The present study reports the first autochthonous cases of infestation with T. callipaeda in dogs from Portugal. Indeed, none of the nine dogs had ever travelled abroad. Interestingly, all T. callipaeda nematodes were identified as h1, thus far the only characterized haplotype reported in Europe. All infested animals lived outdoors at high risk of infestation.
This study involved dogs permanently living in the municipalities of Chaves, Vinhais and Bragança, which lie on the Portuguese-Spanish border. Latitude (between 41’ 00” and 48’ 00” N), climate, altitude and vegetation are similar to those of other European countries and regions where thelaziosis is now considered endemic, e.g. Italy [20, 21], France [11, 12] and Spain . The geographical area of Chaves, Vinhais and Bragança is characterized by a rural environment with many hunting and shepherd dogs, as well as habitats suitable for the development of P. variegata. This scenario is in accordance with the observations collected using a predictive geoclimatic model, which identified this area as suitable for the life cycle of the intermediate host . Although the vector of T. callipaeda in northern Portugal is most likely P. variegata, further studies are required to confirm this hypothesis. Since the nine affected dogs had never travelled abroad, it is presumed that the infestations were of autochthonous origin.
In the Spanish provinces of Orense and Salamanca, 135 and nine cases of canine thelaziosis have been described, respectively . The fact that these provinces are contiguous with the North of Portugal (Orense to the north and Salamanca to the east) supports the hypothesis that this infestation might also be emerging in this country. The existence of a sylvatic life cycle in wild animals, such as foxes and wolves, could be responsible for the spread of the parasite across Europeans countries, because these animals can freely roam through borders and act as reservoirs for this nematode . Under these circumstances, the risk of a spread of the infestation from Spain to Portugal is realistic, for instance through domestic dogs or wild mammals. Moreover, during summer months, a large number of emigrants return home for vacations with their pets from regions of southern and central France where ocular thelaziosis is endemic [11, 12]. In association with the spread of the infestation from Spain, this could provide an additional explanation for the emergence of T. callipaeda in Portugal.
Infestations with T. callipaeda are spreading across Europe and becoming a serious problem in several countries. In Italy, T. callipaeda infests around 60 % of dogs in Potenza, Basilicata . In the region of La Vera, Spain, the prevalence of infestation reached almost 40 % in the canine population . Being limited to clinically suspect cases referred to the two veterinary centers, our report does not allow an estimation of the prevalence of T. callipaeda infestation in the Portuguese canine population to be made. Nevertheless, the number of clinical cases of canine thelaziosis diagnosed from July to November 2011 in the two veterinary clinics involved in this study (i.e. 16 clinical cases out of 2300 dogs brought in for general veterinary consultation) allows an approximate estimation of disease prevalence (i.e. 0.7 %). This projection includes seven additional cases of canine thelaziosis not described in the Methods, Results and Table 1, which were diagnosed only based on clinical presentation and morphological identification of the nematodes collected (unpublished results).
The low estimated prevalence of infestation detected in the present study might be due to the fact that only dogs presenting clinical signs were examined. Thus, it is possible that a considerable number of asymptomatic dogs are also infested. A recent introduction of T. callipaeda in Portugal may also account for this estimated low prevalence. The investigation in the region of La Vera revealed that the proportion of infested dogs presenting clinical signs (~15 %) was considerably lower than that recorded in apparently healthy infested dogs (~85 %) . Under laboratory conditions the transplantation of adult worms into rabbit eyes induced a severe inflammatory response 3–5 hours post-infestation; however, the clinical signs gradually diminished after one week . These findings might contribute to explaining the high number of asymptomatic but infested animals observed in some studies and a similar scenario may also occur in the North of Portugal.
Up until now, there have only been four reported cases of human infestation with T. callipaeda in Europe, two from Italy and two from France . However, in the last two decades the number of cases of human ocular thelaziosis by T. callipaeda in Asian countries has increased . Therefore, it is possible that the number of humans infested in Europe may rise in the future. Due to the potential risk of human infestation, an improved awareness of veterinarians and medical ophthalmologists for this parasite is warranted. In order to control zoonotic eyeworm infestation, correct diagnosis and treatment of infestations in domestic animals are crucial. The administration of broad spectrum macrocyclic lactones on a regular basis as a preventative measure against canine thelaziosis is advisable .