Premature Proposal of the Pine Weevil as a Vector of a Human Pathogen

LETTER

We have with great interest read the paper by Tuuminen et al. (1) published in the July 2014 issue of this journal where the pine weevil (Hylobius abietis) is incriminated as a vector of the potential human pathogen Capnocytophaga canimorsus. The authors base their conclusion on the fact that the man, who was infected by C. canimorsus, claimed that “he had been bitten by a large pine weevil on the lateral side of his neck. The bite mark was a red patch of a diameter of 3 to 4 cm on his skin with a darker red raised center and was noticed when he was admitted to the hospital 3 days later.” It was also stated that he had not encountered any dogs, which are the main carriers of C. canimorsus. Two of us (N.B. and G.N.) have handled pine weevils for several decades. Every year, we collect thousands of them at sawmills for use in research (e.g., see reference 2). The occasional nibbling of our skin by the mandibles of these beetles has never caused any visible injury. And we find it particularly difficult to believe that an infectious dose of viable C. canimorsus could have been transferred from the mouth of a dog or a cat (the normal habitat of C. canimorsus) by a pine weevil, into the skin of this sawmill worker. Pine weevils are attracted to volatiles from fresh conifer wood, and during their migration period in early summer, very large numbers (tens of thousands) may aggregate at sawmills (3, 4), where these beetles may land on almost anything, including people. Being strictly phytophagous insects, pine weevils do not attack people or animals in order to feed on them. The bite mark was on the man's neck; therefore, he could not have observed the organism while it was biting him.

Arthropods, in particular blood-feeding Diptera (mosquitoes, tabanids, etc.) and ticks, are well-known vectors of human pathogens. There are even some instances where beetles may be involved in the transfer of pathogens to humans or other vertebrates (5). Ground beetles (Carabidae) are intermediate hosts of poultry tapeworms, grain-feeding beetles (Tenebrionidae and Dermestidae) are intermediate hosts of worms and potential pathogen disseminators, and dung beetles (Scarabaeidae) and carrion beetles (Silphidae) are also potential disseminators of pathogens (5). We cannot find any previously published information where weevils are incriminated as mechanical or biological vectors of human-pathogenic microorganisms or viruses. The present proposal of the pine weevil H. abietis as a vector of C. canimorsus would be a fundamental shift in how an arthropod is incriminated as a disease vector. In addition, the proposal of pine weevils as vectors of human pathogens could have great implications for how sawmill workers in the taiga biome need to protect themselves against potentially life-threatening infections.

In order to incriminate a particular arthropod as a potential vector of a human-pathogenic microbe it is, inter alia, necessary to isolate a viable and infectious microbe from the suspected vector. In the case described by Tuuminen et al. (1), the pathogen was not even identified by PCR methodology from the putative vector.