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Improved protection of cattle against anaplasmosis in tick-infested areas of Australia
A. marginale is endemic in tropical and subtropical areas of the world where it causes significant economic loss to the cattle industries. Anaplasmosis accounted for about 14% of the confirmed tick fever outbreaks in Queensland in the period 1990 to 2009.
Anaplasma centrale, the current Anaplasma spp strain in trivalent tick fever vaccine, imparts partial and variable immunity against challenge with Australian isolates of A. marginale. A. centrale is not transmitted by Rhipicephalus (Boophilus) microplus or other ticks present in Australia. There are occasional outbreaks recorded of anaplasmosis in herds where serological evidence indicated that A. centrale vaccine had provoked an immune response, but the protection provided was inadequate. Despite this, field evidence suggests that it provides adequate control on most properties against the effects of A. marginale in Australia. A. centrale is not completely benign and is known to cause anaemia, and the principal reason for the very short four day shelf life of the chilled tick fever vaccine is the rapid loss of potency of the A. centrale strain. This limits the availability of the chilled vaccine to users under some circumstances.
The identification of naturally occurring A. marginale isolates that produce mild infections in cattle and protect against virulent challenge has eluded most efforts. The original assessment in 2003 however, of one Australian isolate of A. marginale (Dawn strain) indicated it was as least as mild as A. centrale and provided almost total protection against challenge with heterologous Australian field isolates of A. marginale. This study also indicated it was poorly tick transmissible, as two attempts to artificially transmit this Dawn isolate in pen trials using R. (B.) microplus were unsuccessful. In field trials conducted in 2010, there was every indication that Dawn strain A. marginale was very infective and the infectivity persisted well in the vaccine post-manufacture.
Dawn A. marginale is currently available under Australian Pesticides and Veterinary Medicines Authority (APVMA) Minor Use Permit as a monovalent frozen vaccine. It has been used successfully and without adverse reactions since 2004 on two properties where A. centrale vaccination had failed to prevent anaplasmosis.
Given the mild nature of Dawn strain A. marginale, the advantages it offered in terms of better protection of cattle against anaplasmosis and the potential to extend the shelf life, the intention of this project was to further evaluate this strain in line with APVMA requirements for registration of immunobiologicals (Module 8.3 – Safety and Efficacy); to clarify tick transmissibility; and to find a molecular marker that would allow differentiation of Dawn strain A. marginale in the vaccine from A. marginale field isolates. The aim was to replace A. centrale with Dawn strain A. marginale in the registered chilled and frozen tick fever vaccines.
This page was last updated on 24/07/2017
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