The role of Culex territans mosquitoes in the transmission of Batrachochytrium dendrobatidis to amphibian hosts

Abstract

Background: Mosquitoes are the deadliest organisms in the world, killing an estimated 750,000 people per year due to the pathogens they can transmit. Mosquitoes also pose a major threat to other vertebrate animals. Culex territans is a mosquito species found in temperate zones worldwide that feeds almost exclusively on amphibians and can transmit parasites; however, little is known about its ability to transmit other pathogens, including fungi. Batrachochytrium dendrobatidis (Bd) is a topical pathogenic fungus that spreads through contact. With amphibian populations around the world experiencing mass die-offs and extinctions due to this pathogen, it is critical to study all potential modes of transmission. Because Cx. territans mosquitoes are in contact with their hosts for long periods of time while blood-feeding, we hypothesize that they can transmit and pick up Bd.

Methods: In this study, we first assessed Cx. territans ability to transfer the fungus from an infected surface to a clean one under laboratory conditions. We also conducted a surveillance study of Bd infections in frogs and mosquitoes in the field (Mountain Lake Biological station, VA, USA). In parallel, we determined Cx. territans host preference via blood meal analysis of field caught mosquitoes.

Results: We found that this mosquito species can carry the fungus to an uninfected surface, implying that they may have the ability to transmit Bd to their amphibian hosts. We also found that Cx. territans feed primarily on green frogs (Rana clamitans) and bullfrogs (Rana catesbeiana) and that the prevalence of Bd within the frog population at our field site varied between years.

Conclusions: This study provides critical insights into understanding the role of amphibian-biting mosquitoes in transmitting pathogens, which can be applied to disease ecology of susceptible amphibian populations worldwide.

Keywords: Blood-feeding; Chytrid fungus; Host preference; Mosquito-borne disease; Pathogen transmission.

Fig. 1

A Mosquitoes were collected using a giant bug aspirator around a pond occupied by R. clamitans and other anurans. Female Cx. territans were sorted out, and blood meal analysis was performed. B Blood meal sources of Cx. territans mosquitoes by year between 2018 and 2022

Fig. 2

A Frogs were swabbed five times each on the (1) abdomen, (2) both medial hind thighs, and (3) between the hind digits of both feet. Swabs were (4) sent out for qPCR to confirm Bd presence or absence. B The (1) proboscis and legs of field-caught mosquitoes were removed and (2) placed on a sterile agar plate, which was divided to separate the two body parts. After 4 days, the plates were (3) swabbed and (4) sent out for qPCR and analysis. C Numbers of frogs caught and swabbed per species between 2020 and 2022. D Representation of the numbers of frogs that tested positive for Bd from 2020 to 2022

Fig. 3

A Mosquitoes were (1) tethered to a tungsten rod and (2) placed on a Bd plate with legs touching the agar. After 30 min, the mosquitoes were (3) transferred to a new, individual sterile plate with limbs in contact with the agar for another 30 min and (4) let sit for 4 days. After 4 days, the plates were (5) swabbed and stored at −80 °C until they were (6) sent out for qPCR and analysis. B Representation of negative controls (n = 56). C Representation of Bd-infected plates to confirm the presence of Bd (n = 6). D The number of plates that were positive for Bd 4 days after contact with an inoculated mosquito (n = 123)