Every beekeeper knows varroa is the greatest biological threat to honey bee colonies throughout the world. But in fact, this is only partially true.
Varroa feeds on bee fat bodies, and because fat bodies are essential to the immune system of bees, feeding by varroa weakens bees’ immune response. At the same time, viruses such as deformed wing virus (DWV) are transmitted to bees in the process of feeding by varroa. These viruses, in combination with the bees’ weakened immune response, are what make a particularly lethal 1-2 punch.
Yet we beekeepers focus almost exclusively on varroa control, not virus control. This is evident when you look through any beekeeping supply catalog, whose pages are filled with anti-varroa miticides such as formic acid (Formic Pro and Mite Away Quick Strips), oxalic acid (Api-Bioxal), hop oils (Hopguard), amitraz (Apivar), and more. Have you noticed there’s a conspicuous absence of anti-virus therapeutics in beekeeping catalogs?
Antivirals have been developed for viruses that infect other organisms, including humans. I’m sure everyone has heard of Paxlovid, which many of us have taken over the past few years to treat COVID. I don’t think I’ll ever forget Paxlovid and its awful metallic taste, which sticks with you day and night while you’re taking it!
So, what about antivirals for bees? Is there any evidence that therapeutics can reduce virus levels in honey bees? What are the mechanisms by which antiviral therapeutics might work in bees? And how about their safety; is there any evidence the therapeutics cause unintended harm to bees, or are they safe? These are the topics for the seventieth Notes from the Lab, where I summarize “Potassium ion channels as a molecular target to reduce virus infection and mortality of honey bee colonies,” written by Chris Fellows and colleagues and published in the journal Virology Journal [2023].
For the study, Fellows conducted a series of lab experiments at Louisiana State University under the supervision of Daniel Swale (see Photo 1), followed by field experiments with full colonies at the USDA Baton Rouge lab under the supervision of Mike Simone-Finstrom. They focused their attention on ATP-sensitive inward rectifier potassium (KATP) channels because these channels have previously been shown to play a major role in viral infection of mammals, flies, and more recently, bees.
In the lab, the authors started by testing whether four KATP modulators (i.e., their four putative antiviral therapeutics) were safe for bees. The general setup for these experiments is shown in Figure 1, where bees were kept in cages and allowed access to sucrose feeders containing KATP channel activators (pinacidil or diazoxide), KATP channel inhibitors (glibenclamide or tolbutamide), or an untreated control. The sucrose solution contained a fluorescent tracer to monitor that the bees were consuming the treatments (see Figure 1, panels D & E) and survival was monitored over two weeks.
Next, the authors tested whether KATP channel modulation impacted virus replication in bees. To do this, they inoculated bees with Israeli acute paralysis virus (IAPV), IAPV plus one of the KATP channel activators (pinacidil), or an untreated control, and monitored IAPV levels and bee survival over two weeks.
In addition, they assessed a possible mechanism for how KATP channels slow virus infections: by modulating the production of reactive oxygen species (ROS). To do this, they conducted a suite of experiments with a chemical that causes the production of ROS in bees (paraquat), a KATP channel activator (pinacidil), a KATP channel inhibitor (tolbutamide), and either inoculating with IAPV or collecting bees from low-varroa or high-varroa hives (i.e., bees that were experiencing low or high virus pressure).
Finally, the authors conducted a manipulative field experiment to see if one of the KATP channel activators (pinacidil) could reduce virus levels in full colonies of bees. To do this, they compared the levels of seven viruses — deformed wing virus A (DWV-A), deformed wing virus B (DWV-B), black queen cell virus (BQCV), Lake Sinai virus 1 (LSV1), Lake Sinai virus 2 (LSV2), and IAPV — among untreated control colonies, colonies that were inoculated with viruses, and colonies inoculated with viruses while being treated with pinacidil (see Photo 2). They also used a slick dead-bee collection apparatus placed in front of the hives, which gathered dead workers that were removed from the hive by undertaker bees (see Photo 3).
So, what did they find? Were the four putative antiviral therapeutics safe for bees? Yes, at least in terms of survival of individual bees over two weeks. As can be seen in Figure 1, panel C, there was no difference in survival between bees fed a control sucrose solution and bees fed large doses of each KATP channel modulator. In addition there was no difference in the number of dead bees found in the dead-bee collection apparatus between control colonies and colonies exposed to pinacidil (Photo 3).
Were the KATP channel modulators therapeutic? In other words, did they reduce virus levels in bees? Yes. The authors tested this question with pinacidil, finding that bees consuming pinacidil had much lower levels of IAPV compared to bees that didn’t consume pinacidil. More importantly, nearly four times as many bees survived IAPV inoculation if they consumed pinacidil compared to no consumption of pinacidil. This is a very promising result!
What’s the mechanism by which pinacidil reduced virus levels? Probably by regulating the production of reactive oxygen species (ROS), which subsequently interfered with virus replication. Several experiments revealed that bees treated with paraquat, which induces the production of ROS, were better equipped than untreated bees to combat IAPV and other viruses. This effect was amplified when bees also consumed pinacidil, highlighting its possible role in the regulation of antiviral ROS. Further, consumption of pinacidil caused bees to increase activity of the enzyme glucose oxidase, which has been linked to social immune function in honey bees.
Was pinacidil effective at reducing virus levels in full colonies in the field? Yes, and this is the second very promising result. Colonies inoculated with viruses while being treated with pinacidil had much lower levels of DWV-A, DWV-B, BQCV, and LSV2 compared to colonies inoculated with viruses but not receiving treatment (see Figure 2). In fact, the virus levels in the pinacidil-treated colonies were equivalent to control colonies that didn’t receive virus inoculation. In other words, treatment with pinacidil essentially reset all viruses to background levels.
Well, this sounds excellent. Should I use ….
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