As the numbers of reported cases of tick-borne diseases (TBDs) continue to increase, so do the range of ticks that can carry these diseases, the amount of TBDs that have emerged, and the laboratory approaches to detect them, according to a panel of speakers in a March 21, 2017, webinar presented by the Centers for Disease Control and Prevention (CDC).
TBDs Are Expanding
According to Rebecca Eisen, PhD, from the CDC in Fort Collins, Colorado, despite the rise in infected individuals, just 3 ticks transmit most cases of reported TBDs in the United States: Ixodes scapularis
(blacklegged tick), Amblyomma americanum
(lone star tick), and Dermacentor variabilis
(American dog tick).
“In recent decades, the numbers of many notifiable TBDs have steadily increased,” she said. “For example, the average number of reported Lyme disease cases has roughly tripled from 1992 to 2015. Likewise, the number of reported cases of anaplasmosis, ehrlichioses, and spotted fever group rickettsioses has steadily increased from 2000 to 2015.”
The geographic distribution of several TBDs is also expanding. From 1996 to 2015, the number of counties in which Ixodes scapularis
was considered to be established more than doubled. Other factors also contribute to increases in the number of reported cases of TBDs and their distribution, she explained, including increasing human contact with ticks, landscape changes, a lack of effective prevention strategies, and improved diagnostics and clinical recognition.
No vaccines are available in the United States to prevent TBDs in humans. Current prevention strategies, such as personal protection measures, environmental modification, and tick suppression in animals, vary in efficacy and acceptability. “There is a need to develop effective approaches for preventing TBDs. But perhaps an even bigger challenge is how to deliver effective prevention strategies to large numbers of people and ultimately reduce the trend of increasing TBDs,” Dr. Eisen concluded.
Tick-Borne Spotted Fevers
The etiologic spectrum of tick-borne rickettsioses in the United States has expanded during the past 15 years, said Christopher D. Paddock, MD, from the CDC in Atlanta, Georgia. Rocky Mountain spotted fever (RMSF), caused by Rickettsia rickettsii
, was the first recognized TBD in humans in the United States. The organism has a tropism for vascular endothelial cells, and the disease progresses rapidly to involve the skin, producing the characteristic maculopapular rash. Although RMSF can cause death when it involves the brain and lungs, Dr. Paddock stressed that doxycycline therapy has reduced the fatality rate to about 5%.
Most cases are reported from the central and southeastern United States, where Dermacentor variabilis
is the main vector of the RMSF agent. However, Rhipicephalus sanguineus
, the brown dog tick, more recently emerged as an unexpected vector of RMSF, noted Dr. Paddock. Epidemic levels of RMSF emerged in several American Indian communities in eastern Arizona between 2003 and 2012, with incidence rates approaching 150 times the national average.1
Large populations of these ticks have since expanded among free-roaming dogs in these communities. Fortunately, however, a 2-year pilot integrated tick prevention program was initiated in 2012 in one community with high rates of RMSF. This involved acaricide spraying of yards and placement of tick collars on more than 1000 dogs, and significantly reduced the number of ticks in this area and the number of cases of RMSF. Whereas about 51% of dogs in the community had visible tick infestations at the start of the tick control initiative in April 2012, fewer than 6% of dogs had visible infestations at the end of the project in September 2013.
RMSF was considered the only tick-borne rickettsiosis in the United States for more than 100 years, until Rickettsia parkeri
emerged in 2004. This is transmitted by Amblyomma maculatum
, the Gulf Coast tick, and also causes a skin rash. However, the R. parkeri
rash is typically sparser than that in RMSF and may involve small vesicles or pustules. R. parkeri
infection also produces a distinctive eschar, a small focus of necrotic skin, at the site of the tick bite. Eschars occur only rarely in RMSF, said Dr Paddock, and while RMSF is life-threatening, R. parkeri
rickettsiosis is not.
The percentage of ticks infected with R. parkeri
is also greater than the percentage of ticks infected with R. rickettsii
of R. parkeri
, said Dr. Paddock. About 1 in every 2 to 4 Gulf Coast ticks have been shown to be infected with R. parkeri
. However, estimates suggest that fewer than 1 in 2000 ticks in the eastern United States harbor the bacteria, he added.