In its latest report on Lyme disease patterns, the US Centers for Disease Control and Prevention (CDC) said that illness levels are stable or slightly lower in areas that have typically reported the most cases—the Northeast, mid Atlantic, and upper Midwest—but activity is increasing in neighboring states.
The report, published today Morbidity and Mortality Weekly Report (MMWR),also reveals some demographic differences among confirmed and probable cases.
Cases confirmed in 48 states
Based on data from 2008 through 2015, the report covers 275,589 confirmed and probable cases. The 208,834 confirmed cases were reported in 48 states, and 41 states reported at least one probable case. The CDC noted that the goal of Lyme surveillance system isn't to record every case, but rather to show trends over time.
The number of Lyme cases fluctuated from a low of 30,158 in 2010 to a high of 38,468 in 2009. However, the CDC said the number of reported cases each year stayed above 30,000, making Lyme disease the most commonly reported vector-borne disease in the country.
Of the 14 states classified as high-incidence areas, 7 saw an overall decreasing trend. Of 11 states classified as neighboring areas, however, 8 showed an overall increasing trend in the number of confirmed cases.
In its analysis, the CDC said the decline in high-burden states might mean that disease incidence has stabilized or could be an artifact from changes in case verification practices to minimize the demands of conducting Lyme disease surveillance.
Higher numbers from neighboring states is consistent with a documented increase in the number of counties with established populations of Ixodes scapularis (called deer ticks or black-legged ticks).
Illness and epi findings
For all years, illness-onset peaked in the first week of July. Erythema migrans (a typically bullseye-type rash) was the most common clinical signs in people who were sick from April to November, but in those whose symptoms began during the coldest months, from December to March, arthritis was the most frequently reported symptom.
That the hallmark rash is reported for two thirds of the year is a reminder that adult ticks seek hosts during the fall months and that prevention messages should not focus just on spring and summer when nymphal ticks are feeding, the CDC said.
Though no major demographic or clinical changes were noted, the CDC researchers did see some differences between confirmed and probable cases compared with earlier reports For example, confirmed cases in high-incidence and neighboring states commonly involved male patients and young children, while those in low- incidence states were more likely to be female or a higher age.
Not surprisingly, many of the confirmed cases in low-incidence states probably reflect travel to states where the disease is more common, the CDC said.
Nov 10 MMWR report