Unraveling Host-Parasite-Pathogen Dynamics
In the intricate web of animal disease and health in animals, it is essential to understand how hosts are able to resist parasitic and pathogenic invasion in order to learn about resilience and population viability.
Among such survival processes, *tolerance* has emerged as a central phenomenon. As distinct from resistance, where parasite or pathogen populations are decreased or annihilated, tolerance entails a decrease in the damage done by the invaders without necessarily decreasing their populations. This allows the host to recover from infections with little cost to overall fitness, a concept of growing interest especially in that of honeybees resistant to Varroa mites and viruses they carry and transmit. "
Viruses are readily transmitted between and within honey bee colonies, as well as between other bee and insect species that are found in their vicinity. They are transmitted from drone to queen during mating, from queen to egg, from nurses to larvae during feeding, and between workers during trophallaxis, or via the environment, particularly when the bees consume infested food, e.g., infested honey stores in the colony or infested flowers in the field."
Resistant and tolerant are two opposing yet complementary approaches.
Resistant is when the active defense mechanisms are employed like grooming actions physically removing the mites or immunological response inhibiting viral replication in the host. Tolerance, on the other hand, does not kill the parasite or pathogen directly but reduces the harm that they cause. This may be in the way of enhanced tissue repair, immune modulation to prevent collateral damage, or other physiological adaptations enabling the host to carry on despite ongoing infection. Resistance will keep the parasites and viruses continuously evolving, whereas tolerance will be likely to stabilize at a population level. As the tolerant hosts are not exerting the same selective pressure on the parasites, the interactions can go on long-term without developing more virulent strains and allowing for a mutually harmonious coexistence.
"Honeybees are a very complex system because they are social and microorganisms like mites and viruses have many levels of interaction with individual bees, the colony, the mites, and the viruses."
Varroa mite is also a vector and a parasite to many microorganisms like deformed wing virus, Israeli acute paralysis virus, and several others that are major causatives of colony loss worldwide. These interactions are interconnected: bees, mites, and viruses constitute a group of interactions with colony-level implications for health. Honeybee populations are infected with low infestation levels, which may be indicative of successful resistance in the form of behavior like mite removal or grooming.
Other infested populations have high mite burdens but retain low virus-driven colony mortality. This would mean that they have evolved or established tolerance mechanisms to viral infection, perhaps through immune modulation, greater tissue repair capacity, or some other physiological feature. "Most virus infections in honey bee colonies are not of concern if the honey bee colony is healthy and does not suffer from chronic stress."
Past studies have revealed that some honeybee colonies, especially those in Africa, South America, and some regions of Europe and North America, thrived long with Varroa mites and viruses on board without spending much on mite control actions. Bees can endure relatively heavy mite infestations but incur no or minimal colony loss from viruses.This would involve the development of mechanisms of virus tolerance that are mediated by immune modulation, cell repair mechanisms, or stress response mechanisms that prevent the infection of the cells by the viruses.
Notably, this tolerance appears to be an independent survival mechanism from mechanisms of reduced mite reproduction or infestation numbers. Whether or not this tolerance has come at a cost in fitness, i.e., reduced reproductive success or longevity, is a principal question presently being addressed by researchers. The results suggest that these tolerant bees can maintain colony production and long-term viability under chronic parasitism, a type of adaptation in the interest of resilience rather than elimination.
"Lack of coevolution between the ectoparasitic mites and A. mellifera, when the former parasitized the latter some decades back, is held to be one of the principal reasons for the susceptibility of the honeybee host. In the light of low efficacy and undesirable side effects of drug-based approaches, breeding schemes aim at the selection of drug-resistant honeybee stocks worldwide.".
The immune mechanism which acts as defense for bee colonies and also allows individuals to accommodate to external enemies is plastic in character, and hence artificial selection of honeybee stocks for resistance to main biotic enemies is a critical type of human intervention with the aim of accelerating colony evolution and achieving resistance. Tolerance to virus as a main survival factor discovered offers new possibilities for colony resilience enhancement by means of breeding programs.
Although the resistance traits are advantageous and have a direct capability of controlling mite population, sole application has been proven to make parasites and viruses more virulent. Tolerance offers an alternative by enabling stability in the host-parasite-virus complex. If tolerance characters are maintained at low fitness cost and induce lasting colony health, then they would be a viable long-term method of beekeeping. Of course, if tolerance allows viruses to be sheltered within reservoirs and colonies, then it could also be damaging by enabling the possible transmission of pathogens between colonies.
"Tolerance diminishes host fitness costs of parasitism without reducing parasite abundance, whereas resistance averts parasitism or sustains low parasitism intensity."
Tolerance is an approach that aims to reduce the cost of parasitism to the host by suppressing the pathology of infection. It is intrinsically different from other forms of resistance-based disease defences based on avoidance, control or recovery. Moreover, tolerance evolution is self-strengthening if it increases host infectiousness. Tolerance measures being more prevalent in a population, parasites become more numerous. The force of infection ('parasite pressure') therefore increases, which strengthens the selection pressure on non-tolerant hosts to evolve further upgraded tolerance to the parasite.
"Population monitoring of parasites is crucial during honeybee breeding because the mechanistic difference between parasite tolerance and resistance has significant ecological implications. Although both mechanisms lead to the survival of honeybees, breeding strategies without parasite monitoring can lead to the development of highly tolerant hosts and subsequent 'tragedies of tolerance' which are deadly to wild honeybees."
Miller depicted the initial tragedy of tolerance, and depicted how if host-parasite systems do not evolve towards total commensalism, tolerance strategies can reduce case mortality but overall maximize absolute parasite-caused mortality in the whole population.
"Tolerance can therefore be an evolutionary endpoint where more organisms are dying as a result of a parasite than when the population was fully susceptible to the disease. This pattern in ecology can easily be extended to incorporate spillover, and under the tolerant evolutionary endpoint there are just more infectious definitive hosts present in the environment with which to generate spillover into sympatric species"
It is this balance that has to be maintained, and experiments continue with the aim of establishing whether tolerance is lower or inadvertently preserving infection reservoirs. Virus tolerance has been a prime independent-evolved survival mechanism found by desecrating the complex host-parasite-pathogen relationships in honeybees within those groups that naturally have developed resistance.Incorporating such data into management and breeding has actually transformed the manner in which beekeepers are combating Varroa and viral control presently, shifting from eradication measures to those that introduce resilience.
Evangelia Mavridis
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