Zoological gardens are exceptional environments for studying mosquito ecology and their feeding behavior. They can provide unique insights into how to control mosquito populations and prevent diseases that they can transmit to vertebrates and people in the zoo. A new study looked at the blood nutrition ecology of mosquitoes in two UK zoos.

When examining the ability of mosquitoes to transmit pathogens (vector capacity), three key factors have a significant influence: blood feeding behavior, mosquito population density and mosquito longevity. However, understanding mosquito blood-feeding behavior poses three main challenges:

  • Blood-fed mosquitoes are difficult to catch as they are usually not attracted to traps.
  • Mosquitoes can disperse great distances when looking for and after having a blood meal.
  • Assessing feeding patterns is a positive bias issue because mosquitoes provide information about which vertebrates they have been feeding, but not which.

To meet these challenges, we need to understand more about the environment, the potential hosts available, and employ techniques to control blood-fed mosquitoes.

Some mosquitoes may prefer one host type (specialists), others feed on many different types (generalists), and both strategies can change under the right circumstances. This could turn them into “bridge vectors” for the transmission of pathogens between different vertebrate species. If the pathogen can multiply in the new host (competent host) and be transmitted to more mosquitoes, a reinforcing effect could be observed, while the pathogen would dilute in the community if it were not.

Zoos bring together a wide variety of potential hosts, endemic and exotic, so that mosquitoes can transmit pathogens between species and groups of vertebrates that would not normally be found in natural settings. Hence, it is not surprising that mosquitoes have been found in zoos that feed on zoo vertebrates, free wild vertebrates, and even humans. In some cases, this adaptability of mosquitoes has led to serious health problems, such as the West Nile virus outbreak at the Bronx Zoo / Wildlife Conservation Park. Likewise, mosquitoes can endanger the health of vertebrates in zoos, as in the case of avian malaria caused by Plasmodium spp. and mainly transmitted from Culex spp. Mosquitoes, which are a leading cause of mass mortality in captive penguins.

We worked with Chester Zoo and Flamingo Land to collect mosquitos and analyze the risk of bird malaria transmission to their penguin colonies. As part of this research (2017 and 2018 in Chester Zoo and 2017 in Flamingo Land) we caught blood-fed mosquitoes and decided to search for blood-fed mosquitoes in a third sampling year (2019) at Chester Zoo.

How do I catch blood-fed mosquitoes?

In the first two years we used conventional mosquito traps for host-seeking mosquitoes (BG-Mosquitaire) and gravid mosquitoes (CDC-Gravid trap) and in the third year we added dry ice as a CO source2 to the BG Mosquitaire traps and built-in resting boxes and sucked in the vegetation. Comparing these methods, we found no differences in the proportions of blood-fed mosquitoes caught with the BG Mosquitaire and Gravid traps, but the resting boxes and aspiration had a higher proportion, although the total number of mosquitoes was higher with the traps.

a) BG Mosquitaire trap that has been upgraded with dry ice; b) CDC Gravid trap; c) aspiration of dormant mosquitoes; d) resting box. Pictures by Freya Townsend.

Feeding activity

The distribution of blood-fed mosquitoes was not the same in the sampling areas. It was expected that areas where more mosquitoes are caught would have higher rates of blood donation. However, this has not always been the case, which is why mosquitoes attracted more food in some parts of the zoos. Likewise, mosquito feeding activity was not the same throughout the season and was higher during the summer months. Through generalized linear modeling, we found that temperature was related to an increase in mosquito feeding activity, which is expected since temperature has a strong influence on mosquito physiology.

Proportion of blood-fed mosquitoes

The menu of the mosquitoes: feeding options

We used a barcode technique to analyze the DNA in the mosquito blood meals and adjusted the sequences in databases to identify the vertebrate hosts. From a total of 640 blood-fed mosquitoes that we caught, we identified the vertebrate hosts of 159 mosquitoes. These were 74 birds, 71 humans, and 20 non-human mammals. Native birds such as the Eurasian jackdaw (Corvus monedula) and the Eurasian magpie (Pica Pica), were a popular choice, although zoo birds such as the Schalow turaco (Tauraco schalowi) and Humboldt penguin (Spheniscus humboldti) and non-human mammals of the zoo such as the Bactrian camel (Camelus bactrianus) and Elds Hirsch (Rucervus eldii thamin) were also on the menu.

Culex pipiens, our most common mosquito, prefers to feed on birds and Culiseta annulata (the second most common) prefers mammals, which suits their natural preferences. However, we have found many Cx. pipiens Feeding people previously described, but not in such proportions.

Blood-nourished Culex pipiens

Cx. pipiens belongs to a mosquito complex that is morphologically very similar; For example the Sympatric Cx. Torrentium looks the same and can only be distinguished by dissecting the man’s genitals. On further investigation, we used PCR techniques to precisely identify them and confirmed that they were all present Cx. pipiens. Similarly, this species has two biotypes or forms, Cx. pipiens pipiens and Cx. pipiens molestusThe first prefers to feed on birds and the second on people. We suspected we had caught some Molestus, But that was not the case. Hence, it seems that the high number of visitors (Chester Zoo receives over 2 million annually) may have affected mosquito feeding patterns. This poses a potential risk of transmission to humans and zoo vertebrates Culex spp. are vectors of viruses hosted by wild birds such as West Nile Virus (WNV), Sindbis Virus, and Usutu Virus.

In addition, we found mixed blood meals indicating that mosquitoes had been feeding on two hosts; A bird was involved in all cases and, among other things, humans were paired with Eurasian magpie, a proven natural reservoir of WNV.

Minimum removal of some mosquitoes at Chester Zoo. Orange circles: sampling areas; Red areas: animal exhibitions; Dotted lines: driving routes.

Spread after feeding

Knowing the location of the zoo vertebrates allows us to estimate the minimum range of mosquitoes between animal displays and the location where they were caught. A third of the mosquitoes were caught within 50 meters of the animal exhibits and half within 100 meters, showing that there are more chances of catching mosquitoes closer to their food source. However, we also found mosquitoes feeding on pets that we believe came from nearby homes and farms, allowing mosquitoes to travel greater distances than the maximum we observed (367 m).

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