LIVING WITH PARASITES: AVIAN MALARIA, TELOMERE LENGTH AND LIFE HISTORY TRADE-OFFS

Abstract:
Haemosporidia is a well-studied group of parasites, which infect mammals, reptiles and birds and use blood sucking vectors for their transmission. By conducting natural population studies and experimental infections, We have been able to detect and quantify Haemosporidia from avian blood to investigate how these pathogens affect their avian hosts. We have used life history data of two long-term longitudinal studies of wild bird populations: great reed warbler, Acrocephalus arundinaceus (25 year data) and house martin, Delichon urbica (10 year data) to investigate the relationships between parasite prevalence and parasitemia and host life history traits. By conducting experimental infections in great reed warblers, we demonstrated that the acute phase parasitemia of the avian malaria Plasmodium ashfordi (GRW2) is significantly positively correlated to the subsequent chronic infection level.

We outlined a framework of how candidate Major Histocompatibility Complex (MHC) alleles are associated with infectious diseases in natural populations of animal. We proposed that three types of MHC alleles (qualitative disease resistance, quantitative disease resistance and susceptibility alleles) could be separated by combining data of prevalence and intensity (parasitemia). We tested our hypothesis in a long-term longitudinal data set of great reed warblers, and found a MHC class-I allele (B4b) that was both positively associated with prevalence of severe malaria infection (GRW2) and negatively correlated with GRW2 parasitemia, a pattern typical for a quantitative resistance allele.

Using longitudinal data of great reed warblers, we found that telomere length in great reed warblers is maternally inherited and offspring telomere length is associated with their mother`s age. By using our full pedigree (25 year) great reed warbler data, we found that both early life (at day 9) telomere length and malaria infections (at 1 year age) significantly predicted the individual`s lifespan. Moreover, we found that birds infected with malaria parasites lost significantly more of their telomeres during the first year of life. These results are supported by data from our experimental infection study, where birds infected with the GRW2 parasite showed a higher telomere loss than control birds. Furthermore, we found that parasitemia was positively correlated with telomere loss rate in both natural population (GRW2 and GRW4 parasitemia) and experimental infection (GRW2 parasitemia). Taken together, because malaria parasites increase the telomere attrition rate of the host, it provides a mechanistic explanation for the underlying physiological processes that link the negative effect of a parasite infection to cell ageing and host survival.

Popular Abstract in English

Parasites and their host organisms have engaged in a desperate battle over millions of years to evolve strategies that enable continued survival of their respective species. Parasites require some key metabolites to be able to survive and reproduce inside the host that often results in damage to host, while host organisms attempt to limit or prevent this damage by several ways including behavioral changes and building costly immune defences. Nevertheless, such interactions provide an excellent opportunity not only to study parasite ecology but a wide range of questions including host immunology, genetics, sexual selection, population ecology, behavioral ecology, and evolutionary biology.

Haemospridian blood parasites are well-studied group of parasites as they include the agent of malaria, which still remain the most common human disease in warm climate countries. These parasites infect mammals, reptiles and birds and use blood sucking vectors for transmission. The main reason for the extensive studies of malaria parasites is due the severe impact they have on human populations, with over 500 million clinical malaria cases and up to 2.7 million deaths per year. Avian malaria has been recorded worldwide and in more than 4000 bird species and contains more than 200 described species. The number of morphologically cryptic genetic species might be ten-fold higher. In some population of birds the prevalence can reach very high levels (>90% of the individuals infected). Furthermore, many studies have shown that mixed haemosporidian infections are very common in many bird-parasite systems. There are well documented negative fitness effects of haemosporidin parasite infections in experimental studies. However, in wild population studies there are mixed results. To get a better understanding about the consequences of hameosporidian (plasmodium and Haemoproteus) parasites, we conducted natural population studies as well as infection experiments with caged birds.

We used long term life history data of wild great reed warbler populations (25 year data) to investigate the correlation between parasitemia (parasite intensity) and host life history traits. The great reed warbler is a socially polygynous, long distance migrant passerine bird, which breeds in Europe and Western Asia and winters in tropical Africa. At Lake Kvismaren (south central, Sweden) about 35-65 breeding great reed warblers have been studied annually since 1983. Since 1983, daily visits have been performed throughout the breeding season (May-July) to collect life history data, e.g. arrival day, wing length, tarsus length, body mass, breeding partners, egg laying date, hatching success, fledgling success, recruitment success and overall individual fitness) from this population.

We measured the infection intensity of the three most common parasites Haemoproteus payevskyi (GRW1), Plasmodium ashfordi (GRW2) and Plasmodium relictum (GRW4) in great reed warblers by using qPCR (real time quantitative PCR) and lineage specific primers. This protocol enabled us to quantify the infection intensities of each of these parasites, also in those cases when the co-infected the same individual bird. These parasites have transmission only in Africa and the infected birds must then have carried the chronic infections to the breeding place at Kvismaren. We found that chronic haemosporidian parasite infections have slight negative fitness effects on different life history components (spring arrival, reproductive success and offspring recruitment success) in the wild population of great reed warblers. Furthermore, our study revealed a remarkable pattern that the infected birds kept stable individual-specific chronic infection levels across years. This pattern is still needed to be understood, but might indicate individual-specific tolerance levels again parasite.

We investigated whether multiple malaria infection are more detrimental than single malaria infection in a long-term study of wild population of house martin (Delichon urbica) at Badajoz, southwest, Spain. The house martin is an insectivorous, long distance migratory passerine bird, which breeds in almost all regions in the Western Palearctic. House martins winters in Africa where they become infected with a number of haemosporidian parasite lineages and return to breeding places carrying chronic infections. We found that birds infected with haemosporidia had lower body condition. We also found that birds with double-lineage haemosporidian infection had the lowest growth rate of their tail feathers as compared with uninfected and single infected individuals, but double infection had no effect on feather quality.

We experimentally inoculated juvenile great reed warblers at the Kalimok station (Bulgaria) to study the relationship between acute and chronic malaria infection levels. Our experimental infection study of great reed warblers, revealed a significant positive correlation between acute (primary) peak GRW2 (Plasmodium ashfordi) parasitemia level and subsequent chronic infection level. This finding suggests that the chronic phase parasitemia level can be used to qualitatively infer the parasitemia of the preceding and more sever primary phase.

To study how host build defences against a pathogen, we screened the Major Histocompatibality Complex (MHC) Class-I alleles in our wild population of great reed warblers. We were able to identify an MHC immune-allele B4b, which suppress the severe malaria GRW2 (P. ashfordi) infection. On one hand, this immune-allele is more frequent in GRW2 infected than in non-infected birds, suggesting that it as a susceptibility allele. On the other hand, individuals carrying this allele showed significantly lower infection intensities (quantitative resistance). A likely explanation for this pattern is that birds carrying the B4b allele can mount an immune response that suppresses the acute phase GRW2 infection and are more likely to survive the acute phase of the infection.

Elucidating the physiological mechanisms underlying how parasites affect their hosts is of central importance to evolutionary ecology and may also have very important implications for biomedical research. However, very little is known about the physiological mechanisms that may explain the negative effects of parasite infection. Telomere loss is one particularly important mechanism at the cellular level, which might be involved in host ageing and might underpin life history trade-offs. Some studies in humans and mice suggest that infectious diseases may accelerate telomere attrition, which in turn may increase the risk of mortality.

We first studied the inheritance (heritability) of telomere length in great reed warblers by using a large multigenerational data set from the Lake Kvismaren population in southern Central Sweden. We measured the telomere length using qPCR at early life (9 days old) both in parents and their offspring. We found telomere length is maternally inherited and offspring telomere length is associated with their mother`s age in great reed warblers. This a very interesting and contrasting finding compared to humans, where telomere length is paternally inherited and offspring telomere length is associated with their father`s age. Then, we investigated the relationship between individual longevity and early life telomere length (at 9 days) and malaria infections at later life (1-year age). We found that in great reed warblers, telomere length in early life (at 9 days) and malaria infections at later life (1-year age) significantly predict individual`s lifespan.

We investigated whether malaria infections accelerated the telomere loss, which could provide a mechanistic explanation for the underlying physiological processes that link the negative effects of a parasite infection. We measured the telomere length in blood cells of great reed warblers early in life, i.e., at 9 days after hatching (nestling telomere length). Birds that survived their first winter in Africa and returned to breed at our study site in Sweden were measured again for telomere length when they were 1 year old (1 year telomere length). Furthermore, we used samples from our previously published malaria infection experiment of great reed warblers to investigate if malaria parasites accelerate telomere loss and whether the loss rate is associated with infection intensity. We found that malaria infections accelerate the telomere loss in wild population of great reed warblers and it was also confirmed by our experimental infection study carried out in Bulgaria with captive great reed warblers. Furthermore, we found a positively correlation between telomere loss and malaria infection intensity. Taken together, because malaria parasites increase the telomere attrition rate, it then provides a mechanistic explanation for the underlying physiological processes that link the negative effect of a parasite infection to cell ageing and host survival.