By Cameron Webb
Mosquitoes bite. They need to bite. Female mozzies require the nutritional hit of a bloodmeal to develop eggs. The blood can come from many different animals. Some mosquitoes are fussy with highly specific tastes, while others will feed on whoever, or whatever, is in the wrong place at the wrong time. Understanding these feeding habitats can help better manage outbreaks of mosquito-borne disease.
The outbreak of Zika virus in Brazil has refocused the world’s attention on mosquito-borne diseases. Notwithstanding the unusual route of sexual spread, the transmission of Zika virus is relatively simple. It is generally thought that only one mosquito species is primarily driving the outbreak and that the mosquito, Aedes aegypti, preferentially bites humans. The pathogen is spreading from human to mosquito and to human again. For many other mosquito-borne pathogens, transmission isn’t so simple.
In the case of pathogens such as West Nile virus or Japanese encephalitis virus, the mosquitoes that infect people as they bite have usually bitten an animal first, most often a bird. The mosquitoes pick up the infection from feeding on infected birds and then pass it on to other animals or people. Predicting outbreaks of these diseases can be difficult as it relies on an understanding of mosquito populations, environmental and climatic factors that influence their abundance, and the biology and ecology of the animals that carry the pathogens.
New research from Australia, recently published in the Journal of Medical Entomology, has provided clues about the animals that local mosquitoes are feeding on and the roles these animals may be playing in outbreaks of mosquito-borne disease.
Scientists from the University of South Australia sorted through more than 300,000 mosquitoes collected at various sites along the Murray River and around the South Australia capital, Adelaide. The suburbs of Adelaide may be far from the tropical north of Australia where there are threats of serious mosquito-borne pathogens, including dengue, but that doesn’t mean local mozzies pose any less of a health risk.
Official statistics report the mosquito-borne Ross River virus has infected more than 5,000 South Australian residents in the past 20 years. Fortunately, the disease caused by Ross River virus isn’t fatal, but it can be seriously debilitating. The symptoms include fever, joint pain, rash, and fatigue. In some instances, the illness may persist for many weeks or months.
Predicting outbreaks isn’t easy. The difficulty comes from the fact that around 40 different mosquito species have been implicated in the spread of the virus, each having their own ecological niches and responding differently to changing environmental conditions. From backyards to estuarine saltmarshes and to freshwater wetlands, the mosquitoes transmitting the virus may vary from place to place. They vary too from year to year, depending on climatic conditions.
The next piece of the puzzle is the animals, the reservoir hosts of Ross River virus. For the majority of mosquito-borne viruses, birds or primates are the reservoir hosts. For Ross River virus, the consensus is that kangaroos and wallabies play the most important role. A fitting host for an endemic Australian pathogen perhaps?
Leading the current study was post-graduate student Emily Flies, who explained the logic of their approach to mosquito collections.
“We are primarily interested in mosquito ecology as it relates to humans, and the most important meal a mosquito takes is the one before it feeds on a human,” she said. “Therefore, we conducted trapping in places where people live or recreate so that we can figure out what mosquitoes feed on before, or instead of, feeding on a human.”
The researchers took the blood-fed mosquitoes and extracted DNA that could then be matched to that of potential hosts. Studies of this nature often reflect the abundance of potential hosts, and it can be difficult to elucidate the actual feeding preferences of the mosquitoes. They can even toss up unexpected results.
“I was most surprised by the lack of kangaroo meals,” Emily said. “Kangaroos and wallabies are believed to be important reservoir hosts for the virus, so we expected to see mosquitoes feeding on kangaroos, especially in areas of the state with high Ross River virus transmission. We didn’t find a single kangaroo meal in our study, which suggests that other animals may be acting as reservoir hosts in the areas sampled. Now I know which animals mosquitoes are feeding on, but I still don’t know which animals are infecting mosquitoes with Ross River virus.”
The study certainly expands current knowledge on the feeding preferences of local mosquitoes. Over 200 blood-fed mosquitoes were analyzed, including mosquitoes associated with estuarine wetlands (Aedes camptorhynchus), freshwater wetlands (Coquilettidia linealis and Culex quinquefasciatus) and urban habitats (Aedes notoscriptus and a number of species in the Culex pipiens group).
Wild birds, cattle, and sheep were detected most commonly in the blood-fed mosquitoes. The abundance and diversity of wild birds detected provided some insight into the interaction between common birds and mosquitoes. Blackbirds, white-plumed honeyeaters, noisy miners, and New Holland honeyeaters were most commonly bitten by mosquitoes. Could these birds be reservoirs of Ross River virus or other mosquito-borne pathogens?
The result provides further evidence that there is still much to learn about the local risk factors associated with mosquito-borne disease. If local health authorities are to build better models to predict outbreaks of disease, or develop more strategic surveillance programs, more information on the host-feeding preferences of local mosquitoes is needed.
“Virus reservoirs remain something of a ‘black box’ for us pondering vector-borne disease ecology,” mused Associate Professor Craig Williams, co-author from the University of South Australia, and someone who regularly provides advice to local authorities on mosquito-borne disease management. “For us to better understand the complexity of a disease in the wild, and in turn use this information to predict future disease (and to decrease it), we need to understand all the factors involved in transmission. Bloodmeal analysis doesn’t give us the final answer, but it points the finger strongly at potential animals.”
There is much potential in the testing of wild bird blood for the presence of pathogens. Sentinel chicken flocks kept in many Australian states for the surveillance of pathogens such as Murray Valley encephalitis, and testing blood from wild birds has proven useful in North America for the monitoring of West Nile virus, but in Australia surveillance is likely to continue to be concentrated on catching and testing mosquitoes. Perhaps those blood-fed mosquitoes could be used to provide clues as to virus circulation in animal populations? Could they be tested on a routine basis?
“Surveillance programs need to be based on collection ‘signals’ from the environment that either help tell us about risk level (so we can warn people) and/or enable us to launch preventative action against disease transmission (e.g. killing mosquitoes),” Williams said. “At the moment, bloodmeal analysis cannot be done quickly and cheaply enough to form part of surveillance systems. That said, these studies can inform the design of future surveillance systems.”
Reflecting on the potential gaps in our understanding of mosquito-borne disease transmission cycles that can be filled by tracking the blood-feeding preferences of mosquitoes, it is tempting to look beyond Australian shores to South America. What could we learn by tracking the blood-feeding preferences of mosquitoes, such as Aedes aegypti, driving the outbreak of Zika virus? We already know this mosquito prefers to bite people, but who exactly is being bitten?
“It could be VERY interesting if we could test bloodmeals to distinguish BETWEEN the humans routinely bitten. This could illuminate how diseases like dengue and Zika spread through human settlements,” Williams said.
There is clearly much more to learn about the pathways of pathogen transmission between mosquitoes, people, and wildlife before we’ll be able to take the bite out of mosquito-borne disease permanently.
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Dr. Cameron Webb is principal hospital scientist at NSW Health Pathology and clinical lecturer at the University of Sydney. He studies mosquitoes, their associations with constructed and rehabilitated wetlands, and links to public health concerns. He provides advice on medical-entomology-related health threats to local, state, and federal authorities in Australia. Cameron maintains a blog on mosquito research and management at https://cameronwebb.wordpress.com and can be followed on Twitter at @mozziebites.