Using models to provide rapid programme support for California's efforts to suppress Huanglongbing disease of citrus

Abstract

We describe a series of operational questions posed during the state-wide response in California to the arrival of the invasive citrus disease Huanglongbing. The response is coordinated by an elected committee from the citrus industry and operates in collaboration with the California Department of Food and Agriculture, which gives it regulatory authority to enforce the removal of infected trees. The paper reviews how surveillance for disease and resource allocation between detection and delimitation have been addressed, based on epidemiological principles. In addition, we describe how epidemiological analyses have been used to support rule-making to enact costly but beneficial regulations and we highlight two recurring themes in the programme support work: (i) data are often insufficient for quantitative analyses of questions and (ii) modellers and decision-makers alike may be forced to accept the need to make decisions on the basis of simple or incomplete analyses that are subject to considerable uncertainty. This article is part of the theme issue 'Modelling infectious disease outbreaks in humans, animals and plants: epidemic forecasting and control'. This theme issue is linked with the earlier issue 'Modelling infectious disease outbreaks in humans, animals and plants: approaches and important themes'.

Keywords: Huanglongbing; epidemiology; invasive species; modelling; regulatory response.

Figure 1.

A progression of RBS risk maps for the HLB epidemic in southern California over the period 2013–2018. (Online version in colour.)

Figure 2.

(a) Spatio-temporal cluster analysis identified 41 HLB clusters in southern California when using an 800 m delimitation distance. (b) High-resolution inset image of HLB clusters in Orange County. (c) Reconstructed timeline of HLB-positive tree detections in one infection cluster in Anaheim, CA. The large, yellow circle identifies the original 800 m delimitation survey; the smaller, black circles demonstrate a 400 m delimitation survey, starting with the initial find and iteratively capturing all confirmed CLas+ trees. (d) Summary results for data resampling simulation experiments showing the cumulative percentage of detections achieved by delimitation surveys of different radii using all known detection data from California from the period 2012 to December 2017. The simulations indicate that greater than 90% of all known positive trees would have been detected by delimitation surveys of 400 m radius; the eradication programme operated with a radius of 800 m during this period. (Online version in colour.)

Figure 3.

Estimated minimum (a) and maximum (b) HLB incidence for each STR in southern California. (c,d) Higher resolution of estimated incidence range for Orange County subregion with detections from the RBS identified (dots). (e) Sampling efficacy table predicting the probability of finding at least one CLas+ tree given the sampling effort and detection probability, using the binomial theory. (f) Summary of the statistical methodology for estimating HLB incidence ranges for each STR using the RBS, and subsequent data collection via survey deployment. (Online version in colour.)

Figure 4.

The distribution of density of infected citrus trees within 170 m of newly detected, infected trees. Note that many newly detected trees have no, or few, other infected trees within 170 m at the time of detection, but several have in the order of 20–30 infected close neighbours, and there is a set of trees that have 40 or more infected neighbours. (Online version in colour.)

Figure 5.

(a) Risk matrix for HLB associated with bulk fruit transport between different pairs of quarantine zone within California, depending on risk of infection in the zone of origin and magnitude of potential impact in the zone of destination. (b) Map of California showing the quarantine zone boundaries. The majority of commercial production is located in zone 2. All known HLB cases have been in zone 6. Note that zone 6 is discontinuous, with a small subsection (Riverside) encircled by zone 5. (Online version in colour.)