Tick-Borne Illnesses: Rising Incidence, Rising Concerns

Second only to mosquitoes as vectors of human pathogens worldwide, ticks are responsible for the vast majority of cases of vector-borne infectious diseases in the United States.¹ According to data from the National Notifiable Diseases Surveillance System of the Centers for Disease Control and Prevention, 491,671 cases of tick-borne diseases were reported during 2004 to 2016. Reports of tick-borne diseases more than doubled during this 13-year period, rising from 22,527 cases in 2004 to 48,610 cases in 2016. Lyme disease accounted for 402,502 cases.²

These figures likely represent a fraction of the actual amount of tick-borne disease in the United States. Estimates of incident cases of Lyme disease according to positive tests from large commercial laboratories³ and diagnoses recorded in administrative claims from a large database of insurance records⁴ suggest that Lyme disease alone infects approximately 300,000 individuals in the United States per year.² Even these figures may not accurately indicate the overall frequency of infection. People who are infected with Lyme disease do not consistently present for treatment, and healthcare providers frequently lack clinical suspicion for the disease because of its highly varied clinical manifestations.

Moreover, currently approved laboratory tests are insufficiently sensitive in the early phases of Lyme disease when the antibody response has not yet become detectable.^5,6 At least one model predicts that by 2050, Lyme disease will infect 55.7 million people in the United States, or 12% of the population.⁷

What’s Driving the Spike in Cases?

Numerous explanations have been proposed for the increased number of cases of tick-borne diseases in the last 2 decades, including improvements in diagnostic and surveillance tools, heightened awareness among patients and providers, increased participation in outdoor activities, and the encroachment of residential communities into formerly undeveloped natural areas. Climate change is a likely contributor to the increased risk for tick-borne illness. Warmer temperatures lengthen the survival and activity period of ticks, increase the range of both pathogen reservoirs and tick hosts such as the white-footed mouse and the white-tailed deer, and increase the potential for encounters with ticks over a longer season.^8,9

Alvaro Toledo, PhD, assistant professor in the department of entomology at Rutgers University and co-author with colleague Ilia Rochlin, PhD of a recent review on emerging tick-borne pathogens,¹⁰ told Infectious Disease Advisor that the geographical expansion of Ixodes scapularis, commonly known as the deer tick or blacklegged tick, represents the most important factor in the uptick in tick-borne disease. 

Impact on Public Health

“I. scapularis is the main vector for Lyme disease on the east coast and midwest where you see the majority of Lyme disease cases in the country,” Dr Toledo stated. “In addition, the deer tick is also the vector for human anaplasmosis, human babesiosis and Powassan virus. Therefore, the expansion of this tick species is intimately related to the increase on the number of cases. It is also important to note that this is also the case for other tick species such as Amblyomma americanum, the lone star tick, which is the vector for human ehrlichiosis.”

Anaplasmosis/ehrlichiosis (n=39,959), spotted fever rickettsiosis (n=37,376), babesiosis (n=9631), tularemia (n=2102), and Powassan virus (n=101) comprised 8%, 8%, 2%, 0.4%, and 0.02% of reported tick-borne cases, respectively. The figure for babesiosis may be misleading, because it did not become classified as a nationally notifiable condition until 2011.² Accounting for 82% (n=402,502) of the cumulative reported total of tick-borne disease, Lyme disease presents the greatest risk for the United States population in terms of case numbers but not in terms of severity.²

“Lyme disease is a debilitating disease that if not treated can produce long lasting health issues but it rarely causes death,” noted Dr Toledo. “On the other hand, the number of cases of Powassan virus per year range from a few to a dozen or two. Nonetheless, Powassan virus causes an encephalitis with a mortality rate between 10-15% and over 50% of patients that survive develop long-term neurological sequelae. It is also important to note that we have effective antibiotic treatments for tick-borne diseases caused by bacteria or protozoa but not for those causes by viruses like Powassan.”

Protection Against Tick-Borne Diseases

The increase in the rates of tick-borne disease has made prevention a matter of urgent necessity. LYMErix™, a vaccine against Lyme disease, was approved by the US Food and Drug Administration in 1998. Despite an efficacy rate of approximately 80%, it was withdrawn from the market in 2002.¹¹ Current strategies for limiting the transmission of tick-borne diseases center on environmentally based control measures, such as landscaping modification and the use of acaricide on private properties, and education about protective measures individuals can use to reduce their personal exposure to tick bites, such as performing tick checks or applying tick repellent.¹²

Some investigators have pointed out that the current public health approach to the prevention of tick-borne disease, with its primary emphasis on individual responsibility, stands in stark contrast to the large-scale mosquito control efforts that successfully eradicated malaria and other mosquito-borne pathogens in the United States by the mid-20th century.^10,13,14 Although federal funding for tick-borne diseases totaled >$100 million between 2006 and 2009, >95% was spent on microbiologic and clinical research. Advocates for patients with Lyme disease have likewise focused primarily on its clinical aspects, particularly the debates around the diagnosis and treatment of chronic Lyme disease.¹⁴

“It is clear that control of Lyme and other tick-borne diseases will require a paradigm shift emphasizing measures to reduce tick and host populations and a substantial research and development effort,” wrote Drs Rochlin and Toledo in their review. “At present, clinical assessment and treatment remain the most critical tools in reducing the burden of tick-borne diseases.”¹⁰

References

1. de la Fuente J, Estrada-Pena A, Venzal JM, Kocan KM, Sonenshine DE. Overview: ticks as vectors of pathogens that cause disease in humans and animals. Front Biosci J Virtual Libr. 2008;13:6938-6946.
2. Rosenberg R, Lindsey NP, Fischer M, et al. Vital signs: Trends in reported vectorborne disease cases — United States and territories, 2004–2016. Morb Mortal Wkly Rep. 2018;67(17):496-501.
3. Hinckley AF, Connally NP, Meek JI, et al. Lyme disease testing by large commercial laboratories in the United States. Clin Infect Dis. 2014;59(5):676-681.
4. Nelson CA, Saha S, Kugeler KJ, et al. Incidence of clinician-diagnosed Lyme disease, United States, 2005-2010. Emerg Infect Dis. 2015;21(9):1625-1631.
5. Moore A, Nelson C, Molins C, Mead P, Schriefer M. Current guidelines, common clinical pitfalls, and future directions for laboratory diagnosis of Lyme Disease, United States. Emerg Infect Dis. 2016;22(7):1169-1177.
6. Battaglia G. Improving diagnosis of Lyme disease: laboratory and clinical approaches. ContagionLive. Published November 10, 2017. Accessed August 16, 2020.
7. Davidsson M. The financial implications of a well-hidden and ignored chronic Lyme disease pandemic. Healthcare (Basel). 2018;6(1):16.
8. Bouchard C, Dibernardo A, Koffi J, Wood H, Leighton PA, Lindsay LR. Increased risk of tick-borne diseases with climate and environmental changes. Can Commun Dis Rep Releve Mal Transm Au Can. 2019;45(4):83-89.
9. Garber B, Glauser J. Tick-borne illness for emergency medicine providers. Curr Emerg Hosp Med Rep. 2019;7(3):74-82.
10. Rochlin I, Toledo A. Emerging tick-borne pathogens of public health importance: a mini-review. J Med Microbiol. 2020;69(6):781-791.
11. Nigrovic LE, Thompson KM. The Lyme vaccine: a cautionary tale. Epidemiol Infect. 2007;135(1):1-8.
12. Kilpatrick AM, Dobson ADM, Levi T, et al. Lyme disease ecology in a changing world: consensus, uncertainty and critical gaps for improving control. Philos Trans R Soc B Biol Sci. 2017;372(1722):20160117.
13. Eisen L. Stemming the rising tide of human-biting ticks and tickborne diseases, United States. Emerg Infect Dis. 2020;26(4):641-647.
14. Rochlin I, Ninivaggi DV, Benach JL. Malaria and Lyme disease – the largest vector-borne US epidemics in the last 100 years: success and failure of public health. BMC Public Health. 2019;19(1):804.