Toxoplasma gondii: from animals to humans

Infections by the protozoan parasite Toxoplasma gondii are widely prevalent world-wide in animals and humans. This paper reviews the life cycle; the structure of tachyzoites, bradyzoites, oocysts, sporocysts, sporozoites and enteroepithelial stages of T. gondii; and the mode of penetration of T. gondii. The review provides a detailed account of the biology of tissue cysts and bradyzoites including in vivo and in vitro development, methods of separation from host tissue, tissue cyst rupture, and relapse. The mechanism of in vivo and in vitro stage conversion from sporozoites to tachyzoites to bradyzoites and from bradyzoites to tachyzoites to bradyzoites is also discussed.

Structures of Toxoplasma gondii Tachyzoites, Bradyzoites, and Sporozoites and Biology and Development of Tissue Cysts

In this paper the history of Toxoplasma gondii and toxoplasmosis is reviewed. This protozoan parasite was first discovered in 1908 and named a year later. Its medical importance remained unknown until 1939 when T. gondii was identified in tissues of a congenitally infected infant, and veterinary importance became known when it was found to cause abortion storms in sheep in 1957. The discovery of a T. gondii specific antibody test, Sabin-Feldman dye test in 1948 led to the recognition that T. gondii is a common parasite of warm-blooded hosts with a worldwide distribution. Its life cycle was not discovered until 1970 when it was found that felids are its definitive host and an environmentally resistant stage (oocyst) is excreted in feces of infected cats. The recent discovery of its common infection in certain marine wildlife (sea otters) indicates contamination of our seas with T. gondii oocysts washed from land. Hygeine remains the best preventive measure because currently there is no vaccine to prevent toxoplasmosis in humans.

The history of Toxoplasma gondii--the first 100 years.

SYNOPSIS. The life cycle of Toxoplasma gondii is described from cats orally inoculated with Toxoplasma cysts. Five new structural stages of Toxoplasma designated as “types” A-E were found in the epithelial cells of the small and large intestine. Type A is the smallest of all 5 intestinal Toxoplasma types. It occurs as collections of 2-3 organisms in the jejunum 12–18 hr after infection. Type B organisms are characterized by a centrally located nucleus, a prominent nucleolus and dark blue cytoplasm giving rise to the appearance of bipolar staining with Giemsa. Type B occurs 12–54 hr after infection and appears to divide by simple endodyogeny and by multiple endodyogeny (endopolygeny). Type C organisms are elongate with subterminal nuclei and strongly PAS-positive cytoplasm. They occur at 24–54 hr and divide by schizogony. Type D organisms are smaller than type C and contain only a few PAS-positive granules. They occur from 32 hr to 15 days after inoculation and account for over 90% of all parasites in the small intestine during this time. Three subtypes divide by endodyogeny, schizogony and by splitting of their merozoites from the main nucleated mass without leaving a residual body. Type E organisms resemble one of the subtype D which divide by schizogony, but they leave a residual body. They occur from 3–15 days after inoculation.

Gametocytes occur thruout the small intestine but more commonly in the ileum 3-15 days after infection. Male gametocytes contain on an average of 12 microgametes and comprise 2–4% of the gametocyte population. The prepatent period after cystinduced infection is 3–5 days with the peak oocyst production between 5–8 days and a patent period varying from 7–20 days.

Variable numbers of trophozoites are present in the lamina propria of the small intestine and in the extra-intestinal tissues within a few hr after inoculation. After 9–10 days cysts were seen in the heart and later in the brain.

The lesions of toxoplasmosis are compared in newborn and weanling kittens and in adult cats after oral and subcutaneous inoculation with cysts. After the ingestion of cysts, newborn kittens developed enteritis, hepatitis, myocarditis, myositis, pneumonitis and encephalitis and were moribund by the 9th day. Kittens aged 2 weeks and older developed enteritis, myocarditis, encephalitis and myositis but often survived; adult cats usually remained asymptomatic. After subcutaneous inoculation of cysts, newborn and weanling kittens died of acute toxoplasmosis with severe pneumonia, myocarditis, encephalitis and hepatitis.

Cyst-induced toxoplasmosis in cats

Lymph nodes of the neck.

Earlier work in which the coccidian nature of the protozoan parasite Toxoplasma gondii was revealed has been confirmed and extended in a series of experiments using only specific pathogen free (SPF) cats. All of these cats, prior to our experiments, were negative for both Coccidia and Toxoplasma. Experiments on 17 of these cats are described. 10 were fed with mouse brains containing tissue cysts of Toxoplasma. 8 of these shed Toxoplasma oocysts in their faeces within 5 to 10 days of infection. 2 cats had shed no oocysts before they were killed on the 3rd and 4th day after infection. 5 cats were fed with Toxoplasma oocysts but gametogonic development resulting in the production of oocysts was observed in only 1 individual; oocysts appeared in the faeces 9 days after the infective meal. 2 cats were examined as controls; 1 received no addition to its diet, the other was fed “negative” brains. Neither showed any evidence of infection. Examination of the gut of infected cats showed that shizogonic and gametogonic stages develop usually in the ileum although the entire length of the small intestine can be affected. The parasites were observed in the epithelium of the tips of the villi, developing within vacuoles, and lying between the nucleus and brush border of the cell. All stages of schizogony and gametogony may be found together, occurring in no particular sequence along the epithelium of the villus; multiple infections of the host cells are common. Schizonts are characterized by granular, basophil cytoplasm and vesicular, karyosomic, Feulgen-negative nuclei which become dense and Feulgen-positive when merozoite formation takes place. The merozoites vary from 4 to 29 per schizont and measure 3·5-4·5μ by 1μ in sections; in smears their dimensions are 4·9 (SD = 1·0)μ by 1·5μ. They form clusters about a residual body and may appear to radiate fan-wise from it in sections. The microgametocyte is spherical, about 10μ in diameter and gives rise to 12–32 microgametes which form on its surface, are slender and crescentic and about 3μ in length, excluding the flagella. The macrogametocyte has, throughout development, a vesicular, karyosomic and Feulgen-negative nucleus. At the commencement of development, the cytoplasm contains many basophil and few PAS-positive granules; at its completion this condition is reversed. No granules were seen to be associated with the formation of the oocyst wall. The oocyst itself measures 12·7 (SD = 0·7)μ by 10·4 (SD = 0·8)μ. Sporulation, which does not occur within the gut of the cat, gives sporocysts measuring 7·8 (SD = 0·9)μ by 6 (SD = 0·5)μ. These coccidian stages of Toxoplasma are compared and contrasted with those of Isospora spp. of the cat. Despite its significant resemblances to Isospora, the retention of the existing nomenclature of Toxoplasma gondii is supported.

The life cycle of the coccidian parasite, Toxoplasma gondii, in the domestic cat

Toxoplasma gondii undergoes schizogony and microgametogony in the epithelial cells of the cat ileum. On the basis of cytoplasmic structures, it is impossible to distinguish between the early stages of these processes but nuclear differentiation appears to have occurred. In the early schizont the chromatin is distributed in small patches throughout the nucleus but in the nucleus of the microgametocyte the chromatin is condensed into a few large areas. In the schizont, the first distinguishing cytoplasmic features are the appearance of the dome-shaped membranes of the merozoite anlagen. In the schizogonic process, nuclear division precedes merozoite formation which occurs by multiple internal budding in a manner similar to endodyogeny. The appearance of flagella is the first cytoplasmic feature to distinguish the microgametocyte. Microgamete formation is similar to that reported for other coccidian species. The microgamete consists of a dense elongate nucleus anterior to which is a single mitochondrion. The anterior consists of a perforatorium and two flagella between which 4 microtubules run longitudinally.

J.F. Dunachie

Papers

The life cycle of the coccidian parasite, Toxoplasma gondii, in the domestic cat

Ultrastructural study of early stages of asexual multiplication and microgametogony of Toxoplasma gondii in the small intestine of the cat.

Endogenous development of the coccidian parasite Toxoplasma gondii.

Cats as a source of toxoplasmosis.