Immunology Flashcards
What is a parasite
organism that lives in close association with another organism (the host), diverting the host’s resources to its own fitness, at a cost to the host.
Macro parasites
e.g. worms, ticks, fleas, parasitoids (flies or wasps)
considered as those that induce costs by diverting an individual’s resources directly from their bodies. These can be large ‘macroparasites’ that can be found on the body surface
macroparasites have low virulence, they can be costly, especially if the host is malnourished, but they rarely induce mortality directly. Exceptions are the parasitoids that lay their eggs in or on the body of an insect, with those larvae consuming the insect from the inside out, almost always resulting in mortality
Micro parasites
otherwise known as pathogens. These can also be fairly benign, e.g. the common cold, or incredibly virulent, e.g. Ebola. But costs can come in a form that is unlinked to mortality. An animal’s fitness can be impacted by increased mortality if they are young, but older individuals that have finished reproducing may not lose fitness by dying from infection. However, parasites can also directly or indirectly affect reproduction. Being sick can make you less likely to attract mates or hold a territory. Some parasites can even castrate their hosts so that the energy that would go into reproduction is instead diverted to the parasite.
Two categories of parasite
Classic
Brood
Brood parasites
Many animals provision resources to their offspring after they have detached from the parental body. This parental care provides a further source of energy that a parasite can exploit. The eponymous example of this parasitism of reproductive investment is performed by the common cuckoo (Cuculus canorus), which targets passerine hosts with post-hatching parental care. She removes a host egg upon laying her own in the nest, which hatches more rapidly than its adopted nestmates and promptly forcibly ejects all of the host’s own offspring from the nest, ensuring that all subsequent parental investment is directed exclusively towards the parasite. In contrast with classic parasitism, brood parasitism is a direct attack on the indirect fitness of the parent.
However, like classic parasitism, the costs of brood parasitism vary depending on the strategy of the parasite. For brood parasites, the magnitude of the costs are also a function of the level of parental investment in post-hatching care provided by the host. For example, in many cases of avian brood parasitism by cuckoos (Cuculus spp.) the combination of high levels of parental investment and an extremely virulent attack strategy by the parasite results in high parasite-induced costs for hosts.
Innate immunity
Non-specific defence
Includes cells and humoral components
‘Standing army’ protects an organism immediately upon infection
Adaptive immunology
(vertebrates only)
Memory of encountered infections - pathogen-specific response
Antibodies target pathogens in the bloodstream
T cells detect and kill infected cells
Behavioural defences against parasites- before
Avoidance, concealment, vigilance, grooming, hygienic behaviours, mobbing
Physical defences against parasitism - before
Fur, hair, skin, cuticle, gut lining, melanin, external secretions, nest architecture
Behavioural defences
Before parasitism – keeping them at bay
Avoidance, concealment, vigilance, grooming, hygienic behaviours, mobbing
Avoidance
First, animals can avoid parasites getting close by avoiding contaminated locations. In terms of classic parasites, individuals can avoid infection by shunning contaminated food e.g. grasshoppers [Jaronski 2013] or foraging/sleeping locations with a high risk of parasites e.g. termites [Mburu et al 2009] and badgers [Butler & Roper 1996] or by avoiding infected conspecifics as has been shown in mice [Kavaliers & Colwell 1995]. Some dung beetles specialise on carrion, but behave as they do with dung, collecting chunks and rolling it into a ball. However, carrion is a resource rich in microbial activity and a likely source of pathogens. To avoid this risk, beetles take the ball of meat that they will use for their babies away from the decomposing carcass where the threat of microbes is reduced. Some will roll the ball and tunnel up to 1m below the carcass where the risk of pathogens is greatly reduced compared to the surface.
Concealment and vigilance
Some parasites are mobile and actively seek out their hosts, parasite avoidance isn’t as simple as avoiding other parasitised individuals/locations. For example, a study on the hosts of brown-headed cowbirds found that more detectable hosts (e.g. those with longer nest-building visits, more male vocalisation near nests) were more likely to be parasitized [Banks & Martin 2001 ]. Given brood parasites’ active host searching behaviour, selection might favour hosts that attempt to conceal their locations. This has been shown in the European beewolf, a Sphecid wasp which is parasitised by a cuckoo wasp (Strohm et al 2001). Beewolf activity was temporally shifted during the day relative to cuckoo activity, such that beewolves were more active when cuckoo activity declined during the day (Strohm et al 2001). There is also evidence that the placement of the nest may have evolved to reduce the likelihood of brood parasites detecting or accessing the brood. For example, reed warblers have greater success when they nest further from the observation perches used by cuckoo females [Oien et al 1996], and potential open-nesting host species avoid using areas with vocal cues of cuckoo activity [Tolvanen et al 2017].
Grooming and hygienic behaviours
Grooming is an important anti-parasite measure. Some animals, like cats, are expert ‘self groomers’ whilst others have to enlist the help of friends. Mutual grooming is both an important antiparasite defence, friends can reach locations that you can’t, but has other benefits in terms of social cohesion in groups. Other spp use hired help, who will groom off parasites for the meal, such as oxpeckers and cleaner fish, though these mutualisms can shift to parasitism themselves when some cleaners start to keep wounds open to steal blood. A safer option is to enlist the help of something that won’t expect anything in return, such as a rock or a tree.
Many social insects also perform hygienic behaviours such as removing dead bodies from the nest that could spread disease, or barring others from entering the nest if they are likely to be infected.
Mobbing
So what about brood parasites? They can be deterred before they access the nest. Reed warblers will mob female cuckoos that appear near their nest. This has been tested with models that look like cuckoos or similar sized birds and cuckoos are mobbed more frequently. In addition, birds at greater risk of parasitism mob more than those in low risk areas. Mobbing is effective in that birds that mob more are less likely to end up with a cuckoo chick in their nest.
Melanin
Melanin can be incorporated into skin, cuticle, fur or feathers. Melanin has many functions, protection against UV, use in camouflage and signalling, but also has antimicrobial and antitoxin properties. Black skin is less susceptible to fungal infections than skin with low levels of melanin. This is true in caterpillars too. In fact, some species respond to high population densities, which are predictive of infection risk, by massively increasing the melanin content of their cuticle. This protects them against fungal infections and attack from macroparasites that enter via the skin
External secretions
These physical defences can be further enhanced by chemical protection mechanisms. Sweat and organic acids on the skin, antimicrobials in the respiratory tract, and lysozymes in tears and saliva all help to kill microbes before they enter the body. Birds preen their feathers with a secretion from the uropygial gland, which has been shown to have antimicrobial activity and so protects the feathers from feather-degrading bacteria. Several animal species also use self-produced antimicrobials in the fabric of a nesting structure. For example, tungara frogs cover their eggs in a foam that contains a cocktail of antimicrobial chemicals and burying beetles prepare a vertebrate carcass for their offspring by covering it with antibacterial exudates. For example, the peacock blenny male covers his nest surface with mucus from his anal glands, which contains antibacterial activity, protecting the eggs from infection. Beewolves, a type of digger wasp, house streptomycete bacteria in their antennae, which they smear inside the brood cells. Larva pick up these bacteria and incorporate them into their cocoons, where they produce a suite of antimicrobials, protecting the pupae from fungal and bacterial infection.
Nest architecture
Animals can create nests that parasites cannot enter or which allow for enhanced vigilance. Weaverbird nests, have a narrow entrance that makes it hard for cuckoos to gain access and social insects can have large nests that have few entrances. Many social insects post guards at the entrance to their nests, defending them against intruders, which can include ‘cuckoos’ or infected conspecifics. Just as burying beetles and tungara frogs protect their nests and offspring with cocktail of antimicrobial chemicals, wood cockroaches, that live in family groups inside decaying wood, coat the chambers with faecal pellets with antifungal properties. Other animals use naturally occurring antimicrobials in the structure of their nests. Starlings collect fresh greenery, rich in antibacterial phenolic compounds. Ants and bees collect resins from conifers and incorporate them into the nest to deter microbes.
