Dear Mary
I was asked by Sally Hall of
www.bovinetb.co.uk what I thought of Colin Fink's
letter posted on Warmwell about using triple therapy in oral
bait for badgers to treat M bovis infection in them. My initial response
had been one of rejection but I am glad to have the opportunity to
reconsider. Now I am finding out more about
M bovis in an
effort to understand the infection and disease and to think outside the
box.
I was not previously aware that latent TB could be
treated to prevent its reactivation at any time in future life, (unless
reinfection takes place later (reinfection is documented in humans and when
opportunity for reinfection is high it commonly occurs)). Latent TB can
be treated by 9 months of isoniazid alone (and a shorter course for
chemoprophylaxis after exposure), unless the infecting organism was already
resistant at the time of initial infection. I believe this is what some
camelid owners are doing for exposed camelids. Both camelids and goats as
species seem to be particularly susceptible to M bovis and if
infected develop a progressive infection from which they die relatively
rapidly, within a year.
The infection rate in a confined space such as a
social group or sett of badgers will be between 25 and 50% if comparable to
humans, but genetics and species may make it higher or lower. The diagnosis of
latent M bovis infection in badgers is not accurate in microbiological
terms as yet and would be likely to be underestimated by any tests currently
used on them. Also whatever the badger behaviourists might say if M bovis
DNA is present in the sett latrine then there is M bovis infection in the group
of badgers associated with that sett.
If Isoniazid alone is put out for badgers then the
actively infected badgers that eat it (one cannot exclude actively infected
badgers from eating the bait) are likely to develop resistance and the
M bovis shed will be resistant to isoniazid within weeks or months
and infect other badgers with resistant M bovis, so that strategy won't
work.
In humans active infection is treated with multiple
antibiotics taken every day, isoniazid, rifampicin, pyrazinamide and
ethambutol for example- 2 months on this combination, then depending on the
culture, typing and sensitivity report, reducing to 3 for a further 4
months, but in the case of M bovis dropping pyrazinamide as M
bovis is intrinsically resistant so that in M bovis 3 drug
treatment is given for 9 months in total. If the treatment is taken
reliably then the person is cured of that M tuberculosis or M
bovis infection. For example elephants have been cured of M
tuberculosis infection (they are only rarely infected with M
bovis). (It is unaffordable for farmers to treat cattle).
As you have probably heard multiple drug resistant
and even extremely drug resistant M tuberculosis is now circulating in
humans because they have not taken the full course of treatment, either only
taking it intermittently or for too short a time. These organisms can
infect hitherto uninfected individuals and may be
untreatable and lead to death or indefinite confinement if excretion cannot be
stopped by surgical removal of infected lung for example.
For this reason I have argued against Colin Fink's
idea of putting out triple antibiotic therapy bait for badgers
hitherto.
However if there is essentially a reservoir of
M bovis in badgers and cattle (and deer, camelids, cats etc) that
humans almost never get maybe such an approach together with vaccination of
badgers and birth control of badgers / population reduction would not be such a
bad idea after all. Vaccination of cattle and testing and culling of
infected cattle would have to continue to get rid of M bovis
infection. The cycles of infection would be broken, stopping reinfection
of either badgers or cattle from the other species.
The reasoning would be
1 The closely observed sett would thus have
triple drug treatment of the active and latent M bovis infected badgers
and if this was continued for at least 9 months it could
eliminate infection from that sett. It could also prevent newborn
badgers from becoming infected if treatment was started months before their
birth.
2 All the setts in possible contact with each
other would have to be treated simultaneously.
3 If multiple drug resistant M bovis
developed (part of the monitoring) then that group of badgers would have to be
culled as it could spread to other badgers or the programme halted because it
would be useless.
4. If such resistant M bovis infected a
human which would be very unlikely, that human is anyway very unlikely to pass
it on to another human (though this has been recorded on rare occasion that
M bovis produces lung disease and shedding in sputum). An epidemic of
resistant M bovis in humans would not happen as it is essentially a
preventable zoonosis by pasteurisation of milk and indeed is prevented by this
means in the UK. Another reason an outbreak of resistant M
bovis is unlikely to happen in humans at all is that the resistance of slow
growing mycobacteria, M bovis and M tuberculosis, to
antibiotics is caused by mutations in chromsomal genes. The resistant
organism itself must be acquired as an infection to give rise to an
epidemic. The resistance is not on promiscuous genetic elements that can
be spread to other mycobacteria species.
5 Having small numbers of closely observed
and monitored setts would necessitate reduction in the badger population, at
least of infected setts (ie latrine PCR positive for M bovis).
6 However vaccination with the live
attenuated vaccine BCG would not be successful if treatment was being given
simultaneously (vaccine is killed by treatment), so perhaps vaccination could
follow the treatment in infected areas to try to ensure it does not
return. There is no reason why oral vaccinia with an
immunity boosting M bovis protein should not be put in oral bait
as well. Giving vaccination by aerosol is not something that has been
studied at all for TB but this should be looked at as an ideal method for
animals in a sett to vaccinate them all.
7 Birth control would be a way of reducing
the population without the suffering of culling, the consternation of animal
rights people, and be unlikely to perturb badger society.
A question that I couldn't answer is whether it is
possible to get the badgers to eat sufficient of the bait daily for 9 months so
that the triple drug therapy had an opportunity to work. In a way the
badgers of the treated setts would have to be trained. People have no
difficulty getting badgers to come to their gardens and eat peanuts for
example. If the setts of the badgers are mapped (as they have been in the
North Pembrokeshire Cull area) and badger devotees adopted setts so that
all were accounted for the badgers could be 'trained' or accustomed to
eating a favourite bait near or at their sett entrances every
day, tasty pellets that could then be exchanged for drug containing tasty
pellets.
A method such as this could only be used in a
developed country and would have to have the full co-operation of
people.
I have corresponded with badger trust people in
Wales and they are implacably against population control of badgers by
birth control methods.
I also suspect that DEFRA and the HPA would never
countenance the treatment of badgers with triple drug therapy. The EU
rules would be a problem I am sure.
In conclusion I do think Colin's idea is more
interesting and feasible than I thought as a first reaction. It could
be very effective, better than anything else proposed at present.
The problem is that to do anything other than
killing is obstructed and slowed for lots of different reasons, rules,
bureaucracy, vested interests and scientific difficulties, so that
nothing new can be given a try. And of course the killing of
badgers is also opposed- an exception in the animal world and in the
light of our response to FMD infection one that is understandably held as
valuable. This is where birth control comes in for population
reduction. Such methods are not yet finally developed
for mammals but vaccination against chorionic gonadotrophin may be a safe
method.
Why should the Irish be at the stage of trialling
oral vaccine for badgers and yet we be 5 years away from it?
