Honey Bee Medicine for the Veterinary Practitioner. Группа авторов
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al. 2014). Bees from high propolis colonies also collected more nectar and pollen than bees from low propolis colonies and showed greater hygienic behavior than low propolis colonies (Nicodemo et al. 2013; Borba et al. 2015). Although other factors may impact resin collection and signs of fitness, the health benefits of propolis may be far‐reaching and offer the colony the critical advantage it needs to survive across seasons and reproduce in an environment increasingly dominated by harmful pests and pathogens.
In vertebrates, propolis enhances cellular immune function by increasing the cytotoxic effects of macrophages and the lytic activity of lymphocytes against invading microbes, but few studies have been done on honey bee immune responses with and without propolis. In their study of the health benefits of a complete propolis barrier, Borba et al. (2015) did not see a reduction in bacterial or viral loads in colonies having a natural propolis barrier (by use of propolis traps in Langstroth hives) compared to colonies not having such a barrier, although their methods did not distinguish between pathogenic and commensal organisms. The authors concluded that the reduced investment in immune expression at the level of the individual bee during periods of low pathogen challenge suggests a direct effect of propolis on the bee immune system. And these immune sparing effects were sustained over the entire summer and fall foraging season, only diminishing over the winter when the bees no longer collected resin until such effects were negligible by spring (Simone‐Finstrom et al. 2017).
More is known about the benefits of propolis for the health of humans than for the honey bee with reported antiseptic, anti‐inflammatory, antioxidant, antibacterial, antifungal, antiulcer, anticancer, and immunomodulatory properties (Pasupuleti et al. 2017). Propolis consists of resin (50%), wax (30%), essential oils (10%), pollen (5%), and other organic compounds (5%). The organic compounds from the plant resins are responsible for the health benefits and include primarily phenols, esters, flavonoids, and terpenes. Propolis also contains both vitamins (B1, B2, B6, C, and E) and minerals (Mg, Ca, K, Mn, and Fe) and a few enzymes from the bee saliva. The health benefits of propolis for honey bees has focused on the important bee pathogens causing American foulbrood (P. larvae) and the fungal agent of chalkbrood disease (Ascosphaera apis). Historical studies showed a positive correlation with feeding of propolis to bees in sugar solution, but since bees are not thought to ingest propolis, such models may not reflect the way resin compounds inhibit microbes – by way of a barrier defense at comb edges, colony walls and around the nest entrance – and the use of propolis in sugar solutions could harm beneficial gut microbes or lead to pathogen resistance (Simone‐Finstrom et al. 2017). Colonies having a propolis barrier can still be infected with American foulbrood, but the severity of infection is reduced and the larval food from propolis rich colonies appears to inhibit P. larvae. Propolis may also help control hive parasites. Laboratory studies of propolis extracts demonstrated that propolis has narcosis and lethal effects on Varroa destructor. Yet, since both mites and propolis exist together in a bee colony without apparent varroacidal effects, it may be that the active compounds in propolis are insoluble and unavailable in adequate concentrations within the waxy glue that bees lay down (Garedew et al. 2002).
Self‐medication in Honey Bees
The idea that non‐human animals can self‐medicate – that is, use organic compounds to clear an infection or reduce its symptoms – was long thought to be limited to vertebrates since it was presumed that it required learning (de Roode et al. 2013). However, we now know that self‐medication or “zoopharmacognosy” is widespread in the insect world, in part because insects utilize a wide variety of organic compounds and have evolved methods to medicate their relatives, offspring, or even societal members. Given there are a variety of reasons why an animal might consume an organic substance independent of improving its own health or that of its kin, true self‐medication has a strict definition: the organism must intentionally seek out the compound, the compound must harm the parasite, the compound must benefit the host, and finally, its use must come with a cost to the host if consumed in the absence of an infection (Abbott 2014).
Honey bees exhibit self‐medication both as a way to prevent infection and to treat an acquired infection. While most insects consume organic compounds to protect their own health or that of their offspring, eusocial honey bees collect resins to treat the entire colony rather than the individual bee, a form of mass medication. Simone‐Finstrom and Spivak (2012) observed that honey bees increased their resin foraging in response to exposure to the chalkbrood fungus, A. apis. In their study, rates of pollen collection declined while resin collection increased after honey bees were challenged with chalkbrood. Since chalkbrood is a disease of larvae and not adult bees, the increase in collection of resins in response to a fungal pathogen is a marvelous example of social immunity in which the colony, rather than the individual bee, is the beneficiary of the adaptive behavior (Simone‐Finstrom and Spivak 2012). Curiously, the bacteria causing American foulbrood and another fungus, Metarhizium, failed to elicit increased resin foraging in their investigation.
Pollen plays a key role in brood rearing, worker bee lifespan, and bee resistance to pathogens. In particular, pollen and protein availability influence hypopharyngeal gland development in worker bees and an abundance is associated with lowered infection titers with deformed wing virus (DeGrandi‐Hoffman et al. 2010). Although not a form of self‐medication since bees do not increase pollen collection in response to infection, a pollen rich diet has been shown to provide protective benefits against a variety of pathogens, including the Varroa mite (Annoscia et al. 2017). In particular, the apolar fraction of pollen (that portion of pollen especially high in fatty acids, hydrocarbons, and sterols, and distinguishable in the laboratory from the polar fraction) appears to provide a dietary protective measure against disease. In the case of Varroa mites, the adults penetrate the bee cuticle and increase water loss, feed on the bee's fat body creating a negative energy balance, and vector viral diseases. Pollen is protective by providing a source of hydrocarbons for cuticle integrity, the unsaturated fatty acid component of pollen shows antibiotic activity, and pollens enhance immune function. The authors conclude that in bees infested with V. destructor, access to a pollen‐rich diet increases lifespan and can compensate for the negative effects of the mite (Annoscia et al. 2017). Bumble bees (Bombus impatiens) are known to alter their foraging patterns based on the quality of the nutritional resource, with high Pollen:Lipid ratios of highest attraction (Vaudo et al. 2016). Likewise, the secondary metabolites in floral nectar (alkaloids, teropenoids, and glycosides) have been shown to reduce bumble bee parasite loads (Richardson et al. 2015). Such observations confirm suspicions that changes in bee forage, particularly in agricultural dominated landscapes or in migratory beekeeping practices, likely contributes to colony declines. The important message for the bee doctor from all this research is that colony nutrition is ultimately connected to colony health and that the role of the veterinarian in helping the beekeeper manage disease should always include a thorough evaluation of colony nutrition, including review of local bloom calendars, hive pollen stores, and the use of protein supplements.
Social Fever
It should come as no surprise that the honey bee superorganism can mount a biological “fever” as a direct preventative measure against a heat‐sensitive pathogen. This fever is not mounted in the individual bee but rather in the heart of the colony in the developing brood, and is a remarkable example of convergent evolution between the organism and superorganism. In a fascinating experiment, Starks and colleagues (2000) measured brood comb temperatures in three colonies and one control colony in response to changes in ambient environmental temperatures and following the inoculation of an infective dose of the fungal pathogen for chalkbrood disease (A. apis). Chalkbrood is triggered by chilling of the brood; therefore, it is a seasonal condition most prevalent in the spring of the year or in small colonies that are unable to maintain homeostasis by way of thermoregulation. Normal brood comb temperatures are maintained within a very narrow range from 33 to 36 °C and only vary by small amounts in direct relation to ambient temperature – such a relationship allowed the authors to determine expected brood comb temperatures at each ambient temperature and measure variations from expected results. The brood comb temperature rose 0.56 °C after inoculation with an infectious dose of A. apis (Starks et al. 2000). The authors argue that this small elevation in temperature,