Tag Archives: vaccination

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.