Parasites and Pathogens: Avoidance


Parasites and pathogens are usually harmful, and thus hosts have developed numerous pre‐contact measures to avoid infection in the first place. These avoidance measures range from camouflage to a variety of behavioural measures such as movement and fly‐repelling behaviour, habitat choice and migration, selective foraging, group forming and mate choice. In case avoidance fails, a second line of post‐contact defences (e.g. immune system) comes into place which is treated elsewhere and thus only briefly discussed.

Keywords: parasitism; avoidance

Figure 1.

Examples of pre‐contact avoidance measures as a first line of defence against different types of parasites and pathogens in birds. These are mainly behavioural measures to avoid contact with parasites and pathogens in the first place. Once a host has come into contact with parasites and pathogens, there is a second line of defence in the form of post‐contact defensive measures which can be either behavioural, physiological or immunological. Here we focus on pre‐contact avoidance measures. Figure based on Hart . Copyright of David Thieltges.

Figure 2.

Mechanisms of parasite and pathogen avoidance or defence are costly and thus reduce the fitness of resistant hosts compared to susceptible hosts in the absence of parasites. However, in the presence of parasites, the fitness of resistant hosts is higher than that of susceptible hosts with the latter suffering more from the costs of an infection. Pre‐contact avoidance and post‐contact defensive measures can only evolve if the cost of infection exceeds the cost of avoidance and defensive measures. Copyright of David Thieltges.

Figure 3.

Selective prey selection can help to avoid acquiring parasites and pathogens. Oystercatchers take smaller cockle prey individuals with suboptimal energy intake. Smaller cockles harbour fewer infective stages of trematode parasites (which utilize the birds as final hosts) compared to larger cockles. By preferring smaller cockles the birds thus reduce their parasite intake and avoid acquiring high loads of parasites. Total avoidance of parasites is not possible in this case because all cockle sizes are infected to some extent. Schematic graph based on data taken from Norris . Copyright of David Thieltges.

Figure 4.

Forming groups can help to avoid parasites and pathogens. In birds and mammals, individual hosts suffer from reduced fly bites with increasing group size (dilution) as long as the larger group does not attract more flies due to a higher visibility (encounter). Positioning themselves at the centre of a large group can also help individual hosts to avoid fly bites and potential subsequent infections by the selfish–herd effect. Figure based on Hart , by permission of Oxford University Press (

Figure 5.

Example of post‐contact defensive measures against parasites and pathogens. Invertebrate immune systems can distinguish between self and nonself and eliminate parasites or pathogens for example by encapsulation and melanization. The picture shows an amphipod host in which invading trematode parasites (arrows) have been killed by encapsulation. Copyright of Robert Poulin.



Billing J and Sherman PW (1998) Antimicrobial functions of spices: why some like it hot. The Quarterly Review of Biology 73: 3–49.

Brown CR and Brown MB (2001) Avian coloniality: progress and problems. Current Ornithology 16: 1–82.

Clark L and Mason JR (1985) Use of nest material as insecticidal and anti‐pathogenic agents by the European straling. Oecologia 67: 169–176.

Clayton DH (1990) Mate choice in experimentally parasitized rock doves: lousy males loose. American Zoologist 30: 251–262.

Curtis V, Aunger R and Rable T (2004) Evidence that disgust evolved to protect from risk of disease. Biology Letters 271: S131–S133.

Duncan P and Vine N (1979) The effect of group size in horses on the rate of attacks by blood‐sucking flies. Animal Behaviour 27: 623–625.

Fauchald P, Rødven R, Bårdsen B‐J et al. (2007) Escaping parasitism in the selfish herd: age, size and density‐dependent warble fly infestation in reindeer. Oikos 116: 491–499.

Folstad I, Nilssen AC, Halvorsen O and Andersen J (1991) Parasite avoidance: the cause of post‐calving migrations in Rangifer? Canadian Journal of Zoology 69: 2423–2429.

Hamilton WD and Zuk M (1982) Heritable true fitness and bright birds: a role for parasites? Science 218: 384–386.

Hart BL (1997) Behavioural defence. In: Clayton DH and Moore J (eds) Host‐parasite Evolution: General Principles and Avian Models, pp 59–77. Oxford: Oxford University Press.

Helle T and Aspi J (1983) Does herd formation reduce insect harassment among reindeer? Acta Zoologica Fennica 175: 129–131.

Hulscher JB (1982) The oystercatcher as a predator of the bivalve, Macoma balthica, in the Dutch Wadden Sea. Ardea 70: 89–152.

Karvonen A, Seppälä O and Valtonen ET (2004) Parasite resistance and avoidance behaviour in preventing eye fluke infections in fish. Parasitology 129: 159–164.

Lowenberger CA and Rau ME (1994) Selective oviposition by Aedes aegypti (Diptera: Culicidae) in response to a larval parasite, Plagiorchis elegans (Trematod: Plagiorchiidae). Environmental Entomology 23: 1269–1276.

Milinski M (2006) The major histocompatibility complex, sexual selection, and mate choice. Annual Review of Ecology Evolution and Systematics 37: 159–186.

Mooring MS and Hart BL (1992) Animal grouping for protection from parasites; selfish herd and encounter‐dilution effects. Behaviour 123: 173–193.

Norris K (1992) A trade‐off between energy intake and exposure to parasites in oystercatchers feeding on a bivalve molluscs. Proceedings of the Royal Society London Series B 266: 1703–1709.

Ralley WE, Galloway TD and Crow GH (1993) Individual and group behaviour of pastured cattle in response to attack by biting flies. Canadian Journal of Zoology 71: 725–734.

Rätti O, Ojanen U and Helle P (2006) Increasing group size dilutes black fly attack rate in Black Grouse. Ornis Fennica 83: 86–90.

Slusarenko AJ, Fraser RSS and Loon LC (2000) Mechanisms of Resistance to Plant Diseases. Dordrecht, The Netherlands: Kluwer Academic Publishers.

Spurrier MF, Boyce MS and Manly BFJ (1991) Effects of parasites on mate choice by captive sage grouse. In: Loye JE and Zuk M (eds) Bird‐parasite Interactions: Ecology, Evolution and Behavior. Oxford: Oxford University Press.

Waage JK (1981) How the zebra got its stripes: biting flies as selective agents in the evolution of zebra coloration. Journal of the Entomological Society of South Africa 41: 351–358.

Wobeser GA (2006) Essentials of Disease in Wild Animals. Ames, IA: Blackwell Publishing Professional.

Further Reading

Clayton DH (1991) The influence of parasites on host sexual selection. Parasitology Today 7: 329–334.

Combes (2001) Parasitism: The Ecology and Evolution of Intimate Interactions. Chicago: University of Chicago Press.

Hamilton WJ and Poulin R (1997) The Hamilton and Zuk hypothesis revisited: a meta‐analytical approach. Behaviour 134: 299–320.

Hart BL (1990) Behavioral adaptations to pathogens and parasites: five strategies. Neuroscience and Behavioral Reviews 14: 273–294.

Hart BL (1992) Behavioral adaptations to parasites: an ethological approach. Journal of Parasitology 78: 256–265.

Hart BL (1994) Behavioural defences against parasites: interaction with parasite invasiveness. Parasitology 109: S139–S151.

Moore J (2002) Parasites and the Behaviour of Animals. Oxford: Oxford University Press.

Sheldon BC and Verhulst S (1996) Ecological immunology: costly parasite defences and trade‐offs in evolutionary ecology. Trends in Ecology and Evolution 11: 317–321.

Wakelin D (1996) Immunity to Parasites: How Parasitic Infections are Controlled. Cambridge: Cambridge University Press.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Thieltges, David W, and Poulin, Robert(Dec 2008) Parasites and Pathogens: Avoidance. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0003661]