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Background

Chicken coccidiosis is a widespread parasitic disease caused by several protozoal species of the genus Eimeria (Apicomplexa). These single celled parasites infect the intestinal tract of chickens, damaging the lining of the intestines and disrupting nutrient absorption. Different species of Eimeria affect different parts of the chicken's intestinal tract, and they can lead to varying degrees of illness. The infection is transmitted through a faecal-oral route by the ingestion of parasite ‘eggs’ (oocysts) found in contaminated feed, water, or bedding (see Figure 1). Once ingested, the parasites invade intestinal cells, causing damage and inflammation. This can lead to clinical signs such as watery or bloody diarrhoea, lethargy, decreased appetite, weight loss, dehydration, and sometimes even death in severe cases (see Figure 2).

Eimeria are present in most chicken production settings, but they are most problematic in intensive poultry farming systems where large numbers of animals are kept in close proximity. Overcrowding, poor sanitation, and lack of proper ventilation increase the risk of the disease spreading quickly. In the UK, the financial cost of coccidiosis was estimated to be approximately £100 million in 2016, primarily due to direct costs such as drugs/vaccines for protection and treatment, when protection fails, and indirect costs like reduced productivity (weight loss, lower egg production, and poor feed conversion) and increased mortality. Effective prevention and treatment are crucial to improving chicken welfare and, in doing so managing these costs.

Models for coccidiosis research at the ºÚÁÏÉç

• Chicken models to test anticoccidial vaccines

Vaccination is one of the main methods of controlling chicken coccidiosis, particularly in egg layer and breeder chicken settings. Live attenuated vaccines are used in these chicken populations in the UK but are expensive due to the requirement for live pathogen-free chickens for vaccine production. These vaccines are not commonly used in production of chickens for meat (broilers) due to limitations in production capacity and cost. As a result, the industry demands new alternatives. At the ºÚÁÏÉç, we lead research in the discovery of new antigens with immunoprotective properties against coccidiosis, as well as novel delivery systems to improve mass application in commercial settings. Once selected, antigens and delivery systems must be evaluated in the natural host, necessitating the use of chickens for experimental research. Under the Animals (Scientific Procedures) Act (A(SP)A), we have conducted studies that have significantly advanced the field of coccidiosis vaccine research, for example: ; ; .

• Animal-free models to test anticoccidial compounds

Anticoccidial drugs (ionophores or chemicals) are the primary method for preventing coccidiosis in chickens and is well established in the broiler sector. However, there are growing concerns regarding their application, including the development of resistance in the field, prohibition in certain regions (where ionophores are classified as antibiotics), and increasing public concerns about the use of drugs in production animals. These issues have led to a significant rise in the testing of alternative, environmentally friendly compounds, which has consequently increased the number of chickens used in this area of research.

To accelerate discovery and reduce the use of animals in research, the ºÚÁÏÉç has developed an in vitro (animal free) model of infection (see Figure 3) to pre-screen natural compounds before they are tested with animals: . This model has been widely adopted by companies, resulting in the development of several collaborative projects: ; ; .

It is important to highlight that the parasites responsible for chicken coccidiosis can only replicate in the natural host - the chicken. For this reason, the application of this in vitro model also depends on the use of chickens as a source of parasites under A(SP)A. At the ºÚÁÏÉç we have optimized protocols for parasite replication, balancing production with levels of challenge that do not induce clinical signs in chickens. This approach is based on our experience with the parasites, understanding the replicative potential and known pathogenicity of each species.

One measure adopted to reduce the number of chickens needed to source parasites for the in vitro model has been miniaturization of the model itself, leading to an 80% reduction in the use of chickens to evaluate natural compounds: .

• Novel models for coccidiosis research

Recent advances in new technologies for research outside of animals, such as spheroids, organoids, precision-cut tissue slicing (PCTS) for explant preparation, and organ-on-a-chip, are now a reality and ready to be applied to coccidiosis research. These systems show great promise in supporting coccidian parasite replication, potentially reducing and eventually replacing, the need for chickens to stock parasites for research. Additionally, they can support preliminary and complementary analysis and observations at the host-parasite interaction level that are currently impossible, adding significant value to our existing vaccine and anticoccidial testing models by assessing new strategies in vitro before deciding on the optimal option to take forward to in vivo studies.

At the ºÚÁÏÉç, we have successfully infected spheroids generated from immortalized chicken cell lines: . More recently, chicken intestinal explants obtained by PCTS have supported parasite infections for several days (see Figure 4). Looking to the future, the ºÚÁÏÉç has been awarded funds to develop the first organ-on-a-chip facility for veterinary species. In the near future, we aim to apply the different systems in combination to research chicken coccidiosis, offering opportunities for groundbreaking developments to replace the use of chickens in this area of research.