Host-Shifts and Honey Bees: Lessons From COVID-19
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This global health crisis has certainly hit us all like a ton of bricks, and at the time of this writing (early April 2020) we are still amidst our “social distancing” and mandatory University and state-wide shutdown. Tragedies such as this remind us all of the important things in life, and while we love our honey bees, they are way down on the list of priorities, as family, friends, and loved ones take the only precedent with everything else a distance second. It is my fervent hope that everyone is—and will remain—healthy, happy, and safe.
I’m sure by now everyone has heard (probably more times than they care for) about how COVID-19 came to be. It’s a coronavirus of the virus family Coronaviridae (so named because of its crown-like protein coat), and it is not endemic to humans. [By the way, while honey bees have plenty of viruses, mostly in the families Iflaviridae (e.g., Deformed Wing Virus) and Dicistroviridae (e.g., Acute Bee Paralysis Virus), they don’t have any coronaviruses as far as we know]. What I haven’t heard in the media, however, is the term ‘host-shift,’ which is the biological phenomenon when a parasite or pathogen “jumps ship” from one host species to another. You see, most microbial disease agents (viruses, bacteria, fungi) have a difficult time counteracting the complex immune systems of their hosts, so both host and pathogen have to constantly re-adapt to develop protective measures and counter-measures of resistance, respectively. In doing so, over time most disease agents tend to specialize on a given host—at the cost of being able to infect other hosts. That’s what makes host-shifting so rare (at least compared to within-species spread), but this is exactly what happened with SARS-CoV-2 (the specific virus that causes the disease COVID-19). Horseshoe bats are the predominant mammalian reservoirs of SARS-CoV-2, but genetic sequencing suggests that an intermediate host (a pangolin, which is a scaly ant-eater-looking type of a creature in Asia) is how humans seem to have actually contracted it. Nonetheless, once the virus made this host-shift to humans, the ugly consequences have ensued.
There are three very famous host-shifts of honey bee parasites, and I outline them here in increasing order of importance and severity. In each case, the original host of each parasite was Apis cerana, the Eastern honey bee of Asia (compared to “our” Apis mellifera, the Western honey bee of Europe and Africa then subsequently disseminated worldwide by humans), and the co-mingling of the two species was facilitated by introducing A. mellifera to eastern Asia over 100 years ago (if not before) but particularly after the trans-Siberian railway was constructed. The first parasite that made a host-shift is Nosema cerana, which is a microsporidian that lives in a bee’s gut and one that you’ve likely heard of but have never seen. This new species has all but replaced the original Nosema species in Western honey bees, N. apis, which was mostly a problem during winter confinement. The tell-tale symptoms of this “old Nosema” was the bees got constipation and diarrhea at the same time (their hind guts would get bloated but they don’t defecate in the nest, so they fly outside and litter the hive face with little fecal droplets). N. ceranae, however, seems to infect bees all year long, doesn’t cause overt symptoms, and is pretty much found everywhere with varying degrees of disease (=Nosemosis). It is highly variable, though, both within and among colonies; you can have some individual bees with high levels while other nestmates have few to none. Similarly, while most colonies have low levels, others have a lot and a few have none. Nosemosis, therefore, is also very spotty and difficult to determine, so at present we still don’t really know how major a problem this disease might be for beekeepers.
The second parasite that made its host-shift from A. cerana is the varroa mite, Varroa destructor, and I know you’ve heard of this one since it’s the primary management concern for all beekeepers in North Carolina (and beyond). It actually shifted hosts to A. mellifera about 100 years ago in Asia (Figure 1), but it didn’t start to rapidly spread globally (except for Australia) until the 1960s, being introduced to the US in 1987 and into North Carolina in 1990. Like other non-endemic parasites, our bees don’t have well-developed defenses to varroa, so their impact to NC beekeepers is tremendous. There are two types of honey bee colonies: those that have varroa mites, and those that will have varroa mites. Our new mantra in the NC State Apiculture Program is that while treating for varroa (with synthetic acaricides) may be optional, controlling for varroa (by whatever means) is not.
The third and final A. cerana parasite is, luckily, not here in the US or North Carolina, and we hope it stays that way. It is known as Tropilaelaps (pronounced tro-po-LAY-lapse), and there are several species but the two main ones that can parasitize our bees are T. clareae and T. mercedesae. While the original host is actually the giant honey bee, A. dorsata, the intermediate host that passed it along to A. mellifera is, again, A. cerana (not unlike the pangolin being the intermediate host between bats and humans for SARS-CoV-2). Regardless, its global distribution has been slowly increasing over the last 50 years, but fortunately it has remained mostly within the tropics up until now. While about a third the size of varroa mites (still ovate but skinny rather than oblong; Figure 2), they scurry about the nest very quickly, are hard to detect, and they make varroa infestation look like a decaf latte; Tropilaelaps is actually quite good at controlling varroa mites—because there’s no brood left for varroa to reproduce! This looming threat to beekeeping and the apiculture industry is a major reason why imports of live bees from other countries is strictly prohibited.
Now that our bees have these parasites, what can we do? And, what parallels can we draw from our experience with COVID-19?!
- We’re told to frequently wash our hands to reduce the spread of the coronavirus. Bees can’t wash their hands, of course, but they can exhibit hygienic behavior, which is what handwashing is all about. Bees with particular genes can detect varroa-parasitized brood then remove them from the nest before the next generation of mites matures, and it’s been a popular trait fostered among many different honey bee breeding programs. So the equivalent to frequent handwashing is to buy queens that have been bred for hygienic behavior, if at all possible. If you don’t know if your bees have been selected for this helpful trait, ask the queen producer from whom you’re purchasing your stock!
- We’re also told to physically distance ourselves from others to reduce viral transmission within the entire population. This, too, is something that’s impossible for bees within a colony to do; in fact, their social structure is exactly what makes Nosema, varroa, and Tropilaelaps so problematic for honey bees and beekeepers. While we may not be able to do anything about the sociality of honey bees, we can control the transmission among them. That is, if possible, try to space your hives well apart from each other. The close proximity of colonies in an apiary facilitates the drifting of foragers and especially drones, which can facilitate the spread of parasites and pathogens, so increasing the distance among the hives can dramatically decrease the likelihood of drifting. If you can’t space them further apart, then consider other means by which you can reduce transmission, such as using anti-robbing screens (particularly in the summer), painting your hive equipment different colors (so they can better discern their own hive from others), and/or placing different patterns near the entrances (so foragers can more easily learn what their front door looks like).
- We’ve been asked to quarantine ourselves if we develop COVID-19 symptoms. Bees won’t self-isolate (although they can purposefully fly off and never come back if they’re infected, for the benefit of the colony—called ‘social apoptosis’), but as beekeepers we can impose that at the colony level. If you have a colony with a high Nosema load or varroa count, then put it into quarantine. Ideally, if you can move it to an isolated yard, that would be great, but it’s not necessary. Instead, don’t move frames, brood, honey, or bees from infested colonies to other colonies, even if they’re still strong (for the time being), because you’ll just be spreading the disease. Stay vigilant on the progression of the parasites, and take control measures as necessary. Staying on top of parasite amplification not only helps you save your bees, it will help spread them to your neighbor’s bees as well, which in turn helps the entire honey bee population.
- Finally, and perhaps most importantly, the only way that we’ve been able to keep track of the progression of COVID-19 is through regular screening and testing of the greater population for the causative disease agent, SARS-CoV-2. This, too, is exactly what beekeepers should be doing to their “population” of colonies (no matter how many you have, from 1 to 1,000). Testing for Nosema is a bit tricky because they’re microscopic, but if you suspect a potential problem you can contact your local N.C. Department of Agriculture and Consumer Services Apiary Inspector to come take a sample and they’ll screen it for you at nominal cost. Testing and monitoring for varroa mites, however, is something every beekeeper should know and put to practice very frequently (at least monthly, if not every time you inspect each hive). Doing a ‘sugar shake’ or, ideally, an ‘alcohol wash’ on 1/2 cup of workers from the brood nest (~300 bees) will give you a very good estimate of the percentage of mites within the colony (=mite prevalence). Depending on the time of year, 1-3% is in the “danger zone,” and anything above 5% your bees will likely not recover (but, critically, you should still try to get the mite population under control nonetheless, otherwise you’ll spread them to all of your neighbors even if you don’t know who they are).
Honey bees have been dealing with exotic parasites for decades. Incorporating some of the same strategies that we’ve been practicing for COVID-19 into our beekeeping operation can actually help them cope better and deter their spread. Like in a honey bee colony, there is strength in numbers, so if we all do our part as a population of beekeepers, we can help make a difference for the greater good for the population of honey bees.