Mosquito-borne zoonoses: animal clustering can amplify or dilute outbreak risk

The same livestock and wildlife community can generate very different risks of West Nile, Japanese encephalitis or Rift Valley fever transmission depending on how animals are distributed in space. A new modelling study shows that clustering competent hosts can amplify transmission, while clustering dead-end hosts may instead dilute it, especially when mosquitoes strongly aggregate around host-dense areas.

The study addresses a major challenge in zoonotic arbovirus epidemiology: mosquito-borne viruses often circulate through complex networks involving several vector and vertebrate species, some capable of amplifying infection and others unable to sustain onward transmission.

For Japanese encephalitis virus (JEV), for example, pigs and some birds can act as competent hosts, whereas cattle are generally considered dead-end hosts. Rift Valley fever involves mosquitoes and several ungulate species, while West Nile virus is maintained mainly through mosquito–bird cycles.

Mosquitoes choose hosts at two spatial scales

The researchers developed a theoretical patch model in which mosquitoes first aggregate at longer range toward areas containing more animals, potentially attracted by cues such as carbon dioxide, and then select particular host species at shorter range according to availability and innate preference.

This distinction matters because mosquito abundance is not necessarily evenly distributed across a landscape. Field observations cited by the authors from Bangladesh showed, for example, that each additional cow at a household was associated with roughly a two-fold increase in Culex tritaeniorhynchus and a 1.5–1.7-fold increase in several other common mosquito species.

The model deliberately examined a situation in which mosquitoes strongly preferred dead-end hosts: when competent and non-competent hosts were equally available, only 10% of host choice was directed toward competent hosts.

Clustering competent hosts can increase R0

Despite that strong preference for dead-end animals, clustering competent hosts could still increase transmission and the basic reproduction number, R0, when mosquitoes also aggregated strongly around host-dense patches. Conversely, when dead-end hosts were clustered, mosquitoes were disproportionately drawn into areas where their bites did not contribute to onward transmission, producing a dilution effect.

Landscape fragmentation also produced no simple response. Increasing the number of patches amplified transmission when competent hosts were clustered, but reduced transmission when dead-end hosts dominated those clusters.

Why livestock distribution matters for surveillance

The results challenge surveillance strategies based primarily on total animal numbers or species composition. Two areas containing exactly the same numbers of cattle, pigs or birds could have very different transmission potential simply because those animals are arranged differently and therefore attract different concentrations of mosquitoes.

This has practical implications for JEV, West Nile virus and Rift Valley fever surveillance. Risk assessments may need to integrate the location of farms, livestock density within individual holdings, competent-host clusters and mosquito aggregation rather than relying only on regional host abundance.

The study remains theoretical and does not predict outbreaks in a particular country. But, for mosquito-borne zoonoses, where animals are located may matter almost as much as which animals are present.