Onchocerca spp. have unusually long pre-patent periods of up to 18 months . Although autochthonous transmission of canine onchocerciasis in the UK cannot be excluded, it is highly likely that the first dog was infected as a one-year-old in the Algarve region of Portugal, a known focus of transmission . The pre-patent period noted here of seven years is extremely long but is comparable to a recent observation in Germany in a dog introduced from Greece, which developed eye problems six years following rescue . Such cases of prolonged parasite and nodule development highlight challenges in case diagnosis and management of O. lupi, as well as in monitoring zoonotic disease incursion.
The second case presents an unusually severe pathogenicity, with exophthalmos in one eye and disease progression despite appropriate therapy. To the best of our knowledge, the appearance of new worms following surgical removal of an existing nodule is a novel finding, and the invasive nature of nodule growth necessitating globe removal is an exceptional outcome. However, subsequent nodule development in the previously healthy contralateral eye, a matter of weeks after enucleation of the infected eye, has been described previously  and suggests that nematodes may survive undetected until they form overt nodular lesions.
In both cases, lesions became prominent, with timely veterinary interventions. However, as already mentioned, not all dogs display overt clinical signs, especially when worms do not develop in the external parts of the ocular apparatus . In undetected covert infections of mature worms, microfilariae will accumulate in the skin for a long time, allowing for potential parasite transmission.
The identity of vectors of this parasite remains unclear, but as for most Onchocerca spp., one or more species of Simulium (blackflies) may have a role. In the UK, there are least six species of blackflies recorded as biting both humans and dogs , of which S. reptans (west of England and Wales) and S. tuberosum/S. variegatum (north of England and Scotland) are abundant . Considering that the black fly species composition is similar to that of countries in Europe where O. lupi is endemic and assuming they could act as vectors, we hypothesise that local transmission in the UK could occur. An assessment of the suitability of conditions in the UK for parasite circulation by modelling of climate, ecological and other factors is therefore required.
In conclusion, it is apparent that Onchocerca lupi infection may only become evident in dogs many years following importation. Nodules may be invasive and appear unpredictably with asynchronous development. The popular trend to re-home dogs from O. lupi-endemic regions of Europe will increase the risk of transmission of this parasite in the UK and presents a growing problem of One Health concern.