Tick-borne infections will pose significant threats in the near future, according to infectious disease experts.
Tick, tick, tick.
For some, the phrase marks the passage of time. For public health specialists focused on vector-borne diseases, however, it portends the possibility of the crises that lay ahead. Indeed, although mosquito-borne diseases such as Zika virus and Dengue Fever have generated most of the headlines in recent years, it is believed that bacteria transmitted by ticks will pose the most significant challenges in the not-too-distant future.
“Tick-borne diseases warrant greater attention by clinicians, researchers, and the public alike,” Sarah M. Gunter, PhD, MPH, postdoctoral associate, Baylor College of Medicine and the National School of Tropical Medicine, Houston, told Contagion®. “The geographic distribution of certain species of ticks has been rapidly expanding over the last decade, introducing the potential for tick-borne disease transmission in areas where we have historically not seen it. This expansion of tick-borne disease transmission coupled with a lack of surveillance and poor diagnostics has caused many cases to be misdiagnosed and underreported.”
In a commentary published on August 23 by the New England Journal of Medicine (NEJM), the leadership of the National Institute of Allergy and Infectious Diseases (NIAID) noted that although incidence of Lyme disease—perhaps the most well-known of the illnesses associated with ticks—is on the rise in the United States, so too are tick-borne viral infections such as Powassan virus. Notably, only 20 cases of Powassan virus were reported in the United States prior to 2006; however, from 2006 through 2016, there were 99 confirmed cases of the virus, according to data published in the US Centers for Disease Control and Prevention’s (CDC) Morbidity and Mortality Weekly Report.
“The public health burden of tick-borne pathogens is considerably underestimated,” the authors of the NEJM commentary wrote. “For example, the CDC reports approximately 30,000 cases of Lyme disease per year, but estimates that the true incidence is 10 times that number. Multiple factors contribute to this discrepancy, including limitations in surveillance and reporting systems and constraints imposed by available diagnostics, which rely heavily on serologic assays.”
Meanwhile, Dr. Gunter and her colleagues at Baylor have identified 6 pediatric cases of confirmed or probable spotted fever group rickettsiosis in southeast Texas between 2008 and 2016. This is significant because the tick that carries spotted fever group rickettsiosis is not typically found in the region; in fact, only 1 of the 6 patients “had a history of travel to an endemic area.” Their findings were published on August 27 in the journal Zoonoses and Public Health. CDC estimates suggest incidence of spotted fever group rickettsiosis in the United States has increased from 2 cases per 1 million population in 2000, to 11 cases per 1 million population in 2014.
“Our research indicates that other tick-borne diseases like spotted fever group rickettsia are of growing concern,” Dr. Gunter said. “We have seen an alarming increase in cases of spotted fever group rickettsia in the United States, and… we are seeing transmission in urban areas where we don’t classically think of their being a risk for tick-borne disease transmission.”
Changing patterns of tick distribution across North America were further documented in a “citizen” science-based study published in the journal PLoS One on July 12. The authors of the study had originally budgeted to receive sample ticks submitted by 2400 people who suffered bites between January 2016 and August 2017. In the end, though, they received more than 16,000 submissions. The samples they received suggest that species such as the so-called lone star tick, Amblyomma americanum, indigenous to the eastern United States, are being found in the upper Midwest. In addition, the brown dog tick, Rhipicephalus sanguineous, typically found in warmer climates, are being found in new areas in the western United States.
These changes mean that new tick-borne pathogens such as Anaplasma phagocytophilum and Babesia microti are likely to become more common in affected areas, according to study co-author Nathan C. Nieto, PhD, associate professor, department of Biological Sciences, Northern Arizona University. The former is a gram-negative pathogen linked with human granulocytic anaplasmosis (HGA), while the latter can cause babesiosis, a relatively rare disease (at least in the United States) that is similar to malaria.
“A number of tick species seem to be moving northward, possibly as a result of climate change, including the Ixodes scapularis and the Ixodes pacificus, which are associated with Lyme disease, but also the Amblyomma americanum,” Dr. Nieto added.
Another commentary on Lyme disease published in the Journal of the American Medical Association (JAMA) on August 28 noted that Borrelia miyamotoi, an infection transmitted by the same tick species as Lyme disease, may also be a cause for concern, given that it is often misdiagnosed as Lyme (because the initial symptoms are similar). At present, there is no US Food and Drug Administration-approved test for the infection.
In general, “these diseases… warrant evidence-based and scientific approaches for both research and care of patients who have them,” the authors of the JAMA commentary told Contagion® in an email. “Babesiosis may be less well-appreciated by the public but [it] is of importance to the public’s health… [And], it can also be transmitted by blood product transfusion and from a pregnant woman to the fetus. Powassan virus is also a cause of serious illness requiring hospitalization but is fortunately still quite rare. Infection caused by the subtype of Powassan virus present in the deer tick, known as Deer Tick Virus, however, is increasing in frequency. More research on ways to prevent tick bites and control or reduce relevant tick populations is highly desirable, [and] vaccines that target tick bites per se, as well as vaccines for Lyme disease, are needed.”
And based on recent research, time is clearly of the essence. In other words: Tick, tick, tick.
Brian P. Dunleavy is a medical writer and editor based in New York. His work has appeared in numerous health care-related publications. He is the former editor of Infectious Disease Special Edition.