Tag Archives: measles

Measles, mites and anti-vaxxers

About 11,000 years ago nomadic hunter-gatherers living near the river Tigris discovered they could collect the seeds from wild grasses and, by scattering them around on the bare soil, reduce the distance they had to travel to collect more grain the following year.

This was the start of the agricultural revolution.

They couldn’t do much more than clear the ground of competing ‘weeds’ and throw out handfuls of collected seed. The plough wasn’t invented for a further 6,000 years and wouldn’t have been much use anyway as they had no means of dragging it through the baked-hard soil.

But they could grow enough grains and cereals to settle down, doing less hunting and more gathering. Some grains grew better than others, with ‘ears’ that remained intact when they were picked, making harvesting easier. The neophyte farmers preferentially selected these and, about 10,000 years ago, the first domesticated wheat was produced.

Einkorn wheat (Triticum monococcum), one of the first domesticated cereals

Since they were less nomadic and more dependent upon the annual grain harvest they took increasing care to protect it. They were helped with this by the hunting dogs domesticated from wolves several thousands years earlier. The dogs protected the crops and kept the wild animals, primarily big, cloven-hooved ungulates and the native wild sheep and goats, at a distance.

But those that got too close were trapped and were remarkably good to eat.

And since it was easier to keep animals penned up to avoid the need to actively hunt them it was inevitable that sheep and goats were eventually domesticated (~9,000 years ago) … and the nomadic hunter-gatherers became settled farmers practising recognisably mixed agriculture.

Domestication of cattle

The sheep and goats were a bit weak and scrawny. The large ungulates, the aurochs, gaur, banteng, yak and buffalo 1 had a lot more meat on them.

Inevitably, first aurochs (which are now extinct) and then other wild ungulates, were independently domesticated to produce the cattle still farmed today. This process started about 8,000 years ago.

Auroch bull (left) and modern domesticated bull (right). Auroch were big, strong (tasty) animals.

Cattle were great. Not only did they taste good, but they could be managed to produce milk and were strong enough to act as beasts of burden.

The plough was invented and crop yields improved dramatically because the grain germinated better in the cleared, tilled soil. Loosely knit families and groups started to build settled communities in the most fertile regions.

Bigger farms supported more people. Scattered dwellings coalesced and became villages.

Not everyone needed to farm the land. The higher yields (of grain and meat) allowed a division of labour. Some people could help defend the crops from marauders from neighbouring villages, some focused on weaving wool (from the sheep) into textiles while others taught the children the skills they would need as adults.

Communities got larger and villages expanded to form towns.

Zoonotic diseases

Hunter-gatherers had previously had relatively limited contact with animals 2. In contrast, the domestication of dogs, sheep, goats and cattle put humans in daily contact with animals.

Many of these animals carried diseases that were unknown in the human population. The so-called zoonotic diseases jumped species and infected humans.

There’s a direct relationship between the length of time a species has been domesticated and the number of diseases we share with it.

Domestication and shared zoonotic diseases (years, X-axis)

The emergence of new diseases requires that the pathogen has both the opportunity to jump from one species to another and that the recipient species (humans in this case) transmits the disease effectively from individual to individual.

The nomadic hunter-gatherers had been exposed to many of these diseases as well but, even if they had jumped species, their communities were too small and dispersed to support extensive human-to-human transmission.

Rinderpest and measles

Until relatively recently rinderpest was the scourge of wild and domesticated cattle across much of the globe. Rinderpest is a virus that causes a wide range of severe symptoms in cattle (and wild animals such as warthog, giraffe and antelope) including fever, nasal and eye discharges, diarrhoea and, eventually, death. In naÏve populations the case fatality rate approaches 100%.

Rinderpest outbreak in South Africa, 1896

Animals that survive infection are protected for life by the resulting immune response.

Rinderpest is closely related to canine distemper virus and measles virus. Virologically they are essentially the same virus that has evolved to be specific for humans (measles), dogs (canine distemper) or cattle (rinderpest).

Measles evolved from rinderpest, probably 1,500 to 2,000 years ago, and became a human disease.

Rinderpest was almost certainly transmitted repeatedly from cattle to humans in the 6,000 years since auroch or banteng were domesticated. However, the virus failed to establish an endemic infection in the human population as the communities were too small.

However, by about 1,500 – 2,000 years ago the largest towns had populations of ~250,000 people. Subsequent studies have demonstrated that you need a population of this size to produce enough naÏve hosts (i.e. babies) a year to maintain the disease within the population.

This is because, like rinderpest, measles induces lifelong immunity in individuals that survive infection.

Measles is a devastating disease in an unprotected community. Case fatality rates of 10-30% or higher are not unusual. It is also highly infectious, spreading very widely in the community 3. Survivors may suffer brain damage or a range of other serious sequelae.

Measles subsequently changed the course of history, being partially responsible (along with smallpox) for Cortés’ defeat of the Aztec empire in the 16th Century.

John Enders, Maurice Hilleman and Andrew Wakefield

In the late 1950’s John Enders developed an attenuated live measles vaccine. When administered it provided long-lasting protection. It was an excellent vaccine. Maurice Hilleman, in the early 1970’s combined an improved strain of the measles vaccine with vaccines for mumps and rubella to create the MMR vaccine.

Widespread use of the measles and MMR vaccines dramatically reduced the incidence of measles – in the UK from >500,000 cases a year to a few thousand.

Incidence of measles in England and Wales

If vaccine coverage of 92% of the population is achieved then the disease is eradicated from the community. This is due to so-called ‘herd immunity’ 4 in which there are insufficient naÏve individuals for the disease to be maintained in the population.

Measles cases (and deaths) continued to fall everywhere the vaccine was used.

There was a realistic possibility that the vaccines would – like rinderpest 5 – allow the global eradication of measles.

And then in 1986 Andrew Wakefield published a paper in the Lancet suggesting a causative link between the MMR vaccine and autism in children.

Subsequent studies showed that this was a deeply flawed and biased study. And totally wrong.

There is not and never was a link between autism and measles vaccination 6. But that didn’t stop a largely uncritical press and subsequently even less critical social media picking up the story and disseminating it widely.

Measles and the anti-vaccine movement

Measles vaccination rates dropped because a subset of parents refused to have their kids vaccinated with the ‘dangerous’ measles vaccine.

Several successive birth cohorts had significantly lower than optimal vaccination rates. Measles vaccine coverage dropped to 84% by 2002 in the UK, with regional levels (e.g. parts of London) being as low as 61%. By 2006, twenty years after the thoroughly discredited (and now retracted) Lancet paper vaccine rates were still hovering around the mid-80% level.

As immunisation rates dropped below the critical threshold, measles started to circulate again in the population. 56 cases in 1998 to ~450 in the first 6 months of 2006. In that year there was also the first death from measles for many years – an entirely avoidable tragedy.

In 2008 measles was again declared endemic (i.e. circulating in the population) in the UK.

Similar increases in measles, mumps and rubella were occurring across the globe in countries where these diseases were unknown for a generation due to previous widespread vaccination.

The distrust of the MMR vaccine was triggered by the Wakefield paper but is part of a much wider ‘anti-vaccination movement‘.

“Vaccines are dangerous, vaccines themselves cause disease, there are too many vaccines and the immune system is overloaded, vaccines contain preservatives (thiomersal) that are toxic, vaccines cause sterility etc.”

None of these claims stand up to even rudimentary scientific scrutiny.

All have been totally debunked by very extensive scientific analysis.

The World Health Organisation consider the anti-vaccine movement (anti-vaxxers) one of the top ten threats to global health. Vaccination levels are lower than they need to be to protect the population. Diseases – not just measles – that should be almost eradicated now kill children every year.

Where are the bees in this beekeeping blog?

Bear with me … before getting to the bees I want to move from fact (all of the above) to fantasy. The following few paragraphs (fortunately) has not happened (and to emphasise the point it is all italicised). However, it is no more illogical than the claims already being made by the anti-vaccine movement.

Childhood measles

The inexorable rise of internet misinformation and social media strengthened the anti-vaxxers beliefs further. Their claims that vaccines damage the vaccinees were so widespread and, for the uncritical, naturally suspicious or easily influenced who simply wanting to protect their kids, so persuasive that vaccine rates dropped further. They refused to consider the scientific arguments for the benefits of vaccines, and refused to acknowledge the detrimental effects diseases were having on the community.

The obvious causative link to the inevitable increase in disease rates was not missed – by both the anti-vaxxers and those promoting vaccination. However, the solutions each side chose were very different. Measles remained of particular concern as kids were now regularly dying from this once near-forgotten disease. The symptoms were very obvious and outbreaks spread like wildfire in the absence of herd-immunity 7.

The anti-vaxxers were aware that population size was a key determinant of the ability of measles to be maintained in the population. Small populations, such as those on islands or in very isolated regions, had too few new births annually to maintain measles as an endemic disease.

With the increase in remote working – enabled by the same thing (the internet) responsible for lots of the vaccine misinformation – groups of anti-vaxxers started to establish remote closed communities. Contact with the outside world was restricted, as was the size of the community itself.

A quarter of a million was the cutoff … any more than that and there was a chance that measles could get established in the unprotected population.

Small communities 8 work very well for some things, but very badly for others. Efficiencies of scale, in education, industry, farming and trade became a problem, leading to increased friction. When disease did occur in these unprotected communities it wreaked havoc. Countless numbers of people suffered devastating disease because of the lack of vaccination.

In due course this led to further fragmentation of the groups. They lived apart, leading isolated lives, flourishing in good years but struggling (or failing completely) when times were hard, or when disease was introduced. Some communities died out altogether. 

They chose not to travel because, being unvaccinated, they were susceptible to diseases that were widespread in the environment. Movement and contact between villages, hamlets and then individual farm settlements was restricted further over time.

The benefits of large communities, the division of labour, the economies and efficiencies of scale, were all lost.

They didn’t even enjoy particularly good health.

They had ‘evolved’ into subsistence farmers … again.

OK, that’s enough! Where are the bees?

Anyone who has bothered to read this far and who read Darwinian beekeeping last week will realise that this is meant to be allegorical.

The introduction of Varroa to the honey bee population resulted from the globalisation of beekeeping as an activity, and the consequent juxtaposing of Apis mellifera with Apis cerana colonies.

Without beekeepers it is unlikely that the species jump would have occurred.

Apis cerana worker

Undoubtedly once the jump had occurred transmission of mites between colonies was facilitated by beekeepers keeping colonies close together. We do this for convenience and for the delivery of effective pollination services.

The global spread of mites has been devastating for the honey bee population, for wild bees and for beekeeping.

But (like the introduction to measles in humans) it is an irreversible event.

However, it’s an irreversible event that, by use of effective miticides, can at least be partially mitigated.

Miticides do not do long-term harm to honey bees in the same way that vaccines don’t overload the immune response or introduce toxins or cause autism.

There can be short term side effects – Apiguard stinks and often stops the queen laying. Dribbled oxalic acid damages open brood.

But the colony benefits overall.

Many of the miticides now available are organic acids, acceptable in organic farming and entirely natural (even being part of our regular diet). Some of the hard chemicals used (e.g. the lipid-soluble pyrethroids in Apistan) may accumulate in comb, but I’d argue that there are more effective miticides that should be used instead (e.g. Apivar).

I’m not aware that there is any evidence that miticides ‘weaken’ colonies or individual bees. There’s no suggestion that miticide treatment makes a colony more susceptible to other diseases like the foulbroods or Nosema.

Of course, miticides are not vaccines (though vaccines are being developed) – they are used transiently and provide short to medium term protection from the ravages of the mite and the viruses it transmits.

By the time they are needed again the only bee likely to have been previously exposed is the queen. They benefit the colony and they indirectly benefit the environment. The colony remains strong and healthy, with a populous worker community available for nectar-gathering and pollination.

The much reduced mite load in the colony protects the environment. Mites cannot be spread far and wide when bees drift or through robbing. Other honey bee colonies sharing the environment therefore also benefit.

The genie is out of the bottle and will not go back

Beekeepers (inadvertently) created the Varroa problem and they will not solve it by stopping treatment. Varroa will remain in the environment, in feral colonies and in the stocks of beekeepers who choose to continue treating their colonies.

And in the many colonies of Apis mellifera still kept in the area that overlaps the natural (and currently expanding) range of Apis cerana.

Treatment-free beekeepers may be able to select colonies with partial resistance or tolerance to Varroa, but the mite will remain.

So perhaps the answer is to ban treatment altogether?

What would happen if no colonies anywhere were treated with miticides? What if all beekeepers followed the principles of Darwinian (bee-centric, bee friendly, ‘natural’) beekeeping – well-spaced colonies, allowed to swarm freely, killed off if mite levels become dangerously high – were followed?

Surely you’d end up with resistant stocks?

Yes … possibly … but at what cost?

Commercial beekeeping would stop. Honey would become even scarcer than it already is 9. Pollination contracts would be abandoned. The entire $5bn/yr Californian almond crop would fail, as would numerous other commercial agricultural crops that rely upon pollination by honey bees. There would be major shortages in the food supply chain. Less fruits, more cereals.

Pollination and honey production require strong, healthy populous colonies … and the published evidence indicates that naturally mite resistant/tolerant colonies are small, swarmy and only exist at low density in the environment.

Like the anti-vaxxers opting to live as isolated subsistence farmers again, we would lose an awful lot for the highly questionable ‘benefits’ brought by abandoning treatment.

And like the claims made by the anti-vaxxers, in my view the detrimental consequences of treating colonies with miticides are nebulous and unlikely to stand up to scientific scrutiny.

Does anyone seriously suggest we should abandon vaccination and select a resistant strain of humans that are better able to tolerate measles?


Notes

It is an inauspicious day … Friday the 13th (unlucky for some) with a global pandemic of a new zoonotic viral disease threatening millions. As I write this the UK government is gradually imposing restrictions on movement and meetings. Governments across Europe have already established draconian regional or even national movement bans. Other countries, most notably the USA and Africa, have tested so few people that the extent of Covid-19 is completely unknown, though the statistics of cases/deaths looks extremely serious.

What’s written above is allegorical … and crudely so in places. It seemed an appropriate piece for the current situation. The development of our globalised society has exposed us – and our livestock – to a range of new diseases. We cannot ‘turn the clock back’ without dissasembling what created these new opportunities for pathogens in the first place. And there are knock-on consequences if we did that many do not properly consider.

Keep washing your hands, self-isolate when (not if) necessary, practise social distancing (no handshakes) and remember that your bees are not at risk. There are no coronaviruses of honey bees.

Vaccinating bees

Brace yourselves. There’s some heavyweight science this week.

I’m going to discuss a very recent publication 1 on vaccinating bees against parasites and pathogens.

The paper involves a whole swathe of general concepts many readers will have some familiarity with – vaccines, immunity, infections, parasites, the gut microbiota 2 – which, because the paper is about bees, bear little recognisable relationship in the details.

And the devil is in the detail.

The paper appears to offer considerable promise … but I’ll return to that later.

To start with, let’s begin with measles.

Measles

Measles is a virus. It is highly contagious – typically being transmitted by coughing or sneezing – and causes a characteristic rash. Complications associated with measles infections – pneumonias, encephalitis and other respiratory and neurological conditions – are responsible for a case fatality rate of ~0.3% in the USA, or up to 30% in populations that are malnourished or have high levels of immune dysfunction.

Sixteenth century Aztec drawing of a measles victim

In 1980, 2.6 million people globally died of measles. That’s about five people (mainly kids) a minute 3.

By 2014 this figure had dropped to 73,000 due to a global vaccination campaign.

The measles vaccine is excellent. It is an attenuated (weakened) strain of the virus that is injected. When it replicates it produces all of the measles virus proteins. These are not naturally found in the human body, so the vaccinee 4 recognises them as foreign and produces an immune response that eventually stops the vaccine growing.

The really important thing about the immune response is that it lasts i.e. it has a memory. If the vaccinated individual is exposed to a virulent strain of measles in the future the immune response ‘wakes up’ and stops the virus replicating.

This immune response is effectively lifelong.

One important component of the immune response are antibodies. These are proteins that specifically recognise the measles virus, bind to it and lead to its destruction.

If you’ve been vaccinated (or have survived a previous infection) and subsequently get infected your body produces lots of antibodies which destroy the incoming strain of the measles virus, so protecting you (but this immune response is very specific … the response to measles does not protect you from poliomyelitis or coronavirus or mumps.).

OK, enough about measles 5.

Bees don’t have antibodies

The point about the stuff on measles was to introduce the principles of a protective immune response.

It has several characteristic features, including:

  • highly specific
  • destruction of the incoming pathogen
  • longevity (memory)

In humans, all of the above are provided by antibodies 6.

Bees don’t have antibodies, but they do have an immune response which has all of the characteristic features listed above.

The immune response of bees uses nucleic acids 7 which are common chemical molecules found in the bodies of all living things. Specifically bees use ribonucleic acid (RNA) that interferes with the nucleic acids of invading pathogens.

RNAi

To make ribonucleic acid (RNA) that interferes easier to say it is abbreviated to RNAi 8.

RNA is made up of individual building blocks called nucleotides. There are four nucleotides, with names abbreviated to A, C, G and U. These join together in long strands e.g. ACGUUGUGCAG … the order (or sequence) of which has all sorts of important biological functions we don’t need to worry about for the purposes of vaccinating bees.

Pairs of nucleotides in different strands have the ability to bind together – A binds to U, G binds to C or U. Individually, these bonds are weak. When lots occur close together they are much stronger and therefore very specific.

For example, the sequence ACGUUGUGCAG binds very well to UGCGACGCGUU. In contrast, it binds very much less well to CGUUAGCAUUG (just count the vertical bars which indicate each of the weak bonds between the nucleotides in the two strands. The left hand pair bind tightly, those on the right do not).

                         ACGUUGUGCAG          ACGUUGUGCAG
                         |||||||||||           || | | 
                         UGCGACGCGUU          CGUUAGCAUUG

Finally, these short RNAs interfere when they bind very well to their target sequence.

What does that mean?

In the cartoon above, imagine the text in red represents the RNAi and the text in blue represents part of the RNA genome of deformed wing virus (DWV), the most significant viral pathogen of honey bees 9.

The specific binding of RNAi to its target sequence recruits enzymes that result in either the destruction of the target, or the impairment of its functionality.

RNAi binding to DWV results in the inactivation and eventual destruction of the virus genome.

Virus replication is therefore stopped.

This is a ‘good thing’.

“Foreign”

Before we get on to vaccinating bees I have one final thing to explain.

How does the bee ‘know’ it is infected with DWV (or a similar viral pathogen) and how is the RNAi actually made?

OK, that’s two things, but they’re actually closely related to each other.

I said earlier that our bodies recognise the proteins that the measles virus (or vaccine) produces as ‘foreign’ i.e. something not normally present in the body. It turns out that many organisms – including bees – have evolved specific ways of detecting double stranded RNA as a ‘foreign’ entity.

Double stranded RNA (dsRNA) is made when RNA viruses replicate, but it is never normally present in the cells of a healthy bee. Therefore if the bee detects dsRNA it ‘knows’ it is infected and it induces an immune response … specifically an RNAi-mediated immune response.

The dsRNA is recognised by a protein called Dicer which cuts up the double stranded RNA into smaller duplex RNAi molecules, one of the pair of these then associates with additional proteins (including Argonaute; Ago 10) to form the RNA induced silencing complex (RISC).

RISC, which includes the RNAi, binds to the specific target e.g. the genome of other DWV viruses, and chops it up and destroys it.

The mechanism of RNAi-mediated silencing

Finally, because RNAi is a small molecule it can easily move from cell to cell. So RNAi made in one cell can move to regions of the bee some distance away.

Phew … OK, that’s the end of the whistle-stop introduction to RNAi and insect immunity 11.

Vaccinating bees

It’s been known for some time that you can directly introduce RNAi into bees and reduce the levels of some of the viruses present.

Frankly the data on DWV has not been great, but there are reasonably compelling studies of reductions in Israeli Acute Paralysis Virus (IAPV) levels and even field trails showing benefits at the colony level.

In these studies you either inject individual bees with RNAi, or you feed them large amounts of sugar syrup containing huge amounts (in value) of RNAi.

Neither of these routes is practically or financially viable.

Injecting individual bees takes a very long time 12. You need to anaesthetise the bee with CO2 or by chilling it on ice. It’s pretty tough on the bee and not all survive the anaesthetic or the injection. You need good lighting, good eyesight and a very small needle. It’s obviously a non-starter.

What about feeding? Syrup feeding is incompatible with honey production. It’s also a rather inefficient way to deliver RNAi. RNA is a very sensitive molecule. It is easily damaged. If it has to sit around in syrup for a few days, get collected by the bee, stored in the honey stomach, regurgitated and passed to another bee etc. there’s a risk it will be inactivated.

And it’s very expensive to produce …

The gut microbiota

Which in a really roundabout way brings us to this recent study by Leonard et al., published at the end of January in the prestigious journal Science

Leonard et al., (2020) Science 367, 573-576

In this study, the authors have modified a harmless bacterium normally present in the honey bee gut so that it produces double stranded RNA specific for DWV. This bacterium, specifically called Snodgrassella alvi, is a present in the gut of all bees. It is a core member of the gut microbiota, the bacterial population present in the honey bee gut.

The concept is relatively simple, but the science is pretty cool.

The bacterium sits around in the honey bee gut producing DWV-specific RNAi. If the bee gets infected – through feeding or injection, for example by Varroa – the RNAi (which has diffused around the body of the bee) is ready and waiting to ‘silence’, through RNA interference, the replicating DWV genome.

The bee remains healthy and happy 13.

But there’s more … Snodgrassella alvi is presumably passed from bee to bee during feeding (of larvae or adult workers). Therefore the RNAi-expressing version should naturally spread through a colony, protecting all the bees. In addition, because it is present throughout the life of the bee, a genetically engineered form of the bacterium should provide the longevity that is characteristic of a protective immune response.

So, does it work?

The paper includes lots of introductory studies. These include:

  • demonstrating that engineered Snodgrassella alvi – which for pretty obvious reasons I’ll abbreviate to S. alvi for the rest of this post – colonises the bee gut and could be spread from bee to bee.
  • the introduced bacterium produces double strand RNA (dsRNA) precursors of the RNAi response.
  • that dsRNA produced in the bee gut spreads to other areas of the bee body.
  • and that the presence of dsRNA upregulates components of the immune response.
  • the demonstration that it was possible to control host gene expression using this dsRNA 14.

I’m going to return to some of these points in a future post (this one is already too long) as there are both promising and disturbing features buried within the data.

Let’s cut to the chase …

Symbiont-produced RNAi can improve honey bee survival after viral injection.

Seven day old adult worker bees were fed with S. alvi expressing RNAi to DWV or to an irrelevant target (GFP). Seven days later some were injected with DWV (solid lines in the graph above), others were injected with buffer alone (dashed lines).

In the 10 days after injection about 25% of the bees injected with buffer died. This reflects the ageing of the bees and the attrition rate due to handling in the laboratory.

About 75% of the bees ‘vaccinated’ with S. alvi expressing GFP RNAi or no RNAi died after DWV challenge over the 10 day period.

In contrast, only ~60% of the S. alvi bees expressing DWV-specific RNAi died. This is a relatively small difference, but – because the experiment was conducted with lots of bees – is statistically significant.

Killing mites

The results presented above are promising but the authors also explored the logical extension of this work.

If the RNAi produced by the engineered S. alvi becomes widely distributed in the honey bee, perhaps it also could also taken up when Varroa feeds on the bee?

In which case, if you engineered S. alvi to produce Varroa-specific RNAi’s, perhaps this would help kill mites.

Symbiont-produced RNAi kills Varroa mites feeding on honey bees

It does.

Using a similar ‘vaccination’ schedule as above, only ~25% mites exposed to bees carrying S. alvi expressing Varroa-specific RNAs’s survived 10 days, whereas 50% of mites survived when feeding on bees carrying non-specific engineered strains of S. alvi.

Again, this is encouraging.

Only encouraging?

Yes, at the moment, only encouraging.

Don’t get me wrong, this is pretty fancy technology and the results represent a lot of very laborious and elegant experiments.

At 2100 words this post is already too long … so here are a few things to think about which help justify my qualified enthusiasm for the paper.

  1. Although I didn’t show the data, transmission of engineered S. alvi between bees was rather inefficient. Over 5 days, only 33% of naive co-housed bees demonstrated infection with the modified symbiont. Why might this be an issue? Alternatively, is transmission between adult bees important? When might it be important to not transmit between adult bees?
  2. None of the experiments included any virus quantification. Did the bees that didn’t die after DWV injection challenge have lower DWV levels? If not, why not? What is the mechanism of protection?
  3. Actually, there were some virus quantification studies buried in the Supplementary data. In these the authors showed that virus levels were lower in all bees carrying engineered S. alvi, even those expressing the GFP negative control RNAi. This suggests a non-specific up-regulation of the immune response.
  4. All the challenge experiments were done with 7 day old worker bees. Are these the bees we really need to protect from DWV? Why didn’t they do any studies with larvae and pupae? These are much easier to handle and very much easier to inoculate. And very much more relevant in terms of virus-mediated colony losses.
  5. What other species sharing the environment with honey bees carries S. alvi? Why should this matter? Snodgrassella is a gut symbiont of honey bees and lives in the ileum. Is it present in honey bee faeces?

I’ll post a follow-up in the next few weeks to discuss some of these in further detail.

Congratulations to those of you who have got this far … don’t get rid of your Apivar and oxalic acid stocks just yet 😉


 

Apistan redux†

I’ve discussed Apistan, a pyrethroid treatment for Varroa, in two recent posts. In these I explained in some detail its molecular mechanism of action. I also explained the two major problems associated with Apistan (and the related tau-fluvalinates ) – the widespread resistance of Varroa to Apistan and the residues it leaves in wax.

In this final post I’m going to revisit just how useful Apistan could be if it was used in a more rational manner. I’m going to concentrate on resistance and you’ll probably need to read the previous post on this topic to provide necessary the background. I’ll only really touch on the residues in wax at the end – I’ve already discussed how these can be minimised if you consider them an issue.

This is (another) long post. It draws together the concepts described in previous articles and links the science of Varroa control to potential strategies to benefit practical beekeeping.

How good is Apistan if Varroa are not resistant?

Apistan

Apistan

Exceptionally good. Pyrethroids are some of the most widely used pesticides. They are widely used because they are very effective. Apistan is no exception. When used to treat Varroa populations that are not already resistant it kills over 98% of the mites in the colony when used according to the manufacturers instructions. 98% … that reduces the National Bee Units’ recommended maximum mite load of 1000 to just 20.

Just how effective is emphasised by a quote from the Apidologie paper cited above. “In treated hives, worker pupae and adult bee infestations decreased from 14.2 ± 7.3% to zero and from 15.7 ± 7.3% to zero, respectively. Whereas, in the two control hives, during the first 6 weeks, the average worker pupae infestation increased from 15.9 ± 2.9% to 19.7 ± 3.5%”.

Most mite mortality occurred during the first 4 weeks of treatment and the level of Apistan present at the beginning and end of treatment remained at about 10% i.e. it should be as active at the end of the treatment period as at the beginning.

How good is Apistan in reality

Resistance was first demonstrated in 2002 and is now widespread in the UK. In a recent paper, Ratneiks and colleagues (University of Sussex) demonstrated that Apistan was significantly less effective at killing Varroa when used for a second treatment, four months after the first. In this study they showed only 33% of mites were killed at the second treatment, whereas 58% were killed in colonies treated for the ‘first time in five years’.

This isn’t rocket science … if there are some resistant mites in a population then Apistan will preferentially allow these to survive. Consequently they will make up a greater proportion of the mite population when re-treated.

Since we know the molecular basis of resistance to Apistan it would now be possible to determine – without doing the treatment and counting the corpses – what proportion of mites were resistant in a population before treatment. It would therefore be easy to determine whether treatment would be likely to work.

Equally, it would be possible to determine whether the colonies ‘not treated with Apistan for five years’ still maintained significant levels of Apistan resistant mites. As will become clear, there are studies that contradict this, and the definitive test – the presence of absence of the mutation that confers resistance – was not done in the Sussex study.

Apistan resistance and fitness costs

Mutations, such as the one that confers resistance to Apistan, can – in broad terms – exert three different effects:

  1. Beneficial – the presence of the mutation favours the organism (a fitness benefit), the mutation will be selected for and it’s presence in the population is likely to increase.
  2. Detrimental – the mutations causes a fitness cost and organisms that carry it are likely to reproduce less well, resulting in it being lost from the population.
  3. Neutral – the mutation is neither beneficial nor detrimental.

In the presence of Apistan, the Leucine to Valine mutation at residue 925 (L925V) of the voltage gated sodium channel (VGSC; please see the previous article on the molecular basis of resistance), is a beneficial mutation. Any mites that carry it will not be killed and will be able to reproduce, so increasing it’s prevalence in the population. The same reasoning applies to other Apistan resistance mutations.

The VGSC of Varroa evolved over eons in the absence of Apistan. The mutation is in a part of the protein critical for its function (that’s why Apistan binding blocks function). It’s therefore perhaps unsurprising that in the absence of Apistan selection there is evidence that the L925V mutation is detrimental. In simple terms the VGSC works less well with a Valine at position 925 than a Leucine unless Apistan is present. Where’s the data that supports this?

The influence of prior treatment on Varroa genotype

Table 1. Apistan resistance mutations in Varroa from treated and untreated colonies

Table 1. Apistan resistance mutations in Varroa from treated and untreated colonies

The table above needs a little explanation. Colonies from Henlow and Shillington were treated with Apistan and tested one month later. Colonies from Harpenden, Bishop Stortford, St. Albans and Peterborough had no history of Apistan treatment in the recent past. Unfortunately, the paper does not make clear when the last treatment was, with the exception of a sample from Harpenden which had not been treated for at least 3 years.

Varroa is diploid i.e. there are two copies of the gene for the VGSC. The S and R heading the columns SS, SR, RR, indicates whether the Apistan resistant mutation is absent (S = sensitive) or present (R=present). SR indicates that the mite was heterozygous, one resistant copy and one sensitive. Whether these mites have lower resistance than RR mites has not been determined – for the purpose of the remaining discussion I’m going to lump the SR mites with the RR mites and assume they are resistant§.

Of 279 mites tested, 40 were from Apistan-treated and 329 from -untreated colonies. Of the 40 mites from Apistan-treated colonies, all contained the mutation conferring resistance to the fluvalinate. Of the 239 mites from colonies not recently treated with Apistan, 215 were sensitive and only 25 were resistant.

This suggests that in the absence of Apistan, Varroa sensitive to the fluvalinate replicate better.

Is this a surprise?

No. Partly for the reasons explained above … the Leucine at position 925 is likely to stop the VGSC working as well. More compellingly though is the wealth of data suggesting that insecticide resistance is associated with fitness costs in a range of other insects.

Colorado beetle

Colorado beetle

For example, pyrethroid resistant Myzus persicae (peach-potato aphid) exhibit fitness effects in overwintering survival, response to aphid alarm pheromone and vulnerability to parasitoids; pyrethroid-resistant Cydia pomonella (codling moth) have reduced fecundity, body mass of instars, adult male longevity and larval development; finally, pyrethroid-resitant mutants of the snappily-named Leptinotarsa decemlineata (which you of course know as the stripy-attired Colorado beetle) have reduced fertility and fecundity.

Google will find relevant reference on all the above examples or you can refer to a concise mini-review by Kliot and Ghanim Fitness costs associated with insecticide resistance published in Pest Management Science (2012) 68:1431-37.

Before discussing implications for practical beekeeping I should add that the rate at which the loss of the L925V mutation, and other mutations associated with Apistan resistance, needs to be accurately determined. If, as looks likely, a period of 3+ years results in selection for the sensitive variant of the VGSC, it might be possible to develop rational Varroa treatments that exploit this.

Apistan resistance, rational Varroa control and practical beekeeping

For the sake of discussion, let’s accept the following statement:

  • Apistan is devastatingly effective on sensitive mite populations.
  • Apistan is much less effective (or almost completely useless) on resistant mite populations.
  • Resistance by Varroa is acquired rapidly and lost over the subsequent 2-3 years in the absence of selection.

An effective and rational Varroa control strategy would only use Apistan once every 3-4 years, alternating it with other treatments. To mitigate the transfer of Apistan-resistant mites between colonies due to drifting and robbing, or due to the movement, sale and/or relocation of hives during the season, Apistan use would have to be coordinated. This coordination would have to be both geographical and temporal. There would be no point in the Fife beekeepers using it one year if the Angus beekeepers planned to use it the following year.

“Like herding cats” I hear some mutter …

Perhaps, but the benefits would be considerable. How could it be achieved? Perhaps by restricting the sale of Apistan to certain years, in a formulation or package that meant it had to be used quickly or became inactive.

What about the residues in wax?

I’m not sure whether the level Apistan accumulates to in wax is sufficient to be a selective pressure on the mite population. Apistan strips are 10% Apistan. Nothing like that much accumulates in wax. In a recent study fluvalinate levels ranged between 2 and 200,000 parts per billion in wax (mean ~7500 ppb). However, it is a valid concern and so would necessitate a relatively simple experiment to determine the rate at which Apistan resistant mutations are lost in the presence of absence of trace levels of Apistan in comb.

Herd immunity and the responsibility of the individual

There’s a debate in human healthcare about the necessity to vaccinate individuals in a well-vaccinated population. The chance of an infectious disease spreading to the unvaccinated individual in a protected population is very slight. So, why vaccinate?

Well, what if increasing numbers decided not to vaccinate? Once protection in the population falls below a certain level there is a significant chance that an infectious disease will spread widely. We saw this in the UK after the MMR (measles, mumps and rubella) vaccine was falsely claimed to be associated with autism. Vaccination rates dropped from 90+ percent, to low 80’s and – in parts of the country – to only 60%. Unsurprisingly, measles cases increased and – tragically, for the first time in years – there were childhood deaths due to measles infection.

This may seem a million miles away from looking after our bees, but there are parallels. As beekeepers we have responsibility for our own stock. We also have responsibility to the wider community of beekeepers which – because of the way our bees forage and mingle – happily exchange pests and pathogens.

Beekeepers who do not control Varroa (and consequently virus) levels threaten the viability of their own colonies and those of other beekeepers in the area. The same applies to the foulbroods. This is why the bee inspectors try and check all colonies in the vicinity of an outbreak. This is why standstill orders are placed on apiaries where outbreaks occur.

Perhaps this sort of communal responsibility also applies to Varroa treatment using Apistan? Beekeepers who treat without demonstrating very high levels of susceptibility first in their stocks are simply selecting for resistant mites, reducing the efficacy of treatment for themselves, and others, in the future. Indiscriminate or incorrect use of Apistan has resulted in widespread resistance, thereby compromising Varroa control for all beekeepers.

The coordination and control, geographically and temporally, of Apistan usage would benefit beekeeping and beekeepers.

And … it would also benefit those who chose never to treat with Apistan. Treated colonies in the one year in three Apistan was used would have very low mite levels. Fewer mites would be transferred from these colonies by drifting or robbing … what’s not to like?


 Redux, as in the literary term meaning brought back or restored, derived from the Latin reducere (to bring back).

 This is one compelling reason why Apistan strips should not be left in the colony longer than is recommended. It kills the susceptible mites within the first month or so. After that it effectively selects for resistant mites, allowing them to replicate.

 With apologies to any population biologists who were reading this and have now given up in horror.

§ And I’ll save discussion of the influence of the incestuous lifestyle of Varroa and Varroa levels on the ratio of homozygotes to heterozygotes at different stages of the season for a later post. It’s a fascinating and at the same time rather sordid tale …

 Or 4 or 5 – this would need to be determined empirically.