Discussion

Amongst vector borne helminths (VBH), the eyeworm Thelazia callipaeda is an emergent zoonotic agent of concern to the public health of several European regions. Adult nematodes live in the orbital cavities and associated host tissues of dogs and cats, foxes, wolves, rabbits and humans, causing ocular disease. One of the peculiarities of the life cycle of T. callipaeda is that its vector, Phortica variegata, is a zoophilic fruit fly. Indeed, unlike almost all other arthropod vectors of pathogens, only P. variegata males feed of lachrymal secretions of animals, ingesting first-stage larvae (L1). In the vector, larval development through to infective L3 occurs within 14-21 days from experimental infestation; larvae may also survive in overwintering flies (for up to 6 months) before being transmitted to a receptive host. Although T. callipaeda has not been reported from the US, P. variegata collected in the US can successfully transmit the parasite. At the time when vector identity and biology were elucidated, the infection was confined to remote poor settings in southern Italy; nevertheless, ecological niche models predicted that large areas of Europe (including the United Kingdom) are suitable for the development of the vector, thus highlighting the potential risk of T. callipaeda spreading to other European countries. Indeed, whilst T. callipaeda had exclusively been reported from easternmost countries until two decades ago, it has now been described in both animals and humans from Italy, France, Switzerland, Germany, Belgium, Spain, Bosnia and Herzegovina, Croatia, Hungary, Serbia, Bulgaria, Romania, and Greece. Thus, this parasite can be considered as an emergent vector-borne pathogen in Europe, and zoonotic infections have been diagnosed on several occasions in European endemic areas. In the UK, the infection has been reported in animals with a history of travel were found positive for T. callipaeda and, after ten years since the predictive model was published, a recent updating and re-assessment of that model suggested the presence of suitable conditions for P. variegata in previously undocumented regions of the UK, therefore highlighting the possibility for further spread of the infection. Off-label topical administration of ivermectin may result in irritation of the ocular tissues, and it is therefore discouraged in veterinary practice. Milbemycin oxime/praziquantel tablets as well as the imidacloprid 10% and moxidectin 2.5% spot-on formulation have shown to significantly reduce infection rates in naturally infected dogs.