Tularemia, lawn mowers, and rabbits' nests.

The recent report by Goethert, Shani, and Telford has demonstrated that strains of Francisella tularensis subsp. tularensis (type A) on the island of Martha’s Vineyard are of multiple genotypes (3). In that paper, the authors stated that the mode of transmission of the two outbreaks (summer 1978 and summer 2000) of pneumonic tularemia (1, 5) from this island remain “undescribed” (3). I propose the application of Occam’s razor to explain these pneumonic tularemia outbreaks. To do so requires consideration of two categories of facts. First, epidemiologic investigations have revealed that among the risk factors for the development of pneumonic tularemia was the use of lawn mowers and brush cutters (2). Second, the nesting habits of the major reservoir host, the cottontail rabbit (Sylvilagus floridanus) need to be considered. The cottontail rabbit, after beginning breeding in early spring, creates pocket nests, shallow depressions 10 to 21 cm deep and 10 to 13 cm wide. The nest is lined with grass and fur; the upper opening can be hidden due to this grass lining and overgrowth of vegetation. Nesting locations can be pastures, pine plantations, and even mowed lawns. The female returns to the nest only to nurse her young, which, averaging five per nest, remain in it for 3 to 4 weeks. The average use of the nest is 26 days per brood, with a mean of three (and up to five) broods per summer (4). Rabbit nests therefore are populated with large numbers of growing bunnies most of the summer. In Wisconsin, this writer has had the experience of accidentally mowing over well-hidden broods with unintendedly sad results, and, undoubtedly, has passed over more while mowing lawns. If any such hidden nests have harbored sick cottontail rabbits, aerosolization of F. tularemia could be expected. Therefore, the introduction of infected rabbits by humans to Martha’s Vineyard in 1937 (5), along with the use of modern power lawn mowers, is probably another example of human environmental modification that has caused the emergence of a new route of transmission of tularemia. Supporting data for this hypothesis could be found by a searching out of such rabbits’ nests, as F. tularemia has been found to be viable in straw (and therefore probably rabbit-nest lining) for up to 6 months (1). Experimentally, mowing over infected nests could be shown to aerosolize this organism; however, extreme caution in such experimentation would be advisable. As an aside, the seropositivity for F. tularensis of raccoons and skunks on Martha’s Vineyard probably reflects their known scavenging of dead or dying animals. These predators could acquire infection from such sick or dying rabbits, analogous to the circumstances of prairie dog areas of the American Southwest, where coyotes have been found to be seropositive for Yersinia pestis. Finally, this writer agrees with the authors that Dermacentor sp. ticks are uncommon vectors in propagating tularemia. A more likely tick, the rabbit tick (which also quests for groundfeeding birds), Haemaphysalis leporispalustris, could be studied as a possible route of dissemination of F. tularemia among rabbits on the island; however, this tick seldom bites humans.