Ticks and tick-borne pathogens are increasingly emerging in nonendemic regions. Population growth and rising pet ownership rates have created more opportunities for tick exposure. This article includes major clinical concerns and the role of clinicians in reducing the tick burden and tick-borne diseases.
1. Ticks & Transmitted Pathogens Spreading to New Regions
Changes in the geographic range of major tick species associated with disease in dogs, cats, and humans in North America (Table) have been attributed to changes in climate, distribution of wildlife host species, and land use (especially reforestation).1 Since 1980, the range of Ixodes scapularis (deer tick, black-legged tick) has expanded in the northeastern United States and southeastern Canada, with increased recognition of granulocytic ehrlichiosis and Lyme borreliosis in these regions.2 Ranges of Rhipicephalus sanguineus (brown dog tick), Dermacentor variabilis (American dog tick), Amblyomma americanum (lone star tick), and Amblyomma maculatum (Gulf coast tick) have also been expanding in the United States and Canada.3-6 In contrast, there has been little change in the geographic distribution of Ixodes pacificus (western black-legged tick).7
2. Tick-Borne Pathogens Found in Unexpected Tick Species
Traditionally transmitted by Dermacentor spp ticks, Rh sanguineus–transmitted Rocky Mountain spotted fever (RMSF) caused by Rickettsia rickettsii has emerged as a significant public health concern in the southcentral United States and Mexico.8 In Tennessee, most tick species found in a hot spot for human RMSF-like disease were Amblyomma spp; these ticks contained the DNA of Rickettsia spp other than R rickettsii, suggesting that Amblyomma-transmitted Rickettsia spp may be capable of causing disease that resembles RMSF.9,10A americanum has also emerged as a vector of Francisella tularensis (the cause of tularemia), which is also traditionally transmitted by Dermacentor ticks.11
3. Foreign Tick Species & Tick-Transmitted Pathogens Appearing in New Regions
Human and animal travel (including movement of migratory birds) can result in introduction of foreign tick species and their associated pathogens or can accelerate transmission of existing pathogens in a region. Haemaphysalis longicornis (Asian longhorn tick) is native to east Asia but has spread to Australia, New Zealand, and the United States; can infest a variety of wildlife species, livestock, and companion animals; and has potential to transmit multiple pathogens (eg, Anaplasma phagocytophilum, R rickettsii).5 In the United States, H longicorniswas first recognized on a sheep in New Jersey in 2017,12 but evidence of wildlife infestation was seen in 2010.5 This tick is now widely distributed across the East Coast (New York to Georgia) and as far west as Oklahoma and poses a significant threat to livestock health.5,13 There is currently no evidence of disease transmission to humans or companion animals.
Movement of infected humans and animals can also introduce tick-borne pathogens that can be spread by established tick populations in nonendemic areas. In 2020, Ehrlichia canis (the cause of canine monocytic ehrlichiosis) was detected for the first time in dogs in northern Australia.14 Dogs have since been diagnosed with ehrlichiosis in multiple Australian states, with transmission by Rhipicephalus linnaei (tropical brown dog tick). Whether E canis was introduced through importation of an infected dog or infected ticks is unknown.14
4. Improvements Needed in Diagnostic Tests for Tick-Borne Pathogens
Widely used serologic assays that detect heartworm antigens as well as antibodies to Borrelia burgdorferi, Ehrlichia spp, and Anaplasma spp are useful for detecting exposure but are not effective for diagnosis. Many dogs with ehrlichiosis and anaplasmosis have a negative test result in the first week of illness due to the lag in antibody production; dogs with Lyme borreliosis always test positive because of the long incubation period that ensures sufficient time to mount an immune response before developing clinical signs. Positive results, however, lack diagnostic value because many healthy dogs in endemic regions have antibodies due to widespread subclinical infection, and clinical signs of Lyme borreliosis mimic those of other common diseases (eg, osteoarthritis, primary immune-mediated polyarthritis). For acutely ill dogs, blood PCR assays provide the most accurate diagnoses but can be expensive and take time to receive results. Inexpensive point-of-care organism detection tests (eg, in-clinic nucleic acid amplification tests) are needed.
Even with PCR, many infections likely are undetected because existing vector-borne PCR panels do not detect all species of some pathogens (eg, certain Babesia spp and Rickettsia spp). False negative results can also occur when testing for Rickettsia spp because these pathogens primarily infect endothelial cells. Diagnosis of rickettsiosis may ultimately require acute and convalescent indirect fluorescent antibody (IFA) serology, which is retrospective and only performed by specialized laboratories. These diagnostic challenges have led to overuse of doxycycline and long-acting third-generation cephalosporins, potentially resulting in adverse effects and contributing to antimicrobial resistance.
5. Resistance Evidence Highlights the Need for Improved Parasiticide Stewardship
Parasiticide resistance (ie, genetic pathways to resistance) has been identified in ticks (eg, Rh sanguineus), cat fleas (eg, Ctenocephalides felis), and endoparasites (specifically Ancylostoma caninum, Dirofilaria immitis, Trichuris vulpis, and Dipylidium caninum).15-23 Endoparasiticide resistance is a concern because ectoparasiticides are commonly combined with endoparasiticides for convenience. Growing prevalence of parasiticide resistance suggests that current Companion Animal Parasite Council recommendations for year-round administration of broad-spectrum parasite control with efficacy against heartworm, intestinal parasites, fleas, and ticks may require re-evaluation.24
Widespread parasiticide use in animals has also raised concerns about environmental contamination and impacts on nontarget invertebrates (especially honeybees, fish, and mayflies).25 Returning to more narrow-spectrum products and recommendations based on lifestyle, geography, and seasonality warrants consideration. Other approaches to prevention (eg, tick vaccines) have been slow to advance despite decades of research.26 Pet owner education about responsible parasiticide use is important. The British Small Animal Veterinary Association released a set of resources for pet owners (see Suggested Reading) that may also help North American clinicians more effectively communicate about parasiticide use.
Conclusion
Major concerns relating to ticks and tick-borne diseases include the appearance of ticks in new regions due to climate change and human and animal movement, diagnostic challenges, and concerns about resistance to and environmental impact of parasiticides. Continued surveillance, public education, and development of new diagnostics and approaches to prevention are needed. Clinicians can contribute to these efforts with thoughtful application of existing diagnostic tests, as well as owner education about tick-borne illnesses and lifestyle-based use of preventatives.