positive adult cats, and in 11 newborn kittens after they had been fed mice with chronic toxoplasmosis. Toxoplasma appeared in the feces after 3 to 5 days and persisted for 7 days or longer. Cats which had once excreted toxoplasma in their feces did not reexcrete it within 1 to 5 months when refed with infected mice. Cats also excreted fecal toxoplasma 8 to 10 days after eating mice with acute toxoplasmosis, and 21 to 24 days after eating infected cat feces. Fecal toxoplasma were orally infectious to mice, hamsters, dogs, and cats by the oral route; in mice infectivity was also demonstrated by the intraperitoneal and subcutaneous routes. Cats developed a dye test titer of about 1:100 after 1 month; dogs, mice, and hamsters usually showed 10-fold higher titers. About 45% of stray cats from the Kansas City area had an antibody titer of 1:8 to >1:128. Fecal toxoplasma were not infectious in fresh cat feces but developed infectivity within 1 to 2 days under optimal preservation and oxygenation. They were resistant to acids, alkalies, and to common laboratory detergents, and were killed by ammonia, by drying, and by exposure to a temperature of 55 C. Hutchison (1965) recovered toxoplasma from the feces of a cat which had been fed toxoplasma-infected mice. The feces, which had been subjected to flotation in zinc sulfate solution at a sp gr of 1.180, contained Toxocara cati eggs and Isospora sp. and transmitted toxoplasma to mice even after storage in tap water for 12 months. Fecal infectivity was subsequently observed in two other toxocarainfected cats but not in five toxocara-free cats (Hutchison, 1967). Efforts to separate the fecal toxoplasma infectivity from Toxocara eggs by filtration through a 36-/p wire sieve were unsuccessful; Toxocara ova and toxoplasma infectivity were retained on the sieve. On the basis of these findings Hutchison (1967) suggested that toxoplasma in cat feces is protected within the ova of T. cati. Jacobs (1967) observed that only fecal suspensions containing fully developed ova transmitted Toxoplasma to mice. Dubey (1966) observed that Toxocara eggs retained on a 53-/u wire sieve and washed with 1% formalin to kill toxoplasma trophozoites and cysts and with 6% sodium hypochlorite to remove the chitinous egg shell transmitted toxoplasma to mice; however, the filtrate was not tested for toxoplasma infectivity. Larvae of Toxocara were also found to be associated with Received for publication 15 January 1970. * Supported by grant AI-07489 of the NIAID, Public Health Service. toxoplasma infectivity; the larvae had been kept in tap water for 30 min and the egg culture had been treated with formalin and with sodium hypochlorite. It was concluded that toxoplasma was not trapped in the egg membranes but was inside the larvae (Dubey, 1967). All these observations fitted well with the hypothesis of toxoplasma being transmitted by nematodes (Hutchison, 1967). However, evidence contradicting the role of nematodes also started to accumulate. Jacobs (1967) first observed that six fecal floats from two cats free of Toxocara eggs transmitted toxoplasma to mice. As the cats had been infected with T. cati at some time during transmission experiments and as fecal samples were not titrated for toxoplasma infectivity, the presence of a few eggs could not be ruled out. Dubey (1966, 1968a) also observed that two of 109 fecal samples from cats free of Toxocara eggs transmitted toxoplasma to mice. As the fecal samples were maintained in saline instead of water, the presence of toxoplasma cysts was not ruled out. Dubey (1966) also found that fecal floats with Toxocara eggs incubated in 1% formalin did not transmit toxoplasma to mice, whereas the control inocula maintained in tap water did; however, Toxocara eggs embryonated better in 1% formalin than in water. In addition, eggs removed from the uteri of gravid worms and maintained in tap water did not transmit toxoplasma to mice whereas feces