Accumulation, decumulation and stability trends of avian malaria parasites and their effects on individual blue tits (Cyanistes caeruleus)

Abstract

Global rising temperatures is impacting species interactions worldwide including those between host and parasites. A long-term study on a population of blue tits (Cyanistes caeruleus) linked increases in avian malaria parasites to rising temperatures. Understanding the consequences that these increases in avian malaria parasites are causing is vital. Therefore, data from the past 28 years on this population of blue tits was used to understand the effects that these climate driven increases in malaria parasite prevalences have on individual hosts. The purpose of this study was to look at accumulation, decumulation and stability trends of avian malaria parasites for infected and co-infected individuals, some of which included recaptured individuals. Furthermore, this study also aimed to analyze the effects of Haemoproteus, Plasmodium and Leucocytozoon infections on the breeding performance and morphometric measurement of individual blue tits. These aims were achieved through utilizing blood samples, breeding records and morphometric measurements taken from the population between 1996 to 2024. DNA extractions from the blood samples were analyzed using a PCR analysis to identify the parasite genera present. The results showed that some of these infections are chronic and can last multiple years. Furthermore, facilitation of Leucocytozoon was found in individuals that were previously infected with Haemoproteus. Some negative fitness effects were observed in infected individuals, including reduction in brood size and reduced nestling survival for females with either Plasmodium or Leucocytozoon infections. Furthermore, uninfected females had a larger mass than those with a single infection, while nestling survival (clutch-brood) was higher in females with a single infection compared to females with triple infections. Overall, although this population is showing an increase in parasite prevalences, the negative effects on fitness are not leading to an overall population decrease. As this study only looked at parasite presence, the levels of parasitemia along with the effects that infected parents have on nestlings would provide valuable insight as to the long-term consequences of these parasites and how this may change with further increases in temperature.

Popular Abstract

Climate-driven increases in temperature are affecting species interactions globally, including host-parasite dynamics. Avian malaria parasites, known as Haemosporidian, have experienced increases in prevalence that can be linked to global warming. There are three common genera of avian Haemosporidian which include Haemoproteus, Plasmodium, and Leucocytozoon. Once an infection occurs within a host, there are two phases of infection. The first is referred to as the acute phase, when parasitemia is high which is then followed by a decrease in parasitemia, then classified as the chronic phase. During these two phases, there may be fitness effects for the hosts but the literature if quite contradictory as to what these effects are. Furthermore individuals may harbour multiple infections and how these infections effect the host and interact within a host is unknown.

A long-term study on a population of blue tits (Cyanistes caeruleus) linked increases in avian malaria parasites to rising temperatures. Understanding the consequences that these increases in avian malaria parasites are causing is vital. Therefore, this project utilizes infection data from 1996 to 2024 from a wild population of blue tits, to understand the effects that Haemoproteus, Plasmodium and Leucocytozoon infections have on individuals and determine whether within-host interactions between parasite genera is occurring.

Methods
The blue tit population studied was monitored from 1997 to 2024, utilizing 450 nest boxes distributed across 64 km² in the Revingehed area of southern Sweden. When birds are captured, their sex and age is determined. Furthermore, breeding and morphometric measurements from the individuals were taken, including: the day of first egg, clutch size, brood size, nestling survival, mass, wing length and tarsus length. Along with these measurements, a small blood sample was taken from the individual. In the lab, DNA was extracted from the blood sample and run through a PCR analysis to determine which parasite genera were present.

Using annual parasite prevalence data from the natural population, simulated populations were created to compare trends in parasite accumulation, decumulation and stability within individuals. Furthermore, a second simulated population was created to determine if individuals infected with Haemoproteus gained a secondary infection at a higher rate in the natural population compared to simulated population. When comparing the distribution of the simulated population to the natural population, if the sum from the natural population fell outside of the 95% confidence interval of simulated population, then there was a statistically significant difference in what would be expected given a random accumulation of parasites.

To determine the effects of infection on breeding and morphological measurements of individuals, several analyses were run. Some of the data analysis included looking at differences in measurements between uninfected and infected individuals as well as between individuals with differing numbers of infections.

Results
Simulations
The results showed that individuals are keeping Haemoproteus and Plasmodium infections over multiple years. In terms of Leucocytozoon, it was found that individuals were less likely to acquire an infection, and more individuals never had an infection in the natural population compared to the simulated population. When looking at the accumulation of secondary parasites, individuals infected with Haemoproteus were more likely to gain a Leucocytozoon infection the following year.

Breeding Data Analysis
There was some negative fitness consequences associated with infections that were found in females. Females infected with Plasmodium had significantly lower nestling survival compared to uninfected females. Leucocytozoon infected females also had a lower nestling survival, and a smaller brood size compared to uninfected females.

When looking into co-infections, there were differences in breeding parameters and morphometric data for females with no infection, a single infection, double infections or triple infections. A significantly larger mass was found in females with no infection while nestling survival was significantly higher in females with a single infection.

Conclusions/future studies
Our findings suggest that Haemoproteus and Plasmodium infections are chronic, while Leucocytozoon resistance may be occurring. Furthermore, Haemoproteus infections may facilitate Leucocytozoon infections within hosts. Although this blue tit population has not shown an overall population decrease, some negative fitness effects from infection were found. To fully understand how the increases in prevalences will impact the overall population, future studies looking into the effects that infected parents have on their offspring would further our knowledge of any potential long-term consequences.

Master’s Degree Project in Conservation Biology, 60 credits, 2025
Department of Biology, Lund University
Advisor: Olof Hellgren
Associate Prof/Sr. University Lecturer in Biology
Evolutionary Ecology and Infection Biology

• author: Wright, Lillian
• supervisor: Olof Hellgren
• organization: Degree Projects in Biology
• course: BION03 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9212894
• date added to LUP: 2025-09-19 15:37:30
• date last changed: 2025-09-19 15:37:30