Significance of major international seaports in the distribution of murine typhus in Taiwan

Abstract

Background: International seaports are hotspots for disease invasion and pathogens can persist in seaports even after ports are abandoned. Transmitted by fleas infected by Rickettsia typhi, murine typhus, a largely neglected and easily misdiagnosed disease, is known to occur primarily in large seaports. However, the significance of seaports in the occurrence of murine typhus has never been validated quantitatively.

Methodology/principal findings: We studied the spatial distribution of murine typhus, a notifiable disease, in Taiwan. We investigated whether risk of infection was correlated with distance to international seaports and a collection of environmental and socioeconomic factors, using a Bayesian negative binomial conditionally autoregressive model, followed with geographically weighted regression. Seaports that are currently in use and those that operated in the 19th century for trade with China, but were later abandoned due to siltation were analyzed. A total of 476 human cases of murine typhus were reported during 2000-2014 in the main island of Taiwan, with spatial clustering in districts in southwest and central-west Taiwan. A higher incidence rate (case/population) was associated with a smaller distance to currently in-use international seaports and lower rainfall and temperature, but was uncorrelated with distance to abandoned ports. Geographically weighted regression revealed a geographic heterogeneity in the importance of distance to in-use seaports near the four international seaports of Taiwan.

Conclusions/significance: Our study suggests that murine typhus is associated with international seaports, especially for those with large trading volume. Thus, one of the costs of global trade in Taiwan might be elevated risks of murine typhus. Globalization has accelerated the spread of infectious diseases, but the burden of disease varies geographically, with regions surrounding major international seaports warranting particular surveillance.

Fig 1. International seaports that are currently in use and those that operated in the 19^th century for trade with China but was abandoned since late 19^th century.

Fig 2. Variation in incidence rate (cases per 100000 people per year) of murine typhus among (A) age groups and (B) months in Taiwan during 2000–2014.

Fig 3. Spatial variation in (A) cases and (B) incidence rate (cases per 100000 people per year) of murine typhus among districts in Taiwan during 2000–2014.

Black circles denote international seaports that are currently in use.

Fig 4. LISA (Local Indicators of Spatial Association) map of murine typhus in Taiwan during 2000–2014 and spatial association with international seaports.

See text for definition of map legend. The threshold of significance was set at P = 0.05. Wanhua, Lukang and Anping were regarded as the largest ports in 19^th century in Taiwan.

Fig 5. Relative risk map of murine typhus in Taiwan during 2000–2014 after incorporating fixed and random effects of Bayesian negative binomial CAR model.

Fig 6. Geographically weighted regression pseudo t values for explanatory variable (A) temperature, (B) rainfall and (C) distance to operating ports.

Pseudo t values larger than 1.96 or smaller than -1.96 were deemed significant (P<0.05). Positive and negative pseudo t values represented positive and negative association with murine IR, respectively.