Vaccination of badgers

A report released by DEFRA (Ref 1) states that

"If only 50% of badgers can be trapped and injected with a vaccine which is only 50% effective, and only 50% of farms are involved the disease control benefit becomes rapidly diminished in any given year - 50% of 50% of 50% = 12.5% of the potential available benefit. While there will be a benefit, as any level of vaccination will produce a benefit, it will take substantially longer to appear in terms of reduced cattle breakdowns and vaccination will have to continue for a much longer time in order to accrue the benefit."

On 19th March 2009 DEFRA announced that they were planning to roll out the vaccination to areas where the percentage of infected badgers is likely to be high as inferred in the statement released by DEFRA (Ref 2) and for which an extract is shown below.

"The vaccine will be used in six areas of up to 100km² where there is a high incidence of bovine tuberculosis (bTB) in cattle. Vaccination will start in 2010 and continue for at least five years."

In June 2010 Agriculture Minister James Paice announced that the rollout to 5 of these areas were being cancelled and that it would only be rolled out to Stroud, Gloucestershire.

Realising a benefit from vaccinating badgers where the proportion of diseased badgers is high will take longer than vaccinating in an area where the proportion is low. Reference 1 states the following.

"As the vaccine only benefits uninfected badgers the proportion of the population which is uninfected will influence how quickly benefits are seen. Benefits may take longer to appear in high prevalence⁷ areas as it will take longer for the proportion of uninfected badgers to rise."

DEFRA (Ref 1) further states the following.

As discussed above, vaccination will need to be continued for a number of years in order to maximise the benefits.
The replacement rate for badgers is estimated to be around 30% per year so each year there will be new cubs needing vaccination.
The estimated lifespan of a badger is 3-5 years. An infected badger will not benefit from the vaccine so non-infected badgers need to be protected from infection until infected animals naturally die off.
Some badgers may also be missed in any given cycle of vaccination. Further vaccination cycles provide an additional opportunity to vaccinate these. The duration of immunity is also currently unknown so repeated vaccination may be required to maintain immunity.
The duration of the vaccination programme required to be effective is currently being modelled. The current best estimate is that vaccination will be required on an annual basis for a minimum of five years. Subsequent less frequent vaccination may be required to maintain immunity in the badger population. The longer vaccination is maintained the greater the disease control benefits.

There are further issues associated with vaccinating badgers and these are as follows.

In cattle the vaccine is most effective when given to calves less than 6 weeks old¹ because at this stage the risk of the animal being exposed to bacteria in the environment, which desensitises the animal to the vaccine, is least. If the same applies to badgers, badgers are unlikely to be vaccinated at an age when they are most responsive to the vaccine. Indeed Ref 11 states that cubs do not emerge from their setts until they are around 6-8 weeks old.

Trapping and vaccinating badgers involves very high labour costs. A portable fridge has to be carried to keep the vaccine below a certain temperature. The traps have to be dug in and prebaited and this incurs large staff costs.¹³ Also more cages have to be set to account for a badger recaptures. In fact FERA scientists found that in 2010 and 2011, 226 badgers of the 1169 vaccinated badgers were recaught on the following night¹².

Administering vaccines orally is far more difficult than via injection. The researchers have failed so far to find a formulation for the product that can survive long enough once digested by badgers to take effect.¹⁴

The other big difficulty has proved to be finding a bait that only badgers will eat. The effect would be diluted if another animals ate the bait and it would be a major problem if cattle did so, as this would affect the TB skin test.¹⁴

DEFRA in Ref 1 states that the current best estimate is that badger vaccination will be required on an annual basis for a minimum of five years The British Cattle Veterinary Association has referred to a timescale of 12 years. See Reference 3. Paul Livingstone who is the Technical Manager of the Animal Health Board in New Zealand has referred to a timescale of 20 years. See Reference 4.

Vaccination of cattle

Cattle vaccination is not 100% effective

The following extract taken from a document⁶ compiled in 2008 gives some detail.

Experimental evidence indicates that cattle are most responsive to BCG when the vaccine is administered to neonates (calves less than 6 weeks old). Vaccination early in life also reduces the chance of prior sensitisation to environmental mycobacteria which could affect responsiveness to vaccination. Likewise vaccination is not expected to have any beneficial effect in already infected cattle so earlier vaccination reduces the likelihood of the animal already being infected.

It is unlikely that a cattle vaccine will be developed in the short to medium term (i.e. within the next 5 years) that confers over 80% protection against bTB in the vast majority of cattle although this is currently a longterm research aim.

In the short to medium term what is more probable, is that a BCG vaccine is available that confers full protection against M. bovis infection to 50% of vaccinated animals. Of the 50% that remain susceptible to infection, over half will be partially protected and have a much reduced capability of transmitting M. bovis should they become infected. The benefits of vaccination are likely to last for at least 12 months. ⁶

After the BCG vaccine is administered, detection of infected animals becomes more difficult

The following extracts give some detail. The first extract came from a document⁵ dated September 2010 and the second extract came from a document⁶ compiled in 2008.

The BCG vaccine sensitizes cattle to the mandatory tuberculin skin test for some time after vaccination and can lead to a positive result when an animal is not infected with M. bovis (a false positive). Therefore Defra is also developing a diagnostic test to differentiate infected from vaccinated animals (known as a DIVA test) that could be used alongside the tuberculin skin test, where necessary, to confirm whether the animal is indeed infected. DIVA is an acronym formed from "Differentiating Infected from Vaccinated Animals". ⁵

The DIVA test will be based on the same biological assay as the current 'gamma interferon (IFN-?) test' which is currently used alongside the skin test to improve specificity in certain prescribed circumstances. Experimental evidence suggests that the BCG vaccine is most effective in animals under six weeks therefore the DIVA test will also need to be effective at this age. Although the test will function effectively below this age there may result in some drop-off in accuracy (sensitivity or specificity) compared to older animals. It may therefore be necessary to identify a 'compromise' target age range for vaccination where the vaccine and the diagnostic test are both sufficiently effective. ⁶

Reference 6 points out however that the desensitation to the skin test will subside over a certain period post-vaccination and the animals will then be tuberculin test negative. This time period has to be defined, but current evidence suggests that the majority of animals, greater than 90% will revert to test-negative status within 1 year.

Sensitivity and specificity of the DIVA test

In March 2010, the DIVA test was reported to have a relative sensitivity range of 77.0% to 86.5% at corresponding specificity levels of 100.0% to 77.6%²⁰.

This means that in order to avoid condemning any healthy animal, 23 out of 100 infected animals will be missed when using the DIVA test on vaccinated animals.

At the other end of the range, if it is accepted that 22 out of every 100 healthy animals are condemned, sensitivity of the test would increase so that 13 out of 100 infected animals would be missed instead of 23.

Legislation will need to be changed to allow the trade in live and dairy exports to continue.

The following is an extract from Reference 6.

The use of the tuberculin skin test to define OTF status in trade legislation creates difficulties when using 'sensitising vaccines' such as BCG which give skin test false positives. EU legislation will need to be amended to allow use of a vaccine which sensitises cattle to the tuberculin skin test without trade restrictions being imposed on live animals i.e. through the recognition of a new test which is capable of distinguishing between infected and vaccinated animals, with consequential amendments to domestic legislation.

Milk from animals sensitised to the skin test could not be consumed under current legislation. Changes to EU trade legislation will need to be appropriately worded to ensure that the prohibition in the EU hygiene legislation does not apply to vaccinated but not infected animals.

Reference 8 states that if legislation was not changed this would have a significant impact as our dairy export trade alone is annually worth around £1.1 billion.

In Reference 9, DEFRA state that breaking EU law by vaccinating domestic cattle could include

reputational damage,
loss of EU funding for the UK's TB eradication programme(€27m in 2010),
loss of exports to the EU in both live cattle (negligible) and cattle products (£375m in 2010),
infraction proceedings for which the minimum penalty is now set at €10m.

In response to the following follow-up enquiry to DEFRA

Would you by any chance be able to give any insight into whether or not the loss of EU trade in cattle products is likely to be partial or 100%?

the response was

Intra-Community trade of bovine animals and products is harmonized across the European Union, so we could expect any ban to apply to trade with all other Member states.¹⁰

Widespread use of the vaccine on cattle may pose a risk to human health

In Jan 2013 Tonio Borg from the European Commission in Reference 23 said the following.

BCG is the only vaccine available for humans and its use in cattle may lead to the selection of BCG-resistant strains of bTB that may affect also humans.

However Dr Ruth Watson MBBS, MRCP, FRCPath (retired Clinical Virologist) on 18th Feb 2013 responded as follows to the following questions on Twitter.

bronwyn @terrierview: Any thoughts on the widespread use of BCG in cattle & badgers encouraging resistant strains of TB?

Ruth Watkins @pengraiggoch: I don't believe BCG has been known to give rise to immune resistant TB- BCG does not persist in the body unlike TB itself.

bronwyn @terrierview: Thank you, that's interesting & helpful - I noticed the EU Commissioner mentioned it & I've never really thought about before.

Ruth Watkins @pengraiggoch: Also why I have not heard of vaccine resistant TB is that there is no one target for immunity, thus gain advantage by mutation.

bronwyn @terrierview: Reassuring, thanks: think the query was widespread indiscriminate use of only human vaccine in an uncontrolled/wild situation.

The results of a UK study into cattle vaccination completed in 2012 were disappointing

In 2012 a 7-year, 7-million pound study^17,19 into cattle vaccination carried out at AHVLA, Weybridge, Surrey was completed. The aim of this study is described in Reference 19 as follows.

The aim of this proposal is to establish a facility for generating natural transmission of M. bovis between cattle by assembling reactor cattle in a contained setting. This facility will then be used to determine the efficacy of promising vaccine candidates under conditions of natural transmission. This will be done by introducing sentinel vaccinated and control animals into the reactor herd and leaving them in-contact with reactor animals for 10-12 months. The protective efficacy of vaccine candidates will be determined by comparing disease rates between vaccinated and unvaccinated cattle. The first vaccine to be tested will be BCG given to neonates. Subsequent vaccines to be tested in this way will be prioritised on the basis that they have been shown to induce better protection against experimental challenge in cattle than BCG. This design was presented to and approved by VPAG, which includes a representative from the ISG, at its meeting on 12 May 2005.

In this study on the BCG vaccine it was found that in 3 groups of 20 cattle which were exposed to 39, skin-test-positive cattle, 2 cattle became infected in an unvaccinated group, 4 cattle became infected in a vaccinated group and 2 cattle became infected in a vaccinated and boosted group. Another words, although sample size was very small and this test would need to be repeated many times before results became meaningful, twice as many vaccinated cattle became infected as unvaccinated cattle, and an equal number of vaccinated and boosted cattle became infected as unvaccinated cattle.¹⁸ In the above summary, "infected" means "pathological symptoms had become sufficiently far advanced by the time postmortems were carried out so that they were detectable". Infection was detected in 36 of the 39 skin-test-positive animals.

Vaccination of cattle will incur very high test costs which will probably force the need for field trials in the UK

Prof Glyn Hewinson, who is chief scientist at the Animal Health and Veterinary Laboratories Agency (AHVLA), was reported as saying the following¹⁵.

The duration of immunity with the current vaccine is between one and two years, which would mean annual vaccination would be required at farm level
...
If we are going to generate data to show how effective the vaccine is, we will need to do field trials at some stage to know how effective the vaccines are in the UK, but that's not possible because it's illegal at the moment

These field trials would be expected to take several years in view of the way in which TB in the field changes from year to year. In order to build up confidence that any observed effect is due to the vaccine, as stated on Page 132 of Reference 22, 5 years of field trialling is likely to be required.

In order to help understand why it is necessary to look at data over such a long period, consider the way in which TB levels in Welsh counties have changed over recent years. For example between 2008 and 2010, the proportion of cattle with TB halved in Gwent whereas they stayed the same in Powys. This is shown in Fig 62 of Reference 16. What is striking about this is that Gwent and Powys are neighbouring counties.

Setting up the trials, analyzing the data, reporting them and then putting everything in place for a national roll-out is likely to need at least 10 years if field trials are needed.

Field trials will help to establish how the vaccine will perform in the UK environment and will help farmers decide whether its benefits justifies the financial cost of using it. Going ahead without a field trial risks the uptake being too small for the vaccine to have noticeable impact.

According to Reference 6,

113. The cost of a cattle vaccine is estimated at £8.25 sterling per dose. The perception of whether this is considered affordable and value for money will influence whether farmers are inclined and prepared to pay.

114. The cost of a DIVA test is estimated at £26 sterling per animal based on the cost of the IFN-g test. All vaccinated animals would be subject to the DIVA test if they showed a positive reaction to the skin test. It is estimated that 50% of the vaccinated cattle population will test positive to the tuberculin test at every routine annual test and so DIVA testing is necessary. Who pays for this additional testing cost is likely to strongly influence take up of vaccination by farmers.

Test cost is already very high. In fact in the last 5 years up to financial year 2009/10, test costs have exceeded compensation costs in 4 of these years. These test costs are expected to increase considerably if vaccination and the very expensive DIVA test ever come on stream. Both vaccination and DIVA testing will be applied on top of the current testing regime so will represent additional costs. Reference 6 says the following regarding pre-movement test costs. (Both pre and post-movement testing are currently paid for by the herd owner.²¹)

This issue is potentially even more acute for pre-movement testing where the need for an additional DIVA test could more than triple the cost of an individual test. Consideration would need to be given to how the costs of DIVA testing could be incorporated into the current pre-movement testing framework to ensure this does not become a significant barrier to uptake.

In view of costs, it may not be a case of just rolling out cattle vaccination on a national scale if/when EU laws change. In fact, implementation with adequate take-up may require a further 10 years to allow for field trials.

Note added on 23rd Feb 2013

The EU have stated a need for field trials. In Reference 23 dated 14th Jan 2013 it says the following.

Fundamental scientific information is not yet available on the reliability and feasibility of cattle vaccination accompanied by use of DIVA test(s) that is fundamental for a possible change in the current EU policy on the control and eradication of bTB. Future studies should also address food safety concerns (shedding of vaccine strain in milk), human health concerns (BCG is the only vaccine available for humans and its use in cattle may lead to the selection of BCG-resistant strains of bTB that may affect also humans) and animal health and trade concerns (proper discrimination between vaccinated and infected animals, costs/benefits of vaccination policy, current policy, acceptability of vaccinated animals in international trade).
...
In order to provide answers to the still open scientific questions on bTB vaccination, substantial experimental research and large scale long lasting (possibly 2-5 years) trials, also under EU field conditions, are needed [start 2013, end 2015-2016].

References

If you would like to send to me your comments, please contact me.

Last Modified 27 Apr 2013 07:32

Vaccination of badgers and cattle

Vaccination of badgers

Vaccination of cattle