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Immunity and Resistance in the Bee World

BEE CULTURE – December 2009

Immunity: When a harmful bug has no effect on an organism.

Resistance: An organism tolerates a harmful bug but with little or no economic damage to the organism (100% resistance = immunity).

Humans are immune to a number of bugs, either through long-time exposure or by vaccination. Antibodies (developed through either exposure or vaccination) protect us from being overcome by a disease. The introduction of new diseases into the indigenous populations of the New World by disease-carrying Europeans decimated native populations – some historians make a good argument that these diseases changed the course of history (just as varroa has changed the history of beekeeping). With time, the natives developed requisite antibodies and their populations rebounded. Visitors to Mexico (and some other countries) often get sick if they drink the local water but this same water has no effect on the local population because the locals have built up an immunity (or at the very least a high degree of resistance) to the bugs in the water through generations of exposure.

Home-schooled children are more likely to get sick in college because they have had limited exposure to the myriad of bugs prevalent in public schools – their systems have limited antibody resources. Conscientious mothers that spray home surfaces with disinfectants may be putting the brakes on immune system development in their offspring. And makers of these anti-germ sprays (and of hand sanitizers) may be doing more harm than good by neutralizing a tried and true method of boosting immune systems (just as chemical treatments for varroa and other pests and diseases impede resistance development in honey bees and increase tolerance to the chemicals in the target pest). Currently, health officials tell us that older people are less likely to get the H1N1 flu virus because their systems contain antibodies developed through previous exposure to related viruses. For young people that come down with H1N1, it’s not all bad: they could well be protected from succumbing to a super-virus 50 years hence.

Compared with humans, honey bees have a relatively fragile immune system. A less than robust immune system means greater susceptibility to pests and diseases. A highly developed immune system, however, diverts resources that might otherwise be used to benefit an organism – in the case of bees, more brood rearing and more foraging for pollen and nectar; there is no free lunch. Honey bees use a number of strategies to compensate for deficiencies in their immune systems:

1. When sick, bees altruistically die in the field so that they do not infect others.

2. House-cleaning bees remove dead bees (inoculum sources) from the hive.

3. Bees are good house cleaners, a trait that can be amplified (e.g., hygienic bees).

4. The anti-microbial properties of propolis protect bees.

5. House bees are healthy, nutrition-wise. Old bees die, depleted of nutrients. (Nutrient reserves, diverted to young bees, provide resistance to diseases.)

Honey bee caretakers (keepers of bees, or beekeepers) also allow bees to perform at a high level by providing bees with good pasture (easier said than done) and/or supplemental protein feeding (although no supplemental feed is as beneficial to honey bee health as a multi-colored variety of natural pollens). Beekeeper control of pests and diseases – foul brood, tracheal mites, nosema and especially varroa mites – also allows honey bees to remain healthy in spite of their relatively fragile immune systems (providing beekeepers rotate comb on a regular basis to prevent a buildup of harmful chemicals).

Like humans, honey bees have been challenged by viruses for eons. All bees carry viruses and this virus complex changes over time as new viruses enter the system and old viruses mutate. In 1980 BV (Before Varroa) when a new virus entered a honey bee population, the spread of the virus was gradual, allowing bees ample time to come up with methods of neutralizing the virus, including incorporating resistance into the bee genome. Varroa mites, acting as contaminated hypodermic needles, short-circuited this natural disease-fighting mechanism, overwhelming a colony by rapidly spreading viruses throughout the colony and then throughout an apiary. Honey bees had no defense against varroa mites and current varroa-control measures are less than stellar.

Without an effective transmission agent it is difficult for a disease to establish a toe-hold in a population. The most effective method of controlling some diseases is by attacking the vector that transmits the disease, e.g., killing mosquitoes to control malaria. A consensus is forming that the combination of a virus (or viruses) + varroa (and possibly nosema ceranae) is the cause of current problems with honey bees. Add a robbing environment into the mix and you compound the problem. Without the varroa vector, viruses would cause far less damage. For example, IAPV (Israeli Acute Paralysis Virus) is widespread in Australia but is not considered a major threat (Australia does not yet have varroa). Some feel that the combination of IAPV + varroa represents a threat to U.S. bee colonies and they make a good case for banning the importation of Aussie bees (too late now). Viruses are constantly mutating and some feel that the Aussie strain of IAPV is less deadly than other strains and therefore Aussie imports are not a problem. Or, perhaps, incremental exposure to IAPV by Aussie bees gave them sufficient time to incorporate immunity, or at least some degree of resistance, into their genome. There will always be new viruses — develop resistance (or a vaccine) against one, and another will pop up and take you down.

Some viruses inflict considerably more damage on a population than others. The 1918 flu virus was a super-bug that killed millions of people and on a scale of 1 to 10 (10 being most severe) would rate a 10. Most flu viruses would rate a 1 or 2; H1N1 might currently rate a 5 (subject to change after it has run its course). Past honey bee viruses that caused disappearing bees or collapsing colonies could be similarly rated. Assuming a virus caused CCD in 2007-2008, affected beekeepers might rate this virus a 10. Apiaries that did not (or have not) come down with CCD in recent years likely either were isolated from a virus source or enjoyed robust health when exposed to the virus (and yes, pesticide exposure would compromise colony health). Like humans, honey bees carry chronic viruses and such viruses flare up when the health of a population is impaired. DWV (Deformed Wing Virus) appears to be a chronic bee virus and one that is often associated with collapsing colonies (and with varroa).

How severely a virus affects a population – whether humans or bees — depends on 3 factors:

1. The degree of exposure to (or isolation from) infected individuals.

2. The general health of the population (esp. nutrition-wise).

3. The age distribution of the population (in general, the elderly are more susceptible).

For honey bees, the population of a vectoring agent – mainly varroa, possibly nosema – is also a factor. In the presence of varroa, honey bees must wage a battle on two fronts. An analogy is a man holding his own against a bear attack but succumbing when he is simultaneously attacked by a pack of wolves. Fighting both a virus and varroa is a daunting task for honey bees. In the case of honey bees, a frontal attack on varroa should be more productive than attacking viruses.

Any breeding program that incorporates resistance to mites and diseases comes at a cost. Take an extreme example: posting a guard bee by every brood cell to immediately target and kill varroa could develop a varroa-free population, but at a significant cost – those guard bees would otherwise be foraging bees. We all face tradeoffs in life – work vs. family, mind development vs. body development – honey bees are no different: invest too many resources in combating varroa and colony production will suffer. One strategy for bees to develop varroa resistance is by producing minimal amounts of food – bee brood – for the mites. Reduce brood too far though, and consequent lower worker populations will mean much lower honey production. Biologist Raphael Sagarin put it succinctly: “organisms inherently understand that there is risk in life. The idea that we can eliminate these risks would be selected against quickly in the natural world since any organism that tried to do so would not have enough resources left for reproduction or feeding itself.” (New Scientist, February 9, 2008, p.49). Building a bee with total immunity to pests and diseases would come at too great a cost to the bee.

The Holy Grail in the war against varroa – immunity (or 100% resistance) – is likely impossible. U.S. bee breeding programs aimed at varroa resistance have been hampered by the ever-narrowing number of genes in U.S. bee populations to the point where some feel that our bees are excessively inbred (assuming the relatively recent introduction of African and Australian genes has not been beneficial). The recent importation of drone semen from promising stock in other countries should greatly improve our gene pool*. The current success of Marla Spivak’s Minnesota Hygienic stock shows that good varroa resistance (and resistance to brood diseases) can be obtained without significantly sacrificing honey production. MN Hygienic bees recruit potential foragers for cell cleaning (and mite-biting) duties but apparently not in great enough numbers to affect honey production.

Hygienic bees don’t eliminate mites but reduce mite numbers to levels that can be more easily controlled with minimal (or no) use of chemicals. Yes, these few mites can still carry and transmit viruses, but hopefully mite numbers will be low enough to prevent a virus epidemic in an apiary. Currently, the best method of protecting bees from viruses is the same as protecting humans from the H1N1 flu virus: isolation from others that might be carrying the virus. Admittedly, this is far easier said than done for both bees and humans.

Rather than developing 100% resistance (or immunity) from varroa, a frontal attack on this insidious pest is preferable. Promising work on varroa control includes using odors to lure varroa to their doom or to confuse them so they cannot locate brood cells. Breeding non-pathogenic varroa or inserting a suicide gene into the varroa genome would certainly have benefits. Work to develop additional chemicals to control varroa should continue (although formic acid treatment for varroa control is not new, the new, slow-release, formic acid strip shows promise here; some essential oils also show promise). Until such offensive measures bear fruit, a combination of resistant bees (e.g., hygienic bees), chemical treatments, supplemental feeding, isolation (where possible), nosema control and regular comb rotation will continue to be the best method of keeping healthy bees.

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*Sue Cobey and Steve Sheppard invested significant resources in battling bureaucracies to allow the importation of this semen and in so doing, they have attained a degree of resistance to these same bureaucracies. No one in the history of our country has ever achieved total immunity from bureaucracies but give credit to Cobey and Sheppard for trying.