Ovine toxoplasmosis: transmission, clinical outcome and control

Toxoplasma gondii is a significant cause of abortion in sheep. Infection is picked up from the environment and if initiated during pregnancy may cause fetal mortality. Infected sheep remain persistently infected with tissue cysts in brain and muscle (meat), and are also immune and would not be expected to abort again. The live tachyzoite vaccine (Toxovax) protects against abortion and this allows the suggestion that it may also reduce or prevent tissue cyst development in muscle. If this were so it raises the question of whether the vaccine could be used to make meat safer for human consumption.

Characterization of the immune response in the placenta of cattle experimentally infected with Neospora caninum in early gestation

A serial examination of three groups of cattle infected intravenously (iv) (Group 1, n=8) or subcutaneously (sc) (Group 2, n=8) with live Neospora caninum tachyzoites or with VERO cells (Group 3, n=8) at 70 days' gestation was carried out and the nature of the inflammatory responses in the placenta and the presence of parasite antigen were analysed. Immune cells expressing CD3, CD4, CD8, gamma delta (gammadelta) T-cell receptors (TCR), CD79alpha cytoplasmic (cy) (B cells) and NKp46 [natural killer (NK) cells] antigens were identified immunohistochemically and cells expressing mRNA for interferon-gamma (IFN-gamma) were labelled by in-situ hybridization. Intravenous inoculation caused mortality in all fetuses from 28 days post-inoculation (dpi) onwards. Subcutaneous inoculation caused mortality in 50% of the animals by 28dpi. Pathological changes in the placenta consisted of necrosis of fetal placental villi, necrosis and inflammation in adjacent areas of the maternal septum and inflammation at the base of the maternal caruncle. The inflammatory infiltrate consisted mainly of CD3(+) lymphocytes, dominated by CD4(+) and gammadelta TCR(+) cells, with CD8(+) cells present to a lesser extent. The results from the control group indicated fewer NK cells than those occurring in the placenta of human beings or mice. Infiltration of CD4(+) cells and NKp46(+) cells was observed in the caruncular base and septa 14 days after infection, whereas infiltration of gammadelta TCR(+) cells was observed from 28 dpi onwards. To our knowledge this is the first report on the presence and distribution of NK cells in the bovine placenta. Maternal inflammatory cells expressing mRNA for IFN-gamma were identified in animals inoculated with parasites iv or sc at 14 and 28 dpi, respectively. In the sc-inoculated dams with live fetuses at 28, 42 and 56dpi, there was no evidence of parasite antigen, infiltration of immune cells or production of IFN-gamma, suggesting that the parasite had not reached the placenta. The exact cause of fetal death was not established. Tissue destruction by the parasite may have occurred; in addition, there may have been a T helper 1 (Th-1) immune response to the neospora infection at the materno-fetal interface, resulting in infiltrations of CD4T cells, gammadelta T cells and NK cells and the subsequent production of IFN-gamma. It is possible that a pro-inflammatory Th-1 response early in gestation protects the dam by eliminating the parasite; however, it may lead to destruction of the placental tissues themselves and thus be incompatible with fetal survival.

Role of endogenous transplacental transmission in toxoplasmosis in sheep

To investigate the potential role of endogenous transplacental transmission of Toxoplasma gondii, 31 seropositive ewes presumed to be persistently infected with the parasite and 15 seronegative ewes were mated and monitored throughout pregnancy and lambing. Antibody titres were determined in precolostral sera from the liveborn lambs and in thoracic fluid from the dead lambs. A PCR for the B1 gene of T gondii was applied to the placentas from all the ewes and to the brains of the stillborn lambs. Samples of brain, lung, liver, spleen and heart from the dead lambs were examined by histopathology. No evidence of toxoplasmosis was detected by histopathology or PCR in any of the samples, but low titres of antibody to T gondii were detected in two liveborn, healthy offspring of a seropositive ewe by the immunofluorescent antibody test (3.2 per cent of pregnancies and 4.1 per cent of lambs in the seropositive group). Antibody to specific antigens of T gondii was demonstrated in sera from these two lambs by Western blotting.

Placental pathology associated with fetal death in cattle inoculated with Neospora caninum by two different routes in early pregnancy

Pregnant cattle were inoculated with N. caninum strain NC-1 tachyzoites intravenously (iv) (group 1, n = 8) or subcutaneously (sc) (group 2, n = 8) at 70 days' gestation. Control animals (group 3; n = 8) received uninfected Vero cells iv. Two animals from each group were killed at 14, 28, 42 and 56 days post-inoculation (dpi). Fetal mortality was 100% and 50%, respectively, in groups 1 and 2 from 28 dpi. In group 1 foci of degenerative fetal placental villi were observed at 14 dpi, with clusters of N. caninum tachyzoites in the affected mesenchyme. There was also inflammation of maternal septal tissues, with necrotic cell debris and serum exudate at the interstitium. At 28 dpi pregnancy had ended and the fetal cotyledons had become detached from the maternal caruncles. Immunohistochemically, particulate N. caninum antigen was detected in the cotyledons. At 42 and 56 dpi, fetal tissues had disappeared, the caruncles were greatly reduced in size, and the uterine epithelium had been largely restored. In group 2, lesions were either severe or absent ("all or nothing" response). In one animal carrying a dead fetus at 28 dpi, placentitis was much more severe than that seen in group 1 at 14 dpi. Lesions contained neutrophils, eosinophils and N. caninum antigen. In animals carrying dead fetuses at 42 and 56 dpi, fetal remains were found and the cotyledons contained N. caninum antigen. Antigen was also detected in fetal tissues. No significant pathological changes were detected in group 2 animals carrying live fetuses or any animal in group 3. Thus, N. caninum administered iv or sc in early pregnancy resulted in rapid fetal death, with parasite-associated lesions in the placenta and fetus. Of the two inoculation routes, the intravenous induced the more acute placental lesions and greater mortality.

Maternal and fetal immune responses of cattle inoculated with Neospora caninum at mid-gestation

The humoral and cell-mediated immune responses of pregnant cattle and their fetuses were examined at intervals after infection with Neospora caninum tachyzoites at mid-gestation (day 140). All cattle seroconverted and interferon gamma was detected in supernatants of peripheral blood mononuclear cells stimulated with specific antigen. At day 14 post-inoculation (pi), specific cell proliferation responses were detected in the lymph node draining the site of inoculation and in the uterine lymph node. The peak response was recorded in the majority of maternal lymph nodes by day 28 pi and cells from the maternal retropharyngeal lymph node, which in part drains the central nervous system, showed no specific activity to N. caninum until day 42 pi. This changing pattern of immune responsiveness may reflect parasite invasion and development within different host tissues. Fetal lymph node cells showed mitogen responsiveness from day 14 pi (day 154 of gestation) and also showed N. caninum-specific cell proliferation and interferon-gamma responses by day 28 pi (day 168 of gestation). At day 42 pi, specific cell-mediated immune responses were not apparent; however, N. caninum-specific fetal IgG and IgM antibodies were detected.

The pathogenesis of neosporosis in pregnant cattle: inoculation at mid-gestation

To investigate the pathogenesis of bovine neosporosis, 14 pregnant cattle were each inoculated subcutaneously with either 10(7) or 5 x 10(8) Neospora caninum (strain NC1) tachyzoites at 140 days' gestation. Serial necropsies were then carried out over an 8-week period. In the placenta, Neospora DNA and histopathological changes were observed in samples taken 14 days post-inoculation (dpi), with focal necrosis of maternal caruncular septa and fetal placental villi, serum leakage, and a maternal and fetal inflammatory response. At subsequent samplings, pathological changes in the placenta showed signs of resolution. No parasitaemia was detected in the dams in the two weeks following inoculation. In the fetus, Neospora DNA was detected at 14 dpi, and histopathological changes in the fetal central nervous system at 28 and 42 dpi consisted of small foci of necrosis and inflammation. Resolution of placental lesions during the experiment indicated that the disease was being controlled, and fetal infection, although established, did not appear to be progressing to a fatal outcome. The two doses of tachyzoites produced similar results, but the higher dose elicited earlier and more extensive lesions in the placenta and fetus. Control animals remained negative for all parameters recorded. It is concluded that in bovine neosporosis the placenta plays a central role in the pathogenesis and epidemiology of the infection, and that while primary tissue destruction by the parasite may endanger the fetus, the maternal and fetal inflammatory responses may also be damaging.

Serological analysis of calves experimentally infected with Neospora caninum: a 1-year study

A serological study was conducted with calves experimentally infected with the protozoan parasite Neospora caninum. The animals were inoculated with either a low or high dose of N. caninum tachyzoites and temperature responses monitored daily for the first 2 weeks after inoculation. Blood samples were collected before inoculation, and at regular intervals thereafter for 1 year. Serological analysis was achieved using an indirect fluorescent antibody test (IFAT), an enzyme-linked immunosorbent assay (ELISA) and an IgG avidity ELISA. Injection of Neospora produced a significant rise in rectal temperature in the high dose group. In addition, the lymph node draining the site of inoculation increased in size following injection in all animals, in both infected groups, before returning to normal by day 14 after injection. Both groups given N. caninum produced specific antibody that was detected by the IFAT and the ELISA, which remained elevated for the 12-month duration of the experiment. The specific Neospora antibodies produced did not cross-react in an IFAT for the detection of antibodies to Toxoplasma gondii. IgG avidity increased 2 weeks after inoculation, in both infected groups, until week 12 when infection was well established. There was a little difference between the two infected dose groups. This study demonstrates that the two different doses of N. caninum produced a similar antibody response, and that the higher dose also induced a febrile reaction. The IgG avidity ELISA was successful at distinguishing between recent and long-standing infection in this study. However, in both groups, there was fluctuation in the levels of specific antibody throughout the yearlong study, which accords with similar experiments in pregnant cattle, where it has been suggested that fluctuation may indicate periodic recrudescence of infection and a re-stimulation of antibody production by antigen.

Immunity to experimental neosporosis in pregnant sheep

Neospora caninum is an important cause of fetal loss in cattle but has also infrequently been shown to cause disease in sheep and goats. Experimental infection of pregnant sheep with N. caninum causes clinical and pathological changes very similar to those of neosporosis in cattle. An experiment in sheep was undertaken to examine whether infection with N. caninum before pregnancy conferred immunity to subsequent challenge with the parasite during pregnancy. Primary inoculation of NC1 tachyzoites subcutaneously, either before or during pregnancy, caused a significant temperature response in ewes, while those given a secondary challenge at 90 days gestation (dg) did not show such a response. Primary infection of 12 ewes during pregnancy resulted in the loss of all fetuses while a further 12 ewes inoculated with NC1 tachyzoites before mating and subsequently challenged with the same dose at 90 dg produced nine live and seven dead lambs. There were no fetal deaths in ewes only infected with Neospora before mating although there was serological evidence of vertical transmission in four of their clinically normal offspring while Neospora DNA was detected in the cerebrospinal fluid of a fifth healthy lamb. Thus an experimental primary infection with N. caninum during pregnancy killed all the fetuses while inoculation before pregnancy did not cause any mortality but did provide a degree of protection against subsequent challenge with Neospora during pregnancy.

Detection of T. gondii in tissues of sheep and cattle following oral infection

It has been reported in the literature that cattle are more resistant to toxoplasmosis than sheep. Congenital disease due to T. gondii infection is rarely reported in cattle whereas the parasite is a major cause of abortion and neonatal mortality in sheep. It is believed that sheep remain chronically infected for life. Undercooked meat from infected sheep is an important source of infection for man. In contrast cattle are thought to harbour fewer parasite tissue cysts which may not persist for the lifetime of the host. Therefore, cattle are believed to pose less of a risk for human infection. In this study we examined the presence of T. gondii within a range of tissues in sheep and cattle at 6 weeks and 6 months following oral infection with 10(3) or 10(5) sporulated oocysts of T. gondii. The presence of parasite was determined by bioassay in mice and using polymerase chain reaction (PCR). The results from this study show that T. gondii was more frequently and consistently detected in sheep, in particular within brain and heart tissues, whereas parasites were not detected in the samples of tissues taken from cattle. T. gondii was more frequently detected in sheep given the higher dose of T. gondii. Examination of tissues at either 6 weeks or 6 months after infection did not appear to affect the distribution of T. gondii. The polymerase chain reaction has more specificity and sensitivity when detecting the presence of T. gondii in large animals than histological detection.

An IgG avidity ELISA to discriminate between recent and chronic Neospora caninum infection

The avidity of IgG antibodies directed to Neospora caninum was measured using an IgG avidity enzyme-linked immunosorbent assay (ELISA) employing N. caninum proteins incorporated into immunostimulating complexes as antigen. In this ELISA, low-affinity antibodies were eluted by adding an incubation step with urea after the serum incubation. The antibody titers obtained with and without incubation with urea were then used to calculate the IgG avidity values. Analysis of sequential sera collected from experimentally infected calves revealed that the avidity increased during the course of infection. Three weeks after infection, the IgG avidity was 9-18%, and 24 weeks later it had increased to 58-76%. Cattle naturally infected for more than 6 months all had avidities >50%. The results in this study, however preliminary, indicate that the IgG avidity ELISA can be used to discriminate between recent and chronic N. caninum infections and may therefore be a valuable complement to IgG assays in epidemiologic studies of N. caninum infection in cattle.

Cellular immune responses in cattle experimentally infected with Neospora caninum

Neospora caninum has recently been identified as an important cause of infectious abortion in cattle. The parasite is closely related to Toxoplasma gondii, but the two species are antigenically distinct. To examine cell proliferative responses and the induction of IFN-gamma in experimentally infected cattle, four 2-4 months old calves were subcutaneously inoculated with N. caninum tachyzoites. Peripheral blood mononuclear cells were collected regularly and stimulated in vitro with a crude lysate of N. caninum or T. gondii tachyzoites. Significant proliferative responses to N. caninum antigen were recorded in all calves from days 4-6 postinoculation. This response was accompanied by production of high levels of IFN-gamma. Although the calves remained seronegative to T. gondii, while seroconverting to N. caninum, stimulation with T. gondii lysate resulted in cell proliferation of a similar magnitude as that obtained using the N. caninum lysate. However, the T. gondii lysate appeared less effective than the N. caninum lysate to stimulate IFN-gamma production. Cells taken from uninfected control animals did not show any significant proliferation to either N. caninum or T. gondii antigen and no IFN-gamma was produced. These results suggest that the two parasites may possess cross-reacting T-cell epitopes, but that the T cells specific for N. caninum may have a different functional capacity. This highlights the need to investigate the antigen specificity and cytokine profile of T cells from infected animals to help understand their role in immunity to N. caninum.

The pathogenesis of experimental neosporosis in pregnant sheep

Three groups of eight pregnant sheep were inoculated with tachyzoites of the NCl isolate of Neospora caninum at 45 (group 1), 65 (group 2) or 90 (group 3) days' gestation. A further six animals (group 4) served as controls. Fourteen of the infected ewes developed a fever, which in two cases was biphasic. In six ewes in group 1, the fetuses died and were resorbed, and in the other two the fetuses were aborted. In group 2, one ewe resorbed her fetus, six aborted dead fetuses and one produced a live lamb. In group 3, six ewes aborted and two produced one live and one stillborn lamb each. Thus, the stage of gestation influenced the outcome of infection. All but one of the ewes "seroconverted", as shown by enzyme-linked immunosorbent assay, and 10 of 13 fetal sera examined by an indirect immunofluorescent antibody test were positive. The polymerase chain reaction was also used to detect DNA of N. caninum in aborted tissues. Immunohistochemical examination showed that the parasite had invaded the placentas of all cases examined, displaying an apparent predilection for fetal chorionic epithelium and fetal placental blood vessels, as well as inducing thrombosis in some maternal caruncular blood vessels. Organisms were associated with fetal vasculitis, focal degeneration and inflammation of the chorioallantois, and widespread, severe focal necrosis in the placentome. Characteristic lesions were seen in the fetal brains, in addition to focal leucomalacia, thought to be due to anoxia resulting from the placental damage. The six control sheep in group 4 remained clinically healthy and produced normal uninfected lambs.

Neosporosis and bovine abortion in Scotland

Serum samples were collected over a two-year period from aborting cows and their fetuses from throughout Scotland; 465 maternal sera were examined using an immunofluorescent antibody test for IgG antibody to Neospora caninum and 547 fetal sera were examined for IgM and IgG antibody to the parasite; 355 of the maternal sera were paired with their respective fetal samples. The maternal titres ranged from 1/8 to 1/16,384, with 59 per cent (275) having titres of 1/128 or 1/256. Of the 547 fetal sera, 9.9 per cent had IgM titres equal to or greater than 1/64, 11.2 per cent had IgG titres equal to or greater than 1/64 and 15.9 per cent had IgM and/or IgG titres equal to or greater than 1/64. It is concluded that a fetal IgM or IgG titre of 1/64 or more is evidence of fetal exposure to N caninum but that the examination of maternal sera is less reliable on an individual basis, although valuable for indicating the degree of infection in a herd. Although fetal seropositivity does not necessarily provide proof that N caninum was the cause of death in a given case, the observation that 15.9 per cent of aborted fetuses had antibody to the parasite shows that vertical transmission from dam to fetus is relatively common and may be an important cause of fetal loss in cattle in Scotland.

Experimental infection of non-pregnant and pregnant sheep with Neospora caninum

In an initial experiment, 21 sheep in groups of five or six were inoculated subcutaneously (sc) with 10(8), 10(6) or 10(4) Neospora caninum tachyzoites (Liverpool isolate), or with control inoculum, and monitored for clinical signs and for "seroconversion". Animals given the two higher doses showed febrile responses and all three groups inoculated with the parasite showed seroconversion. In a second experiment, 12 pregnant sheep were each inoculated sc at 90 days' gestation with 10(6) tachyzoites, and at 25, 40 and 53 days post-inoculation (dpi) groups of four were killed for examination of the fetuses and placentas. Appropriate control ewes were included in the study. All fetuses were alive immediately before their dams were killed, except for one, which was found to be mummified at 40 dpi. Histopathological lesions were found consistently in both fetal central nervous system (CNS) and placental tissues. In the latter, focal necrosis, which was mild at 25 dpi, was much more severe at 40 dpi and much less severe at 53 dpi. Lesions in the fetal CNS consisted of focal microgliosis (with or without central necrosis), lymphoid cuffing and non-suppurative meningitis. Lesions were also found in fetal liver, heart and lung. Neospora antigen was demonstrated in fetal brain and placental tissues and, at 25 dpi, in single samples of fetal liver and heart. The prescapular lymph nodes did not differ in size from those of control fetuses but were more mature in that they contained a significantly greater number of secondary follicles. Both IgM and IgG antibodies to N. caninum were detected in the serum of fetuses from infected ewes. Thus, N. caninum readily infected pregnant ewes and caused lesions in fetal tissues and placentas which resembled those of ovine toxoplasmosis. In addition, the changes were similar to those of bovine neosporosis; the infected pregnant ewe therefore offers a good model for the bovine disease.

A mouse model of recrudescence of Toxoplasma gondii infection

Dexamethasone was given to mice infected with Toxoplasma gondii to provide a model of recrudescence of infection in immunocompromised patients and to permit investigation of the interaction between parasite and host.

Decoquinate and the control of experimental ovine toxoplasmosis

Decoquinate was tested for its ability to reduce the effect of experimentally induced toxoplasmosis in pregnant ewes. Sheep were given decoquinate in their feed daily at either 2 mg or 1 mg/kg bodyweight from 10 days before an oral challenge with Toxoplasma gondii oocysts at 90 days of gestation, until lambing. Feeding decoquinate at the higher rate caused a delay in the onset of the febrile response to infection, reduced the overall severity of the fever and delayed the production of antibodies to the parasite. This treatment also reduced the placental damage caused by the parasite, lengthened the mean gestation period and increased the number and weight of live lambs, in comparison with ewes not fed decoquinate but challenged with T gondii oocysts. The treatment with 1 mg of decoquinate had smaller effects.

Pathogenesis of Chlamydia psittaci infection in sheep: detection of the organism in a serial study of the lymph node

Sixteen sheep were inoculated subcutaneously over the left prefemoral lymph node with an abortifacient strain of Chlamydia psittaci. Groups of four animals were killed after 3, 6, 12 and 18 days. Four of eight sheep which received a control inoculum were killed on day 6 and four on day 18. The left and right prefemoral lymph nodes were removed and weighed and portions taken from each for examination by the polymerase chain reaction (PCR), by culture, and by histopathological and immunohistochemical methods. The left prefemoral lymph nodes enlarged after the injection of C. psittaci, with the group mean weight on day 6 being the greatest and that on day 18 being normal. Examination by "nested" PCR showed samples from these nodes to be positive, except for one animal killed on day 3 and one on day 12. Live organisms, however, were not cultured from any of the samples collected. C. psittaci antigen was detected immunohistochemically in three of four nodes on day 3, in each of four on day 6, and in two of four on both days 12 and 18. Nodes from the contralateral side remained normal, as did those from unchallenged control sheep, and no antigen or DNA was detected in them.

The importance of interferon-gamma in an early infection of Chlamydia psittaci in mice

Athymic mice (nu/nu) and their hairy littermates (nu/+) were infected experimentally with Chlamydia psittaci and the role of endogenous interferon-gamma (IFN-gamma) on the resolution of the infection was studied. The pathological changes produced in the spleen, liver and lung were exacerbated by administration of a monoclonal antibody (mAb) to IFN-gamma and an increased number of viable chlamydiae were recovered from the tissues of both nu/+ and nu/nu mice treated in this way.