What is the best time of year to distribute baits in the field?

There are two aspects to this question: (i) vaccine and (ii) issues related to the target species.
(i) Vaccine: all commercially available vaccine baits for foxes contain a liquid vaccine suspension. High temperatures are especially harmful to liquid vaccines, and exposed to high temperatures, vaccines rapidly lose infectivity (loss of potency). There are differences among the vaccine viruses depending on the virus strain used and the stabilisers incorporated into the vaccine.

However, in general, most oral rabies vaccine viruses lose their infectivity within several days if they are exposed to temperatures above 35°C. Therefore, distribution of baits between late spring and early autumn should be discouraged for regular vaccination campaigns. It is important to remember that measured air-temperature is completely different than to the temperatures on the ground.

Also bait distribution should not be conducted during the winter in areas where it is very likely to freeze (<0 °C) If the vaccine virus is frozen, the vaccine will not be released into the mouth cavity when the fox perforates the capsule and bait uptake results in an unsuccessful vaccination attempt. It is also important to remember that in open areas night temperatures <0°C can occur regularly in late autumn and early spring in the northern hemisphere.

(ii) A further point to consider is the target species. Generally, it is advisable to distribute baits when natural food resources are less abundant. If the young fox population is targeted, it is important to keep in mind that these young animals have relatively restricted movements, and distributing baits at standard flight line distances could considerably limit access of young foxes to the baits (see case study here). To circumvent this problem, baits could be distributed directly in and around fox dens. However, it is important to take into consideration the fact that maternal antibodies could interfere with immunization, and that only a fraction of fox dens are known to hunters. Finally, bait competition with non-target species during certain periods can have a profound effect on bait availability to the target species, and this needs to be taken into consideration when planning campaigns.

In general, under moderate climate and sub-tropical conditions (e.g. Turkey) two ORV campaigns a year in early spring and late autumn, and one during winter, respectively, have been shown to be optimal.

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Version 1 - Last updated November 2012