Temporal patterns and multiple infections of avian haemosporidian parasites in great reed warblers (Acrocephalus arundinaceus)

Abstract

The primary (acute) haemosporidian infection has the greatest negative effect on an individual’s fitness. Weak individuals are most likely eliminated from the population, while those that survive, develop a chronic infection when parasite infection intensity is kept at low levels with minor effects on the condition of the host’s life history traits. It is still unknown whether low levels of haemosporidian parasites persist in blood and internal organs throughout an individual’s life, or if some of these chronic infections ultimately are cleared out by the host’s immune system and replaced with new infections. Moreover, the usual PCR and Sanger sequencing approach, based on the parasite mitochondrial DNA and frequently used to detect haemosporidians in birds, are often unable to detect more than one parasite lineage in a sample, showing only the most prevalent one. Here I have used multiplex PCR to detect haemosporidian parasites on a genus level and a modified version of the nested PCR and next generation sequencing (NGS) to sequence and detect haemosporidian parasites on a species/lineage level. I have used DNA samples extracted from blood of great reed warblers during the chronic stage of infection. NGS data have increased the efficiency of haemosporidian parasite detection in co-infections as well as have revealed a greater number of co-infections of the same parasite lineages present in the analyzed samples. I have investigated parasite infections within individuals within their breeding season in Sweden to show that parasite prevalence reduced in the late season, however the probability of having an infection declined only in males. There were no significant age differences in parasite prevalence, however there was a numerically higher proportion of individuals gaining infections in the second breeding year compared to old individuals and it was the opposite trend for losing the infections. These findings might suggest that young individuals have higher chance of contracting the infections due to their naïve immune system, while with age it becomes more efficient in recognition and clearing out due to accumulated immunological memory from previously encountered parasites. Additionally, only advantageous disease resistant or tolerant genotypes could have been positively selected and remained in the breeding population or those that by chance avoided contracting the infection.

Popular Abstract

Avian malaria in great reed warblers – not as deadly as in humans?

Great reed warblers are long distance migratory birds that winters in tropical Africa and breeds in Europe during spring and summer. In Africa, these birds are highly exposed to blood sucking insects that may transmit avian malaria-like parasites that infectred blood cells and internal organs. Since the late 1800s, bird malaria has been studied as a model system that can be applied to better understand malaria infections in humans and improve medical treatments. The first exposure to malaria parasites has the largest negative impact on an individual’s health and may result in reduced mobility leading to starvation, exposure to predators and possibly death. In addition, it is possible that more than one species of malaria-like parasites is present in the blood of a bird individual, called multiple infection, which could be expected to have more severe negative effects on the bird. In general, it can be predicted that weak individuals are more likely to be eliminated from the population. The individuals that survive the initial infection develop chronic infections when parasite prevalence is kept at low levels with minor effects on the host’s health and is often undetectable by frequently used PCR analyses for detecting malaria parasites in bird blood. However, in cases of multiple infections, the malaria parasite/-s of lower prevalence are often “hidden” by the main one due to reasons arising from the PCR-based technique to detect avian malaria from blood samples. Therefore, the actual prevalence, diversity and effects of hosts of avian malaria parasites may be underestimated.

In my study, I have tested a modified PCR protocol in combination with DNA sequencing that are more sensitive in detecting all parasites present in the DNA samples extracted from the blood of great reed warbler during the chronic stage of infection. There are many different sequencing technologies available and the advantage of the one I used is that it generates thousands of reads which increases the chance of getting DNA sequences of all parasites from the sample, even those that are present at very low intensities. Preliminary studies on great reed warblers have indicated that parasite prevalence and the intensity of infection become reduced at the end of the breeding season (in late summer), possibly due to lack of suitable vectors in Sweden, supporting the conclusion that all malaria parasites in this host species are contracted during migration to and from or when wintering in Africa. To investigate this with the more sensitive new methods to detect malaria parasites, I have investigated the probability of having a parasite infection or not over the breeding season in Sweden (May-August). I found that there was a higher occurrence of malaria earlier in spring and a decline over the season, but in males only. In contrast, females had lower probability of infection in the beginning and a mild increase over the breeding season. This could indicate sex differences in the efficiency of the immune system in dealing with infections. Maybe the immune system of males is more suppressed in the beginning of the season, because of higher stress levels resulting from rapid migration, early spring arrival, territory occupation and mate attraction behaviors. In contrast, the immune system of females may possibly be more weakened during the egg laying and incubation periods. I have also analyzed if the age of a bird may affect infection probability. Contrary to my prediction, older birds did not have a higher chance of being infected with malaria. Instead, there was an opposite trend where the youngest individuals (in their first breeding year) had the highest risk of becoming infected when they came breed as two-year-old. In contrast, a larger proportion of four-year and older individuals have exterminated the infection in the following breeding year. These findings suggest that young individuals have a higher chance of contracting the infections due to their naïve immune system, while with age it becomes more efficient in recognizing and clearing out the infection, possibly due to accumulated immunological memory from previously encountered parasites. Yet, it remains unknown whether low levels of parasites persist in blood and internal organs throughout an individual’s life and are below the detection threshold, or if some malaria parasite strains can actually be completely cleared out by the host’s immune system, as well as to which extent these are replaced with new infections. More studies are required to track the infection patterns of each parasite species separately.

Master’s degree project, 45 credits
Department of Biology, Lund university
Main supervisor: Dennis Hasselquist
Co-supervisor: Staffan Bensch

• author: Krol, Evelina
• supervisor: Staffan Bensch
• organization: Degree Projects in Biology
• course: BION02 20221
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9102922
• date added to LUP: 2022-11-07 16:04:52
• date last changed: 2022-11-07 16:04:52