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SASTM Newsflash - Lyme disease: USA - Increased incidence


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SASTM Newsflash





The incidence of Lyme disease has increased approximately 80 percent in the United States between 1993 and 2007, although rates vary between states, according to a new study in "CMAJ Open" [Canadian Medical Association Journal online open-access journal]. Latitude and population density were correlated with higher increases, with states in the north seeing increases and southern states seeing stable or declining rates.


Lyme disease and other tick-borne illnesses have been increasing and spreading with ticks moving from warmer areas into more northern latitudes. It is projected that this trend will continue with warmer temperatures associated with climate change.



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Background: Tick-borne illnesses represent an important class of emerging zoonoses, with climate change projected to increase the geographic range within which tick-borne zoonoses might become endemic. We evaluated the impact of latitude on the rate of change in the incidence of Lyme disease in the United States, using publicly available data.


Methods: We estimated state-level year-on-year incidence rate ratios (IRRs) for Lyme disease for the period 1993 to 2007 using Poisson regression methods. We evaluated between-state heterogeneity in IRRs using a random-effects meta-analytic approach. We identified state-level characteristics associated with increasing incidence using random-effects meta-regression.


Results: The incidence of Lyme disease in the US increased by about 80 percent between 1993 and 2007 (IRR per year 1.049, 95 percent CI [confidence interval] 1.048 to 1.050). There was marked between-state heterogeneity in the average incidence of Lyme disease, ranging from 0.008 per 100 000 person-years in Colorado to 75 per 100 000 in Connecticut, and significant between-state heterogeneity in temporal trends (p less than 0.001). In multivariable meta-regression models, increasing incidence showed a linear association with state latitude and population density. These 2 factors explained 27 percent of the between-state variation in IRRs. No independent association was identified for other state-level characteristics.


Interpretation. Lyme disease incidence increased in the US as a whole during the study period, but the changes were not uniform. Marked increases were identified in northern-most states, whereas southern states experienced stable or declining rates of Lyme disease.


Lyme disease is caused by the bacterium _Borrelia burgdorferi_, and is transmitted to humans by the bite of infected _Ixodes_ ticks. In areas of the northeastern, mid-Atlantic and north-central United States where Lyme disease is endemic, the white-footed mouse is the principal reservoir for _B. burgdorferi_ and _I. scapularis_ is the tick vector. The western black-legged tick (_I. pacificus_) is the main vector on the west coast, where only 2-4 percent of _I. pacificus_ are reported to be infected with _B. burgdorferi_, whereas over 50 percent of _I. scapularis_ are infected in hyper-endemic areas in the northeastern states


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The content and opinions are neither pre-screened nor endorsed by the SASTM. The content should neither be interpreted nor quoted as inherently accurate or authoritative.

The information provided in SASTM Newsflashes is collected from various news sources, health agencies and government agencies. Although the information is believed to be accurate, any express or implied warranty as to its suitability for any purpose is categorically disclaimed. In particular, this information should not be construed to serve as medical advice for any individual. The health information provided is general in nature, and may not be appropriate for all persons. Medical advice may vary because of individual differences in such factors as health risks, current medical conditions and treatment, allergies, pregnancy and breast feeding, etc. In addition, global health risks are constantly evolving and changing. International travelers should consult a qualified physician for medical advice prior to departure.

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