Background: Feather lice are small parasitic insects that attack birds. There ar

Question

Background: Feather lice are small parasitic insects that attack birds. There are many species of feather lice, each usually attacking just one or a few species of bird host. Clayton et al. (2013) studied the interaction between pigeons and their feather lice and discovered that preening (running the feathers through the bills to remove debris) is an important defense against the lice. They discovered that larger species of lice were removed by preening, but smaller species of lice fit between the barbs (hair-like branches) of the birds’ flight feathers and remain attached during preening. The smaller lice that stay attached can then parasitize the pigeon.

1. Does Clayton et al.’s result (that pigeon sizes are correlated with the sizes of their lice) suggest that there’s a cost to host defense? Does it suggest that there’s a cost to the parasite of evolving to be better and better at attacking the host? Explain.

Explanation / Answer

Blood-feeding arthropods can harm their hosts in many ways, such as through direct tissue damage and anemia, but also by distracting hosts from foraging or watching for predators. Blood-borne pathogens transmitted by arthropods can further harm the host. Thus, effective behavioral and immunological defenses against blood-feeding arthropods may provide important fitness advantages to hosts if they reduce bites, and in systems involving pathogen transmission, if they lower pathogen transmission rate.

Blood-feeding arthropods and the pathogens they transmit are key contributors to the infectious disease burdens that decrease human health, agricultural productivity, and the health of wild species In nature, only a fraction of arthropods carry pathogens, and so vertebrate hosts are most often exposed to the bites of uninfected individuals. Yet, even if no pathogens are transmitted, blood feeding irritates and distracts hosts, often triggering behavioral defenses (such as grooming) With every bite, arthropods deliver salivary compounds that change the local physiological conditions at the bite site, creating an interface where the host immune system can interact with the suite of compounds in the saliva . Salivary compounds may enhance acquisition of host blood by blocking hemostasis, causing vasodilation and reducing inflammation Certain compounds are potent antigens that stimulate the host immune system to act against the arthropod, e.g. causing inflammation and scabbing in response to bites . This interaction between host and arthropod is a key bottleneck in the transmission of blood-borne pathogens and, as such, it is an attractive target for disease control

Interactions between host and arthropod have the potential to modulate pathogen transmission. While effects of behavioral defenses on pathogen transmission have yet to be demonstrated, they have the potential to reduce pathogen transmission by preventing or reducing vector biting, or by decreasing the duration of vector blood feeding. For example, ciconiiform birds with more defensive behavior, such as foot stamping or head shaking, were better at preventing mosquitoes from feeding on them . The same effect was shown for passeriform and galliform species Behavioral defenses may have indirect benefits if the energetic costs to the host associated with reducing vector fitness are offset by the benefits of smaller vector populations, and a reduced rate of pathogen transmission.

Immune defenses against blood feeding arthropods may shape host disease dynamics through interactions among the host immune system, arthropod salivary compounds, and pathogens transmitted to the host by feeding. In many cases parasites benefit from the injection of vector saliva during vector feeding, reaching higher numbers than they would by injection without saliva present. However, if hosts are exposed to salivary compounds alone prior to parasite infection, immune responses to these compounds can have a protective effect by altering the physiological conditions at the bite (transmission) site

Immune defenses against vectors may reduce pathogen transmission by interrupting the vector-induced physiological changes at the bite site that allow blood-feeding, or by indirectly reducing the overall number of vectors (through higher vector mortality and/or lower fecundity). Immune defenses against longer-term feeders, such as ticks, were discovered decades ago The idea that the immune system can also protect against shorter-term feeders (e.g. sandflies and mosquitoes) is less intuitive, but in some cases immune defenses against short-term feeders can also be effective. Immunoglobulins specific to salivary compounds can decrease feeding, acting within minutes ]; proteolytic compounds released by basophils and eosinophils ingested with the blood meal can damage the gut of feeding vectors; these act more slowly as the meal is digested

Immunologically “priming” pigeons against flies by exposure to uninfected flies significantly increased their anti-fly antibody levels compared to naïve controls However, birds that began the experiment without such priming were able to “catch up” in their anti-fly IgY antibody levels about 2 weeks following exposure to infected flies (between time 1 and time 2; The latter were indistinguishable from “primed” birds after this point, indicating that they became just as immunologically responsive as primed birds Flies on primed birds initially were killed at a higher rate than those on birds that were not primed; however, only female flies were significantly affected The immune defense increased female fly mortality by 15.5% compared to female flies on birds without prior exposure to flies.

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