The movements of workers, service providers and visitors between agricultural operations can contribute to the spread of plant diseases, causing an initially localized outbreak to reach properties tens or even hundreds of kilometers away. A new study shows that patterns of human mobility can reproduce the spread of a bacterial disease that affected kiwifruit production in New Zealand from 2010.
The researchers used movement data between agricultural properties to build a mobility network and simulate the spread of Psa-V, a bacterium that affects kiwifruit plants. The model was developed based on about 86,600 movements registered among 2,281 horticultural properties in New Zealand during 2022 and subsequently tested against the real outbreak that occurred a decade earlier.
The results indicate that most new infections occur near an already affected property. More than 90% of the properties infected in the model were located within 10 kilometers of another infected property.
However, a small portion of the movements between properties traveled much farther distances. Although infrequent, trips of more than 20 kilometers were recorded and some reached about 1,000 kilometers. According to the researchers, these long-distance movements can act as “sparks” capable of carrying a disease to a region not yet affected and triggering new foci.
The model managed to reproduce not only the geographic distribution of the Psa outbreak, but also the timing when the disease began to gain momentum and the month it reached its peak. For the researchers, this result shows that the network of contacts between properties can be relevant for understanding the dynamics of agricultural diseases.
The calendar also matters
The intensity of the spread does not depend only on where people move, but also on when those movements occur. Agricultural work varies throughout the year, and periods of greater activity can increase the number of connections between properties.
According to the researchers, a disease introduced during a period of higher mobility can give rise to a more severe outbreak than a similar introduction during a period of lower activity.
This information could be used to identify properties that serve as linking points between different regions and guide surveillance to locations or routes with greater potential for spread.
The approach could also support responses to future diseases, allowing the pre-emptive testing of different propagation scenarios and assessing where surveillance or movement-restriction measures may be most effective.
The study was developed by a team from Indiana University in collaboration with the New Zealand company Onside and published in a peer-reviewed journal. The researchers emphasize that biosafety is not limited to border disease control: after a pathogenic agent enters a territory, the way people and agricultural activities move can play a significant role in its spread.
