CHAPTER I INTRODUCTION
VI. Brain
histopathologic damage in the diaphragm compared to control mice.
The presence of tissue cysts in the diaphragm at WK-3 PE is in congruence with earlier findings (Dubey, 1997b; Dubey, 1996;
Feldman, 1994). Moreover, Folkers (1974) and Frenkel (1973) noted presence of parasites in the spleen and other lymphoid organs for two weeks, in the liver and lungs for 10 weeks. Once immunity develops, they find their way into the muscles and brain where they form tissue cyst and may stay there for two years PE.
Plate 26. Histopathology in brain tissue. A & B. Normal brain tissue.
C & D. Gliosis (gli). E & F. Necrosis (nec) of the brain. Mg: A & E=100x, B, C & F=400x; D=40x
100µ 40µ
45µ 200µ
95µ 50µ
Plate 27. Histopathology of brain tissue. A. Normal brain tissue. B. Non- suppurative meningoencephalitis (nsm). C & D. perivascular cuffing (pc). Mg. 400x.
20µ 28µ
20µ 25µ
BTH+/RES. Intracellular parasites in tachyzoites and tissue cysts were detected in the brain of R.r.m.-BTH+ exposed mice at WK-4 PE (Plates 28A & B). All other dead mice showed no convincing brain tissue pathology.
Brain lesions become apparent as the parasites continue to multiply, and the lesion formation depends on the vulnerability/
sensitivity of brain tissue to lesion formation is due to its highly non-regenerative neurons. The observed cases of acute toxoplasmosis with necrosis, meningoencephalitis or mass lesions in thepresent study are manifestations similar to those earlier reported by Sandritter and Thomas (1997); Dubey et al. (1986) and Frenkel (1974). The cellular nodules formed in necrotic areas are essentially as granulomas comprising lymphocytes, plasma cells, histiocytes and glial cells (Dubey, 1995).
Tissue cysts were observed in the brain of R.r.m.-BTH+/RES at WK-4 PE, indicative of chronic phase of infection. Tachyzoite migration to the brain from the visceral tissues approximately 1-3 weeks have been earlier reported in mice, goat, sheep, horses and other mammals (Dubey and Beattie, 1988; Guimaraes et al., 1990;
Feldman, 1994) as tissue cysts. Slow proliferating parasites (=bradyzoites) are confined within thin-walled cyst to reduce parasite proliferation, and protect them from immunologic
Plate 28. T. gondii parasites (arrows) A. Tachyzoites. B. tissue cyst.
15µ 15µ
response of the host. The works of Kikuchi et al. (2001) and Petr et al. (2002) have provided information on the factors that can influence tissue cyst formation. Using virulent RH strain and avirulent ME49 strain, cyst formation was detected at WK-2 post- infection (PI), and the number of cysts remained constant until 56 days PI producing little or no inflammatory response. Mice exposed to RH strain-inoculated mice succumbed to death 5 days PI. The high mortality in mice exposed to RH virulent T. gondii strain was suggestive of the extensive proliferation of the virulent versus the avirulent or non-virulent strain (Kikuchi et al., 2002).
The detection of brain tissue cysts in R.r.m.-BTH/RES at WK-4 PE in the present study is in contrast to the findings of Feldman(1994) and Dubey and Beattie (1988), where tissue cysts were detected as early as 1-3 WK PE. However, in view of the presence of tissue cysts in the cardiac muscle and diaphragm as early as WK-3 PE, and the less severe histopathology in the organs of R.r.m.-BTH+-exposed mice, the T. gondii strain isolated from R.
rattus mindanensis may represent a non-virulent or low-virulent form compared to R. norvegicus. The absence of tissue cysts in all the organs of sero+ R. n./BTH+ is suggestive of a highly proliferative T. gondii, making it difficult on the host to mount a sufficiently- strong humoral and cell-mediated host response to slow down parasite proliferation as early as WK-4 PE. Extending the
duration of infection beyond WK-4 PE may result in either tissue cyst formation or host death.
CHAPTER V
SUMMARY, CONCLUSIONS AND RECOMMENDATIONS A. Summary of Findings:
Rat seropositivity was 55.0% in 157 rats caught in the agricultural (AGR), commercial (COM) and residential (RES) sites. Sixty percent (60.0%) for R. norvegicus (anti-T. gondii Abs titre: 1:126-1:2048) and 50.0% for R. rattus mindanensis (anti-T. gondii Abs titre:1:126-1:1024).
Sixty-six percent (66.0%) of the seropositive (sero+) R. norvegicus and 34.0% of R. rattus mindanensis were chronically infected.
Ninety-three (=64.0%) of brain tissue homogenate (BTH+)- inoculated mice were sero+. Seropositivity was significant (p≤0.05) in mice exposed to 0.75-1.0ml BTH+. Regardless of the rat species used as source of BTH+, seropositivity was significantly lower (p≤0.05) in BTH+/RES-inoculated mice relative to those exposed to BTH+/AGR and COM. Toxoplasma gondii parasites were detected in 15 (=16.0%) of the blood smear and 7 (=8.0%) of intraperitoneal exudates smears of R.n. and R.r.m.-BTH+-inoculated mice within WK-1 post- exposure (PE) and prior to euthanization. Manifestations of T. gondii- associated physical and behavioral changes were hunching, ruffled hair, wasting, loss of appetite, loose bowel movement, sensitivity to touch, and withdrawal. Weight change per week in sero+ mice was
significantly (p≤0.01) lower compared to sero- and control mice.
Percent mortality was 11.0% in sero+ mice.
Histopathologic conditions such as inflammatory cell infiltration, vasculitis and congestion and dilatation of the blood vesels were consistently manifested by the organs examined. As early as WK-1 PE, histopathologic manifestations were noted in the lungs, liver, spleen, heart, diaphragm and brain, particularly pronounced in the lungs and liver. With the progression of infection to WK-4 PE, tissue damage became more evident in the spleen, heart, diaphragm and brain.
Histopathologic manifestations consisted of interstitial pneumonia, atelactasis of the alveoli and pulmonary congestion due to hemorrhages;
hepatic congestion, necrosis of the liver tissue and cells; hyperplasia of the macrophages in the spleen and necrosis; myocarditis of the heart;
myositis of the diaphragm; and non-suppurative meningoencephalitis of the brain, gliosis and perivascular cuffing. Tissue cysts were found in the heart, diaphragm and brain as early as WK-3 PE in sero+ mice inoculated R.r.m.-BTH+/RES. In general, histopathology in mice exposed to BTH+ of R. norvegicus and R. rattus mindanensis caught in AGR and COM sites was more severe relative to mice exposed to BTH+/RES.