Arno N. Vermeulen.
Intervet International PO Box 31 5830 AA Bosmeer The Netherlands arno.vermeulen@intervet.com

Introduction
The family of Coccidian parasites comprising genera such as Eimeria, Toxoplasma, Neospora and Sarcocystis is more or less particular, because to date effective vaccines have been developed against each of its members. Having vaccines available might suggest that problems are solved and other diseases can be tackled; however, this is an oversimplification of the complex nature of immunity and disease control in different animal species and against different members of each genus.
Present vaccines may be improved with respect to better control of parasitemia and easier and more cost-effective production and distribution.
A major disadvantage of an immunological approach is that for each species to be controlled a separate and specific vaccine needs to be developed. This may seem obvious, but it is a drawback when compared to the development of a drug.
Most anti-coccidial drugs developed in the second half of the 20th century are active against multiple species and even genera and can act in different, albeit not all, hosts (examples are ionophores and anti-folates).
The occurrence of resistance makes it still necessary to develop new drugs, preferentially targeting common pathways of Coccidia or even Apicomplexa, of which some are even absent in vertebrate hosts. Coombs (1) reviews such targets exemplified as the functional apicoplast, the shikimate pathway, mannitol cycle and the polyamine metabolism. Recently developed organotypic cultures of neural tissue may be of benefit in the selection of active compounds (2). It is to be foreseen that although more and better vaccines become available, several animal species suffering from the parasites mentioned above, will remain dependent on drug therapy because vaccines can’t handle the many different species involved (10 species of Eimeria are attacking young lambs) (3).
Reduction of the use of drugs in consumption animals however also drives the generation of biological solutions (vaccines).

Eimeria vaccines
Eimeriosis, often designated as coccidiosis, is the disease caused by Eimeria parasites resulting in severe mucosal damage, weight loss and sometimes even death. The disease is widespread and many species occur in poultry, livestock and small animals such as rabbits. Especially the intensification in poultry production (broiler farms) could only be achieved by the concurrent development and application of effective drugs (4).
Since chickens readily develop immunity from natural infection, vaccines were developed based on virulent strains of the most frequently occurring species E. tenella, E. acervulina and E. maxima. These vaccines were shown to be effective when administered at low dose, early in life.
Attenuation of strains could be achieved by adaptation to development in fertilized eggs. Passage of strains through eggs rendered them less pathogenic, although for instance E.acervulina could not be adapted to the egg (5). Livacox ® contains an embryo-adapted E. tenella line. Precociousness is a characteristic of most Eimeria referring to a naturally occurring population of parasites that complete their lifecycle from sporozoite to oocyst 20-30 hrs faster than their fellow parasites from the same parent. This is a selectable trait and sometimes this is accompanied with a decrease in proliferative capacity and pathogenicity. Paracox® was the first line of vaccines that utilized this feature for live vaccines with a better safety profile (6). Recent publications report on newly developed precocious vaccines in different parts of the world (4,7,8). Kawazoe et al. (7) reported that as a result of the selection for precociousness one strain appeared less pathogenic, whereas the other did not differ from the parent line in that respect. This emphasizes the fact that the trait precociousness is not unambiguously linked to the aspect of low pathogenicity.
Strains selected for naturally occurring low pathogenicity were included in NobilisCox ATM® (9). Whereas most vaccines allegedly contain sensitive strains, the latter comprises strains with a defined tolerance for specific drugs. Thereby it allows the concomitant use of certain ionophores until immunity is fully developed. The strains are, on the other hand, fully susceptible to chemical drugs such as diclazuril and toltrazuril, which allows removal if preferred. Li et al. (8) recently selected ionophore-tolerant, precocious strains for similar purpose, and Kawazoe et al. (7) demonstrated that a certain degree of ionophore tolerance is a natural feature of different Eimeria strains, even if they did not have contact with these drugs before.
Antigenic variability becomes more and more evident, especially when live E. maxima vaccine strains are applied (10). Only Paracox® and NobilisCox ATM® have included antigenically different strains. The two strains in the latter vaccine appear to act synergistically (4).
Vaccine failures were often attributed to highly virulent strains. However, with improved administration of live oocysts, chickens develop immunity more readily as a consequence of early cycling of the parasites.
Vaccine spray on day-old birds is nowadays widely considered as the best method to convey the infection to the chicks. In ovo application of live coccidial vaccines (such as Inovocox®, Embrex) may also turn out to be effective. It at least ensures that all birds receive the same dose. Now that more and more vaccines of similar kind enter the scene, objective criteria need to be set for these products as presently there are no studies regarding safety and efficacy requirements.
Being realistic, we estimate that the vaccination of forty billion broilers each year will be very difficult if using live oocysts produced by chickens. Thus, there is a great need for mass production and mass application of effective vaccines. Subunit vaccines are generally not suited for such purpose, although a maternal vaccine Coxabic® was licensed. This vaccine is based on E. maxima gametocyte-derived antigens gam56 and gam82, which should protect the offspring by the action of maternal antibodies present in the yolk. An Emax250kD antigen was recently cloned, and predominantly recognized by maternal sera infected with E. maxima (11). It is not clear what the relation between these approaches is, but they could work in concert.
Recombinant vaccines should be the only long term solution for the problem in the future, especially since more drugs will be banned and requirements for live vaccines will become stricter.
It was shown that target antigens are involved in essential parasite functions. Surface-expressed and apical complex-associated proteins are prime targets for antibody attack (12). Effective protection using invasion-related proteins has not been demonstrated so far. In our lab, we have extensively tested EtMic1 and EtMic5; protein or DNA-based antigens EtMic2 and EtMic4 have been tested and showed some protective effect (13,14). Presentation is of essential importance and may influence the level of the effect observed.
The discovery of an array of 23 different genes (variants), encoding a SAG family of GPI-anchored proteins (no homology to Toxoplasma this time.) expressed on the surface of E. tenella merozoites, cannot be currently be evaluated in relation to vaccine development, but may underline the complexity of our quest to find a vaccine based on only few components covering the major species of Eimeria in chickens (15).
Immunogenic soluble/cytoplasmic proteins, such as LDH and enzymes from the anti-oxidant pool (SOD and 1Cys-peroxidoxin), have been shown to be promising candidates, since they are recognized by T-cells, CD4+, and CD8+, producing IFN-g. Partial protection was elicited in vaccinated chickens using Salmonella typhimurium expressed genes (16).
As said above, the presentation of all candidate antigens to the host immune system is decisive for inducing good levels of protection.
Lillehoj and coworkers have concentrated their work on this issue as reviewed in (13). A series of cytokine genes were reported to enhance the effect of the E.acervulina 3-1E or EtMic2 vaccination, although there was apparently no consensus cytokine that promoted protective effects for both antigens.
DNA application is more frequently reported. Song et al (17) and Min et al. (18) used pcDNA3-1E plasmid and co-injected it with cytokine genes in day-old chicks; however, protection was minimal (<30% oocyst reduction). Wu et al. (19) found Et1A and TA4 genes effective due to improved weight gain and >60% reduction in oocyst output.
DNA plasmid deposition was applied using Salmonella typhimurium bacteria (14), achieving a 50% oocyst reduction after challenge with a pcDNA5401 plasmid.
Scientifically, these approaches will stimulate our thinking about immune mechanisms involved, the role of cytokines in maturation of an adaptive response, and antigen processing. Practical cost-effective solutions for a mass application of Eimeria vaccines will most probably not come from these approaches. Viral vector vaccines are, in my opinion, the best solution, but still a number of obstacles have to be overcome. Herpes and Pox viruses are able to harbor the insert sizes needed to express multiple genes, which are necessary to control the multiple species involved. Fowl pox and Herpes virus of turkeys (HVT) are possible candidates (4,20). We have designed cassettes containing three Eimeria genes, each under the control of its own promoter in HVT, whereby each of the genes was expressed efficiently.
This indicates that we have the tools to construct vector vaccines for multiple antigen expression. Proof of principle to find the effective combination needs further study.

Toxoplasma gondii vaccines
In contrast to the vast experience with vaccination against Eimeria in chickens, only limited data are published on the induction of protection against Toxoplasma-related disease in mammals.
Toxoplasma infections occur in nearly all warm-blooded animals, including man. Apart from felids that can act as definitive host (sexual cycle producing oocysts) and intermediate host, the remaining hosts are only intermediate hosts acquiring the infection by ingestion of oocysts or tissue cysts contained in food (muscle, brain or organs of chronically infected animals). The tachyzoites are the pathogenic stage of the parasite causing high fever, strong inflammatory reactions, and inducing abortion or birth of the offspring with neurological symptoms by the passage from the pregnant host to the fetus. Due to the innate immunity response of the host, the tachyzoite stage converts into bradyzoites embedded in tissue cysts, after which a life long immunity is established.
Immunedepression may cause recrudescence, but in man this is only seen in HIV patients or in patients under post-transplantation treatments, and this seems to be caused by vast impairment of macrophage function in the brain (21). There is vertical transmission of the parasite, but in sheep, for instance, solid immunity is induced after the primary infection and subsequent pregnancies do not induce recrudescence (22). Some conflicting papers (23) detected high percentage of familial abortion in sheep, but no association to Toxoplasma was evidenced.
When a primary infection is acquired during pregnancy, the embryo is seriously at risk due to a very efficient infection of placenta and fetal tissue.
In humans it is generally believed that most ocular toxoplasmosis is caused by congenital infection, but recent studies have shown that ocular lesions are seen mostly in postnatally acquired infection (24,25).
In animals, the main health problems are abortions in sheep and goats (26). In UK and Spain, the prevalence of Toxoplasma in sheep is high and it is responsible for 25% of abortions (26,28). This is probably true in most sheep-producing countries. Cattle seem to be less efficiently infected by Toxoplasma gondii (27), and cats usually do not develop clinical symptoms. Prevalence in pigs is very variable, and very much dependent on management and facility sanitation. In Argentina, indoor reared pigs only had 4% seroprevalence, whereas outdoor reared sows were 100% positive (29).
It is generally accepted that the main source for human infection is eating infected pork or lamb. Since indoor farming started to be practiced in Europe, the prevalence in pigs and humans has dramatically decreased, with only a population of young women at risk, since most are seronegative up to child-bearing age (30).
A risk factor for acquiring Toxoplasma infection also seems to be pregnancy itself. Dramatic changes in the CD4/CD8 ratio and reduced functionality of macrophages and NK cells especially in the third trimester of gestation render a state of high susceptibility for infections such as T. gondii (31). In a recent multicenter study in Europe 62% of women seroconverted during pregnancy, with 18.5% of the children acquired congenital toxoplasmosis (32).
Vaccine research for farm animals has focused on the control of abortions in sheep and goats, and the only commercial vaccines available are live vaccines comprising the S48 strain of T. gondii (Ovilis®Toxovax, Intervet) (Toxovax, Agvax New Zealand) (33). This strain is peculiar, as it does no longer produce tissue cysts ,and it is regarded as a deficient strain, that can be safely used as a live vaccine for livestock. The product is registered for use in sheep, and reduces the risk of abortion due to T. gondii infection (33). It is applied either intramuscularly or subcutaneously no later than three weeks prior to mating. In a challenge study it was shown that the duration of immunity was of at least 18 months (59). This could indicate that the parasite deposited antigenic materials at remote sites, which are able to sustain the immunity. The S48 strain was attenuated by serial passages for over 30 yrs. Mutagenesis has also been used and has resulted in temperature-sensitive variants, which optimal proliferation temperature is 28-32 C. These strains (TS4 mutant) (34) could be used as a vaccines, but have not been developed commercially.
A bradyzoite-based vaccine was developed using a mutant T263, deficient in sexual replication. This product could be used as a vaccine to reduce oocyst production in cats. It needed storage in liquid nitrogen and was applied orally to cats by a straw.
Vaccines for humans probably will not be based on live attenuated parasites. Recombinant approaches could result in defined products of sustainable quality. In this respect, progress can be made along the lines of immunology and the functional role of proteins in the life cycle of the parasite.
The completion of the Toxoplasma genome, the availability of extensive EST databases, and further detailed studies of stage conversion mechanisms have elucidated different host-parasite interactions that could be of value for the future development of vaccines based on individual proteins, genes or combinations of these.
Some success is reported from latest studies using DNA-encoding granule-dense antigens in conjunction with SAG1 DNA-plasmids (35). Almost 90% reduction of mortality after a lethal challenge was achieved in mice, and a significant reduction of tissue cyst establishment was produced. Similarly, the use of SAG1 DNA-plasmids reduced cysts numbers, but no effect was detected on vertical transmission (36). The latter indicates that even few tissue cysts can be responsible for the generation of fetus infection.
Although such studies are performed in animal models, this is certainly encouraging for future development of vaccines for women during their reproductive phase of live. However, the progress mentioned in molecular systems driving stage conversion, such as the role of Hsp90, especially in the conversion of tissue cysts to tachyzoites, may also provide alternative ways to fight the disease. Also, drugs that could interfere in this process could be developed. The main challenge will be how to cross the blood-brain barrier (37).

Neospora caninum vaccines
Neospora caninum has been recognized as the most commonly diagnosed cause of abortion in cattle, after its discovery in 1984 (38). Its life cycle is very similar to that of T. gondii, with asexual multiplication through tachyzoites and bradyzoites in different mammals – though mainly ruminants – and sexual multiplication in canids, such as the dog. It was demonstrated that coyotes are part of a sylvatic cycle (39). Although extensively studied, there is no indication that human infections occur on any significant scale. Very low seroprevalence is detected in man and no relation to any pregnancy problems could be found (38).
Cattle do mount an effective immune response protecting the fetus from aborting, although repeated abortions may occur in 5% of the animals. Previously infected animals have a greater chance of abortion than seronegative animals due to recrudescence of existing infection during pregnancy, whereas exogenous infections from dog-spread oocysts are relatively rare (40). The efficiency of transplacental transmission is over 90%, which makes this disease hard to control, since no drugs are available against the acute phase, and certainly not the tissue cyst stage.
The negative effects of N. caninum infection are not only abortion, but also embryo mortality, reduction in milk production, higher culling rate, birth of calves with congenital abnormalities, and decreased growth rate (60). The consequences of N. caninum infection depend on the time of gestation in which the parasitemia occurs (44). Infection during late gestation seldom results in abortion, but in the birth of congenitally infected calves or calves with congenital abnormalities.
Control measures are focused on reducing the chance of infection during gestation and on preventing the vertical spread of infection by not breeding infected cows. Intensive systems of screening of serum or milk tank antibody levels have been developed to identify farms at risk, but these systems require guidance and are very costly.
Vaccination therefore seems to provide the best tool for long lasting control of neosporosis.
As with the use of live vaccines against Toxoplasma-induced abortions, it was also shown that experimental infection of naïve animals prior to pregnancy can reduce or even prevent abortion if challenged at 10 weeks of gestation (41), and can even reduce vertical transmission if animals were challenged around 130 days of gestation (42). However, no effect was seen on the recrudescing infection during mid-gestation in already infected animals (41).
This indicates that vaccination with live tachyzoites is feasible in seronegative animals. However, since no attenuated strain of Neospora caninum that would not induce a chronic infection is available, no such vaccine was developed. Some naturally low pathogenicity strains may be used for this purpose, as reported for the Nowra strain in Australia (43). Moreover, the main target is to develop a vaccine that prevents vertical transmission of the parasite in previously infected animals and not only in naïve animals.
Since the disease is associated to immunological changes that occur during gestation, the main focus of research has been on the immunological responses of both the cow and the fetus during the whole gestational period in relation to the effects needed to control an active or reactivated infection of N. caninum. Innes et al (44) reviewed the state of the art in this subject, and concluded that the reduction of maternal T-cell mediated immunity or the tilting of the balance towards the Th-2 type of response reduces the ability of the mother to generate IFN-gamma, TNF-alpha and other typical responses of the anti-inflammatory repertoire, which clears the way for the parasite to proliferate more or less uncontrolled and pass to the foetus.
The timing of this passage during gestation determines the outcome of infection. Early infection leads to high innate IFN-g response, resulting in abortion or mummification. Infection during mid-gestation is accompanied by a reduction in maternal response and the outcome is mainly abortion. When the infection occurs late in gestation, the immune system of the fetus is already able to cope with the infection and a healthy but infected calf is born (44).
Congenital acquired infection leads to life-long infection as a postnatal infection would, but the immune status of the animal may differ according to the timing of infection acquisition. There may be a point during which tolerance converts into responsiveness.
In Bovine Viral Diarrhoea (BVD), disease-tolerant calves are known to carry BVD virus when infected before gestational day 120; these calves do not produce antibodies, but shed virus during their entire life (45).
The immunological studies have been mainly carried out in rodent models for the availability of different cytokines, antibodies, and knockout strains. These models confirm the consensus hypothesis that the key cytokines during pregnancy are IL-4 and IL-10, which modulate the response towards a Th-2 bias (46).
Cattle were vaccinated with killed tachyzoites, and this was found to be effective in reducing the chance of abortion. The commercial vaccine resulting from these studies (Bovilis® Neoguard or in USA called NeoGuard, Intervet) consists of killed tachyzoites with Havlogen adjuvant in an oil-in-water emulsion given subcutaneously to 1-3 months pregnant cattle (47). Romero et al. (48) applied this vaccine in farms in Costa-Rica and found a 50% reduction in the risk of abortion in vaccinated animals (total n=876). Most abortions occurred during 5-6 months of gestation, which is consistent with earlier observations. Heuer et al. (47) used the same vaccine in New Zealand, and demonstrated similar efficacy. Interestingly, Heuer was able to detect a vaccination effect in seropositive cattle as well as seronegative cattle, although sample size was too small to be significant. Killed tachyzoite preparations were also efficacious in reducing transplacental transmission in pregnant mouse studies (49), whereas no data are reported that this is also true for cattle.
Rodent studies showed that recombinant antigens can be effective either as proteinaceous vaccine for SRS2 (50), for combinations of SAG1, SRS2 and DG1 and DG2 (51) or as plasmid-DNA for SAG1 and SRS2 (52), or presented by vaccinia virus for SRS2 (53). The extrapolation of such studies to the problems in cattle needs to be further studied.
In conclusion, N. caninum is an important cause of abortion and congenital infection of cattle. Although a killed vaccine is available, which reduces the chance of abortion, prevention of transplacental infection is the final challenge. Such vaccine could derive from defined antigens, which should be tested in the final host. A vaccine for dogs has limitations, since other canids or stray dogs can also transmit the infection, and are sources which are hard to control.

Sarcocystis neurona vaccine for horses
Sarcocystis neurona is the causative agent of a neurological disease in horses known as EPM (equine protozoal myeloencephalitis), observed especially in the Americas. The definitive host for this coccidian parasite is the opossum. Horses acquire the disease from sporocysts spread by roaming opossums. Although until recently parasites were detected only in horses that were severely immunocompromised (54), Rossano et al (55) described the culture of viable merozoites from the blood of an immunocompetent horse artificially infected with sporocysts obtained from opossum. The incidence of clinical EPM in the USA is rather low (<0.15%), but over 40% of horses are seropositive in areas where the opossum is prevalent (56). It is suggested that stress factors induced by transportation or heat can elicit the clinical phase of the disease, but that has not been proven yet. Cutler et al. (57) showed that dexamethasone-treated horses developed neurological symptoms when infected, but the parasite could not be detected as the unambiguous cause of the diseased state.
Notwithstanding the low chance of clinical disease, a vaccine has been conditionally launched in USA consisting of killed merogonic stages.
The efficacy is not documented since no challenge model is established. Field serology seems to be interfered by vaccination titers (58).

So, in conclusion, coccidial parasites are highly immunogenic and, due to their role in causing disease in animals and man, have been target for development of vaccines based on either live or killed parasites. Due to their diversity in species and hosts, vaccination not always the first method of choice, and new drugs are needed. However, there has been progress in the development of the current solutions into more sustainable products for the future. The increasing knowledge of molecular processes by unraveling of the genomic organization of the first three genera will pave the path for the development of more defined therapeutics and vaccines.

ACKNOWLEDGEMENTS
I would like to thank Drs. T. Schetters, R. Koopman, and J. Munoz-Bielsa for their helpful comments on this manuscript. I thank the organizers of the 9th International Coccidiosis Conference, Foz do Iguaçu, Brazil, for inviting me to present this review paper.


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