Peer Reviewed

Parasitology

Diagnostic Testing for Tick-Borne Diseases: Recommendations and Interpretation of Results

Treatment is not always required based on the results of point-of-care tests, and clinical judgment needs to be used in their interpretation.

December 9, 2024 |

Issue: January/February 2025

Noppadon stocker/shutterstock

Abstract

Tick-borne diseases, including those caused by members of the genera Anaplasma, Borrelia, Ehrlichia, and Rickettsia, pose significant risks to pets. Left undetected, infections with these pathogens may result in severe disease necessitating intense treatment or hospitalization. Many diagnostic modalities are available to aid in pathogen detection, but interpretation of their results can be complex. This article describes key strategies for performing tick-borne disease diagnostic tests and interpreting the results.

Take-Home Points

Tick ranges are expanding, and the prevalence of tick-borne diseases is increasing in many areas as tick encounters become more common for people and pets.
When evaluating patients for suspected tick-borne disease, it is important to consider the vector capacity, seasonality, and geography of the many tick species found in the United States.
Multiple testing modalities, including point-of-care serologic testing and reference laboratory testing options, may be used to detect tick-borne diseases in symptomatic or asymptomatic patients. Each modality has advantages and limitations.
No individual point-of-care test is effective at confirming infection, and, in many cases, correlation of results with a patient’s history, clinical signs, and additional testing should be included in diagnostic interpretations before treatment is pursued.

In recent decades, tick-borne diseases in companion animals and humans have emerged as a growing concern throughout North America. After mosquitoes, ticks are considered the most important vectors of pathogens worldwide, including many important bacterial, protozoal, and viral agents.¹ In the United States, 95% of the vector-borne diseases reported annually are vectored by ticks, with dogs acting as sentinels for potential human exposure.²

As the range and prevalence of vector tick species continue to expand—aided by climate change and host movement—the need for quick, accurately interpreted diagnostic testing has increased.³ Many diagnostic tools are available to veterinary clinics in the United States; however, given the complicated nature of tick-borne disease epidemiology and vector species phenology, interpretation of easily accessible diagnostic tests is not without challenges. It is thus important to recognize that no single diagnostic test alone is sufficient to diagnose tick-borne disease. This article aims to provide clinically applicable advice regarding testing procedures and interpretations of common benchtop pathogen tests for tick-borne illness.

Vectors and Associated Pathogens

Many ticks of veterinary and medical importance are frequently found parasitizing domestic dogs and cats in the United States, including species of the genera Amblyomma, Dermacentor, Ixodes, and Rhipicephalus.⁴ Pathogens of concern have some degree of specificity to vector species, which are in turn associated with preferential host species and environmental conditions, and pathogen–vector–host dynamics are responsible for maintenance of diseases in wildlife reservoirs. Seasonality and geography of vector species are important factors to consider when assessing a patient’s risk of tick-borne disease, and species identification of ticks can be an important guide for diagnostic differentials and plans.

Tick-borne pathogens include numerous species of bacteria, protozoa, and viral agents. BOX 1 features a selection of commonly tested pathogens of veterinary importance and their vectors in North America.⁵

BOX 1 Common Tick Species in the United States and Select Vectored Pathogens

Pathogen Transmission Times

Experimental evidence regarding the transmission time for various pathogens is convoluted and often difficult to accurately determine; as an example, transmission times for Borrelia burgdorferi have been reported to be between 17 and 72 hours.^6-9 Differences in transmission time vary according to pathogen, tick species and life stage, environment, and type of host. For example, viruses often require no incubation period within the tick and are transmitted almost instantly with the feeding process, whereas many bacterial species rely on chemical mechanisms associated with tick saliva and host blood during feeding to facilitate transmission into the host.¹⁰ It has been documented that interrupted feeding by ticks can hasten the transmission rate for some pathogens.¹¹

When to Test

Surveillance and testing for vector-borne diseases should be year-round.¹² In addition to routine testing of patients that are at risk based on their geographic location, travel history, or lifestyle, diagnostic testing should be pursued in patients that are symptomatic. Positive serologic results may have different implications for sick patients than for asymptomatic patients, and additional diagnostic modalities may be recommended.

Diagnostic Testing Modalities

Point-of-Care Testing

The accessibility of point-of-care tests (POCTs) continues to expand, with multiple tests being commercially available.^13,14 These tests often require a small amount of blood or serum and provide quick results. TABLE 1 describes commercially available in-clinic tests, all of which are labeled for use in dogs.

Except for Dirofilaria immitis antigen tests, most serologic POCTs for vector-borne pathogens detect antibodies, which often indicates previous exposure rather than clinical infection (FIGURE 1 AND BOX 2). Correct interpretation according to history and clinical signs is crucial and can help discern various stages of infection.¹⁵

BOX 2 Case Example: Positive Ehrlichia Result in a Healthy Dog

A 5-year-old male castrated German shorthaired pointer presents to a veterinary clinic in central Oklahoma for an annual wellness visit. The dog has a history of hunting with the owner and spends a lot of time roaming wooded areas during hunting season. The owner reports no concerns about the patient’s health.

Physical examination and routine blood testing (complete blood count and serum biochemistry testing) are within normal limits. As part of routine wellness care, a point-of-care test for detection of heartworm antigen and tick-borne disease antibodies is performed. Results indicate Ehrlichia species antibody detection.

Using the algorithm in FIGURE 1, what is a reasonable course of action for this patient? Based on the lack of clinical abnormalities but a history of access to tick habitats in a region where vectors for Ehrlichia species could be present, previous exposure to Ehrlichia species can be suspected. However, the patient is clinically healthy and no treatment is currently warranted.

Because antibody tests alone cannot differentiate between exposure and active disease, additional or ancillary testing to confirm the diagnosis is often required before initiating treatment based on a positive result (BOX 3). Additionally, acute infections may have negative serologic testing results, as the delay between infection and seroconversion can be up to 3 weeks.¹⁵ Some of the commercially available POCTs for vector-borne pathogens provide diagnostic algorithms to assist veterinarians with effective diagnostic interpretation and management plans.

BOX 3 Case Example: Negative Antibody Results in a Symptomatic Dog

A 1-year-old female Labrador retriever presents to a veterinary clinic in Chester, Pennsylvania, due to a 4-day history of lethargy, inappetence, and lameness. The dog is on a heartworm, flea, and tick monthly preventive, but the owner mentions that he sometimes forgets to give the preventive on time.

Physical examination reveals a weight of 31 kg (68 lb), temperature of 40 °C (103.4 °F), heart rate of 140 beats/min, respiratory rate of 30 breaths/min, and a capillary refill time of 2 seconds. No integument abnormalities are noted, but the patient is painful on palpation of hips and elbows. Laboratory testing reveals complete blood count numbers within reference ranges except for a platelet count of 67 000 cells/µL. Serum biochemistry analysis reveals mild hypokalemia and hypoalbuminemia. A point-of-care test to detect tick-borne pathogens is performed, but no antibodies are detected.

Using the algorithm in FIGURE 1, what is a reasonable course of action for this patient? Although no antibodies were detected for tick-borne pathogens, this patient’s history and clinical findings indicate the possibility of an early tick-borne infection. Treatment with doxycycline can be initiated and a whole blood sample submitted for a tick-borne pathogens polymerase chain reaction panel.

Reference Laboratory Testing

Diagnostic reference laboratories can perform additional serologic or molecular testing for tick-borne diseases. Sample submission to referral laboratories is recommended when POCT results need to be confirmed or when conflicting results have been obtained in-clinic.

Compared with POCTs, the advantage of reference laboratory testing is the use of diagnostic methods with increased specificity and sensitivity, such as:

Molecular testing, including polymerase chain reaction (PCR) testing, quantitative PCR testing, and next-generation sequencing (can be useful to confirm an active infection detected by an immunochromatographic lateral flow test previously performed in-clinic)
Microimmunofluorescence assay, which can detect immunoglobulin M and immunoglobulin G antibodies and evaluate seroconversion using acute and convalescent serum samples
Enzyme-linked immunosorbent assay (ELISA)
Immunofluorescent assay, which can detect antigens or antibodies

Waiting time and additional cost may be disadvantages.

Test Results and Interpretation

When antibody tests for tick-borne diseases are used, results should be interpreted in light of the patient’s clinical signs; preventive use; history of tick exposure; geographic location; travel history; potential for coinfection; and, in some instances, response to antibiotic treatment, as well as vector phenology and transmission capacity. FIGURE 1 can be used as an aid in interpreting POCT results when infection with Anaplasma species or Ehrlichia species is suspected.

Molecular testing varies in specificity and sensitivity according to target, host tissue, and test. Detection of pathogen DNA by molecular testing methods does not necessarily indicate active infection; therefore, clinicians should always make treatment decisions according to clinical presentation. Cytauxzoon felis and Babesia species are examples of tick-borne pathogens that can cause chronic infection without clinical signs but can be detected by molecular testing (TABLE 2).

Detection of Lyme borreliosis can be difficult, and there is no test that can absolutely confirm it. Acute and convalescent serum samples are not needed for Lyme disease detection in dogs due to development of clinical signs before seroconversion in most cases. However, when additional tick-borne diseases are suspected and, often, are the cause of clinical signs, paired serologic testing can be helpful in detecting coinfections. Detection of the antibody to C₆ peptide can be helpful in detecting antibodies earlier than whole-cell ELISA, as it is more specific and does not detect dogs vaccinated with OspA (outer surface protein A; TABLE 3). Nevertheless, correlation with clinical signs should still be considered before treatment.

SUMMARY

The continuous expansion of ticks and tick-borne diseases continues to drive the development of new and improved POCTs. However, it is important to understand that treatment is not always required based on the results of these tests, and clinical judgment needs to be used in their interpretation. Additional testing, such as serum biochemistry testing, urinalysis, or diagnostic laboratory testing, can be helpful when conflicting results are observed.