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 4m® 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.