Ben Ho, MD
WACEP Board Member
For many of us summer in Wisconsin means time spent outside, whether on a trail, by the lake, in our yards, or if we are lucky, somewhere on a tropical vacation. As we enjoy the activities that we love, however, we knowingly expose ourselves to a number of seasonal environmental factors: heat, UV light, and of course, ticks and mosquitoes. We may know these arthropods best as nuisances that crash our barbecues and hitchhike on our trail walks. As vectors for important communicable diseases, however, their significance goes much beyond this.
Earlier this spring, the Department of Health and Human Services and the Center for Disease Control published a review of vectorborne pathogen transmission in Morbidity and Mortality Weekly Report. Using data from the National Notifiable Disease Surveillance System, the authors reported on trends in both tick- and mosquito-borne illnesses from 2004, the first year that arboviral illnesses became notifiable, to 2016, within the United States and its Territories. The report not only indicates a global increase in the incidence of vectorborne illnesses throughout the United States, but also identifies unique patterns of transmission at the local level. Two general patterns are evident. Tickborne illness reports rose steadily, doubling over the 13-year study period, and their greatest impact was on the eastern half of the country. While Lyme disease accounted for more than 82% of tickborne illness reports, other diseases such as anaplasmosis, ehrlichiosis, Rocky Mountain Spotted Fever, and babesiosis also increased in identification. The occurrence of mosquito-borne illnesses, on the other hand, was punctuated by epidemics, and was much more geographically dispersed. While endemic outbreaks of Zika, Dengue, and chikungunya were limited to the Territories, local transmission in the Southeast was likely due to infected travelers returning home from these areas.
A number of messages can be taken from this report. Although Lyme disease might be the illness that we consider most commonly in Wisconsin, the world of vectorborne diseases includes many more pathogens. In fact, if the next twelve years is anything like the last, we should expect increasing incidences of all kinds of different tick- and mosquito-borne illnesses. The epidemiology of these diseases is unique, largely because transmission depends heavily on environmental factors such as the geographical range of their hosts, the feeding cycles of their vectors, temperature, and rainfall patterns. Regions that were once too cold for ticks may eventually see Lyme disease; mosquito-borne illnesses once limited to tropical climates may creep further into the continental U.S., encouraged by heat waves, deforestation, flooding patterns, and human travel. It already appears that the physical range of A. aegypti is increasing, setting the scene for outbreaks of dengue, Zika, and chikungunya in years to come. To make matters worse, there are not many vaccines available to protect us (Rosenberg et al., 2018). As the world around us changes, so will the diseases that we see and treat.
Despite these challenges, there are several things that we can do as physicians and health advocates to address the growing threat of vectorborne illnesses. As with everything in medicine, awareness of these diseases and having appropriate clinical suspicion based on history and physical exam will not only aid in diagnosis and treatment of our patients, but also improve reporting of disease transmission. We can also reinforce common-sense strategies to limit transmission of these diseases to our patients: wearing long-sleeves and pants in tick and mosquito habitat, avoiding exposure during mosquito feeding times, keeping outdoor spaces clear of leaf litter and standing water, performing frequent tick checks on fellow humans and pets, and protecting ourselves with Deet and permethrin. The world may indeed be changing, but our process of limiting risks and exposures to vectors have not.
As a final point – have you ever wondered how effective the combination of Deet and permethrin is in deterring mosquito bites? A study done in the 1980’s by the U.S. Air Force compared mosquito bite rates on control subjects wearing no protection verses subjects treated with 35% Deet and/or permethrin at a remote test site in Alaska. Subjects using both 35% Deet and permethrin-treated clothing experienced only one mosquito bite per hour; subjects using no protection experienced on average 1,188 mosquito bites per hour, with a maximum rate reported at 3,360 bites per hour (Lillie et al., 1988). There are two clear take-aways from this study: 35% Deet combined with permethrin offer excellent protection against mosquitoes, and never volunteer as a research subject for the Air Force.
Rosenberg R, Lindsey NP, Fischer M, Gregory CJ, Hinckley AF, Mead PS, Paz-Bailey G, Waterman SH, Drexler NA, Kersh GJ, Hooks H, Partridge SK, Visser SN, Beard CB, Petersen LR. 2018. Vital signs: trends in reported vectorborne disease cases – United States and Territories, 2004-2016. MMWR-Morbid Mortal W. 67(17): 496-501.
Lillie TH, Schreck CE, Rahe AJ. 1988. Effectiveness of personal protection against mosquitoes in Alaska. J Med Entomol. 25(6): 475-478.