Letter to the editor: diagnosis of a single imported dengue case who had travelled to Japan - how serious is it for travellers?

Letters Letter to the editor: Diagnosis of a single imported dengue case who had travelled to Japan – how serious is it for travellers? Citation style for this article: Ueno R, Nishiura H. Letter to the editor: Diagnosis of a single imported dengue case who had travelled to Japan – how serious is it for travellers?. To the editor: We enjoyed reading the meticulous clinical report of an imported dengue case in a German traveller returning from Japan [1]. It is, however, unclear what level of risk the 'autochthonous' infection of a single case in Japan represents. By investigating the epidemiological aspects of one imported dengue case, we would like to discuss how serious the implications of autochthonous transmission are for future travellers. The diagnosed case travelled to Japan in August 2013, during which time the dengue virus infection is believed to have occurred. We would like to estimate how many primary cases there were and how transmissible the dengue virus was. Let I t and R t represent the number of primary cases and the effective reproduction number, respectively, at a generation t (i.e. the mean number of secondary cases generated by a single primary case at generation t). Supposing there were St susceptible individuals who can be infected with dengue virus, the probability of producing I t+1 =k secondary cases through a single generation interval of dengue (i.e. the time from infection in a primary human case to infection in a secondary human case caused by the primary case through the mosquito vector) is given by [2,3]: (1) If the diagnosed German patient represents all infected cases, k=1. However, dengue was not at the forefront of Japanese physicians' attention before the case report. If there were other undiagnosed cases in the same generation , k≥2. As can be seen from Equation 1, the reproduction of k cases in generation t+1 depends on three unknown epidemiological parameters, i.e. I t , R t and S t. The negative loglikelihood of observing k secondary cases reads as follows: (2) By allocating plausible values for a part of three unknown parameters, we can examine hypothetical situations in which a transmission event in a German traveller could have occurred. The relationship between the effective reproduction number and the number of primary cases with three possible values of k (k=1, 3 and 5) and S t =50 is shown (panel A of the …