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This report describes the presence of Isospora belli unizoic cysts in mesenteric lymph nodes and of gametocytes in the gallbladder epitelium of a 26 year-old Brazilian male patient with Acquired Immune Deficiency Syndrome. This patient had received treatment for several times with sulfamethoxazole-trimethoprim. It is discussed the significance of I. belli tissue cysts as possible foci of resistance of the parasite and their association with the infection relapse even post-treatment with anticoccidian medication.

Infection and immunity to toxoplasmosis induced by the RH strain of Toxoplasma gondii was compared in Sprague-Dawley (SD) and Wistar rats and in outbred Swiss Webster mice. All rats injected with up to 1,000,000 RH-strain tachyzoites remained clinically normal, whereas mice injected with only 1 live tachyzoite died of acute toxoplasmosis. Rats could be infected with 1 tachyzoite of the RH strain as shown by antibody development and by bioassay in mice. However, after 8 days, RH-strain organisms were recovered only inconsistently from SD and Wistar rat brains. Contrary to a report of sterile immunity to T. gondii infection in rats after immunization with live RH tachyzoites, we found infection immunity after challenge with the VEG strain. Toxoplasma gondii tissue cysts of the VEG strain could be recovered from most SD and Wistar rats, first injected with live RH-strain tachyzoites and then challenged with oocysts of the VEG strain. Our RH strain, and probably many others, passed for 50+ yr as tachyzoites has lost not only the capacity to form oocysts, but also shows a marked reduction or absence of tissue cyst (bradyzoites) formation.

Twenty-two HIV+ patients with encephalitis were studied. Of these, 7 had meningoencephalitis due to Toxoplasma gondii (MT) and 15 due to Trypanosoma cruzi (MC). Pathologic and computerized axial tomography (CAT) changes were compared. We found that focal necrotizing encephalitis due to Toxoplasma involved the cerebral cortex and the basal ganglia, whereas lesions due to Trypanosoma cruzi were centered in the white matter, sometimes extending into the cortex. Hemorrhages, myelin lesions and organisms were more pronounced in chagasic than in toxoplasmic encephalitis. These findings are consistent with the literature reviewed.

A cohort of more than 500 children from Panama City, Panama was studied prospectively over five years for acquisition of antibody to Toxoplasma gondii. The direct agglutination test showed that 72 of 571 children seroconverted between one and six years of age, for a cumulative incidence of 12.6%. Children were examined by pediatricians quarterly, and illnesses that had occurred in the interval and their activities were noted on questionnaires. Thirty-eight variables were examined for their role as risk factors for seroconversion. There was a higher correlation between children's seroconversion and contact with dogs than with cats. Combinations of significant predictors without dogs explained only 67% of the seroconversions, but the same factors with dogs explained 90%. On the other hand, ingestion of raw or rare meat or eggs appeared to play no role in transmission. Cats were examined and 110 (45.6%) of 241 had Toxoplasma antibody on the first bleeding. Only two (0.5%) of 383 cat fecal specimens, when tested in mice, resulted in seroconversion. Ten (1.1%) of 924 soil samples resulted in seroconversion in mice that had been injected. Antibody to Toxoplasma was found in 52 (23.3%) of 226 rats (Rattus norvegicus) and two (0.035%) of 571 mice (Mus musculus). Two hundred sixteen birds of 16 different species were bled. Antibody to Toxoplasma was found in 13.4% of these birds, mostly in grackles, blue-gray tanagers, and doves. The rate of isolation of Toxoplasma was low: one of 23 in rats and three of 201 in birds. High relative risks (RRs) of transmission to children were predicted by contact histories with nursing dogs (RR = 5.8), weaned dogs (RR = 4.7), many flies (RR = 3.6), 6-12-month-old dogs (RR = 3.4), weaned cats (RR = 3.0), 6-12-month-old cats (RR = 2.7), nursing cats (RR = 2.5), much garbage (RR = 2.4), and many roaches (RR = 2.2). The high statistical correlation of dog contact with seroconversion in children suggests the possibility that dogs, by eating and rolling in cat feces, are instrumental in mechanically transmitting Toxoplasma infection. In addition, flies, and to a lesser extent, cockroaches, may have practically important roles in transmission.

We conducted a survey of 760 Amerindian children 2-12 years of age in the Bayano and San Blas areas of Panama in 1991 to determine the prevalence of serum antibodies to Toxoplasma gondii and the importance of hypothesized risk factors in human-induced native and sylvatic conditions, which have had few environmental changes, as opposed to rural and urban areas in Panama previously studied. The overall prevalence of infection ranged between 0% and 42.5%. No age curve was detected, indicative of nonconstant transmission. Only two hypothesized risk factors, floor type and having cats inside the house, were significantly associated with the presence of antibodies in some of the communities. Antibody prevalence appeared to be associated more with the community of residence than with any specific behavior. The risk factor of importance may be the level of oocyst contamination, since infection by tissue cysts in meat was excluded. On three of the nine islands studied, no antibody was detected in the children or the cats. It would appear that T. gondii is not present on these islands. Although the data did not support the importance of many of the hypothesized risk factors, the study is consistent with the theory of transmission by oocysts and the importance of cats in transmission.

We describe a variety of toxoplasmic lesions in seven patients with the acquired immunodeficiency syndrome. The first patient had multiple small-intestinal ulcers associated with Toxoplasma tachyzoites and high antibody titers; he died of disseminated histoplasmosis. The second patient, who died of tuberculosis, also had an inactive chronic Toxoplasma infection, with tissue cysts in the brain that were associated with glial nodules. A third patient died of Toxoplasma encephalitis, manifested by multiple foci of necrosis associated with Toxoplasma tachyzoites, cysts, and hypertrophic arteritis. A fourth patient had been treated for toxoplasmic encephalitis with co-trimoxozol (trimethoprim-sulfamethoxazole combination) for 3 to 4 days and showed degenerating tachyzoites associated with necrotic areas. A fifth patient, treated for toxoplasmic encephalitis with co-trimoxazol for 14 days, had necrotic lesions associated with Toxoplasma antigen and a few cysts. A sixth patient with encephalitis and Toxoplasma tachyzoites and young cysts in the biopsy showed healed brain lesions after 22 days of treatment. A seventh patient, diagnosed radiologically and serologically with Toxoplasma encephalitis, was treated for 7 months; his ring-enhancing lesions subsided, and he died of a central nervous system lymphoma. Toxoplasma could not be isolated from the brain, although toxoplasmic DNA was detected in the brain and heart by polymerase chain reaction. The pathogenesis of the range of these lesions, their diagnosis, and the possibility of terminating Toxoplasma infection by prolonged chemotherapy are discussed.

Sera from 273 wild mammals from Missouri and Kansas (USA), collected between December 1974 and December 1987, were tested for the presence of antibodies to Toxoplasma gondii using the Sabin-Feldman dye test. Sixty-five (24%) had antibodies at titers of > or = 1:8, including 38 (66%) of 58 carnivores, 14 (15%) of 94 omnivores, 13 (11%) of 117 herbivores, and none of four insectivores. The prevalence of antibodies in mice (Mus musculus and Peromyscus spp.) and rats (Rattus norvegicus and Sigmodon hispidus) was low (3%), while medium sized herbivores such as squirrels (Sciurus spp.), rabbits (Sylvilagus floridanus), and muskrats (Ondatra zibethicus) had prevalences of about 18%. Red foxes (Vulpes fulva) and mink (Mustela vison) had the highest prevalence of antibodies with frequencies of 90 and 66%, respectively. In 32 attempts to isolate Toxoplasma gondii from wild mammals with positive (> or = 1:4) titers, only six (19%) were successful: a gray squirrel (Sciurus carolinensis), a beaver (Castor canadensis), an opossum (Didelphis marsupialis), a red fox and two mink. These findings are consistent with the hypothesis that the probability of infection with Toxoplasma gondii, and therefore prevalence of antibodies in wildlife, is greatest in carnivores.

Old World psittacines experienced an acute fatal illness in outdoor breeding collections in South Florida. Toxoplasma-like organisms were found histologically in pulmonary capillaries and elsewhere. Because the organisms underwent schizogony and could not be transmitted to mice, we looked for a cause other than Toxoplasma gondii. An opossum was trapped on the premises of 1 facility and was found to be shedding sporocysts similar to Sarcocystis falcatula in its feces. Cockroaches were prevalent and suspected as transport hosts. Cockroaches that had ingested opossum feces and subsequently were fed to cockatoos induced an identical fatal illness. Obstruction of pulmonary capillaries by developing schizonts and pulmonary edema were the most important pathologic findings. The epidemic was stopped by biological insect control employing flightless chickens to reduce cockroach populations and by an electric fence restricting access of opossums to these outdoor psittacine breeding facilities.

A 24-year-old homosexual man with acquired immunodeficiency syndrome presented with intractable diarrhea and fever. Examination of a rectal biopsy specimen and stool revealed Cryptosporidium. Approximately 4 months after admission he developed respiratory failure and died. Postmortem examination revealed cryptosporidiosis involving the entire gastrointestinal tract as well as the tracheobronchial tree. To our knowledge, this is one of the rare presented cases of tracheobronchial cryptosporidiosis documented histologically.

The capacity of hamster peritoneal cell populations to control viability and growth of Besnoitia and Toxoplasma organisms was assessed in vivo and in vitro. Immunized hamsters reduced the homologous organisms 100- to 10,000-fold over a 5-day period, but the heterologous infection increased 100- to 1,000-fold in numbers, similar as in the nonimmune controls. Passively administered antibody was ineffective although lytic cofactors were supplied by hamsters. In cultures, peritoneal cells from Besnoitia-immune hamsters delayed the growth of homologous parasites to an average of 38.5 h per division; however, in Toxoplasma-immune and nonimmune cells, Besnoitia divided every 12.8 h. Specificity of immunity was pronounced against both infections. With cross-infections, Toxoplasma-immune cultures did not effectively delay Besnoitia growth; however, Besnoitia-immune cultures reduced Toxoplasma growth by one-half. Co-cultivation experiments demonstrated that specifically committed lymphocytes could instruct macrophages to reduce the homologous organism 10-fold, whereas heterologous organisms were reduced only 2-fold. Lymphocyte supernatants initiated hypersensitivity as indicated by macrophage activation and giant cell formation in culture. However, these supernatants did not transfer infection immunity. Lymphokines could account for the hypersensitivity phenomena, but cell-mediated infection immunity in this model required close lymphocyte-macrophage proximity. These studies indicate that a number of distinct processes including delayed hypersensitivity, macrophage activation, and specific cellular immunity are acting simultaneously during latent Besnoitia infection of hamsters. All three processes are mediated by lymphoid cells and appear to be specifically induced. Although activated macrophages develop some heightened nonspecific capabilities, these were several orders of magnitude below the specific effects.

Mice were immunized with live organisms of the different stages (i.e., tachyzoites, bradyzoites, or sporozoites) of Toxoplasma gondii, or with killed tachyzoites with or without adjuvants. The adjuvants used were liposomes, anhydrides of myristic or lauric acid, levamisole and Freund's complete or incomplete adjuvant. The following strains of T. gondii were used: RH, M-7741, the nonpersisting, temperature-sensitive mutants ts-1, ts-4, or ts-5, and the "back mutant" of ts-1 (Pfefferkorn and Pfefferkorn, 1976). The protection afforded was measured by challenge with the pathogenic M-7741 strain. Killed tachyzoites alone, or with adjuvants, offered only slight protection against challenge with M-7741 and no protection against challenge doses that were lethal to all control mice. Chronic infection and live nonpersisting vaccines conveyed a strong immunity to challenge, except strain ts-1. Because it was less pathogenic and did not require chemoprophylaxis, strain ts-4 best fulfilled the requirements for a good vaccine; its effect in hosts other than the mouse remains to be determined. The immunity induced by tachyzoites, bradyzoites, or sporozoites appeared equally strong when challenged with sporozoites.

Development of immunity to the shedding of oocysts was examined in 75 kittens that survived infection with the three stages of Toxoplasma gondii. Of 16 kittens fed bradyzoites in cysts, 94% were immune and did not shed oocysts. Of seven injected with tachyzoites 86% were immune. Of 18 fed sporozoites only 11% were immune, but following injection, 54% of 12 were immune. After the administration of either bradyzoites or tachyzoites from nonoocyst-producing strains, only 9% of 22 were immune. Considering all inocula, immunity was present in 93% of kittens that had previously shed oocysts, 25% of those that only develop antibody, and none that had neither shed nor developed an antibody titer. After a second challenge with a different isolate, a similar percentage of immunity was observed. Infection with killed tachyzoites, alone, or together with Freund's complete or incomplete adjuvants was followed by immunity in only one of 24 kittens. Eighty-five percent of 13 kittens were immune, after they had been treated prophylactically with 200 mg/kg monensin, or 60 mg/kg cat sulfadiazine combined with 1 mg/kg cat of pyrimethamine; oocyst shedding had been suppressed in all. It is concluded that cats can be immunized against oocyst shedding by infections where oocysts are produced, or where developmental stages are suppressed by chemoprophylaxis, but not if enteroepithelial stages are absent, as in the oocyst-less strain examined.

Immunity to the intracellular protozoan, Toxoplasma gondii, in mice is of the premunition type. Chemoprophylaxis with sulfadiazine normally permits mice to develop immunity to virulent organisms. However, nude (nu/nu) mice, which lack the thymus, failed to develop immunity to toxoplasma during 3 weeks of drug therapy while their hirsute littermates developed immunity during this period. When nude mice were injected intraperitoneally with thymus cells from hirsute littermates, they became able to develop immunity to toxoplasma during drug prophylaxis. The injection of bone marrow cells or high-titered specific antibody did not prolong survival after sulfadiazine was discontinued. Therefore, it appears that immunity to toxoplasma in mice is dependent upon active cellular immunity while the role of antibody is uncertain.

Predominant muscle parasitism, and an obligatory two-host cycle (cat-mouse-cat), distinguishes an otherwise similar organism from Toxoplasma. The presence of multiplicative stages in the cat gut separate it from Sarcocystis. Antibody that cross reacts with Toxoplasma antigen is developed in mice and other experimental intermediary hosts, but not in cats, the final host. Recognition of the two-host cycle is essential for the experimental isolation and transmission of the parasite, and for prevention of the infection.

Sarcocystis muris, obtained from laboratory and house mice, was transmitted experimentally to cats. When cysts in the skeletal muscle of mice were ingested by cats, bradyzoites developed directly into gametes in the intestinal mucosa of the cats. After five to 11 days, infectious sporulated sporocysts (8.5 x 10.3 mum) were shed by cats for up to three months. After ingestion of the sporocysts, S. muris multiplied first in the livers and then in the skeletal muscle of mice. Cysts in skeletal muscle attained a length of 5-6 mm and were visible to the naked eye. With heavy infections the cysts outlined major muscle groups. Lesions in mice were apparently the result of degeneration or rupture of cysts, with myositis and sometimes muscle necrosis, leading to locomotor impairment in heavy infection. S. muris required approximately 76 days of development in mice before becoming infectious for cats. S. muris is perpetuated by an obligatory two-host cycle (mouse-cat). Both cats and mice could be infected repeatedly. Rats, hamsters, and guinea pigs did not become infected.

Certain temperature-sensitive mutants of the RH strain of Toxoplasma gondii and a "back mutant," all maintained in human fibroblasts, were studied in mice. Most mutants gave rise to acute, fatal infections, and after sulfonamide prophylaxis rarely persisted as chronic infections in mice. However, the ts-4 strain was nonfatal in doses up to 10(3) to 10(5) tachyzoites, elicited high titers of antibody, and did not persist beyond 2 mo. No Toxoplasma cysts were found. There was no evidence that a febrile reaction of the mice was restrictive, because the highest temperatures, 37.9 to 38.4 C, occurred 3 days after infection, whereas the organisms were recoverable for 16 to 32 days. Because doses of Toxoplasma, survived by 11 of 12 normal BALB/c mice, and by one of five thymic transplanted athymic mice, killed six of six athymic mice, it appears that the limited persistence of ts-4 is related to the immunologic response.

The role of the cat (Felis domestica) as a definitive host for Sarcocystis cuniculi of European rabbits (Oryctolagus cuniculus) was confirmed. It was shown that after dosing with sporocysts from cats, rabbits developed sarcocysts and these became infective for cats at not less than 93 days post-infection (p.i.). The earliest infection detected was at 142 days p.i. Infected muscle from an experimental rabbit did not transmit Sarcocystis when fed to other rabbits. Microscopically, sarcocysts in European rabbits (O. cuniculus) were morphologically indistinguishable from those in cottontail rabbits (Sylvilagus floridanus).

Specific immunity developed by mice against protozoan (Toxoplasma gondii and Besnoitia jellisoni) and bacterial (Listeria monocytogenes) infections was compared with nonspecific protection conferred by prior infections. The results indicated that homologous immunity protected mice from more than 10-5 LD50 of T. gondii or B. jellisoni, but from only 10-2 LD50 of L. monocytogenes. Heterospecific protection among these organisms was for 10-0.4 minus 10-1.2 LD50. In studies in hamsters specific immunity to protozoan (T. gondii and B. jellisoni) and viral (equine Herpesvirus type 1 and Oriboca virus) infections was compared with nonspecific protection conferred by prior infections with several heterospecific agents: T. gondii; B. jellison; equine Herpesvirus type 1; Oriboca, Ossa, vesicular stomatitis, yellow fever, and Newcastle disease viruses; L. monocytogenes; and the bacillus Calmette-Guerin strain of Mycobacterium tuberculosis. The results indicated that homologous immunity in hamsters was effective against 10-6 minus 10-7 LD50 of T. gondii, B. jellisoni, equine Herpesvirus type 1, or Oriboca virus. Prior infection with Newcastle disease virus protected (probably by interferon induction) against 10-3 LD50 of equine Herpesvirus type 1. Heterospecific protection among other agents was for less than 10 LD50. This insignificant heterospecific protection in infections in which cellular immunity plays a role suggests that both the induction phase and the expression phase are specific.

Toxoplasma oocyst shedding by cats was suppressed by 0.02% monensin incorporated in dry cat food. Ten of 12 cats that had not shed oocysts were immune when reinfected with mice chronically infected with Toxoplasma. The medicated food, which was well accepted and tolerated by kittens, could be useful to minimize risks of infection for pregnant women and small children from pet cats defecating in soil close to homes. However, in the presence of stray cats a false sense of security may be engendered.

Mice challenged with a pathogenic strain of Toxoplasma gondii develop fatal infections. However, if such mice are initially treated with sulfadiazine (SD), they develop immunity and survive with chronic infections. The role of antibody (Ab) in establishing protective immunity against acute parasitemias and in maintaining chronic infections was investigated using B-cell-deficient (immunoglobulin M-suppressed), T-cell-deficient (athymic), and normal BALB/c mice. All mice not receiving SD treatment rapidly died (mean 7.5 days) after infection, but the majority (80%) of intact mice developed immunity during SD treatment and survived for over 5 months with chronic toxoplasmosis. Athymic mice rapidly died (mean 6.0 days) after the removal of SD treatment. Although all SD-treated immunoglobulin M-suppressed mice eventually died, they lived considerably longer (18 to 83 days) in the complete absence of antitoxoplasma Ab than unprotected mice (7 to 9 days). Histopathological sections of liver, lung, brain, and other tissues showed that toxoplasma organisms gave rise to fatal lesions in all nonsurviving animals. The injection of Ab into acutely infected and athymic mice imparted no protection, but transfer of antitoxoplasma Ab (titer greater than 1:8,000) to immunoglobulin M-suppressed mice after SD treatment resulted in elimination of the parasites in 50% of the mice. Results of this study suggest that Ab may not be decisive in acute infections, but may be important in controlling long-term toxoplasmosis.

Cysts of Sarcocystis muris develop within muscle cells and each is bounded by a parasitophorous vacuole membrane. Closely spaced spherical blebs formed from this membrane extend into the muscle cell cytoplasm. A dense substance fills the cavity of the bleb and occupies the vacuolar space immediately adjacent to the membrane. The remainder of the vacuole is filled with a moderately dense matrix within which the parasites develop. At 40 days after infection only metrocytes are present, characterized by their ovoid shape, lightly stained cytoplasm, amylopectin-like granules, and lack of micronemes. Metrocytes divide by a process resembling endodyogeny and eventually produce bradyzoites. By 78 days after infection, at which time the cyst is infective for cats, the few remaining metrocytes are located at the cyst periphery but most organisms are elongated and contain organalles characteristic for bradyzoites including micronemes, dense granules, and amylopectin. Structures indicative of division were not seen in bradyzoites. Rhoptries are few in number. Numerous vesicles of smooth endoplasmic reticulum accumulate in the cytoplasm of muscle cells adjacent to the periphery of the enlarging cyst but significant destruction of muscle fibers containing cysts with viable organisms was not seen in specimens fixed between 40 and 325 days after infection. Unusual lamellar structures were seen in some parasitized muscle cells and intracystic tubules occurred in some cysts.

To assess the roles of the German cockroach (Blatella germanica) and the American cockroach (Periplaneta americana) in the transmission of Sarcocystis muris and of 3 other coccidia of cats-Toxoplasma gondii, Isospora felis, and Isospora rivolta, cockroaches exposed to feces containing these coccidia were periodically fed to mice, as was a portion of the fecal matter. Sarcocystis muris sporocysts, which in feces remained infectious for at least 20 days, were also transmitted to mice by P. americana for at least 20 days and by B. germanica for 5 days after exposure to infectious feces. Toxoplasma gondii oocysts were transmitted by P. americana intermittently up to 10 days, but by B. germanica only immediately after exposure to feces. Oocysts of 2 species of Isospora, when associated with fecal matter, remained infectious for 20 days. Those of I. rivolta were transmitted by both cockroach species for 10 days, but I. felis was transmitted only by by B. germanica, and for only 2 days.

The inhibitory effects of cortisol on cellular immunity were studied in vitro by using hamster peritoneal exudate cells. Two obligate, intracellular protozoa--Toxoplasma gondii and Besnoitia jellisoni-- were used to control for specificity of effects. Results indicate that immune lymphocytes specifically confer immunity to (or "arm") macrophages that specifically express immunity. This arming can be inhibited by 5 microng of cortisol per ml. Macrophages that have been armed already will continue to express immunity (by limiting parasite growth specifically) in the presence of 5 microng of cortisol per ml. Cortisol levels of 20 microng/ml are required to inhibit the expression of immunity by armed macrophages. It was also found that lymphocytes, from hamsters given 20 mg of cortisol subcutaneously 2 days before the harvest of cells, did not arm macrophages, whereas macrophages from these same animals could be armed by immune lymphocytes from untreated hamsters. Therefore, it was concluded that in relation to cellular immunity, lymphocytes are more sensitive to cortisol than are macrophages. Since antibody to these parasites is almost always present in vivo, we also tested the effects of cortisol on the disposition of antibody-modified organisms by activated (not armed) macrophages, and found that 50 microng of cortisol per ml was needed to inhibit macrophage effects on antibody-treated organisms.

Lymphocytes from mice immunized against Toxoplasma gondii protected T. gondii-infected macrophage and kidney cell cultures. After contact with antigens, supernatants of such immune lymphocytes, also contained a factor protective for T. gondii-infected macrophages and kidney cells. Supernatants were protective only when the lymphocytes and kidneys cells were isogeneic. Protection was specific in that supernatants from only T. gondii-immune, but not Besnoitia jellisoni-immune, lymphocytes provided protection against toxoplasmosis. Sixteen to 24 h were required for an appreciable amount of protective factor to be secreted; a similar absorption time was necessary for kidney cells to be protected. Peritoneal lymphocyte lysates, prepared as transfer factor, contained protective substances with a potency similar to that of lymphocyte supernatants, which were also strain restricted in their effect.

We studied the specificity of protection conferred by Toxoplasma gondii immune lymphocytes and their supernatants on infected hamster kidney cells, using Besnoitia jellisoni immune lymphocytes as a nonspecific control. The intracellular growth of the organisms was measured by [3H]uracil incorporation, and inhibition of multiplication was used as a measurement of immunity. Although the immune lymphocytes restricted principally the multiplication of homologous organisms, partial protection, expressed against the heterologous organism, was found. This was true for either parasite with intact lymphocytes or their supernatants. Exposure of immune lymphocytes to antigen for 18 to 24 h and treatment of kidney cells with supernatant fluids for 18 to 24 h were required for maximal protection. The specific protective mediator in supernatants of immune lymphocytes was characterized by dialysis as having a molecular weight between 3,000 and 12,000 and was found in the 3,000 to 5,000 peak after Sephadex G-50 chromatography. Nonspecific protective activity was greater than 12,000 by dialysis; it chromatographed in the excluded peak, measuring over 43,000, and was destroyed by exposure to pH 2. In vitro production of lymphokines from toxoplasma immune lymphocytes was first detected 7 to 10 days after vaccination of hamsters. At about the same time, hamsters began to resist challenge infection with the pathogenic RH strain of T. gondii and were able to prevent its multiplication in lungs, liver, spleen, and the subcutaneous infection site. The expression of tissue immunity and the production of toxoplasma-immune lymphokines appear to be time-related events.

Two of five opossums examined from the Kansas City area were found to be infected with Besnoitia darlingi. Inoculation of cysts resulted in acute lethal infections in mice and hamsters. Chronic infections with development of tissue cysts were obtained in mice by prophylaxis with sulfadiazine. Cysts were fed to cats and a dog which resulted in the shedding of isosporoid oocysts by cats. The prepatent period was 11 to 14 days. Unsporulated oocysts averaged 11.9 by 12.3 micrometer and sporulated in 48 to 72 hr. Mouse-to-mouse tramsmission was achieved by injecting triturated tissue containing cysts. This is the third species of Besonitia to be cyclically transmitted by cats.

Isosporan oocysts, measuring 13 by 16 micrometers, from a cat in Hawaii produced Besnoitia cysts in tissues of mice and rats. Feeding these cysts to cats led to oocyst shedding after 11 to 13 days, continuing for a mean of 11 days. This indicates a two-host cycle for Besnoitia, adding an intestinal phase and oocyst production by a carnivore to the already known tissue stages. Thus a representative of Besnoitia, similar to other species in cattle, horses, reindeer, impala, other mammals, and reptiles, has been shown to be a coccidian of cats, capable of being spread by fecal contamination. Besnoitia is the fourth mammalian tissue parasite, together with Toxoplasma, Hammondia, and Sarcocystis, found to produce isosporan-type oocysts.