Neospora caninum in cattle: experimental infection with oocysts can result in exogenous transplacental infection, but not endogenous transplacental infection in the subsequent pregnancy

The role of genetic variability of the host on the resistance to Neospora caninum infection in cattle

Neospora caninum is one of the most frequently diagnosed abortifacient pathogens in cattle. There is abundant genomic information about the parasite itself, but very little is known about the genetic variability of resistance in the most common intermediate host. The aim of this review was to compile all the available information about the genetic variability associated with the resistance to N. caninum both between and within cattle breeds. We systematically searched for published studies that investigated the influence of genetics of the host on the prevalence of N. caninum and risk of abortion. Beyond the potential confounding effects of feeding systems, management and animal density, some lines of evidence suggest that Holstein, the most popular breed for milk production, has a comparatively higher risk of abortion due to infections by N. caninum, whereas some beef breeds from Continental Europe seem to be more resistant. It is still not clear if different genetic mechanisms of resistance are involved in the two known routes of infection: postnatal ingestion of oocysts or transplacental transmission from the infected dam to the fetus. Genomic information associated with susceptibility to infection and risk of abortion in different cattle breeds is still scarce. The information reported here could be useful to identify new research alternatives and to define novel strategies to deal with this major problem of animal production.

Experimental Neospora caninum Infection in Pregnant Cattle: Different Outcomes Between Inoculation With Tachyzoites and Oocysts

Neospora caninum is a globally distributed abortifacient protozoan of cattle. Experimental infections with N. caninum in cattle have provided valuable information on host-parasite interaction and immunopathogenesis. Experimental infection of pregnant cows has been reported in about 20 articles, with most studies using cultured parasite tachyzoites as the inoculum. Only three experimental studies have been conducted in pregnant cows using the parasite's oocysts which are shed by dogs, in large part because transmission experiments using oocysts take more time and are more complex and expensive than experiments using tachyzoites. In this minireview, we discuss differences between N. caninum tachyzoites and oocysts as inocula for experimental infection of pregnant cows, as well as the route animals are inoculated.

Modeling the Ruminant Placenta-Pathogen Interactions in Apicomplexan Parasites: Current and Future Perspectives

Neospora caninum and Toxoplasma gondii are one of the main concerns of the livestock sector as they cause important economic losses in ruminants due to the reproductive failure. It is well-known that the interaction of these parasites with the placenta determines the course of infection, leading to fetal death or parasite transmission to the offspring. However, to advance the development of effective vaccines and treatments, there are still important gaps on knowledge on the placental host-parasite interactions that need to be addressed. Ruminant animal models are still an indispensable tool for providing a global view of the pathogenesis, lesions, and immune responses, but their utilization embraces important economic and ethics restrictions. Alternative in vitro systems based on caruncular and trophoblast cells, the key cellular components of placentomes, have emerged in the last years, but their use can only offer a partial view of the processes triggered after infection as they cannot mimic the complex placental architecture and neglect the activity of resident immune cells. These drawbacks could be solved using placental explants, broadly employed in human medicine, and able to preserve its cellular architecture and function. Despite the availability of such materials is constrained by their short shelf-life, the development of adequate cryopreservation protocols could expand their use for research purposes. Herein, we review and discuss existing (and potential) in vivo, in vitro, and ex vivo ruminant placental models that have proven useful to unravel the pathogenic mechanisms and the host immune responses responsible for fetal death (or protection) caused by neosporosis and toxoplasmosis.

Neosporosis in bulls: potential for venereal transmission, and effect on semen quality and production

Neospora caninum caninum is a protozoan parasite that can cause abortions, birth of congenitally infected calves, and reduced milk production in cattle. Neosporosis in cattle can be transmitted through vertical transplacental transmission from dam to calf, or through horizontal transmission between animals. While it is well established that horizontal transmission from dogs (the definitive host) to cattle (an intermediate host) occurs, it is not clear whether horizontal transmission may occur between cows and bulls during mating. Transmission via infected semen of seropositive bulls was suggested when N. caninum DNA was found in bull semen. The primary objective of this article is to critically evaluate the evidence for venereal transmission of N. caninum between cattle, to determine whether this transmission route is likely to occur. A secondary objective is to review and evaluate published evidence for whether neosporosis in bulls may affect semen quality and production. N. caninum DNA can be detected by PCR in occasional batches of semen straws from bulls that are seropositive for N. caninum. Insemination of heifers with semen experimentally infected with 5 × 10⁴ N. caninum tachyzoites has caused persistent seroconversion in some animals. However, the highest concentration of parasites in the semen of naturally infected bulls was estimated, using quantitative PCR, as 10 tachyzoites. Thus it appears that, whether used for natural or artificial insemination, the semen of bulls naturally infected with N. caninum, does not contain the necessary intrauterine dose of tachyzoites to cause persistent seroconversion in a heifer or cow. Thus, based on the available evidence, the venereal transmission of N. caninum via bull semen is extremely unlikely. Nevertheless, neosporosis in bulls may influence semen quality and production. Epididymal semen concentration, viability, and motility appeared "to be" significantly decreased in N. caninum-seropositive bulls compared to seronegative bulls. Furthermore, semen production could be altered if neosporosis affects the thyroid gland. The thyroid hormones such as triiodothyronine and thyroxine (T4) may affect testicular development. Significantly lower concentrations of T4 were observed in the blood of N. caninum-seropositive, compared with those of seronegative bulls. Longitudinal studies are needed to confirm the long-term effect and significance of neosporosis on semen quality and thyroid hormone concentrations in bulls.

Spatial distance between sites of sampling associated with genetic variation among Neospora caninum in aborted bovine foetuses from northern Italy

BACKGROUND

Neospora caninum, a coccidian protozoan, represents an important cause of bovine abortion. Available N. caninum strains show considerable variation in vitro and in vivo, including different virulence in cattle. To which extent sexual recombination, which is possible in the intestines of domestic dogs and closely related carnivores as definitive hosts, contributes to this variation is not clear yet.

METHODS

Aborted bovine foetuses were collected between 2015 and early 2019 from Italian Holstein Friesian dairy herds suffering from reproductive problems. A total of 198 samples were collected from 165 intensive farms located in Lombardy, northern Italy. N. caninum samples were subjected to multilocus-microsatellite genotyping using ten previously established microsatellite markers. In addition to our own data, those from a recent study providing data on five markers from other northern Italian regions were included and analysed.

RESULTS

Of the 55 samples finally subjected to genotyping, 35 were typed at all or 9 out of 10 loci and their individual multilocus-microsatellite genotype (MLMG) determined. Linear regression revealed a statistically significant association between the spatial distance of the sampling sites with the genetic distance of N. caninum MLMGs (P < 0.001). Including data from this and a previous North Italian study into eBURST analysis revealed that several of N. caninum MLMGs from northern Italy separate into four groups; most of the samples from Lombardy clustered in one of these groups. Principle component analysis revealed similar clusters and confirmed MLMG groups identified by eBURST. Variations observed between MLMGs were not equally distributed over all loci, but predominantly observed in MS7, MS6A, or MS10.

CONCLUSIONS

Our findings confirm the concept of local N. caninum subpopulations. The geographic distance of sampling was associated with the genetic distance as determined by microsatellite typing. Results suggest that multi-parental recombination in N. caninum is a rare event, but does not exclude uniparental mating. More comprehensive studies on microsatellites in N. caninum and related species like Toxoplasma gondii should be undertaken, not only to improve genotyping capabilities, but also to understand possible functions of these regions in the genomes of these parasites.

Treatment of Toxoplasmosis and Neosporosis in Farm Ruminants: State of Knowledge and Future Trends

Toxoplasmosis and neosporosis are closely related protozoan diseases that lead to important economic impacts in farm ruminants. Toxoplasma gondii infection mainly causes reproductive failure in small ruminants and is a widespread zoonosis, whereas Neospora caninum infection is one of the most important causes of abortion in cattle worldwide. Vaccination has been considered the most economic measure for controlling these diseases. However, despite vaccine development efforts, only a live-attenuated T. gondii vaccine has been licensed for veterinary use, and no promising vaccines against ne-osporosis have been developed; therefore, vaccine development remains a key goal. Additionally, drug therapy could be a valuable strategy for disease control in farm ruminants, as several drugs that limit T. gondii and N. caninum proliferation and dissemination have been evaluated. This approach may also be relevant to performing an initial drug screening for potential human therapy for zoonotic parasites. Treat-ments can be applied against infections in adult ruminants to minimize the outcomes of a primo-infection or the reactivation of a chronic infection during gestation or in newborn ruminants to avoid infection chronification. In this review, the current status of drug development against toxoplasmosis and neosporo-sis in farm ruminants is presented, and in an effort to promote additional treatment options, prospective drugs that have shown efficacy in vitro and in laboratory animal models of toxoplasmosis and neosporosis are examined

Occurrence of abortions induced by Neospora caninum in dairy cattle from Santa Catarina, southern Brazil

Abstract The aim of the present study was to investigate the occurrence of N. caninum associated with abortions of dairy cattle from Santa Catarina state, southern Brazil by using enzyme-linked immunosorbent assay (ELISA), immunohistochemistry (IHC), and polymerase chain reaction (PCR). Blood from dairy cows that aborted along with intrathoracic fluid and tissue samples (brain, heart, liver, and lung) from their fetuses were collected and used for serology; PCR, histopathological, and immunohistochemistry (IHC) evaluations were also conducted. Twenty-one cows (51.2%) out of 41, and eight fetuses (26.7%) out of 30 were ELISA (HerdCheck, IDEXX) positive for N. caninum. Dams > 36 months of age had a higher risk of being serum positive than younger animals. PCR and IHC revealed that 38.8% (14/36) and 25.0% (9/36) of the fetuses were positive for N. caninum, respectively for each of the tests. Seropositive cows had a higher frequency of fetuses that were also positive by either intrathoracic fluid, PCR, or IHC. In summary, the present study observed a high frequency of N. caninum in abortions from dairy cows from southern Brazil, with a higher N. caninum prevalence found in cows that were older than 36 months. In addition, serology, PCR, and IHC should be used all together for better diagnosis of neosporosis in cattle.

Neospora caninum: Chronic and congenital infection in consecutive pregnancies of mice

Neospora caninum, the causative agent of bovine neosporosis is the major cause of abortion in cattle worldwide. The principal route of transmission is via in utero infection of the offspring. Congenitally-infected dams remain persistently infected for life and might undergo abortions in consecutive pregnancies. In the present study, the effect of N. caninum in chronic and congenital infection was examined. CD1 mice were infected intra-peritoneally with live tachyzoites of the NcIs491 isolate, while non-infected mice served as a control. There were no clinical signs of infection observed following inoculation, but high titers of specific anti- N. caninum antibodies were detected. A month after infection, when chronic-infection was established, mice were mated. Fertility, litter size and mortality rate were monitored within two generations of four consecutive pregnancies. During a nine months period of the study all females maintained high level of antibodies, while the non- infected control mice remained seronegative. There was no difference in the fertility rate of the dams, or in the litter size of infected and control mice. Mortality of offspring of the first and second generations of the infected dams was observed within the two first weeks of life. The vertical transmission was analyzed by PCR assay of offspring brains. PCR positive results were found in all 13 litters of the first generation tested during four consecutive pregnancies. The rate of vertical transmission slightly decreased in successive pregnancies being 74.2%, 59.5%, 48.1% and 40% for the first to fourth pregnancies respectively. In the second generation 21 out of 28 litters were found positive and the overall rate of vertical transmission was 28.5%. In chronically and congenitally infected dams N. caninum infection was maintained during all successive pregnancies for about 9 months. The results show that CD-1 outbred mice are a suitable model for studying chronic and congenital neosporosis.

Secreted effectors in Toxoplasma gondii and related species: determinants of host range and pathogenesis?

Recent years have witnessed the discovery of a number of secreted proteins in Toxoplasma gondii that play important roles in host-pathogen interactions and parasite virulence, particularly in the mouse model. However, the role that these proteins play in driving the unique features of T. gondii compared to some of its nearest apicomplexan relatives (Hammondia hammondi and Neospora caninum) is unknown. These unique features include distinct dissemination characteristics in vivo and a vast host range. In this review we comprehensively survey what is known about disease outcome, the host response and host range for T. gondii, H. hammondi, and N. caninum. We then review what is presently known about recently identified secreted virulence effectors in these three genetically related, but phenotypically distinct, species. Finally we exploit the existence of genome sequences for these three organisms and discuss what is known about the presence, and functionality, of key T. gondii effectors in these three species.

Experimental ruminant models for bovine neosporosis: what is known and what is needed

At present, bovine neosporosis is an important worldwide concern because of its wide geographic distribution and economic impact. Abortion is the main clinical sign of bovine neosporosis in both dairy and beef cattle. Ruminant challenge models are critical to evaluate potential vaccine candidates to help tackle bovine neosporosis and to study pathogenesis and host responses to infection. Several research groups have developed ruminant models of Neospora caninum infection independently of others, resulting in a high degree of variability due to the use of different species of animals, breeds, strains/isolates of N. caninum, doses, routes and times of inoculation. Standardization is greatly needed to advance research in a more collaborative, timely and efficient manner. In the absence of widely accepted international guidelines, this manuscript serves to summarize and discuss the different models and parameters currently in use. Parameters essential for the development of non-pregnant and pregnant ruminant models are outlined and the main knowledge gaps are identified. This information could act as the basis to develop a consensus for international standard guidelines for ruminant models of neosporosis that would be helpful for researchers in this field worldwide.

Control options for Neospora caninum--is there anything new or are we going backwards?

Recent work has highlighted and enumerated the economic annual losses due to Neospora caninum abortions worldwide, which should provide strong motivation for the control of bovine neosporosis. However, with the recent withdrawal from sale of the only commercially available vaccine, control options for N. caninum have become more restricted. While researchers continue to work on developing alternative efficacious vaccines, what are the control options presently available for the cattle industries? At the practical level, recommendations for 'Test-and-cull', or 'not breeding from seropositive dams' stand diametrically opposed to alternative options put forward that suggest a primary producer is better advised to keep those cows in the herd that are already seropositive, i.e., assumed to be chronically infected, and indeed those that have already aborted once. Treatment with a coccidiostat has been recommended as the only economically viable option, yet no such treatment has gained official, regulatory approval. Dogs are central to the life cycle of N. caninum and have repeatedly been associated with infection and abortions in cattle by epidemiological studies. Knowledge and understanding of that pivotal role should be able to be put to use in control programmes. The present review canvasses the relevant literature for evidence for control options for N. caninum (some of them proven, many not) and assesses them in the light of the authors' knowledge and experience with control of N. caninum.

Vaccines against neosporosis: what can we learn from the past studies?

Neospora caninum is an intracellular apicomplexan parasite, which is a leading cause of abortion in cattle; thus neosporosis represents an important veterinary health problem and is of high economic significance. The parasite can infect cattle via trans-placental transmission from an infected cow to its fetus (vertical transmission), or through the oral route via ingestion of food or water contaminated with oocysts that were previously shed with the feces of a canid definitive host (horizontal transmission). Although vaccination was considered a rational strategy to prevent bovine neosporosis, the only commercialized vaccine (Neoguard®) produced ambiguous results with relatively low efficacy, and was recently removed from the market. Therefore, there is a need to develop an efficient vaccine capable of preventing both, the horizontal transmission through infected food or water to a naïve animal as well as the vertical transmission from infected but clinically asymptomatic dams to the fetus. Different vaccine strategies have been investigated, including the use of live attenuated vaccines, killed parasite lysates, total antigens or antigen fractions from killed parasites, and subunit vaccines. The vast majority of experimental studies were performed in mice, and to a certain extent in gerbils, but there is also a large number of investigations that were conducted in cattle and sheep. However, it is difficult to directly compare these studies due to the high variability of the parameters employed. In this review, we will summarize the recent advances made in vaccine development against N. caninum in cattle and in mice and highlight the most important factors, which are likely to influence the degree of protection mediated by vaccination.

Mucosal and systemic T cell response in mice intragastrically infected with Neospora caninum tachyzoites

The murine model has been widely used to study the host immune response to Neospora caninum. However, in most studies, the intraperitoneal route was preferentially used to establish infection. Here, C57BL/6 mice were infected with N. caninum tachyzoites by the intragastric route, as it more closely resembles the natural route of infection through the gastrointestinal tract. The elicited T-cell mediated immune response was evaluated in the intestinal epithelium and mesenteric lymph nodes (MLN). Early upon the parasitic challenge, IL-12 production by conventional and plasmacytoid dendritic cells was increased in MLN. Accordingly, increased proportions and numbers of TCRαβ+CD8+IFN-γ+ lymphocytes were detected, not only in the intestinal epithelium and MLN, but also in the spleen of the infected mice. In this organ, IFN-γ-producing TCRαβ+CD4+ T cells were also found to increase in the infected mice, however later than CD8+ T cells. Interestingly, splenic and MLN CD4+CD25+ T cells sorted from infected mice presented a suppressive activity on in vitro T cell proliferation and cytokine production above that of control counterparts. These results altogether indicate that, by producing IFN-γ, TCRαβ+CD8+ cells contribute for local and systemic host protection in the earliest days upon infection established through the gastrointestinal tract. Nevertheless, they also provide substantial evidence for a parasite-driven reinforcement of T regulatory cell function which may contribute for parasite persistence in the host and might represent an additional barrier to overcome towards effective vaccination.

Genetic diversity and geographic population structure of bovine Neospora caninum determined by microsatellite genotyping analysis

The cyst-forming protozoan parasite Neosporacaninum is one of the main causes of bovine abortion worldwide and is of great economic importance in the cattle industry. Recent studies have revealed extensive genetic variation among N. caninum isolates based on microsatellite sequences (MSs). MSs may be suitable molecular markers for inferring the diversity of parasite populations, molecular epidemiology and the basis for phenotypic variations in N. caninum, which have been poorly defined. In this study, we evaluated nine MS markers using a panel of 11 N. caninum-derived reference isolates from around the world and 96 N. caninum bovine clinical samples and one ovine clinical sample collected from four countries on two continents, including Spain, Argentina, Germany and Scotland, over a 10-year period. These markers were used as molecular tools to investigate the genetic diversity, geographic distribution and population structure of N. caninum. Multilocus microsatellite genotyping based on 7 loci demonstrated high levels of genetic diversity in the samples from all of the different countries, with 96 microsatellite multilocus genotypes (MLGs) identified from 108 N. caninum samples. Geographic sub-structuring was present in the country populations according to pairwise F_ST. Principal component analysis (PCA) and Neighbor Joining tree topologies also suggested MLG segregation partially associated with geographical origin. An analysis of the MLG relationships, using eBURST, confirmed that the close genetic relationship observed between the Spanish and Argentinean populations may be the result of parasite migration (i.e., the introduction of novel MLGs from Spain to South America) due to cattle movement. The eBURST relationships also revealed genetically different clusters associated with the abortion. The presence of linkage disequilibrium, the co-existence of specific MLGs to individual farms and eBURST MLG relationships suggest a predominant clonal propagation for Spanish N. caninum MLGs in cattle.

Mice congenitally infected with low-to-moderate virulence Neospora caninum isolates exhibited clinical reactivation during the mating period without transmission to the next generation

Endogenous transplacental transmission (EnTT) is the major transmission route of Neospora caninum in cattle. Thus, the development of a standardised experimental model of EnTT is needed for more appropriate testing of parasite biology and control strategies. A recent study reported up to 40-50% EnTT rates in chronically infected dams with either high or low-to-moderate virulence isolates, although low fertility rates were observed in dams inoculated with the high virulence isolate. Therefore, low-to-moderate virulence N. caninum isolates (Nc-Spain 3H; G1 and Nc-Spain 8; G2) that previously showed high TT rates versus low mortality and morbidity rates in a congenital mouse model were inoculated into BALB/c dams (first generation). The new approach followed in the present study aimed to start with a high number of congenitally infected mice (second generation), which allowed a more efficient EnTT from congenitally infected dams to their progeny (third generation). Interestingly, a reactivation of infection occurred in several congenitally infected non-pregnant females (second generation) from both infected groups. This fact was evidenced by neosporosis-associated clinical signs after mating accompanied by an increase of specific antibody levels (IgG1, IgG2a and specific antibodies against rNcGRA7) (P<0.0001; one-way ANOVA). Moreover, a higher number of PCR-positive mice compared to pregnant females were observed (P<0.05; Fisher's exact test). These results support the hypothesis that only mice without clinical signs and with a low parasite burden in the brain became pregnant, which may explain the posterior failure in inducing EnTT from the second to the third generation. These findings confirm that this mouse model is not a suitable experimental EnTT model for testing the efficacy of drugs and vaccine candidates against EnTT. The employment of other putative suitable species with a similar placenta structure, such as small ruminants, should be taken into consideration.

Low rates of Neospora caninum infection reactivation during gestation are observed in both chronically and congenitally infected mice

Endogenous transplacental transmission (EnTT) of Neospora caninum is the most common route of infection in cattle and occurs as a consequence of a reactivation of N. caninum infection that may lead to abortion or to the birth of congenitally infected calves. The reactivation of N. caninum infection was studied during the gestation of chronically infected dams and, for the first time, in their congenitally infected pups. BALB/c mice were infected with Nc-Spain 7 (Group 1) or Nc-Spain 3H (Group 2), high virulence isolates and low-to-moderate virulence isolates, respectively. The mice were mated after 90 days post-infection, and the morbidity, mortality, vertical transmission and humoral immune responses were recorded for 2 consecutive generations. In the first generation, higher morbidity and mortality rates were observed in G1 before mating than in G2 (P < 0·0001). In the second generation, low vertical transmission rates were observed in both inoculated groups (7·7% and 17·1% in G1 and G2, respectively) and were significantly diminished in the third generation (8·7% in G2 versus 0% in G1). Low rates of reactivation of N. caninum infection were induced in chronically infected mice and decreased in subsequent generations regardless of the isolate employed in the inoculations. Thus, further studies are needed to improve this reactivation mouse model.

A review of the infection, genetics, and evolution of Neospora caninum: from the past to the present

This paper is a review of current knowledge on Neospora caninum in the context of other apicomplexan parasites and with an emphasis on: life cycle, disease, epidemiology, immunity, control and treatment, evolution, genomes, and biological databases and web resources. N. caninum is an obligate, intracellular, coccidian, protozoan parasite of the phylum Apicomplexa. Infection can cause the clinical disease neosporosis, which most notably is associated with abortion in cattle. These abortions are a major root cause of economic loss to both the dairy and beef industries worldwide. N. caninum has been detected in every country in which a study has been specifically conducted to detect this parasite in cattle. The major mode of transmission in cattle is transplacental (or vertical) transmission and several elements of the N. caninum life cycle are yet to be studied in detail. The outcome of an infection is inextricably linked to the precise timing of the infection coupled with the status of the immune system of the dam and foetus. There is no community consensus as to whether it is the dam's pro-inflammatory cytotoxic response to tachyzoites that kills the foetus or the tachyzoites themselves. From economic analysis the most cost-effective approach to control neosporosis is a vaccine. The perfect vaccine would protect against both infection and the clinical disease, and this implies a vaccine is needed that can induce a non-foetopathic cell mediated immunity response. Researchers are beginning to capitalise on the vast potential of -omics data (e.g. genomes, transcriptomes, and proteomes) to further our understanding of pathogens but especially to identify vaccine and drug targets. The recent publication of a genome for N. caninum offers vast opportunities in these areas.

Transplacental transmission in cattle: is Toxoplasma gondii less potent than Neospora caninum?

We compared the transplacental-transmission ability of Toxoplasma gondii and Neospora caninum in cattle. One uninfected pregnant heifer served as control, while three were inoculated with N. caninum K9WA strain and four with T. gondii RH strain at their midgestational period. Both infected groups showed clinical signs and antibodies either to N. caninum or T. gondii, while the control animal was normal. Two (50%) Toxoplasma dams aborted on days 6 and 11 postinoculation. T. gondii tachyzoites were found in various organs of those dams that had abortions but not in their fetuses. Two Neospora dams did not abort but gave birth to subclinically infected calves. The remaining two Toxoplasma dams and one from Neospora group became recumbent. Those two dams and their fetuses showed disseminated Toxoplasma DNA, but no Neospora DNA was found. Our findings suggest that maternal toxoplasmosis could be a cause of abortion and congenital toxoplasmosis in cattle, especially when they are infected by virulent strains.

Experimental infection of dogs (Canis familiaris) with sporulated oocysts of Neospora caninum

Neospora caninum is widely distributed in the world and this parasite is one of the major causes of abortion in cattle. Dogs and coyotes are definitive hosts of N. caninum and several species of domestic and wild animals are intermediate hosts. Dogs can become infected by the ingestion of tissues containing cysts and then excrete oocysts. It is not yet known whether sporulated oocysts are able to induce a patent infection in dogs, i.e. a shedding of N. caninum oocysts in feces. The objective of this study was to experimentally examine the infection of dogs by sporulated oocysts. The oocysts used in the experiment were obtained by feeding dogs with brain of buffaloes (Bubalus bubalis) positive for anti-N. caninum antibodies by indirect fluorescent antibody test (IFAT ≥200). Oocysts shed by these dogs were confirmed to be N. caninum by molecular methods and by bioassay in gerbils, and sporulated N. caninum oocysts were used for the oral infection of four dogs. The dogs were 8 weeks old and negative for antibodies to N. caninum and Toxoplasma gondii. Dogs 1 and 4 received an inoculum of 10,000 sporulated oocysts each; dog 2 an inoculum of 5000 sporulated oocysts and dog 3 received 1000 sporulated oocysts of N. caninum. The total feces excreted by these dogs were collected and examined daily for a period of 30 days. No oocysts were found in their feces. The dogs were monitored monthly for a 6-month period to observe a possible seroconversion and when this occurred the animals were eliminated from the experiment. Dogs 1 and 4 seroconverted 1 month after the infection with titer, in the IFAT, of 1600 and 800, respectively; the other two dogs presented no seroconvertion during the 6-month period. Dogs 1 and 2 were euthanized 180 days after infection and were examined for the detection of N. caninum in tissues (brain, muscle, lymph node, liver, lung, heart and bone marrow) by immunohistochemistry and PCR with negative results in both techniques. Bioassay in gerbils with brain of these dogs was also performed and again the results were negative. In conclusion, dogs infected with sporulated oocysts of N. caninum were not able to shed oocysts in feces. However, a higher dose of infection stimulated the production of antibodies against N. caninum in the dogs.

Endogenous and exogenous transplacental transmission of Neospora caninum - how the route of transmission impacts on epidemiology and control of disease

Vertical transmission of the protozoan parasite, Neospora caninum is highly efficient and can take two forms - endogenous transplacental transmission resulting from activation of the quiescent bradyzoite stage during pregnancy or exogenous transplacental transmission resulting from ingestion of oocysts during pregnancy. Calves born carrying infection derived from either endogenous or exogenous transplacental transmission are capable of infecting their offspring when they start to breed. This review considers firstly the frequency with which exogenous and endogenous transmission occur, secondly the role of the immune response in controlling N. caninum infection and thirdly how the parasite persists in an immune-competent host and is re-activated during pregnancy.

Experimental infection with a low virulence isolate of Neospora caninum at 70 days gestation in cattle did not result in foetopathy

The Nc-Spain 1H isolate of Neospora caninum, which was newly obtained from the brain of a congenitally asymptomatic infected calf, demonstrated a reduced in vitro tachyzoite yield and viability rate, as well as low virulence in mouse models. The objective of the present study was to determine the ability of this isolate to induce foetal death in a pregnant bovine model. For this purpose, 13 naïve pregnant heifers were divided into three groups and were experimentally challenged with either 10⁷ tachyzoites of Nc-1 (group 1, n = 5), Nc-Spain 1H (group 2, n = 5) isolates or phosphate-buffered saline (group 3, n = 3) intravenously at 70 days of gestation. After inoculation, pregnancy was monitored and dams were sacrificed when foetal death was detected. The remaining animals were slaughtered at 45 days post-infection. Maternal and foetal samples were collected for examination by histology and parasite DNA detection. Parasitaemia, specific anti-N. caninum IgG and interferon γ responses were also studied. At 3–4 weeks after infection, foetal death was detected in 3 out of 5 Nc-1-infected dams. However, no evidence of foetal death was observed in either Nc-Spain 1H-infected or control groups during the period studied. The most severe histopathological lesions were observed in the placenta and foetal organs from Nc-1-infected cattle that exhibited foetal death. It was in these animals that N. caninum DNA was more frequently detected. Parasitaemia was observed in all Nc-1-infected dams and in only 3 out of 5 Nc-Spain 1H-infected animals. The magnitude of the immune response was significantly higher in the Nc-1-inoculated group than in the group inoculated with the Nc-Spain 1H isolate. These data reveal the reduced virulence of the Nc-Spain 1H isolate in cattle.