Risk analysis of ectoparasites acting as vectors for chronic wasting disease

Prion diseases are rare neurodegenerative diseases of humans and animals with a lethal evolution. Animal prion infections, such as chronic wasting disease (CWD) and scrapie (sheep) have shown a pattern of horizontal transmission. CWD is an endemic disease that has been affecting thousands of domestic and wild cervids in US for the last three decades. The mode of contamination is not known, although direct contact between infected and non-infected animals via saliva, urine and feces have been considered. Increasing spread of CWD has raised concerns about the potential transmission to humans and the conversion of human prion protein by CWD-associated prions has been demonstrated in laboratory experiments. Fly larvae exposed to brain infected material were able to readily transmit scrapie to hamsters. Prion rods were identified in both larvae and fly pupae. New lines of evidence confirmed that adult flies are also able to express prion proteins. The most prevalent species of myiasis in cattle, sheep and wild cervids (Hypoderma spp.) present a very different life cycle from human myiasis, with a long contact with neurologic structures, such as the spinal canal and epidural fat, that are potentially rich in prion rods. Considering the huge amount of fly larvae that affects each animal, it is important to discuss the possibility that these ectoparasites could theoretically act as reservoirs and vectors for CWD and other prion diseases. It is critical to recognize all the possible factors involved in CWD transmission since ectoparasites could be handled in an easier way than the environmental persistence of infectious prions.