Development of a histological scoring model for use in
interpreting the immune response

D.G. Richards*, Eimeria Research Project, Werribee, Australia
*medichick@ava.com.au

A number of the stages of Eimeria growth can be identified visually using gross lesions. These observations are the tried and true method of intestinal lesion scoring. Histology can identify lesions not visible to the eye and can visualise infections at an earlier stage of the life cycle. Some lesions uniquely identify the species. Therefore, it should be possible to identify mixed infections using histology. This hypothesis was investigated using a mixed infection of Eimeria with a high chance of being differentiated using histology. In addition, it was hypothesised that with the onset of an immune response, the number of microscopic lesions in tissues would decrease. EimeriaVAX 4mŽ is a 4 species precocious coccidiosis vaccine of Australian origin and contains Eimeria acervulina, E maxima, E necatrix, and E tenella. This vaccine was used to validate the accuracy of histology. A trial was designed in 2 parts: the first inoculated ten (10) 7-day-old chickens with a times-10 dose of vaccine. At 5 and 7 days post-inoculation, tissues were fixed and stained with PAS or H&E. The number of chickens showing lesions was enumerated, and the species were identified. This trial was repeated with different batches of vaccine ten times. A second trial examined the relationship between the histological lesions present and the onset of immunity following eye-drop vaccination with EimeriaVAX 4m. These birds were challenged with a times-80 dose of the same vaccine.
In trial 1, 100% of chickens were infected following an oral dose of vaccine. In trial 2, an immune response was developed which resulted in the dramatic reduction in the number of lesions detectable in immune birds. As the result of this data, a simplified scoring model was developed and is used for vaccine testing.
The results indicate that histology detects consistent infections, is a useful technique for examining multiple species infections, and has the potential to be used to examine the onset of an immune response. Unlike faecal oocyst counts, histological slides can be archived, which facilitates Good Laboratory Practice (GLP) compliance.
The conclusion is that histological lesion scores are repeatable. This paper also presents arguments in favour of the use of the homologous challenge models to simplify the traditional challenge models using wild Eimeria sp.