Biology subjects

Couper, L.

Publications and source records attributed to Couper, L..

11 recordsLinked to original sources

bioRxiv2025

Climate warming and urbanization may expand dengue transmission risk in California

BackgroundWhile primarily a disease of tropical and subtropical regions, dengue outbreaks are increasing in non-endemic regions due to environmental change and increasing travel and trade. For these non-endemic regions, estimating the risk of dengue is challenging as transmission is driven by both local environmental conditions and the introduction of viremic travelers. In this study, we aimed to estimate current and future dengue risk in California, USA--a region that has recently experienced its first cases of locally-acquired dengue. MethodsWe modeled dengue risk as the product of three key components needed for local transmission--vector presence, temperature-suitability for pathogen transmission, and viral introductions via travel-associated cases--estimated using vector and case surveillance, sociodemographic, and environmental data. We estimated risk for locations and months where local transmission was reported in 2023-2024 to define a threshold level of risk. We then projected monthly, census tract-level risk under both current conditions and future scenarios of climate warming and urban expansion. FindingsApproximately 18.2 million (95% CI: 17.9-18.3) California residents--primarily in the Central Valley and the Los Angeles and San Diego metropolitan areas--currently live in areas where peak monthly dengue risk exceeds levels estimated during observed local transmission. Under moderate scenarios of climate warming and urban expansion, an additional 4.1 million (95% CI: 3.7-4.6) California residents may be at risk by mid-century, with the largest increase in risk estimated for September and for the Sacramento Valley and coastal southern California regions. Outside the summer months and beyond the Central Valley and southern California, current and future risk remains low due to one or more major bottlenecks to transmission. InterpretationOur study identifies the specific regions and months conducive to dengue transmission in the non-endemic setting of California. At present, this covers a substantial portion of the state and is projected to expand under on-going climate warming and urbanization. Our results underscore the need for sustained vector control, and timely detection and management of travel-associated cases. Research in ContextO_ST_ABSEvidence before this studyC_ST_ABSDengue is considered endemic in over 125 countries and rapidly expanding its range, aided by climate warming, urbanization, and global travel and trade. Estimating transmission risk in newly emerging regions is critical for public health preparedness and depends on both local environmental conditions and the introduction of viremic travelers. We searched PubMed from database inception to May 8, 2025, for articles published in English using search terms "dengue", "model", "non-endemic", and their common textual variants. We identified 75 relevant studies modeling dengue transmission risk in non-endemic settings. However, nearly all were focused on one or two major determinants of transmission (eg, climate, vector population dynamics, or case importations) and/or did not include future projections. We found no studies that developed and validated a model of dengue transmission risk in non-endemic settings that incorporated vector, pathogen, and human suitability factors, and applied this model to project future risk. Added value of this studyThis study provides a novel approach to model dengue transmission risk in emerging regions that integrates the major factors driving transmission--vector presence, temperature suitability, and travel-associated cases. We apply this model to California--an emerging center of transmission risk in the continental USA--to identify the times and regions where risk exceeds levels observed during recent local transmission. We found that approximately 18.2 million California residents may be at risk based on this threshold, with an additional 4.1 million potentially at risk by mid-century under a moderate scenario of warming and urban expansion. Implications of all the available evidenceOur study identifies the hotspots of dengue transmission risk at a fine spatial and temporal resolution (census tract, month) in a highly populous and globally-connected region of emerging dengue risk. These risk estimates, and the regionally-specific bottlenecks to transmission that we identify can inform targeted disease surveillance and prevention strategies. Further, our findings have implications for other emerging regions including the southern USA and southern Europe, suggesting that the risk of local dengue transmission may increase under ongoing climate warming, urbanization, and global travel.

ecology↗

bioRxiv2025

A systematic review of climate-change driven range shifts in mosquito vectors

As global temperatures rise, concerns about shifting mosquito ranges--and accompanying changes in the transmission of malaria, dengue, and other diseases--are mounting. However, systematic evidence for climate-driven changes in mosquito ranges remains limited. We conducted a systematic review of studies documenting expansions or contractions in medically important mosquito species. In total, 178 studies on six continents identified range expansions in 118 mosquito species. While over a third of these studies cited warming as a driver, fewer than 10% performed statistical tests of the role of climate. Instead, most expansions were linked to human-aided dispersal (e.g., trade, travel), land-use changes, and urbanization. Although several studies reported poleward or upward expansions consistent with climate warming, none demonstrated warm-edge contractions driven by rising temperatures, which are theoretically predicted in some settings. Rather than expanding into newly suitable areas, many expansions appear to be filling preexisting thermally suitable habitats. Our findings highlight the need for long-term mosquito monitoring, rigorous climate-attribution methods, and better documentation of confounding factors like land-use change and vector control efforts to disentangle climate-driven changes from other anthropogenic factors.

ecology↗

bioRxiv2024

Detection of airborne Coccidioides spores using lightweight portable air samplers affixed to uncrewed aircraft systems in California's Central Valley

Coccidioidomycosis is an emerging fungal infection caused by inhalation of Coccidioides spp. spores. While airborne dispersal is critical to Coccidioides transmission, limited recovery of the pathogen from air has hindered understanding of the aerosolization and transport of spores. Here, we examine uncrewed aircraft systems (UAS) with portable, active air samplers as a novel means of capturing aerosolized Coccidioides and characterizing emissions and exposure risk. We sampled in September 2023 in eastern San Luis Obispo County, California, in an area with confirmed Coccidioides immitis in soils. We completed 41 20-minute flights across 14 sites using UAS equipped with an 8 L/min bioaerosol sampler and a low-cost particulate matter sensor. We sampled source soils and air under ambient conditions using one UAS at 1-10 m above ground level, and under a simulated high-dust event using two UAS, one at <2 m height and one at 5-12 m. We detected Coccidioides DNA in two of 41 air samples (4.9%), both under ambient conditions at 8 m above ground level, representing the highest known height of airborne Coccidioides detection. Spatially explicit UAS-based sampling could enhance understanding of Coccidioides aerobiology and enable detection in hard-to-reach or hazardous air masses, including dust storms and wildfire smoke. SynopsisUAS-based air sampling for bioaerosols, including pathogenic Coccidioides spp., opens new possibilities for characterizing the aerosolization and transport of fungal spores and other airborne pathogens.

ecology↗

bioRxiv2024

Characterizing the soil microbial community associated with the fungal pathogen Coccidioides immitis

Coccidioidomycosis is a fungal disease affecting humans and other mammals, caused by environmental pathogens of the genus Coccidioides. Understanding the ecological factors that shape the distribution of Coccidioides in soils is important for minimizing the risk of human exposure, though this remains challenging due to the pathogens highly variable spatial distribution. Here, we examined associations between the soil microbial community and Coccidioides immitis presence within the Carrizo Plain National Monument--a minimally disturbed grassland ecosystem, and the site of a longitudinal study examining the effects of rodents and their burrows on C. immitis presence in soils. Using internal transcribed spacer 2 (ITS2) and 16S sequencing to characterize the soil fungal and bacterial communities, we found over 30 fungal species, including several other members of the Onygenales order, that co-occurred with Coccidioides more frequently than expected by chance. Coccidioides-positive samples were significantly higher in microbial diversity than negative samples, an association partly driven by higher Coccidioides presence within rodent burrows compared to surface soils. Soil source (i.e., rodent burrow versus surface soil) explained the largest amount of variation in bacterial and fungal community diversity and composition, with soils collected from rodent burrows having higher microbial diversity than those collected from adjacent surface soils. While prior evidence is mixed regarding associations between Coccidioides and microbial diversity, our study suggests that favorable microhabitats such as rodent burrows can lead to a positive association between soil diversity and Coccidioides presence, particularly in otherwise resource-limited natural environments.

ecology↗

bioRxiv2024

Small mammals and their burrows shape the distribution of Coccidioides in soils: a long-term ecological experiment

BackgroundFor nearly all human fungal pathogens, the environmental constraints on their distributions remain poorly understood, hindering disease management. Here, we investigated the role of zoonotic host presence, soil conditions, and their interaction on the presence of Coccidioides immitis - an emerging fungal pathogen and causative agent of coccidioidomycosis. MethodsA long-term experimental study initiated in 2007 in the Carrizo Plain National Monument, California excluded rodents from certain areas (20 by 20-meter exclosures; n = 20) via the installation of submerged mesh fences, leaving emptied burrows intact. We collected 1,988 soil samples over four sampling periods spanning April 2021 to April 2022 using a factorial design that crossed burrows and surface soils (i.e., top 10 cm of soil) with exclosures (i.e., rodent absence) and non-exclosures (i.e., rodent presence). We measured in situ soil moisture, temperature, and vegetation, and analyzed soils for C. immitis DNA. ResultsThe proportion of samples containing C. immitis was significantly higher (p <0.0001) in burrow samples taken from outside rodent exclosures (i.e., rodents present; 28.5% positive) compared to burrow samples taken from within exclosures (i.e., rodents absent; 19.7%), surface soils outside of exclosures (3.6%) and surface soils within exclosures (0.5%). In generalized linear mixed models adjusting for soil conditions, rodent presence, and sample type (burrow vs. surface soil), we estimated that the odds of detecting C. immitis were 19.2 (95% CI: 11.3 - 36.7, p < 0.0001) times higher in soils taken from burrows compared to surface soils, and 2.6 (95% CI: 1.0 - 5.6, p = 0.049) times higher in soils where rodents were present compared to absent. Soil moisture was the only abiotic factor associated with C. immitis detection (odds ratio per 1% increase in water content: 0.83, 95% CI: 0.70 - 0.89, p = 0.0002). In mediation analyses, we estimated that 73.7% (95% CI: 68.9, 78.5) of the total association between rodents and Coccidioides detection effect was attributable to rodent creation of burrows. ConclusionsRodent creation and occupation of burrows substantially increases C. immitis detection. Synergy between reservoir host presence and environmental conditions may exist for other human fungal pathogens.

ecology↗

bioRxiv2024

Plasticity in mosquito size and thermal tolerance across a latitudinal climate gradient

Variation in heat tolerance among populations can determine whether a species is able to cope with ongoing climate change. Such variation may be especially important for ectotherms whose body temperatures, and consequently, physiological processes, are regulated by external conditions. Additionally, differences in body size are often associated with latitudinal clines, thought to be driven by climate gradients. While studies have begun to explore variation in body size and heat tolerance within species, our understanding of these patterns across large spatial scales, particularly regarding the roles of plasticity and genetic differences, remains incomplete. Here, we examine body size, as measured by wing length, and thermal tolerance, as measured by the time to immobilization at high temperatures ("thermal knockdown"), in populations of the mosquito Aedes sierrensis collected from across a large latitudinal climate gradient spanning 1300 km (34-44 {degrees}N). We find that mosquitoes collected from lower latitudes and warmer climates were more tolerant of high temperatures than those collected from higher latitudes and colder climates. Moreover, body size increased with latitude and decreased with temperature, a pattern consistent with James rule, which appears to be a result of plasticity rather than genetic variation. Our results suggest that warmer environments produce smaller and more thermally tolerant populations.

ecology↗

bioRxiv2024

How much warming can mosquito vectors tolerate?

Climate warming is expected to substantially impact the global landscape of mosquito-borne disease, but these impacts will vary across disease systems and regions. Understanding which diseases, and where within their distributions, these impacts are most likely to occur is critical for preparing public health interventions. While research has centered on potential warming-driven expansions in vector transmission, less is known about the potential for vectors to experience warming-driven stress or even local extirpations. In conservation biology, species risk from climate warming is often quantified through vulnerability indices such as thermal safety margins - the difference between an organisms upper thermal limit and its habitat temperature. Here, we estimated thermal safety margins for 8 mosquito species (including Aedes aegypti and Anopheles gambiae) that are the vectors of malaria, dengue, chikungunya, Zika, West Nile and other major arboviruses, across their known ranges to investigate which mosquitoes and regions are most and least vulnerable to climate warming. We find that several of the most globally important mosquito vector species have positive thermal safety margins across the majority of their ranges when realistic assumptions of mosquito behavioral thermoregulation are incorporated. On average, the lowest climate vulnerability, in terms of both the magnitude and duration of thermal safety, was just south of the equator and at northern temperate range edges, and the highest climate vulnerability was in the subtropics. Mosquitoes living in regions largely comprised of desert and xeric shrubland biomes, including the Middle East, the western Sahara, and southeastern Australia have the highest climate vulnerability across vector species.

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bioRxiv2023

Temperature and intraspecific variation affect host-parasite interactions

Parasites play key roles in regulating aquatic ecosystems, yet the impact of climate warming on their ecology and disease transmission remains poorly understood. Isolating the effect of warming is challenging as transmission involves multiple interacting species and potential intraspecific variation in temperature responses of one or more of these species. Here, we leverage a wide-ranging mosquito species and its facultative parasite as a model system to investigate the impact of temperature on host-parasite interactions and disease transmission. We conducted a common garden experiment measuring parasite growth and infection rates at seven temperatures using 12 field-collected parasite populations and a single mosquito population. We find that both free-living growth rates and infection rates varied with temperature, which were highest at 18-24.5{degrees}C and 13{degrees}C, respectively. Further, we find intraspecific variation in peak performance temperature reflecting patterns of local thermal adaptation--parasite populations from warmer source environments typically had higher thermal optima for free-living growth rates. For infection rates, we found a significant interaction between parasite population and nonlinear effects of temperature. These findings underscore the need to consider both host and parasite thermal responses, as well as intraspecific variation in thermal responses, when predicting the impacts of climate change on disease in aquatic ecosystems.

ecology↗

bioRxiv2023

A mosquito parasite is locally adapted to its host but not temperature

Climate change will alter interactions between parasites and their hosts. Warming may affect patterns of local adaptation, shifting the environment to favor the parasite or host and thus changing the prevalence of disease. We assessed local adaptation in the facultative ciliate parasite Lambornella clarki, which infects the western tree hole mosquito Aedes sierrensis. We conducted laboratory infection experiments with mosquito larvae and parasites collected from across a climate gradient, pairing sympatric or allopatric populations across three temperatures that were either matched or mismatched to the source environment. L. clarki parasites were locally adapted to their hosts, with 2.6x higher infection rates on sympatric compared to allopatric populations, but were not locally adapted to temperature. Infection peaked at the intermediate temperature of 13{degrees}C. Our results highlight the importance of host selective pressure on parasites, despite the impact of temperature on infection success.

ecology↗

bioRxiv2022

Temperature impacts on dengue incidence are nonlinear and mediated by climatic and socioeconomic factors

Temperature can influence mosquito-borne diseases like dengue. These effects are expected to vary geographically and over time in both magnitude and direction and may interact with other environmental variables, making it difficult to anticipate changes in response to climate change. Here, we investigate global variation in temperature-dengue relationship by analyzing published correlations between temperature and dengue and matching them with remotely sensed climatic and socioeconomic data. We found that the correlation between temperature and dengue was most positive at intermediate (near 24{degrees}C) temperatures, as predicted from the thermal biology of the mosquito and virus. Positive temperature-dengue associations were strongest when temperature variation and population density were high and decreased with infection burden and rainfall mean and variation, suggesting alternative limiting factors on transmission. Our results show that while climate effects on diseases are context-dependent they are also predictable from the thermal biology of transmission and its environmental and social mediators.

ecology↗

bioRxiv2021

Lizard feeding enhances Ixodes pacificus vector competency

A vectors susceptibility and ability to transmit a pathogen-- termed vector competency--determines disease outcomes, yet the ecological factors influencing tick vector competency remain largely unknown. Ixodes pacificus, the vector of Borrelia burgdorferi (Bb) in the western U.S., feeds on rodents, birds, and lizards. While rodents and birds are reservoirs for Bb and infect juvenile ticks, lizards are Bb-refractory. Despite I. pacificus feeding on a range of hosts, it is undetermined how larval host bloodmeal identity affects future nymphal vector competency. We experimentally evaluate the influence of larval host bloodmeal on Bb acquisition by nymphal I. pacificus. Larval I. pacificus were fed on either lizards or mice and after molting, nymphs were fed on Bb-infected mice. We found that lizard-fed larvae were significantly more likely to become infected with Bb during their next bloodmeal than mouse-fed larvae. We also conducted the first RNA-seq analysis on whole-bodied I. pacificus and found significant upregulation of antioxidants and antimicrobial peptides in the lizard-fed group. Our results indicate that the lizard bloodmeal significantly alters vector competency and gene regulation in ticks, highlighting the importance of host bloodmeal identity in disease transmission and upends prior notions about the role of lizards in Lyme disease transmission.

ecology↗