Apicomplexans, including species ofEimeria, pose a real threat to the health and wellbeing of animals and humans.Eimeriaparasites do not infect humans but cause an important economic impact on livestock, in particular on the poultry industry. Despite its high prevalence and financial costs, little is known about the cell biology of these ‘cosmopolitan’ parasites found all over the world. In this review, we discuss different aspects of the life cycle and stages ofEimeriaspecies, focusing on cellular structures and organelles typical of the coccidian family as well as genus-specific features, complementing some ‘unknowns’ with what is described in the closely related coccidianToxoplasma gondii.

Recently, the availability of protocols supporting genetic complementation of Eimeria has raised the prospect of generating transgenic parasite lines which can function as vaccine vectors, expressing and delivering heterologous proteins. Complementation with sequences encoding immunoprotective antigens from other Eimeria spp. offers an opportunity to reduce the complexity of species/strains in anticoccidial vaccines. Herein, we characterise and evaluate EtAMA1 and EtAMA2, two members of the apical membrane antigen (AMA) family of parasite surface proteins from Eimeria tenella. Both proteins are stage-regulated, and the sporozoite-specific EtAMA1 is effective at inducing partial protection against homologous challenge with E. tenella when used as a recombinant protein vaccine, whereas the merozoite-specific EtAMA2 is not. In order to test the ability of transgenic parasites to confer heterologous protection, E. tenella parasites were complemented with EmAMA1, the sporozoite-specific orthologue of EtAMA1 from E. maxima, coupled with different delivery signals to modify its trafficking and improve antigen exposure to the host immune system. Vaccination of chickens using these transgenic parasites conferred partial protection against E. maxima challenge, with levels of efficacy comparable to those obtained using recombinant protein or DNA vaccines. In the present work we provide evidence for the first known time of the ability of transgenic Eimeria to induce cross protection against different Eimeria spp. Genetically complemented Eimeria provide a powerful tool to streamline the complex multi-valent anticoccidial vaccine formulations that are currently available in the market by generating parasite lines expressing vaccine targets from multiple eimerian species.

Coccidiosis caused by Eimeria species parasites remains a major threat to poultry production, undermining economic performance and compromising welfare. The recent characterisation of three new Eimeria species that infect chickens has highlighted that many gaps remain in our knowledge of the biology and epidemiology of these parasites. Concerns about the use of anticoccidial drugs, widespread parasite drug resistance, the need for vaccines that can be used across broiler as well as layer and breeder sectors, and consumer preferences for ‘clean’ farming, all point to the need for novel control strategies. New research tools including vaccine delivery vectors, high throughput sequencing, parasite transgenesis and sensitive molecular assays that can accurately assess parasite development in vitro and in vivo are all proving helpful in the ongoing quest for improved cost-effective, scalable strategies for future control of coccidiosis.

Neospora caninum, a tissue cyst-forming parasite, is the causative agent of bovine neosporosis. It is considered to be one of the most important transmissible causes of reproductive failure in cattle; abortion and neonatal mortality result in significant economic losses within the cattle industry worldwide. The balance between acute (mediated by the tachyzoite stage) and persistent (mediated by the bradyzoite stage) phases of the infection is influenced by the immune status of the animal, and for pregnant cows (the intermediate host) immune status is critical for transplacental (i.e. vertical) transmission of the parasite and associated disease outcomes. The horizontal route of transmission from the definitive host, the dog, occurs via ingestion of oocysts containing sporozoites, and plays a minor but important role in transmission of the infection to cattle. Despite the importance of this disease, there is no vaccine or treatment available currently, and at the present the only control measure to reduce the impact of disease is informed management on the farm. The development of vaccines, targeting key biological processes such as invasion and persistent infection, is needed urgently for the control of this widespread parasite.

Eimeria species are parasitic protozoa that cause coccidiosis, an intestinal disease commonly characterised by malabsorption, diarrhoea and haemorrhage that is particularly important in chickens. Vaccination against chicken coccidiosis is effective using wild-type or attenuated live parasite lines. The development of protocols to express foreign proteins in Eimeria species has opened up the possibility of using Eimeria live vaccines to deliver heterologous antigens and function as multivalent vaccine vectors that could protect chickens against a range of pathogens. In this study, genetic complementation was used to express immunoprotective virus antigens in Eimeria tenella. Infectious bursal disease virus (IBDV) causes Gumboro, an immunosuppressive disease that affects productivity and can interfere with the efficacy of poultry vaccination programmes. Infectious laryngotracheitis virus (ILTV) causes a highly transmissible respiratory disease for which strong cellular immunity and antibody responses are required for effective vaccination. Genes encoding the VP2 protein from a very virulent strain of IBDV (vvVP2) and glycoprotein I from ILTV (gI) were cloned downstream of 5’Et-Actin or 5’Et-TIF promoter regions in plasmids that also contained a mCitrine fluorescent reporter cassette under control of the 5’Et-MIC1 promoter. The plasmids were introduced by nucleofection into E. tenella sporozoites, which were then used to infect chickens. Progeny oocysts were sorted by FACS and passaged several times in vivo until the proportion of fluorescent parasites in each transgenic population reached ~20 % and the number of transgene copies per parasite genome decreased to < 10. All populations were found to transcribe and express the transgene and induced the generation of low titre, transgene-specific antibodies when used to immunise chickens. E. tenella can express antigens of other poultry pathogens that are successfully recognised by the chicken immune system. Nonetheless, further work has to be done in order to improve the levels of expression for its future use as a multivalent vaccine vector.