B.AHE.0060-Improved anaplasmosis protection of cattle in tick-infested areas of Australia (Dawn A marginale strain) | 2023

Summary

A. marginale causes significant economic loss to the cattle industries and causes Anaplasmosis. The current vaccine for anaplasma spp uses the Anaplasma centrale strain, and only provides partial and variable immunity against the Australian isolates of A. marginale. Furthermore, A. centrale is not completely benign and can cause anaemia, and also rapidly loses potency.

An isolate of A. marginale (Dawn strain) indicated to be as mild as A. centrale and provide almost total protection against Australian field isolates of A. marginale. Given the mild nature of Dawn strain, 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 evaluate this strain for registration; 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.

Trial steers were inoculated with three different batches of both experimental chilled and frozen trivalent vaccines. Safety, virulence, infectivity, and efficacy were compared to the current registered vaccine. Whilst there was no difference in safety and virulence of the experimental vaccines, infectivity of the A. marginale was poor in some of the vaccines, and unexplained lack of virulence makes interpretation of some aspects of the trial difficult. Dawn strain A. marginale was also shown again to be poorly tick transmissible. It will not be possible to register Dawn strain A. marginale as a vaccine strain without further evaluation.

Objectives

The intention of this project was to further evaluate this strain in line with Australian Pesticides and Veterinarian Medicine Authority (APVMA) requirements for registration of immunobiologicals.

Based on the APVMA registration requirements, the following objectives were developed.
• further evaluate the safety and efficacy of Dawn strain A. marginale in line with the requirements to achieve full registration of this strain with APVMA as a component of trivalent chilled and frozen tick fever vaccines
• validate the shelf life of Dawn strain A. marginale in chilled and frozen tick fever vaccines
• further investigate the tick transmissibility of Dawn strain A. marginale
• develop a marker which allows discrimination of vaccine and field strains of A. Marginale

Key findings

There is some corroboration of a previous study where Dawn strain A. marginale proved to be as safe as A. centrale and no more virulent. Molecular studies identified a missing gene in Dawn strain A. marginale which may be useful to distinguish it from other field isolates and the poor tick transmissibility of Dawn strain A. marginale was confirmed.

However, poor infectivity of Dawn strain A. marginale was not easily explained given previous results; and apparent lack of virulence in the challenge trial using Gypsy Plains strain has made any interpretation of efficacy of the vaccines difficult. Reasons for poor infectivity and the effect of Theileria spp infection on response to A. marginale infection need to be further investigated. The challenge study needs to be repeated before consideration is given to registration of Dawn A. marginale as a vaccine strain.

Benefits to industry

Anaplasmosis accounted for about 14% of the confirmed tick fever outbreaks in Queensland in the period 1990 to 2009. The current widely used vaccine uses live A. centrale and only imparts partial and variable immunity against the Australian isolates. This vaccine can cause anaemia and reduction of weight gain and only has a four-day shelf life, limiting availability for some users. The Dawn strain showed potential for being milder, providing better protection, and extended shelf life.

Future research

Reasons for poor infectivity of Dawn strain A. marginale and apparent lack of virulence of Gypsy Plains strain A. marginale in this trial and the effect of Theileria spp infection on subsequent A. marginale infection needs further evaluation. Additionally, cut-off points and sensitivity and specificity of the VMRD Anaplasma spp cELISA need to be discussed with the manufacturer of the test kit; and further evaluation conducted under Australian field conditions.