Biology subjects

Johnson, P. C. D.

Publications and source records attributed to Johnson, P. C. D..

9 recordsLinked to original sources

bioRxiv2026

Implications of the trade-offs between negative density-dependence and Allee effects for vector control

BackgroundUnderstanding population dynamics is fundamental to predicting species persistence and extinction, yet remains challenging due to the complex interplay between ecological, environmental and anthropogenic factors. Population dynamics are regulated by intrinsic factors such as negative density-dependence and Allee effects. While negative density-dependence is a well-understood process, Allee effects have received less attention but can have important implications for conservation and species management. For control of disease vectors, negative density-dependence and Allee effects can drive vectors to elimination or to rebound after control. For malaria mosquitoes, negative density-dependence at the larval stages is well known to limit population growth, but the implications of Allee effects and the trade-offs between negative density-dependence and Allee effects remain unknown. It is hypothesised that, depending on the vector control strategy, Allee effects could be triggered and push populations closer to extinction. MethodsA stochastic state-structured population simulation model was developed, which followed a simplified mosquito life cycle. Negative density-dependence and Allee effects were included as parameters influencing larval survival and total fecundity, respectively. The aims were addressed by varying the strength of negative density-dependence and Allee effects parameters, independently and simultaneously, under different vector control scenarios: control, sustained and shorter-term interventions (here, immolating larvicides) that reduced the larval population. ResultsWhile in isolation, the strength of negative density-dependence and Allee effects did not have a long-term impact on the population dynamics, their combination accelerated population extinction as both the strength of negative density-dependence and Allee effects increased. As Allee effects act on small population sizes, sustained interventions were able to activate Allee effects, increasing the probability of population extinction, but short-term interventions can lead to populations rebounding, driven by negative density-dependence. ConclusionUnderstanding less-studied regulatory processes like Allee effects can support vector control by highlighting aspects of the vectors life cycle that are either resilient or vulnerable to interventions. If present, Allee effects could potentially be harnessed to accelerate the elimination of vectors and diseases such as malaria.

ecology↗

bioRxiv2025

Risk factors, patterns and seropositivity of inter-epizootic Rift Valley fever virus of cattle in northern Tanzania.

1.1BackgroundRift Valley Fever (RVF) is a zoonotic viral disease affecting both animals and humans, with significant public health and economic implications. In Tanzania, a series of RVF outbreaks in livestock have been reported. The epidemiological information about hosts exposures status, time, location and risk factors that contribute to virus maintenance between one epizootic and the other, is important for preparedness and intervention plans. This study investigated the seroprevalence and potential determinants of RVFV exposure in cattle in Babati, Hai, and Moshi Districts between the last RVF outbreak in 2007 and the anticipated El Nino rains of 2024, which could precipitate the next outbreak. MethodsA total of 1,627 cattle serum samples (790 from Babati, 518 from Moshi, and 319 from Hai), were collected between August and October 2023 from both urban and rural areas, were analyzed using a competitive ELISA to detect RVFV-specific antibodies. Data on cattle biodata, management practices, environmental factors, were obtained through structured questionnaires and field records. Sero-prevalence was determined by descriptive statistics, while risk factors were quantified by multivariable logistic regression and RVF seropositivity was plotted on administrative boundaries of Babati, Moshi, and Hai districts by using QGIS. ResultsThe overall RVFV seroprevalence was 15.0% (95% CI: 13.26 - 16.73). Seropositivity was 22.2% (95% CI: 18.62-25.78) in Moshi, 12.2% (95% CI: 8.63-15.82) in Hai and 11.4% (95% CI 9.18-13.61) in Babati. Age (OR = 1.22, 95% CI: 1.15-1.3, p<0.001), sex (OR = 0.53, 95% CI: 0.36-0.79, p = 0.0017), herd size (OR=0.2201; 95% CI: 0.095-0.5101; p=0.0004), water access along irrigated crop fields (OR=3.7181; 95% CI: 1.5868-8.7121; p=0.0025), and geographical location (OR = 2.23, 95% CI: 1.63-3.06, p < 0.001) were significant predictors of seropositivity. Spatial visualization revealed clustering of seropositive herds in villages near lake Manyara and Tarangire National parks in Manyara region, also seropositive clusters in villages near irrigated areas, Nyumba ya Mungu dam and floodplains in lower Moshi. ConclusionThis study highlights ongoing inter-epizootic exposure of cattle to RVFV in northern Tanzania. The RVFV is not randomly distributed during inter-epizootic periods, but rather forms seropositive clusters. Most significant determinants of seropositivity includes age, sex, herd size, water access along irrigated crop fields and geographical location Moshi showing higher seropositivity.

ecology↗

bioRxiv2024

A randomized controlled trial of the effectiveness of a community-based rabies vaccination strategy

BackgroundApproximately 60,000 people die from dog-mediated rabies annually. Low and heterogeneous coverage reduces the effectiveness of dog vaccination campaigns that can eliminate rabies. Campaigns typically involve teams travelling annually to villages to deliver cold chain stored vaccines from centralized headquarters. Thermotolerant vaccines enable novel decentralized delivery of locally-stored vaccines by communities throughout the year. We compared the effectiveness of annual team-based versus continuous community-based dog vaccination strategies. MethodsWe conducted a cluster randomized controlled trial across Mara region, Tanzania. Trial clusters were administrative wards (112, on average four villages each). For the team-based arm vaccinators hosted annual static-point clinics, whilst for the community-based arm, a ward-based animal health officer with a village community leader managed vaccinations using vaccines stored within the ward. We measured vaccination coverage, the primary outcome, twice annually per cluster (month 1 and 11) through household surveys over three years (November 2020 to October 2023) and examined spatial and temporal coverage variations as secondary outcomes. FindingsCommunity-based delivery achieved significantly higher coverage (49-62%) than team-based delivery (22-46%), and consistently exceeded the critical threshold for herd immunity (40%), Odds ratio (OR): 1.48-3.49. The lower less uniform coverage achieved through team-based delivery had a higher monthly probability of falling below the critical threshold (0.6, 95% CI: 0.38-0.81) vs 0.18 (95% CI: 0.04-0.40). Greater declines in coverage over the year were recorded in the team-based arm compared to the community-based ConclusionCommunity-based mass dog vaccination achieves higher more consistent coverage than team-based delivery across settings typical of many sub-Saharan African countries. This approach could play an important role in national rabies elimination programmes aiming to end human rabies deaths by 2030 as part of the global zero by 30 strategy. FundingDepartment of Health and Human Services of the National Institutes of Health (R01AI141712), Wellcome Trust (207569/Z/17/Z, 224520/Z/21/Z) and MSD Animal Health. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

immunology↗

bioRxiv2024

Modelling resource-driven movements of livestock herds to predict the impact of climate change on network dynamics

In East Africa, climate change is likely to profoundly impact livestock management and the potential spread of infectious diseases. Here, we developed a network model to describe livestock movements to grazing and watering sites, fitted it to data from the Serengeti district of Tanzania, and used it to explore how projected changes in resource availability due to climate change could impact future network structures and therefore infectious disease risks, using 2050 and 2080 as exemplar scenarios. Our modelled networks show increased connections between villages in grazing and watering networks, with connectivity increasing further in the future in correspondence with changes in vegetation and water availability. Our analyses show that targeted interventions to efficiently control regional disease spread may become more difficult, as village connectivity increases and disease vulnerability becomes more evenly distributed. This analysis also provides proof of principle for a novel approach applicable to agropastoral settings across many developing countries, where livestock trade plays a crucial role in maintaining local livelihoods but also in spreading disease.

ecology↗

bioRxiv2022

Modelling the effectiveness of targeting Rift Valley fever virus vaccination using imperfect network information

Livestock movements contribute to the spread of several infectious diseases. Data on livestock movements can therefore be harnessed to guide policy on targeted interventions for controlling infectious livestock diseases, including Rift Valley fever (RVF) -- a vaccine-preventable arboviral fever. While detailed livestock movement data are available in many countries, such data are generally lacking in others, including many in East Africa, where multiple RVF outbreaks have been reported in recent years. Available movement data are imperfect, and the impact of imperfect movement data on targeted vaccination is not fully understood. Here, we used a network simulation model to describe the spread of RVF within and between 398 wards in northern Tanzania connected by cattle movements, on which we evaluated the impact of targeting vaccination using imperfect movement data. We show that pre-emptive vaccination guided by only market movement permit data could prevent large outbreaks. Targeted control (either by the risk of RVF introduction or onward transmission) at any level of imperfect movement information is preferred over random vaccination, and any improvement in information reliability is advantageous to their effectiveness. Our modelling approach demonstrates how targeted interventions can be carefully applied to inform animal and public health policies on disease control planning in settings where detailed data on livestock movements are unavailable or imperfect due to a lack of data-gathering resources.

genetics↗

bioRxiv2021

Heritability and phenotypic plasticity of biting time behaviors in the major African malaria vector Anopheles arabiensis

Use of Insecticide Treated Nets for malaria control has been associated with shifts in mosquito vector feeding behavior including earlier and outdoor biting on humans. The relative contribution of phenotypic plasticity and heritability to these behavioural shifts is unknown. Elucidation of the mechanisms behind these shifts is crucial for anticipating impacts on vector control. We used a novel portable semi-field system (PSFS) to experimentally measure heritability of biting time in the malaria vector Anopheles arabiensis in Tanzania. In PSFS assays, the biting time of F2 offspring (early: 18:00-21:00, mid: 22:00-04:00 or late: 05:00-07:00) was significantly associated with that of their wild-caught F0 grandmothers, corresponding to an estimated heritability of 0.30 (95% CI: 0.20, 0.41). F2 from early-biting F0 were more likely to bite early than F2 from mid or late late-biting F0. Similarly, the probability of biting late was higher in F2 derived from mid and late-biting F0 than from early-biting F0. Our results indicate that variation in biting time is attributable to additive genetic variation. Selection can therefore act efficiently on mosquito biting times, highlighting the need for control methods that target early and outdoor biting mosquitoes.

evolutionary biology↗

bioRxiv2020

Emergence of Lyme disease on treeless islands in Scotland, UK

Lyme disease (LD) is typically associated with forested habitats but has recently emerged on treeless islands in the Western Isles of Scotland. This has created a need to understand the environmental and human components of LD risk in open habitats. This study quantified both elements of LD risk and compared these between treeless islands with high and low LD incidence. We found high LD incidence was linked to higher prevalence in ticks (6.4% vs 0.4%) and increased human tick bite exposure. Most reported tick bites (72.7%) were within 1km of the home address and commonly in gardens. Residents on islands with high LD incidence reported increasing problems with ticks and suggested changing deer distribution as a potential driver. This study highlights the benefits to public health of an integrated approach to understand the factors contributing to LD emergence and a need to evaluate LD ecology in treeless habitats more broadly.

ecology↗

bioRxiv2020

Rapid ageing and species identification of natural mosquitoes for malaria surveillance

The malaria parasite, which is transmitted by several Anopheles mosquito species, requires more time to reach its human-transmissible stage than the average lifespan of a mosquito. Monitoring the species-specific age structure of mosquito populations is critical to evaluating the impact of vector control interventions on malaria risk. We developed a rapid, cost-effective surveillance method based on deep learning of mid-infrared spectra of mosquitoes cuticle that simultaneously identifies the species and the age of three main malaria vectors, in natural populations. Using over 40,000 ecologically and genetically diverse females, we could speciate and age grade An. gambiae, An. arabiensis, and An. coluzzii with up to 95% accuracy. Further, our model learned the age of new populations with minimal sampling effort and detected the impact of control interventions on simulated mosquito populations, measured as a shift in their age structures. We anticipate our method to be applied to other arthropod vector-borne diseases.

zoology↗

bioRxiv2020

Classification and characterisation of livestock production systems in northern Tanzania

Livestock keepers in sub-Saharan Africa face a growing range of pressures, including climate change, land loss, restrictive policies, and population increase. Widespread adaptation in response to such pressures can lead to the emergence of new, non-traditional typologies of livestock production. We sought to characterise livestock production systems in northern Tanzania, a region undergoing rapid social, economic, and environmental change. Questionnaire and spatial data were collected from 404 livestock-keeping households in 21 villages in Arusha and Manyara Regions in 2016. Multiple factor analysis and hierarchical cluster analysis were used to classify households into livestock production systems based on household-level characteristics. Indicators of vulnerability, including household-level reports of hunger, illness, livestock loss, land loss and crop losses were compared between production systems. Three distinct clusters emerged through this process. The ethnic, environmental and livestock management characteristics of households in each cluster broadly mapped onto traditional definitions of pastoral, agro-pastoral and smallholder livestock production in the region, suggesting that this quantitative classification system is complementary to more qualitative classification methods. Our findings also suggest that traditional systems of livestock production continue to persist in northern Tanzania. Nonetheless, we found indicators of substantial change within livestock production systems, most notably the adoption of crop agriculture in the majority of pastoral households. Smallholder households were less likely than either pastoral or agro-pastoral households to report hunger, illness, and livestock, land or crop losses. Livelihoods that rely solely on livestock are relatively rare in northern Tanzania, which represents an important shift in production in the region, particularly among pastoralists. Policy initiatives to improve household and community well-being should recognise the continuing distinctiveness of traditional livestock production systems in the region.

systems biology↗