From Dr Ruth Watkins BSc Hons, MSc, MBBS, MRCP, MRCPath - 30 October 2007
" I thought other farmers in East Anglia might like to know what they might do to mitigate infection (no advice from Defra)...."
What will happen? What should we do? (revised version for Warmwell)How can we control this outbreak of bluetongue serotype-8?
I believe that this year there will be more cases over East Anglia and that October will be the peak month for the number of new cases (different herds and flocks) and that cases will continue at a lower rate through to the end of December.
This is based on what happened in Northern Europe in 2006.
There may be hundreds by the beginning of January particularly if farms in the restriction zone are screened for infection even without reporting sick animals.
Next year it will recommence in earnest in July.
I believe that figures in authority are being dishonest with farmers when they say that a cold winter may eliminate the infection; not in Britain even if it is a freezing winter through January and February. It was certainly not eliminated in Northern Europe.
This year the clinical cases are likely to be few per herd or flock in the UK. The intensity of infection will be greater next year.
An important reason why the disease, Bluetongue, has affected so many more animals this year in Northern Europe so that more have been severely ill and died is that not only has it been a good year for midges but greater numbers of midges are infected.
This means that when an animal is infected it is bitten by many virus infected midges and not just one by chance. This also means that almost every ruminant on an infected farm will be infected. Also that each infected animal is likely to be infected with a large dose of virus which is more likely to give a severe or overwhelming infection.
This year in Northern Europe in the restriction zones of endemic infection hardly any farms will be spared infection.
The midges are on the move not only "diffusing" by spreading locally by about 2 km per day but also "jumping" when there are appropriate conditions of light warm humid breezes they can spread much further in a day down wind.
Our topography is unlikely to represent much obstruction to its spread westwards as well as north and south.
Cattle in particular may remain infectious to the biting midge for about 2 months, the exact time seems a little hazy from what I have read. This is because the virus remains sequestered in red blood cells and their half life in blood is 180 days in the cow so that virus can be detected by RT-PCR for instance for 6 months after infection.
However long before 6 months is up the blood cells have been found to lose their infectivity (though not their virus) for feeding midges and other ruminants by experimental inoculation.
When an animal is infected, the virus not only infects many different cells of the immune system and the blood, but also the cells lining the blood vessels, the endothelial cells and can be found in the serum. The formation of antibody is prompt but of course does not reach intracellular virus. Antibody remains for life and if the animal recovers from the infection it is protected for life from the same serotype of bluetongue as it was infected with.
The only method of control will be vaccination. A killed vaccine for BTV-8 has been trialled and found to be protective and its use can be fast forwarded on the basis of the success with killed vaccines to BTV-2 and BTV-4 in Corsica and the Balearics. An immunity by vaccination of greater than 80% of all domestic ruminants has proven successful in control and even elimination. Inactivated or killed virus vaccines cannot recombine with wild virus and are safe to use in pregnant animals etc.
I am not sure what sort of problem our deer would represent, and whether they might be large enough in population with the addition of new susceptible fawns each year, to sustain BTV-8 at a low level. But it may be a reasonable hope that if we were able to vaccinate >80% of domestic ruminants we may be able to eliminate bluetongue before it is established widely in a high intensity of infection and becomes indemic in deer.
We may have to vaccinate for several years. On the other hand it may be that bluetongue vaccination to BTV-8 may have to become the norm- with an annual booster after the initial full vaccination, probably 2 doses.
Doramectin and Ivermectin pour-ons may prove useful in killing the midges that suck the blood of treated animals and in killing the midge larvae as they hatch from eggs laid in their dung. However I have not seen any papers to show that the doses normally given are effective in the case of the C obsoletus complex and the C dewulfi (assuming that it will be the same midges in the UK that are responsible for spread of BTV-8 as in Northern Europe), it may be that the levels achieved are too low (as a paper on a Canadian species of midge has shown). A contact in Germany in the midst of the epidemic is using ivomec pour-on for all her longhorn cattle 3 times a year and immediately a cow is sick with bluetongue she gives her an additional pour-on treatment, presumably to reduce the likelihood of local amplification of virus through the production of a newly infected swarm of female midges.
I am not sure at what point a more frequent use of ivermectin than 3 times a year would become neurotoxic. The use of pour-ons would have to be set against expense and simply a reduction in the number of infected midge bites and possibly infected female midges and not elimination of the infection risk.
Many of her cattle have been infected and some became ill but none were lost.
The midges are attracted by CO2 in the expired breath which cannot be changed. Ivermectin can exert a repellent effect on the midge for sheep by modifying the attractive chemicals, lactic acid and butanone, on the sheep's wool.
Also deltamethrin can be used as a repellent, again as a pour-on or perhaps as eartags; this is the method favoured by my german contact.
I am not sure if deltamethrin is available in eartag form in the UK.
Midges bite everywhere (as anyone knows who has visited Scotland and suffered an attack by C impunctatus). Any protection afforded by a pour-on generally does not reach the feet and the midge likes to bite the lower leg above the hoof. The two species of midge implicated in the spread of BTV-8 in Northern Europe are associated with animal housing and breed in dung, dewulfi also breeding in horse dung (horses themselves are not infected).
I am concerned about passive transmission by reusing needles in the case of multi-use needles and syringes for injecting herds or flocks e.g. heptavac-P vaccination of a flock of sheep. However this would be on the one flock or herd by the farmer.
Of greater concern would be tuberculin testing, the bovine TB skin test, as the vet uses the same syringe (very expensive) and needle going from herd to herd. The needle is dunked in alcohol or surgical spirit. The orbivirus (bluetongue is an orbivirus) is very robust with a double protein shell and it should be checked out as to whether the surgical spirit or type and concentration of alcohol used would inactivate the virus.
Clearly these needles are not used to draw blood, but in the case of an acutely infected animal with a high viraemia potentially virus could be transmitted in this way. (In human medicine blood borne viruses can be transmitted in this way, hepatitis B and C for example.)
I have not been able to draw any conclusion on discussion of this possibility but will raise it again at the European Livestock Association meeting in Brussels in October.
Both Merial and Intervet need to have their places of vaccine manufacture licensed and to start production in October to make a great number of doses of inactivated BTV-8 vaccine by next spring - even then there may not be enough doses. The Intervet site in Europe is I understand not in production yet. And Merial at Pirbright is on standstill since August- that site is used to produce both FMD and bluetongue vaccines.
Do you think Defra will see sense and join our European compatriots in vaccinating against BTV-8?
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