Certain specific behaviors have been shown to be associated with the transmission of hantaviruses and arenaviruses within host populations. Venereal transmission, which has been suggested for Machupo virus , implies a certain seasonality (assuming that breeding in host populations is seasonal) and thus predictable variation in risk for human populations. Transmission of some other arenaviruses (e.g., Junin virus) and hantaviruses in host populations seems to be associated with a different behavioral mechanism. Antibody in host populations is more common in males than in females, and is more common in larger, older animals [1,5,14,29], implying horizontal transmission by a mechanism that favors males. Field studies have provided data that may identify that specific mechanism. Mammalogists frequently use the presence of scars as indicators of aggressive interactions among individuals. Field studies have shown that males more frequently have scars than do females and antibody-positive males are much more likely to have scars than are antibody negative males [14, 29]. This suggests that a frequent mechanism of transmission of these viruses in host populations is by fighting and inflicting bite wounds.
As might be expected from the hypothesized route of transmission, seroconversions to hantaviruses occur during the breeding season in many areas. In Arizona male brush mice seroconvert to Limestone Canyon virus throughout the breeding season, but only rarely in winter . In high-altitude areas in Colorado, there is a second peak in seroconversions during mid winter . This suggests a second behavioral mechanism of transmission, perhaps associated with communal nesting and mutual grooming during cold weather. An understanding of these different mechanisms of transmission is important if we are to develop accurate models of virus transmission and human risk.
A second behavioral characteristic that has important implications for human risk is habitat selection, which can be viewed on a regional scale, or on a micro (local) scale. On a regional scale, we found that deer mice were found in every major biome represented in the southwestern United States, from desert to alpine tundra. Furthermore, at least some deer mice infected with SNV were found in all of these habitat types. Nevertheless, the relative density of deer mice and especially the relative density of antibody-positive deer mice varied widely among habitat types. The lowest densities and prevalences were found in the altitudinal and climatic extremes (desert and alpine tundra) and the highest densities and prevalences were found in the middle altitude habitats, such as pinyon-juniper woodland and great basin scrub . Although I have placed this example in the behavioral category, much of the pattern may also be due to physiological tolerances. Regardless, knowledge of these differences allows a more accurate prediction of risk to humans living or traveling in various habitat types.
On a micro scale, habitat selection can be an important determinant of viral transmission among rodent hosts and from rodent hosts to humans. I have discussed the propensity of some rodents (such as deer mice) for peridomestic habitats, where they are more frequently antibody-positive than they are in sylvan habitats. Argentine hemorrhagic fever (AHF), caused by Junin virus, is associated with farming activities in rural Argentina. Descriptions of the epidemiology of AHF have stated that farmers are infected while working in crop fields . However, during a three-year longitudinal study, we found that the reservoir for Junin virus, Calomys musculinus, was largely restricted to the more stable weedy roadsides and fencerows between crop fields. Its congener, C. laucha was frequently found in crop fields . This pattern of habitat partitioning between the two closely related species may be very important epidemiologically. It may help explain lack of infection with Junin virus in C. laucha, it suggests a specific high-risk habitat for contracting AHF, and it suggests a potential mitigation practice -cutting or burning the weeds along the roadsides and fence lines that separate crop fields.
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