Do Bees Feel Sick?
Jay Evans, USDA Beltsville Bee Lab
In our own species, disease can be swift and devastating. In the U.S. alone, SARS-CoV2 has killed at least one million loved ones in two years, behind only heart disease and cancer as our leading cause of death. Much of the time, however, disease can be chronic, setting us back somewhat but not, at the time, causing death. Many illnesses have more favorable outcomes, with time and our immune systems erasing all signs in a few days. The same mix of devastatingly swift disease versus long-term illness versus, one hopes, full recovery, holds true for the many plants and animals we cherish and/or rely upon. A lot of our work as researchers goes toward better understanding the long-term effects of disease on colony productivity, from lost days as a nurse, to some sort of brain fog when trying to find flowers or return home, to slightly shorter lifespans that are rarely noticed aside from their additive effects on colony size and productivity.
In musing about this for honey bees, I wondered if they ever take sick days, if they move just a bit slower in the hive or on the wing during or after an illness, and if they ever fully recover from an infection picked up in the cozy confines of a colony. We can get some insights into these effects by looking at both historical and recent work on nosema disease. Here is my attempt to get some clarity on long-nosema and the impacts of chronic disease in bees.
Nosema is regarded as a metabolic disease, a case of the parasite growing to such numbers in the cells lining bee guts that the energetic demands of keeping this alien population alive become taxing. This could be said for many parasites, of course. Those that do not tear risky lesions on skins, or release toxins during infection, normally cause harm by simply being dedicated mooches, taking what they can from their hosts. Nosema, and others in the Microsporidia, have taken this theft to a new level for cellular parasites, foregoing any pretense of generating the energy their cells needed when they dropped having mitochondria in their cells millions of years ago. As a result, even the transfer of nutrients to the molecules that make cells respire and function is left to their bee hosts.
In one of the first papers to tackle the impacts of Nosema ceranae on bee health, Christopher Mayack and Dhruba Naug used clever experiments to test “Energetic stress in the honey bee Apis mellifera from Nosema ceranae infection” (2009), Journal of Invertebrate Pathology 100; 185–188, doi:10.1016/j.jip.2008.12.001. First, they proposed that bees infected by an energy-sapping parasite would be eager to replace that energy. They confirmed this prediction in that infected bees were both less selective of syrup concentrations and ate more. They also found that the impacts of nosema on infected bees could be reduced by abundant sugar supplies.
Further making the case that nosema is an energy pirate, these two researchers next showed that blood sugar levels in infected bees were lower than in healthy bees of the same age (Mayack C and Naug D “Parasitic infection leads to decline in hemolymph sugar levels in honey bee foragers” (2010), Journal of Insect Physiology 56: 1572-1575, https://doi.org/10.1016/j.jinsphys.2010.05.016). Ignoring the challenge of assessing blood sugar in worker honey bees (hint: blood tests are harder on bees than the infection itself), it would appear that reduced blood sugar in bees that rely on sugar to fuel adult activities would make those bees ‘sick’ in ways that hurt productivity. There are at least two possible outcomes to nosema-induced pressures on worker bees in colonies. First, infected bees could take a sick leave and tone things down, flying less far and perhaps taking fewer trips daily. Second, infected bees could return more frequently to honey stores, removing the valuable resources that they should be collecting. Do sick bees indeed fly less often? A paper by Trish Wells and colleagues in the United Kingdom (“Flight performance of actively foraging honey bees is reduced by a common pathogen” (2016) Environmental Microbiology Reports, doi:10.1111/1758-2229.12434 suggest this is not the case for nosema infection. So perhaps the real cost is in over-consumption of colony food stores. Interestingly, this paper did find that bees infected with viruses made poor foragers, travelling one third as far as healthy bees and spending half as much time overall foraging, indeed a direct sickness cost to the colony. Given that bee viruses also seem to cause a form of brain fog that affects the abilities of foragers to find food and return home safely (Iqbal J and Mueller U “Virus infection causes specific learning deficits in honey bee foragers” (2007) Proceedings of the Royal Society B: Biological Sciences 274: 1517-1521, doi:10.1098/rspb.2007.0022 assessing the colony impacts of viral sickness is a great topic.
As far as nosema, there is one more recent paper that hints at an underlying sickness of bees carrying this parasite. As if it weren’t enough to take the energetic building blocks of bees, nosema is also extremely good at scavenging iron from the cells of their bee hosts. In a paper co-led by my USDA colleague Judy Chen, Cristina Rodriguez-Garcia showed that not only does nosema scavenge iron from bee hosts, they enlist the bee’s own transferrin protein (an iron vehicle) as an accomplice (“Transferrin-mediated iron sequestration suggests a novel therapeutic strategy for controlling Nosema disease in the honey bee, Apis mellifera” (2021), PLoS Pathogens 17(2): e1009270. https://doi.org/10.1371/journal.ppat.1009270. Exploiting this vehicle, Judy, with some of us at the USDA-ARS Bee Research Lab, is trying to knock down the transferrin protein in order to iron out nosema disease.
So, it is clear that bees can get sick and that the effects per bee range from drop-dead bad to subtle. Bees have a strong individual and ‘social’ immune system and the benefits of food buffers to help nurse sick bees back to productivity. How far a wave of nosema goes to reduce productivity will probably depend on colony genetics, nutrition, and the co-occurrence of other pathogens and stresses. Colony-level studies using tagged healthy and sick bees will be key for firmly assessing the rate of sick days in a honey bee colony, and the impacts of management schemes aimed at keeping workers healthy.
