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 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.

Furthermore, 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.

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 document¹ 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. ¹

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.

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 on the UK Agri-Food Sector and the economy as a whole as this trade is annually worth around £1.06 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

Why is it necessary to spend money addressing bovine TB when bovine TB is currently considered to be a very small risk to human health?

Prof Christl Donnelly replied to this on the BBC's Farming Live program on 28th July 2011 as follows.

If there was no control of the animal health disease then bovine TB would increase dramatically and then it would be a huge occupational health risk. Also any failure of systems could then have potential for public health risks.

References

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Last Modified 08 Apr 2012 11:51

Vaccination of badgers and cattle

Vaccination of badgers

Vaccination of cattle