Concentration of Toxoplasma gondii in the Brain Tissue of Animals: A Histological Study Confirmed by Biological Isolations

Abstract

Eighty to 97% of the cysts of Toxoplasma gondii in brain tissues of mice, rats, rabbits, and pigs are distributed in the forebrain, i.e., olfactory lobe and cerebrum. Forebrain tissue selectively is a better material for isolation of the parasite by the mouse inoculation method, and correlates well with the dye test titers of the host in case of an active infection. Serologic disagreements with biological isolation in earlier reports could be due to (1) long lasting dye test antibodies without any evidence of active infection and/or (2) failure to obtain proper postmortem or biopsy material for isolating the parasite in active, acute, or chronic infection despite the dye test titer. Isolation of toxoplasma from the tissues of men and animals provides the best evidence of infection. Although the Sabin-Feldman methylene blue dye test (D.T.), the complement fixation test, and the dermal sensitivity test, are of value they do not necessarily indicate continuing infection. Isolation of Toxoplasma gondii from a suspected case may be attempted by obtaining biopsy material, inoculating laboratory animals, and recovering the parasite from the inoculated animals. The most important source of infective material for this purpose is the brain (Jacobs et al., 1952; Eyles et al., 1957). Various strains of T. gondii are conventionally maintained over extended periods of time in laboratory animals by serial passage of whole brain tissue suspension from previously infected animals. The RH strain of T. gondii which is lethal to mice but not to rats can be recovered from the infected rat's brain tissue for about 2 years (Ruchman and Fowler, 1948). The postmortem diagnosis of toxoplasmosis in man and large animals is generally made by inoculation of their brain tissues into mice and recovery of the parasite. However, use of whole brains of man or large animals for isolation of the parasite presents a problem. In most instances a small postmortem sample has been used. None of the 30 human brains obtained at postmortem by Krause (1955) were Received for publication 26 October 1966. * Study supported by Medical College of Georgia Professional Research Fund. t The Continental Research Institute, 25 Cedar Street, New York, New York 10038. t Medical College of Georgia, Augusta, Georgia 30902. positive for the parasite. He used a 5-g random sample of brain tissue from each patient tested. Walls et al. (1963) in their attempts to isolate Toxoplasma gondii from 15 human brains were unsuccessful in 14 cases. The only positive isolation was one in which the entire organ was used. They, therefore, suggested that the smaller samples may have missed areas which harbored a few cysts. These observations stimulated us to make a systematic study of the brains of experimentally infected animals to ascertain the area most likely to con-