Placental thrombosis in acute phase abortions during experimental Toxoplasma gondii infection in sheep

Abstract

After oral administration of ewes during mid gestation with 2000 freshly prepared sporulated oocysts of T. gondii isolate M4, abortions occurred between days 7 and 11 in 91.6% of pregnant and infected ewes. Afterwards, a further infection was carried out at late gestation in another group of sheep with 500 sporulated oocysts. Abortions happened again between days 9 and 11 post infection (pi) in 58.3% of the infected ewes. Classically, abortions in natural and experimental ovine toxoplasmosis usually occur one month after infection. Few experimental studies have reported the so-called acute phase abortions as early as 7 to 14 days after oral inoculation of oocysts, and pyrexia was proposed to be responsible for abortion, although the underline mechanism was not elucidated. In the present study, all placentas analysed from ewes suffering acute phase abortions showed infarcts and thrombosis in the caruncullar villi of the placentomes and ischemic lesions (periventricular leukomalacia) in the brain of some foetuses. The parasite was identified by PCR in samples from some placentomes of only one sheep, and no antigen was detected by immunohistochemical labelling. These findings suggest that the vascular lesions found in the placenta, and the consequent hypoxic damage to the foetus, could be associated to the occurrence of acute phase abortions. Although the pathogenesis of these lesions remains to be determined, the infectious dose or virulence of the isolate may play a role in their development.

Figure 1

Mean rectal temperature following T. gondii oocysts challenge. Infected ewes, either showing abortion of not, showed elevation of rectal temperature between days 4 and 7 pi, with a peak of fever, over 40.5°, at day 6. Mean temperatures of control animals remained under 39.5° for the whole experiment.

Figure 2

Histological lesions of placenta and foetuses from infected ewes. a: Coronal section of a placentome from an acute case abortion. Several triangular areas of coagulative necrosis, involving both caruncular and cotyledonary villi (dotted line). HE. Bar = 2 mm. b: Thrombosis in the placenta of an acute abortion case. The thrombus is partially occluding the lumen of the vessel in the caruncular septa. HE. Bar = 100 μm. c: The presence of fibrin forming the thrombus is shown by the red staining in a serial section from the same placentome shown in b. MSB. Bar = 100 μm. d: Placentome from an acute abortion case. Detail from the margin of an infarct. The picture is divided in two areas by a caruncular villi showing congestion of the septal vessels (asterisk). At the left side there is abundant cellular debris between the denuded caruncular and foetal villi (white arrowhead). Caruncular villi in this area are necrotic, while cotyledonary villi show mesenchymal hyalinization. Note the relatively spare cotyledonary villus at the right side of the picture, with clear mesenchyme and intact trophoblasta layer (black arrowhead), and also the caruncular villus at the right superior corner with no evident lesions. HE. Bar = 200 μm. e: Area of leukomalacia (dotted line) in the white matter of a foetal brain from an acute phase abortion. Note numerous axonal spheroids (arrowhead) within the focus of malacia. HE. Bar = 100 μm. f: Necrotic in the placentome of an infected ewe culled at 21 dpi. The lesion affects both caruncular and cotyledonary villi. Bar = 25 μm. g: Focus of coagulative necrosis in the foetal white matter from an infected ewe culled at day 21 pi. Note the abundant infiltration of glial cells surrounding the necrotic focus. HE. Bar = 100 μm.

Figure 3

Kinetics of the antibody production in infected and control ewes. Infected animals showed an increase on serological antibodies from day 16 pi. Differences between ewes suffering or not acute phase abortions were not significant.

Figure 4

Kinetics of the specific IFN-γ release by PBMC stimulation assay against T. gondii antigen. Although differences between ewes suffering or not acute phase abortions were not significant, the peak of IFN-γ release appeared sooner in the former than in the latter. Values of the O.D. index in the control group remained low over the whole experiment.