We investigated the degree of geographical shifts of transmission areas of vector-borne avian blood parasites (Plasmodium, Haemoproteus and Leucocytozoon) over ecological and evolutionary timescales. Of 259 different parasite lineages obtained from 5886 screened birds sampled in Europe and Africa, only two lineages were confirmed to have current transmission in resident bird species in both geographical areas. We used a phylogenetic approach to show that parasites belonging to the genera Haemoproteus and Leucocytozoon rarely change transmission area and that these parasites are restricted to one resident bird fauna over a long evolutionary time span and are not freely spread between the continents with the help of migratory birds. Lineages of the genus Plasmodium seem more freely spread between the continents. We suggest that such a reduced transmission barrier of Plasmodium parasites is caused by their higher tendency to infect migratory bird species, which might facilitate shifting of transmission area. Although vector-borne parasites of these genera apparently can shift between a tropical and a temperate transmission area and these areas are linked with an immense amount of annual bird migration, our data suggest that novel introductions of these parasites into resident bird faunas are rather rare evolutionary events.

Detecting shifts of transmission areas in avian blood parasites: a phylogenetic approach.

Immunosuppressive effects of testosterone lead to a rediction of male-biased parasitism. To test this prediction, prevalences of blood parasites were compared between male and female birds using statistically correct vote counts of data from 33 studies. We found no overall difference in prevalence between males and females, in either breeding or non-breeding birds. However, infections by Haemoproteus (the most common genus of blood parasite found) were significantly more common among breeding females than breeding males, Restricting the analysis to breeding birds of polygynous species, females again were more likely than males to be infected by blood parasites: this result held for an intra-family comparison that controlled for phylogenetic effects. In comparison, measures of sexual size dimorphism did not relate to sex biases in parasitism as predicted, after controlling for phylogeny using independent comparisons. Because testosterone is often implicated in suppressing the immune system, female biases in parasitism are unexpected. Female biases in parasitism by blood parasites could result from differential exposure of the sexes to vectors, or from oestrogen-based effects on immunity.

/pdf/sex-biased-parasitism-of-avian-hosts-relations-to-blood-6ut57f9hre.pdf

Sex-Biased Parasitism of Avian Hosts: Relations to Blood Parasite Taxon and Mating System

Variation in the prevalence of blood parasites among species of birds has been used to test hypotheses about the effects of sexual selection and parental investment on disease resistance, and how vector abundance influences infection. However, the factors causing this variation are still poorly understood. We assessed the statistical effects of biogeographic, plumage-related and life-history traits on the prevalence of the blood parasites Plasmodium, Haemoproteus, Leucocytozoon and Trypanosoma in European passerine birds. Most of the variation in parasite prevalence occurred at low taxonomic levels. Brighter male plumage and greater host body mass were associated with higher prevalence, explaining 32% of the total variation. Male plumage brightness remained a significant factor when we controlled for phylogenetic effects. These relationships were driven primarily by simuliid-transmitted parasites (Leucocytozoon, Trypanosoma), which were more frequent in species with northern distributions. Host species with greater maximum longevity and shorter nestling periods had higher prevalences of Plasmodium; however, the effect was not stable after controlling for phylogeny using pairwise contrasts. Coevolution between hosts and parasites appears to create temporal and spatial variation that disconnects haematozoan prevalence from evolutionarily conservative life-history traits while creating some positive associations with traits that are phylogenetically labile. Clearly, ecologists should be cautious in relating patterns of variation in haematozoan prevalence to particular host traits.

http://www.umsl.edu/~ricklefsr/Reprints/Scheuerlein2004.pdf

Prevalence of blood parasites in European passeriform birds

An important pattern in host-parasite assemblages is a higher intensity of parasites in juveniles than in adults, but the reasons for these differences remain obscure. Three non-mutually exclusive hypotheses have been proposed: (1) heavily parasitized juveniles die before being recruited into the adult population ('selection' hypothesis); (2) the development of an acquired immunity by the host in front of the parasite reduces the intensity of the parasite in adult hosts ('immunity' hypothesis); and (3) differences in behavior makes adults less exposed to the parasite than juveniles ('vector exposure' hypothesis). Having rejected the 'vector exposure' hypothesis in a previous study, here we tested the 'selection' and 'immunity' hypotheses in feral pigeons (Columba livia) infected by the blood parasite Haemoproteus columbae. In agreement with the 'selection' hypothesis, young (but not adult) pigeons that were highly parasitized had a lower probability of surviving until adulthood, independent of their body condition. However, selection was not strong enough to account for the observed differences in parasite intensity between age-classes, and after selection parasite intensity of survivors still remained 85% higher in juveniles than in adults. In contrast, the 'immunity' hypothesis offered a greater explanatory power. The intensity of blood parasites in young pigeons, but not in adults, decreased over time so dramatically that by the time they had become adults their intensities were indistinguishable from that typically seen in adults. Therefore, while selection against highly parasitized juveniles can contribute to some extent to a reduction in parasitism seen in the adult population, age-specific blood parasitism in feral pigeons is best explained as a transitory phase just before the host develops an effective immune response.

Parasite mediated mortality and host immune response explain age-related differences in blood parasitism in birds

Prevalence and intensity of blood parasites are known to vary in space within a same species, yet the causes underlying such variation are poorly known. Theoretically, blood parasites variation can be attributed to differences to exposure to parasite vectors and/or to differences in host susceptibility. Here, we show that prevalence of Haemoproteus columbae in feral pigeons Columba livia varied among five near-by populations (range 15%-100%), paralleled by variation in the abundance of its main vector, the louse flies Pseudolynchia canariensis. Geographic variation in intensity of blood parasites did not covary with abundance of vectors. Within populations, older individuals had a higher probability of being parasitized than younger ones, whereas younger birds, when infected, suffered higher intensities. Furthermore, we found no evidence of sex-related differences neither in prevalence nor in intensity of blood parasite infections. To demonstrate that geographical variation in prevalence was actually due to differences in vector exposure, we conducted two experiments based on translocation of unparasitized pigeons from a vector-free area to an area where both the parasite and vector were abundant. With the first experiment, we demonstrated that unparasitized pigeons were not resistant to the parasite because when transmission was possible pigeons became parasitized in a few months. With the second experiment, in which half of the pigeons were prevented from contacts with the vector, we ruled out the posibility that pigeons we considered as unparasitized would have suffered from latent infections. Therefore, both observational and experimental evidence supports the view that vector abundance is the major factor influencing the spatial variation in prevalence of H. columbae in pigeons.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1600-0587.2000.tb00286.x

Geographical variation in blood parasites in feral pigeons: the role of vectors.

Three Fenno-Scandian populations of Pied Flycatchers Ficedula hypoleuca were sampled for their haematozoan parasites over the period 1989-1992. Pied Flycatchers from the environs of Uppsala, Sweden had lower prevalence and intensities of infections than those from Meltaus, Finland. In addition, birds from Uppsala were not parasitized by leucocytozoids and were virtually free from trypanosomes; conversely, birds from Meltaus were heavily parasitized by both groups. Birds from Konnevesi, Finland, showed an intermediate prevalence and intensity of parasitism. This study clearly shows the role of ecologically diverse conditions in determining the composition, transmission and prevalence of a blood parasite fauna presumably through their effect on vector composition and population density.

A comparison of the blood parasites of three Fenno-Scandian populations of the Pied Flycatcher Ficedula hypoleuca

Leucocytozoon spp. and Trypanosoma spp. blood parasites in the redstart (Phoenicurus phoenicurus) were studied during spring migration 1994 in southern Finland (53 individuals) and the breeding season 1992–1994 in northern Finland (69). Parasite prevalence was higher during the breeding season (48%) than during the migration period (13%), with no age or sex differences in the breeding site birds. In both periods, redstarts were infected by the same blood parasites Leucocytozoon shaartusicum (46% prevalence at the breeding site and 71% during the migration period) and Trypanosoma avium complex (58% and 43%, respectively). One individual at the breeding site had contracted L. dubreuili and one at the stop-over site had T. everetti. Our results may support the assumption that tissue-hidden parasites relapse during the breeding season when birds may have diminished immune response related to egg production and brood rearing. Another explanation could be that the high abundance of ornithophilic vectors enhance...

https://bioone.org/journals/Journal-of-Wildlife-Diseases/volume-35/issue-3/0090-3558-35.3.603/Leucocytozoonosis-and-Trypanosomiasis-in-Redstarts-in-Finland/10.7589/0090-3558-35.3.603.pdf

Deborah Squires-Parsons

Papers

A comparison of the blood parasites of three Fenno-Scandian populations of the Pied Flycatcher Ficedula hypoleuca

Leucocytozoonosis and trypanosomiasis in redstarts in Finland.