Delineation of an endemic tick paralysis zone in southeastern Australia

Tick paralysis has a major impact on pet dog and cat populations in southeastern Australia. It results from envenomation by Ixodes holocyclus and Ixodes cornuatus ticks, the role of Ixodes cornuatus in the epidemiology of this disease in Australia being unclear. The aim of this study was to describe the geographical distribution of tick paralysis cases in southeastern Australia using data from a national disease surveillance system and to compare characteristics of “endemic” cases with those reported outside this endemic zone (“sporadic” cases). Data were collated and a proportional symbol map of all cases by postcode was created. A 15-case isopleth was developed based on descriptive spatial statistics (directional ellipses) and then kernel smoothing to distinguish endemic from sporadic cases. During the study period (January 2010-December 2015) 12,421 cases were reported, and 10,839 of these reported by clinics located in 434 postcodes were included in the study. Endemic cases were predominantly reported from postcodes in coastal southeastern Australia, from southern Queensland to eastern Victoria. Of those cases meeting selection criteria, within the endemic zone 10,767 cases were reported from 351 (88%) postcodes and outside this zone 72 cases were reported from 48 (12%) postcodes. Of these latter 48 postcodes, 18 were in Victoria (26 cases), 16 in New South Wales (28 cases), 7 in Tasmania (9 cases), 5 in South Australia (7 cases) and 2 in Queensland (2 cases). Seasonal distribution in reporting was found: 62% of endemic and 52% of sporadic cases were reported in spring. The number of both endemic and sporadic cases reported peaked in October and November, but importantly a secondary peak in reporting of sporadic cases in April was found. In non-endemic areas, summer was the lowest risk season whilst in endemic areas, autumn was the lowest risk season. Two clusters of sporadic cases were identified, one in South Australia (P = 0.022) during the period 22 May to 2 June 2012 and another in New South Wales (P = 0.059) during the period 9 October to 29 November 2012. Endemic and sporadic cases did not differ with respect to neuter status (P = 0.188), sex (P = 0.205), case outcome (P = 0.367) or method of diagnosis (P = 0.413). However, sporadic cases were 4.2-times more likely to be dogs than cats (P < 0.001). The endemic tick paralysis zone described is consistent with previous anecdotal reports. Sporadic cases reported outside this zone might be due to a history of pet travel to endemic areas, small foci of I. holocyclus outside of the endemic zone, or in the case of southern areas, tick paralysis caused by I. cornuatus.