Tick fever disease and the use of vaccines

Bull breeders and their clients will be well aware that bulls destined for the “ticky’ regions of Queensland, Western Australia and Northern Territory should be vaccinated against tick fever. Buyers paying for ‘preferred’ genetics will be looking for some insurance that a purchased bull makes it past its first encounter with the local cattle ticks.

Standard advice is:

• A single vaccination is usually sufficient. The tick fever vaccine is an attenuated (ie. made mild) live vaccine derived from parasites isolated from field cases. We know in the case of Babesia bovis (which causes most of our outbreaks of tick fever) and Anaplasma centrale that the animals are likely to remain infected for life – the organisms continue to circulate in the bloodstream at a very low level, and therefore continually prime the immune system. So once infected or successfully vaccinated (unless treated), we think the animal should always be infected and therefore immune.
• Vas weaners to reduce the risk of vaccine reactions. Although in most cases there is no clinical effect of the vaccine, it is not completely devoid of risk and sometimes results in a clinical reaction. Weaners rarely show any reaction to the vaccine. The risk of a reaction to the vaccine (just as with tick fever in the field) increases in animals vaccinated as yearlings or adults, and as the Bos indicus content of the cattle decreases. The usual reaction is a very mild form of tick fever disease which would not be noticed just by looking at the animal – maybe a small temperature rise, some reduction in red cell count (that is, slight anaemia) and perhaps some reduction in weight gain. In rare instances however the reaction is more severe and the animals must be treated just as if they had contracted disease in the field. It is an unfortunate fact of life that if we take all the “sting” out of the vaccine (which we can do), it loses some of the ability to protect against the variety of tick fever parasites the animal may encounter in the field. Vaccine reactions occur between 7 – 21 days for Babesia and 30 – 60 days for Anaplasma. This means that monitoring for vaccine reactions, and the effect of vaccine reactions, can take place for 2 months from the time of vaccination. Weaners need little if any monitoring after vaccination.
• Vaccinate well away from sale time and Bull Breeding Soundness examinations. Bulls are perhaps more susceptible to the risk of vaccine reactions; and although the incidence of sustained high fever and other vaccine reactions is very low, there may be an effect on appearance at sale (weight loss) and a temporary effect on subsequent fertility. Vaccine reactions might also affect results at pre-sale bull breeding soundness examinations.
• Vaccinate at least 2 months before the bulls are introduced to the cattle tick areas so that immunity to all three parasites has developed prior to introduction; if that is not possible, allow 3 weeks for immunity to the Babesia spp at least to develop.

In most cases, a single successful vaccination allows introduction of bulls to tick areas without problems. However, this is not always so, and we occasionally investigate reports of bulls (and others) developing tick fever after introduction, despite having been vaccinated. Why does this occur, and what can we do about it?

There are two main issues:

1. Anaplasma centrale does not provide complete protection.
A centrale is the organism incorporated into the trivalent tick fever vaccine to protect against field infection with Anaplasma marginale. It is not perfectly protective and in some cases, where there is heavy tick challenge and a virulent A marginale organism, then clinical disease results. Even where there is not evidence of clinical disease, it would not be uncommon to find changes to red blood cell counts and evidence of organisms in blood smears when the bulls first encounter the field strain of A marginale in cattle tick infested areas.

We are currently in the process of evaluating an isolate of A marginale (named Dawn strain – after the cow!), with the intention to register this organism with APVMA as the vaccine strain to replace A centrale. Initial work indicated that Dawn strain was not only a “milder” organism than the current vaccine strain (Anaplasma centrale) in terms of vaccine reactions, but also provided better protection against virulent field strains of A marginale. Further to this in field trials conducted in 2010, Dawn strain proved highly infective in an experimental vaccine. There is substantial work yet to be done to prove safety, efficacy, and lack of virulence and assess tick transmissibility, but we are working on those issues in 2011; hopefully this will lead to improved protection against anaplasmosis.

2. Does one vaccination dose provide adequate immunity?
We know that once the animal develops immunity that it is likely to be lifelong, particularly with Babesia bovis and Anaplasma spp. This occurs because the vaccine contains live organisms which establish a lifelong infection in the animal, simmering away at a level that is undetectable when we examine blood smears, but nevertheless present and providing some constant interaction with the immune system. This immunity may also be broadened by challenge from field strains of parasites encountered through tick bites. So, once vaccinated and successfully infected with the vaccine strains, there is no advantage in giving a second dose. This is quite a different scenario to many other cattle vaccines where a live persistent infection does not follow vaccination, and so two initial doses and regular boosters are needed to keep the immunity primed.

However, our experience and extensive trial work tells us that a small number of cattle will not become immune to one or other of the organisms in the tick fever vaccine, but we expect greater than 95% of animals to be immune to each of the organisms. So after one vaccination, a few animals will not have protection against all three organisms. A booster vaccination in valuable animals like bulls (and valuable cows, ET recipients etc) might be considered, particularly if they are born and raised in the cattle tick free area, to increase the chance that the animal develops immunity to all three organisms. Ideally, this should be given some months prior to a sale. For those animals which are raised in cattle tick area, the risk of not developing immunity to one or other organism is reduced because there is the chance of exposure to tick fever organisms in infected ticks, in addition to the immunity conferred by vaccination.

So, the recommendation is that a second vaccination is strongly considered for all susceptible animals (but especially bulls and other valuable animals) coming into the tick area from the cattle tick-free areas. This means that there is a second chance for the animal to develop immunity to any organism where infection (and immunity) failed to establish after the first dose of vaccine.

How should this be handled? There seems to be a number of options. Ideally the bulls would have the second dose well before going in to the ticky areas. So regimes might include an initial dose at weaning and a second dose prior to sale; or an initial dose with a second dose after sale for those animals destined for the tick areas; or it may even be that the vendor chooses to give only one vaccine dose, but passes on the advice that a second dose might be warranted. The actual way this is handled and the appropriate regime will probably vary for each property’s circumstances.

What is the risk of a vaccine reaction when giving a booster? For bulls that have developed immunity to all three organisms from previous vaccination, the risk is virtually nil. The risk of a vaccine reaction is only present in the few animals where immunity has not developed to all three tick fever parasites; and only for those parasites to which immunity has not developed. The risk then is the same as if they were encountering that particular parasite in the vaccine for the first time. All other risk conditions apply – the age of the animal, the breed, and so forth. These non-immune animals however would also be very susceptible to a field infection of tick fever.

Can we test to see if immunity has developed? The short answer is that tests are available, but it is not a service that is routinely offered, except for investigation of vaccine problems and R&D work. The testing is reasonably expensive (currently over $24 per head to cover the B bovis and Anaplasma tests without the cost of sample collection), labour intensive for small numbers and there are some inaccuracies with the test (and indeed most diagnostic tests) which can make interpretation of results difficult for each individual bull. It is cheaper to give a second dose.

Further general information about tick fever can be found at our website or by contacting the Tick Fever Centre:
Tick Fever Centre
Biosecurity Queensland
280 Grindle Road Wacol Qld 4076
Phone: 07 3898 9655
Fax: 07 3898 9685
Email: tfc@deedi.qld.gov.au
Business Information Centre 13 25 23
Visit www.biosecurity.qld.gov.au and search for ‘tick fever’