A study carried out in the Camargue region in the south of France suggests that lower bird diversity and evenness are linked to higher detection rates of the Usutu virus in Culex pipiens mosquitoes. This has important implications as factors such as climate change and biodiversity loss are increasing the range & spread of mosquito-borne viruses. Insights such as these could help feed into One Health strategies for surveillance and control. 

Birds of a feather - Usutu and West Nile Virus spreading further north.

Both Usutu and West Nile Virus are flaviviruses (family Flaviridae) with a sylvatic (wildlife) lifecycle involving birds as the definitive hosts, mosquitoes as the vectors, with other mammals, such as humans, horses etc as dead-end hosts, meaning they are not likely to be involved in transmission, but may become ill due to infections.  

The Usutu virus, originally detected in Africa, has been spreading across Europe for decades. In the UK, scientists have recently observed its presence in blackbirds, raising concerns of its establishment further north and its impact on wild bird populations. The virus, transmitted by Culex mosquitoes, has been linked to declines in bird populations, particularly blackbirds. 

Adding to these concerns, West Nile Virus has now been detected in the UK for the first time, in Aedes mosquitoes. This virus can be a significant public health concern. The UK Health Security Agency (UKHSA) has highlighted that the danger to human health in the UK is low, however surveillance is key to prevent and control the virus. The detection in UK Aedes mosquitoes suggests that changing environmental conditions may be enabling the virus to establish itself in new regions. Rising temperatures, altered rainfall patterns, and habitat changes are creating conditions that favour mosquito survival and virus transmission. Warmer climates allow mosquitoes to expand their range into new areas, including regions previously unsuitable for their survival. Extreme weather events, such as alternating floods and droughts, can also create breeding grounds for mosquitoes, further increasing disease risk. And then there are the impacts on the natural predators or competitors of mosquitoes, which may be declining due to unfavourable conditions created by climate change and environmental degradation. A final consideration is the impact of biodiversity and the availability of susceptible avian hosts, on virus transmission. A recent study undertaken in the Camargue region in the south of France sheds light on the relationship between bird diversity and flaviviruses such as Usutu and West Nile Virus.  

Methods and findings 

Researchers used nine locations in three study sites for mosquito sampling between May and November 2021 (mosquito season). Mosquitoes were captured over 24 hours once every 3 weeks at each study site and morphologically identified to species level. Unfed male and females were separated and stored in pools of up to 50 individuals per pool (average of 18 individuals per pool) before storing at -80 until viral detection. Viral RNA was extracted from pools of unfed Culex pipens and Culex modestus female mosquitoes and tested for orthoflavivirus genomes using real-time PCR. Positive samples were then characterized using a generic RT-Nested PCR followed by Sanger sequencing of amplicons for phylogenetic analysis. 

A total of 39,631 mosquitoes (36,163 females and 3468 males) were collected, and 15 species identified, the most abundant being Cx. pipiens. Of these 14,377 unfed female Cx. pipiens grouped in 535 pools and 1381 unfed female Cx. modestus in 84 pools, were tested for the presence of flavivirus. Of these 7 Cx. pipiens pools were found to be positive for the Usutu virus infection across July, August and October 2021.

A professional ornithologist carried out bird surveys at each of the three habitats (Natural, Agricultural and Periurban) in the three study areas, in July, September and October 2021. The Shannon diversity index (diversity of species in the community), bird richness (number of species) and evenness (proportional representation by each species) were calculated and the researchers used generalised linear models to analyse the factors associated with the probability of finding the Usutu virus infection. A total of 11,221 individual birds belonging to 130 species were identified, the most bird abundance was found in agricultural habitat, whereas natural habitat had a slightly lower number of birds with the highest number of species (bird richness was highest here). And the peri-urban habitat had the lowest abundance and least diverse population of birds. 

The generalised linear models showed that bird evenness (proportional representation by each species) and bird diversity were negatively associated with Usutu detection in mosquitoes, with bird evenness being the most important factor. Bird abundance and richness were positively associated with Usutu virus detection. 

Key Findings:  The authors conclude that if a local population of birds has a diverse and balanced distribution of species , there is a lower probability of Usutu virus infections in local mosquitoes. They suggest that the avian population composition in a habitat, namely species evenness and diversity, are critical factors impacting the risk of Usutu virus detection in mosquitoes. However they do highlight that this study reports on associations, rather than cause and effect.   

The findings reported in this study support the dilution effect hypothesis, which theorizes that greater biodiversity can reduce disease transmission by limiting the number of susceptible hosts and disrupting the transmission cycle. 

The Dilution Effect Theory - How Bird Diversity Limits Disease Transmission

Bird diversity could play a crucial role in reducing the spread of mosquito-borne viruses by 

• Reducing the probability of encountering a susceptible host.
  Some bird species are more susceptible to flaviviruses like Usutu and West Nile Virus. In ecosystems with high evenness (proportional representation by each species) and diversity (species richness + abundance), mosquitoes will feed on a mix of species, some of which may not support viral infection or viral replication - thus disrupting transmission cycles and lowering infection rates.
  
• Increasing competition among birds
  In diverse ecosystems, competition among bird species can limit the population of highly susceptible hosts. If a single species dominates, it may provide an abundant reservoir for viruses, increasing transmission rates.
  
• Regulation of Mosquito Populations
  Some bird species actively prey on mosquitoes, helping to control their numbers. Fewer mosquitoes mean fewer opportunities for disease transmission.
  
  

The Role of Bird Diversity in Disease Prevention

The findings from this study supports the importance of maintaining bird communities and bird diversity to mitigate the spread of mosquito-borne diseases. A rich and varied bird population may help limit the transmission of viruses like Usutu and West Nile Virus by reducing the number of highly susceptible hosts and interrupting transmission cycles. Evidently biodiversity conservation could have a strong role to play in the control of vector-borne diseases and should be integrated into One Health approaches for disease surveillance and response strategies. 

As researchers continue to monitor these developments, conservation efforts aimed at preserving bird diversity and controlling mosquito populations could be crucial in managing the risks associated with these emerging diseases in Europe.

Main image is of a Eurasian Blackbird. Image credit: Zeynel Cebeci, CC BY-SA 4.0 via Wikimedia Commons.