Avian malaria spilling over from wild to zoo-kept birds

About avian malaria

Avian malaria is caused by various parasites of the genus Plasmodium which are transmitted by different mosquito species and are distributed globally except in Antarctica. Due to global warming and consequential increase in vector* distribution, further spread of these parasites is inevitable. 

The impact of avian malaria varies among bird species. Infected native birds may be asymptomatic but, in some cases, the disease may affect host* fitness. When the disease is newly introduced in a bird population, non-adapted birds may succumb to infection. This scenario has been seen in the 20^th century in Hawaii, where avian malaria contributed to the decline of native bird species. 

Avian malaria is one of the major causes of death in zoo-kept penguin populations. In Switzerland, fatal cases of avian malaria have been reported in zoo-kept and wild birds. The same parasite species found in Switzerland are also reported in neighbouring countries, hence there is a need for constant monitoring of infections.

Our research on avian malaria

At the Institute of Parasitology, UZH (University of Zurich), we focus on obtaining data about:

1. occurrence

2. transmission 

3. and pathological effect of avian Plasmodium parasites on zoo-kept penguins and puffins, and wild passerines including sparrows, blackbirds, and great tits. We work in collaboration with Zoo Zurich and Tierpark Bern to establish how avian malaria parasites are transmitted between zoo-kept and chronically infected wild bird species that may be a source of disease transmission. 

1. Vector analysis: Searching for the number one suspect, mosquitoes.

To assess in which vector parasites occur, we identify mosquitoes transmitting the parasites and perform xenodiagnosis* to identify hosts of blood-fed mosquitoes. To this end, we set mosquito traps at both zoos during spring and summer season. First, we identify and dissect collected mosquitoes. Then, we identify the parasite species and lineages* with the help of PCR*. 

Result: Our study has shown that Culex pipiens is the main vector of avian Plasmodium parasites in accordance with a previous study in Switzerland (Glaizot O et al. 2012. PLoS One). Also, we have identified up to four different avian Plasmodium lineages in mosquitoes, with Plasmodium relictum SGS1 being the most common one.

2. Natural reservoir hosts: The transmission chain across birds

To detect and identify avian Plasmodium parasites in potential zoo-kept bird hosts, we performed PCR on blood and tissue samples. In our research, we have detected the same parasite species in penguins and puffins as in mosquitoes, suggesting a plausible transmission path. The most common parasites in penguins and puffins were P. matutinum LINN1 and P. relictum SGS1. 

Result: Our analysis in wild birds (same procedure as for zoo-kept birds) has identified sparrows and blackbirds as potential wild reservoir hosts of avian Plasmodium species. We found in a unique case, P. relictum GRW11 in a wild-adopted pet pigeon (Muchaamba G et al. 2024. Malar J). This information is important as these wild reservoir hosts bridge the gap of transmission between wild migratory birds and zoo-kept birds. 

3. Pathological effects: Complicated diagnosis

Furthermore, we examined tissues from the infected birds to determine the pathological effect of the parasites in the different hosts. Like human malaria infection, avian Plasmodium parasites may cause destruction of red blood cells and anaemia. However, in avian malaria infections, the parasite undergoes a second stage of development within various tissues of the bird hosts, which may also result in disease. These tissue stages are responsible for maintaining the infection in chronic/latent phase and may lead to the reoccurrence of infection. This scenario is poorly understood and studied but information gained from dead birds will help us unveil the host-parasite interaction. 

Result: Our analysis has shown that in penguins, infection is detected mostly in the tissues and scarcely in the blood, which complicates diagnosis through blood samples. Development of serological* assays to determine the level of antibodies against parasites may facilitate diagnosis of avian malaria. 

How can we combat avian malaria?

Control of avian malaria should be centered around vector control (mosquitoes) and monitoring/reducing reservoir hosts (wild birds). The latter is not practical in a zoo set-up as pest birds and other wild birds are abundant. To reduce transmission, prophylactic* measures are advised and implemented during the peak vector season including draining stagnant waters, treatment of mosquito larvae with an insecticide and indoor housing of penguins and puffins to reduce mosquito bites. 

A DNA-based vaccine has been trialled in penguins before, and it was shown to reduce the number of parasites in the blood of infected birds. However, the vaccine produced only short-term immunity in the subsequent years and is not currently used. 

In addition to these preventive steps, together with the two zoos, we help to monitor infections by screening birds to ensure timely treatment with malarone (atovaquone and proguanil). This combination is used in human and avian malaria and is known to effectively clear parasites in the blood. However, in instances where the parasites are in the tissues, better treatment options like Tafenoquine which target the parasites in the tissues need to be tested. For more information, please visit here. 

Involved teams: 

Muchaamba Gillian, Marti Matthias; Institute of Parasitology, Vetsuisse Faculty, UZH; https://www.paras.uzh.ch/de/research/Malaria-Transmission-Lab.html

Hatt Jean-Michel and Kummrow Maya;  Clinic for Zoo Animals, Exotic Pets and Wildlife, Vetsuisse Faculty, UZH.

Hetzel Udo, Kipar Anja, Dervas Eva; Institute of Veterinary Pathology, Vetsuisse Faculty, UZH.

Davis Leyla; Zoo Zurich; https://www.zoo.ch/en/conservation-animals/research-zoo/mosquitoes-vectors-avian-malaria

Huwiler Meret; Tierpark Bern 

Albini Sarah; Departments of Veterinary Bacteriology and Poultry and Rabbit Diseases, Vetsuisse Faculty, UZH

Keller Saskia and Imlau Michelle; Institute of Fish and Wildlife health (FIWI), Vetsuisse Faculty, Bern

*Glossary: 

Host: An organism that serves as a temporary or permanent habitat for another organism or provides it with food, protection or the opportunity to reproduce. Humans, for example, are a host for mosquitoes, which feed on their blood, or for malaria parasites, which reproduce in them.

Lineage: Plasmodium genetic lineage based on cytochrome b gene sequencing. 

PCR: Abbreviation for polymerase chain reaction. This method is used to amplify and detect a precisely defined short DNA segment in a sample.

Prophylactic: Intended to prevent disease.

Reservoir host: an organism that harbors a parasite, serves as a source of infection, and transmits it to a potential host. Reservoir hosts are often asymptomatic.

Serological: Serological tests are used to identify antibodies and antigens in a patient's sample, usually in the blood serum.

Vector: Carrier of pathogens. In the case of malaria, mosquitoes carrying Plasmodium parasites that cause disease.

Xenodiagnosis: Xenodiagnosis is a method for detecting whether a host is infected with a pathogen. Tissue of the animal suspected of being infected is exposed to a vector. The vector is then analysed to determine whether it has ingested pathogens. In the present case, potential reservoir hosts (birds) are screened by analyzing host blood ingested by mosquitoes for the presence of pathogens (Plasmodium).