Evolution of pathogen and parasite avoidance behaviours

Employing strategies to survive and successfully reproduce is arguably the most important theme in animal biology. Because of the ever-present threat of disease-causing organisms for animals and ancient humans living in nature, avoiding, controlling and/or ridding the body of pathogens and parasites is essential to surviving and reproducing. Disease-control strategies of animals and ancient humans can be categorised as: 1) physical avoidance and removal of pathogens and parasites; 2) avoidance or peripheralization of conspecifics that could be carrying potential pathogens or parasites; 3) herbal medicine to prevent or treat an infection or infestation; 4) potentiation of the body immune system and healing processes; and 5) care of sick or injured family and/or group members. There are multiple examples from animals living in nature of each of these strategies – from biting flies and intestinal parasites to sexually-transmitted diseases – but the examples are spread out among species with just a couple or so strategies evident per species. In contrast to animals, in humans all the disease control strategies are evident and have now been consolidated into an extensive medical system. A hypothesis that explains this difference of more frequent use of disease control strategies in humans is that they are infected or sick more often than animals. This disease prevalence difference has been attributed to an evolutionary dietary transition in humans from mostly natural vegetation to a meat-based diet, with an increase in health-eroding free radicals and a dietary reduction of free-radical-scavenging antioxidants.

Discrimination among pathogenic and beneficial microbes is essential for host organism immunity and homeostasis, and increasingly the nervous system of animals is being established as an important site of bacterial recognition. For the nematode Caenorhabditis elegans, the role of the nervous system in microbial detection is tied to food foraging behaviors, as C. elegans is a bacterivore that feeds on microbes in decaying organic matter. However C. elegans in the environment and the laboratory can be infected with fungal and bacterial pathogens, and are capable of mounting a protective immune response that includes behavioral avoidance. How does the nervous system of C. elegans discriminate between beneficial and pathogenic microbes, and how is this detection translated into a long-term behavioral response? We show that chemosensory detection of two secondary metabolites produced by Pseudomonas aeruginosa modulates a neuroendocrine signaling pathway that promotes C. elegans avoidance behavior. Specifically, secondary metabolites phenazine-1-carboxamide and pyochelin activate a G protein-signaling pathway in the ASJ chemosensory neuron pair that induces expression of the neuromodulator DAF-7/TGF-. DAF-7, in turn, activates a canonical TGF-signaling pathway in adjacent interneurons to modulate aerotaxis behavior and promote avoidance of pathogenic P. aeruginosa. This study provides a chemical, genetic, and neuronal basis for how the behavior and physiology of a simple animal host can be modified by the microbial environment, and suggests that secondary metabolites produced by microbes may provide environmental cues that contribute to pathogen recognition and host survival.

Similar to avian brood parasites, social parasites exploit the social behaviour of other species. Slavemaking ants are social parasites, which steal worker brood from host colonies, to use them as slaves. These slave raids are highly destructive for host colonies, which not only lose their annual production of new workers and their nest, but queens and workers that are killed during defense. Host defences become increasingly costly as parasites breach successive defence lines. In a series of studies, the Foitzik group investigated the interplay between host defence portfolios and social parasite pressure by comparing 17 populations of two Temnothorax ant species. Under low parasite pressure, host colonies responded to intruding slavemakers with collective aggression, which prevents them from escaping and recruiting nest-mates. However, as parasite pressure increased, ant colonies of both species became more likely to flee rather than fight. Aggression against conspecifics also co-varied with parasite pressure and was elevated in highly parasitized populations. Indeed, experiments show better defense against intruding slavemakers in more aggressive colonies. Finally, the degree of behavioral specialization varied with slavemaker presence: unparasitized host colonies in locales with slavemakers exhibited a lower degree of specialization. To show that this trait is associated with parasite defense, the group manipulated the degree of specialization in host colonies and exposed them to slave raids. As expected, colonies with less specialized workers were better able to respond to an attack, saved more brood and killed more slavemakers. Hosts of slavemaking ants therefore exhibit complex defense portfolios that shift with social parasite pressure.

Behaviour is key to limiting the risk of acquiring and spreading infection. Infection avoidance behaviours, in particular, are the first line of defence against pathogenic encounters. While clearly beneficial to hosts, completely avoiding infection is rarely possible. Foraging and feeding, for example, are vital aspects of host ecology, reproduction and fitness, but are also major routes of pathogen transmission. Foraging and feeding are especially important to holometabolous insect larvae, which devote most of their time to these behaviours. Beyond foraging during the larval stage, choosing where to oviposit or rear offspring is another important life-history decision, but can be risky if individuals are unable to identify and avoid potentially infectious environments. The environment in which adult insects choose to oviposit is therefore a major determinant of both offspring environmental quality and infection risk.  Using the fruit fly Drosophila melanogaster as a model of how infection affects host behaviour, I will discuss experimental results showing how larval and adult fruit flies exhibit behavioural plasticity in foraging and oviposition behaviours in response to internal and external cues of infection risk, and that infection-induced changes in these behaviours may carry important fitness costs.

Honey bees, Apis mellifera, have many diseases. Hygienic behaviour (HB) is a social defence against brood diseases in sealed cells. That is, diseases of fully-fed larvae and pupae whose cell has been sealed with wax. HB is heritable. It is possible to screen colonies with a simple freeze-killed-brood test to quantify colony levels of hygienic behaviour and to breed hygienic bees.  Hygienic workers remove the wax cappings from cells containing dead or diseased brood and remove the infected individuals from the colony. This reduces the spread of the pathogen in the colony. Previous research in the USA has shown that HB is highly effective at controlling the bacterial disease American foulbrood and the fungal disease chalkbrood. Research in my laboratory has shown that HB helps control varroa mites, reducing their annual population growth in a colony by >50%, and deformed wing virus, reducing viral levels 10,000 times. This is important as varroa and DWV are the two most important pests/pathogens affecting honey bees today. It is even possible to save the life of a colony with gross symptoms of DWV by requeening it with a hygienic queen, whose eggs develop into hygienic workers. HB is naturally occurring but rare. We do not know why it is rare. It is not harmful to the colony—hygienic colonies do not mistakenly remove more healthy brood. It may be rare as it is one of two alternative adaptive peaks, the other one being ‘leave dead brood sealed’.

Seawater is an effective medium for distributing pathogens. It explains the rapid spread and wide distribution of many marine pathogens relative to their terrestrial counterparts. While few terrestrial pathogens spread more than 1000km in a year, it is the norm in the marine realm. The same properties of seawater that lend it so well to pathogen transport also make it effective for transporting chemosensory cues.  Marine organisms, and particularly crustaceans, have developed sensitive chemosensory receptors and use them for a multitude of ecological functions that include avoiding predators and competitors, locating mates and conspecific aggregations, foraging and, for spiny lobsters at least, avoiding infection.  The Caribbean spiny lobster Panulirus argus is normally social, sharing crevice shelters with healthy conspecifics during the day.  This behavior is mediated by chemical cues found in lobster urine and confers typical advantages of sociality such as reduced predation risk.  However, the Caribbean spiny lobster is also able to detect and avoid urine-based cues from conspecifics infected with the lethal virus PaV1 (Panulirus argus Virus 1). This adaptive behavior reduces the infection risk for individuals and reduces the likelihood of an epizootic by breaking the expected host density – transmission relationship. Although reports of pathogen avoidance behavior among marine animals is rare, this may be more a function of our inability to detect avoidance than a lack of its occurrence.

Birds have diverse behavioral adaptations for avoiding parasites.  These include the active choice of parasite-free mates, microhabitats, and nests.  Parasite-mediated mate choice, which has received a good deal of attention, has been demonstrated in several groups of birds.  Birds avoid parasites in their nests through careful inspection, as well as sanitation and fumigation behavior.   If all else fails, birds are also known to desert parasitized nests. Social birds that choose to nest in the center of breeding colonies may be less exposed to parasitic flies (selfish herd effect).  Some birds also have very effective fly repelling behavior, which reduces the direct threat of parasitic flies, as well as the indirect threat of flies as vectors of other parasites.  Birds have sleeping or resting postures that may offer protection from flying insects.  When the first line of defense - parasite avoidance - fails, birds have a backup arsenal of behavioral resistance mechanisms.  These include preening, scratching, sunning, dusting, and self-anointing behavior, all of which function by reducing parasite abundance.  Some birds also have behavioral tolerance mechanisms, which compensate for parasite damage without reducing parasite abundance.  For example, parasitized nestlings of some species increase begging rates, which triggers an increase in parental feeding rates, which replaces the energy lost to parasites by nestlings. In this talk I will review examples of these different forms of anti-parasite behavior, and illustrate some of them with data from my group’s work on the anti-parasite behavior of Rock Pigeons, Darwin’s Finches, and Galapagos Mockingbirds.

Pathogen and parasite avoidance is a complex and flexible process that is context specific, and affected by various social factors (e.g. own or others’ health/pathogen status, social  behaviours of others ). Social responses and choice involves the acquisition and cognitive processing of information about others (i.e. social recognition) as well as information originating from others (i.e. social learning), accompanied by the exploitation and application of that information in subsequent decision making.  In rodents odours are particularly important determinants of social behaviour, providing information on species, sex, individual and class identity and kinship of the owner, as well as information on an individual’s current reproductive, social, and  health and infection status.   Odour-driven pathogen  avoidance involves the very quick detection and recognition of potential partners and a similarly rapid neural and hormonal mediation of the behavioural  (e.g. avoidance )  and physiological  (e.g. stress) responses to them. These rapid responses involve a variety of neurobiological regulatory mechanisms associated with social recognition and social learning.  These include evolutionarily conserved neurotransmitters, opioid systems, sex steroid hormones ( in particular, estrogens), nonapeptide neurohormones (oxytocin, arginine-vasopressin) and their receptors. As these neuromodulatory systems and responses are dynamic and vary between individuals they allow for flexible environmental and social influences on the expression of the various components of  pathogen detection and avoidance. Supported by NSERC.

Macropodid marsupials (kangaroos and wallabies) harbour an extraordinary range of gastro-intestinal helminth parasites, probably more than any other group of mammalian hosts. Eastern Grey Kangaroos (Macropus giganteus) defaecate as they forage, releasing a peak of nematode eggs in spring and summer. The eggs hatch and develop on the grassy sward, resulting in a peak of infective larvae the following winter. Kangaroos cannot detect these microscopic larvae, instead using the presence of faecal pellets as an imperfect cue to the risk of infection. An open-field test of actual versus random foraging paths suggests that kangaroos avoid risky, contaminated patches simply by biting less and moving more when they encounter faeces, but kangaroos treated with an anthelmintic did not change their foraging behaviour when compared with untreated controls. A chequerboard of cells with and without faecal pellets showed that more wild kangaroos spent more time in the uncontaminated cells; this preference was strongest in juveniles, the most susceptible age class. Because fertilisation with faeces and avoidance of contaminated patches both encourage grass growth, kangaroos then face a potential trade-off between forage intake and parasite risk. An exclosure experiment, which also manipulated faecal load, demonstrated that wild kangaroos rejected this trade-off: they preferred tall, uncontaminated grass over tall, contaminated patches, and over short patches whether contaminated or not. Taken together, these findings indicate that kangaroos are influenced by their susceptibility, but not their infection status, in making conditional foraging decisions based on sward height and faecal contamination, to reduce their risk of infection.

To what extent do primates – our closest phylogenetic relatives and thus the most relevant to understanding the origins of human hygiene practices – exhibit counterstrategies when faced with risk of infection? To address this, we conducted feeding-related infection-avoidance experiments with 5 species of Papionini and Hominini: Macaca fuscata fuscata, Macaca fascicularis, Mandrillus sphinx, Pan troglodytes troglodytes, and Pan paniscus. First, we found that free ranging Japanese macaques vary in their sensitivity to infection risk during foraging under both experimental and natural conditions, and the ‘hygienic tendencies’ of individuals were good predictors of their current levels of geohelminth infection. Then, we expanded our experimental protocol to include visual, olfactory and tactile cues of feces and other contaminants such as blood, semen, rotten meat and rotten fruit with captive chimpanzees, semi-free-ranging mandrills, group-housed long-tailed macaques and semi-free-ranging bonobos. Results indicate that subjects demonstrated risk-sensitivity to these potential contaminants, manifest as increased latencies to consumption of food rewards, maintenance of greater distances from contaminants, and/or outright refusals to consume food in test versus control conditions. Current work is testing whether risk-averse individuals with greater tendencies to avoid potential sources of contamination are less prone to infection and thus characterized by better general health than risk-prone individuals. These studies are aimed at better understanding behavioral immunity to infection among primates, which is fundamental to the understanding of the origins of human hygiene.