Mosquitoes and soft ticks cannot transmit Lyme disease spirochetes

Abstract

Investigators frequently assume that the discovery of a pathogen in a blood-feeding arthropod provides conclusive proof that these arthropods participate in the enzootic cycle of that pathogen. This assumption may lead to the conclusion that soft ticks and mosquitoes are vectors of the agent of Lyme disease, Borrelia bugdorferi sensu lato. This reasoning oversimplifies the requirements of any such transmission cycle. The ability to serve as a host to a pathogen is the most elementary requirement of the transmission cycle of a vector-borne infection; some minimal degree of vector competence is essential. In the case of Lyme disease, a competent vector must be able to acquire the pathogen from a host, permit its development, support its multiplication and must then be able to deliver the pathogen to a reservoir host (Matuschka and Spielman 1986). To serve as a vector of the agent of Lyme disease, it follows that a mosquito or a soft tick must be capable of more than spirochete acquisition. Indeed, the presence of Lyme disease spirochetes has been demonstrated in various blood-feeding arthropods other than ixodid ticks on both sides of the Atlantic (Magnarelli et al 1986; Halouzka et al. 1998; Hubalek et al. 1998). We lack evidence, however, that such arthropods can transmit the pathogen to another level in the cycle of transmission. To our knowledge, only a few investigators have examined the possibility that insects may serve as competent vectors for the agent of Lyme disease (Magnarelli et al. 1986, 1987; Magnarelli and Anderson 1988). Mechanical transmission of these pathogens by Aedes mosquitoes was suggested when spirochetes were found in the heads of such insects after they had ingested spirochete-infected blood through a membrane. In another experiment, field-collected Aedes mosquitoes were permitted to feed on competent but spirochete-free reservoir hosts. Although spirochetes were later detected in a tenth of these mosquitoes, none of the hosts on which they engorged acquired an infection. Mosquitoes fail to transmit spirochetes to a second level of hosts. No attempts to transmit Lyme disease spirochetes with the Argas ticks that feed on pigeons have been recorded. Extensive seroepidemiologic studies on these birds and microbiological observations on thousands of their argasid parasites, however, suggest that pigeons and their ticks play no role in the enzootic transmission cycle of the agent of Lyme disease (Fabbi et al. 1995; Thormahlen et al. 1997; Dautel et al. 1999). In nature, few pigeon ticks feed on hosts competent for Lyme disease spirochetes. Even smaller is the chance that such ticks might subsequently feed on a person. The agent of Lyme disease is exquisitely adapted to its tick and vertebrate hosts. Their cork-screw like form permits them to move more effectively through the relatively viscous skin of their host than through its blood (Kimsey and Spielman 1990). As a result, spirochetemia is rare and of short duration. Ixodid vector ticks, such as the wood tick (Ixodes ricinus) in Europe and the deer tick (Ixodes dammini) in North America, insert their relatively short mouthparts into their host’s skin and feed first on lymphatic fluid and then on the blood that collects in the resulting feeding pool. Their prolonged feeding period, between 2 and 8 days, permits spirochetes to become trapped in the feeding cavity that is formed in the skin. Argasid ticks, on the other hand, feed mainly on blood, and they do so much more rapidly, with nymphs and adults generally engorging in no more than 10 min. Mosquitoes feed from capillaries and do so even more rapidly than Argas ticks, becoming replete within about 90 s. Ixodid ticks, therefore, are far more likely than an argasid tick or a mosquito to ingest Lyme disease spirochetes. Parasitol Res (2002) 88: 283–284 DOI 10.1007/s00436-001-0584-1