[Advances in etiology, epidemiology and genetic diversity of Thelazia callipaeda]

Thelazia callipaeda, T. californiensis and T. gulosa are three causative agents of human thelaziasis. Most of the reported cases were caused by T. callipaeda, occurring in the old world, particularly in Asian and European countries. T. californiensis and T. gulosa have rarely been reported infecting humans in North America. T. callipaeda has long been called the oriental eye worm, referring to its traditional distribution across eastern and southeastern Asia (i.e., China, Korea, Japan, Indonesia, Thailand, and India) where infection is endemic in animals and humans, usually in poorer rural areas and mainly among children and the elderly. The identification of the parasite was mainly based on the characteristics of oral and genital organs. In Asia and Europe, vectors for this nematode are male Phortica okadai and P. variegata drosophilids respectively, which feed on ocular secretions of hosts and transmit infective stage larvae to domestic and wild carnivores, lagomorphs, and humans. China probably has the largest number of cases with thelaziasis in the world, and lots of cases have been existed in other Asian countries such as Japan and Korea. Although a few of human cases have been reported, there were high infection rates of wild animals and domesticated dogs and cats in most of European countries. Based on the cox1 gene, a total of 21 haplotypes were identified in the samples from worldwide, in which, one circulated only in European countries (h1), while the other 20 haplotypes were distributed in Korea, Japan and China. In general, the Chinese clinical isolates of T. callipaeda expressed high genetic diversity. The population differences between Europe and Asian countries were greater than those among China, Korea and Japan. The T. callipaeda populations from Europe and Asia should be divided into two separate sub-populations. These two groups started to diverge during the middle Pleistocene.