Contributed
Papers: Posters
Vaccines |
THE USE OF
A VIABILITY ASSAY TO FORMULATE A NEW COCCIDIOSIS VACCINE
Julia J. Dibner, Marco A. Quiroz, and Chris D. Knight
Novus International, Inc., 20 Research Park Dr., St.
Charles, MO 63304
marco.quiroz@novusint.com
Determination of coccidial
oocyst viability has traditionally been done using
in vivo methods such as lesion score or oocyst shedding
following the oral delivery of a dose determined by
microscopic enumeration. The microscope is readily
used to determine number of sporulated vs non-sporulated
oocysts per ml, but the determination of % live oocysts
is not possible using a microscope. This introduces
a source of uncontrolled variability when multiple
suspensions of sporulated oocysts are used to formulate
a live coccidiosis vaccine. An in vitro assay (VIACYST®
assay) has been developed to measure the viability
of sporocysts of E. tenella, E. maxima and E. acervulina.
The assay is based on the use of a non-vital stain,
ethidium bromide. The assay requires the excystation
of sporocysts and depends on the permeability of sporocysts
from Eimeria or any other sporocyst-forming protozoan
to this non-vital dye. Preliminary experiments confirmed
that vaccine from a suspension of E. tenella oocysts
assessed non-vital by ethidium bromide staining were
also not capable of generating resistance to a coccidiosis
challenge. As the proportion of oocysts from the non-vital
suspension increased in the vaccinating dose, there
was a concomitant decrease in the resistance of the
bird to oral challenge as indicated by lesion scores.
When the immunizing inoculum contained 1000 viable
oocysts or less, lesion scores were not different
than non-vaccinated controls. This study confirmed
that the in vitro viability assay correlates to the
ability to confer resistance to a coccidiosis challenge.
A second experiment was done to test the use of the
viability assay to formulate vaccines from suspensions
of oocysts that ranged from 24 to 52 weeks old and
16 to 82% viability. Results indicated that suspensions
of E. acervulina varying widely in viability and age
were all capable of generating resistance to an oral
challenge providing the vaccine was formulated on
a viable oocyst basis.
Ethidium bromide assay detects non-viable sporocysts
that reflect the original oocyst population and that
formulation of the vaccine based on viable sporulated
oocysts/bird will yield consistent efficacy across
a wide range of age and viability.
®VIACYST
is a service mark of Viridus Animal Health, LLC, and
is registered in the United States and other countries.