UNCLASSIFIED TOXOPLASMA GONDII-LIKE ORGANISM
(Figures 1-15 to 1-17)
ETYMOLOGY: This organism has not been named.
HISTORY: This parasite was first reported in the early 1990's (Dubey et al., 1992; Dubey and
Carpenter, 1993; Dubey and Fenner, 1993). It closely resembles Toxoplasma gondii, but the
tissue cysts of this organism are about twice as large as those of T. gondii.
GEOGRAPHIC DISTRIBUTION AND PREVALENCE: Dubey and Carpenter (1993)
found this organism in 3 of 103 cats examined in a retrospective study of feline toxoplasmosis.
The subjects had been examined at necropsy at the Angell Memorial Animal Hospital, Boston,
Massachusetts between 1952 to 1991.
DIAGNOSIS: A tentative diagnosis can be made on the large size of the tissue cysts.
Transmission electron microscopy is needed to confirm the diagnosis. Bradyzoites of the
unclassified organism have numerous micronemes that are arranged in rows, while micronemes
in bradyzoites of T. gondii are fewer in number and arranged randomly (Dubey et al., 1992;
Dubey and Fenner, 1993). Infected cats may be T. gondii antibody positive and tissue cysts may
react weakly with anti-T. gondii serum in immunohistochemical tests.
CLINICAL SIGNS AND PATHOGENESIS: Infected cats have ranged in age from 3 to
17 years. Clinical signs attributable to parasitism have been associated with only 1 of 6 cats
reported to have been infected with this organism (Dubey and Fenner, 1993). The cat was three
years-old and was presented for lameness. Neurologic and ophthalmologic abnormalities were
present on physical examination. The neurologic examination indicated lesions in the spinal
cord at C6-T2. A complete necropsy was done and gross and microscopic lesions were confined
to the spinal cord. Grossly, a focal translucent grayish area 1 x 0.3 cm laterally between
cervical nerve roots C5 and C6. In addition, the spinal cord widened unilaterally from C4 to C6.
Microscopic lesions consisted of focal granulomatous myelitis involving both gray and white
matter and focal nonsuppurative meningitis including radiculoneuritis. Numerous protozoal
tissue cysts were associated with the lesions.
Three of the other 5 infected cats have had concurrent lymphoid disorders and may have
been suffering from varying degrees of immunosuppression. The association of this parasite and
the immune status of the host needs to be better defined.
Dubey JP, Peters D, Brown C. 1992. An unidentified Toxoplasma-like tissue cyst-forming
coccidium in a cat (Felis catus). Parasitol Res 78:39-42.
Dubey JP, Carpenter JL. 1993. Unidentified Toxoplasma-like tissue cysts in the brains of three
cats. Vet Parasitol 45:319-321.
Dubey JP, Fenner WR. 1993. Clinical segmental myelitis associated with an unidentified
Toxoplasma-like parasite in a cat. J Vet Diagn Invest 5:472-480.
Figure 1-15. Toxoplasma-like organism. Brain of a cat with a large cyst of this Toxoplasma-
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Feline Clinical Parasitology – Chapter 1
Figure 1-16. Toxoplasma-like organism. Higher power view of the cyst in which can be seen
the many bradyzoites that are present.
Figure 1-17. Toxoplasma-like organisms. Electron micrograph of the Toxoplasma like
organism showing the many micronemes and rhoptries present in this organism.