Efficacy of unregulated minimum risk tick repellent products evaluated with Ixodes scapularis nymphs in a human skin bioassay.

BACKGROUND: The majority of vector-borne disease cases in the USA are caused by pathogens spread by ticks, most commonly the blacklegged tick, Ixodes scapularis. Personal protection against tick bites, including use of repellents, is the primary defense against tick-borne diseases. Tick repellents registered by the Environmental Protection Agency (EPA) are well documented to be safe as well as effective against ticks. Another group of tick repellent products, 25(b) exempt or minimum risk products, use alternative, mostly botanically derived, active ingredients. These are considered to pose minimal risk to human health and therefore are exempt from EPA registration; efficacy testing is not mandated for these products. METHODS: We used a finger bioassay to evaluate the repellency against I. scapularis nymphs for 11 formulated 25(b) exempt products together with two positive control DEET-based EPA registered products. Repellency was assessed hourly from 0.5 to 6.5 h after product application. RESULTS: The DEET-based products showed ≥ 97% repellency for all examined timepoints. By contrast, an average of 63% of ticks were repelled in the first 1.5 h after application across the 11 25(b) exempt products, and the average fell to 3% repelled between 2.5 and 6.5 h. Ten of the 11 25(b) exempt products showed statistically similar efficacy to DEET-based products at 30 min after application (repellency of 79-97%). However, only four 25(b) exempt products maintained a level of repellency similar to DEET-based products (> 72%) at the 1.5-h mark, and none of these products were effective in repelling ticks at the timepoints from 2.5 to 6.5 h after application. CONCLUSIONS: Neither the claims on the labels nor specific active ingredients and their concentrations appeared to predict the duration of efficacy we observed for the 25(b) exempt products. These products are not registered with the EPA, so the methods used to determine the application guidelines on their labels are unclear. Consumers should be aware that both the level of efficacy and the duration of repellency may differ among unregulated 25(b) exempt repellent products labeled for use against ticks. We encourage more research on these products and the 25(b) exempt active ingredients they contain to help determine and improve their efficacy as repellents under different conditions.

Comparison of in vitro and in vivo repellency bioassay methods for Ixodes scapularis nymphs.

BACKGROUND: Numerous bioassay methods have been used to test the efficacy of repellents for ticks, but the comparability of results across different methods has only been evaluated in a single study. Of particular interest are comparisons between bioassays that use artificial containers (in vitro) with those conducted on a human subject (in vivo) for efficacy testing of new potential unregistered active ingredients, which most commonly use in vitro methods. METHODS: We compared four different bioassay methods and evaluated three ingredients (DEET [N,N-Diethyl-meta-toluamide], peppermint oil and rosemary oil) and a negative control (ethanol) over a 6-h period. Two of the methods tested were in vivo bioassay methods in which the active ingredient was applied to human skin (finger and forearm bioassays), and the other two methods were in vitro methods using artificial containers (jar and petri dish bioassays). All four bioassays were conducted using Ixodes scapularis nymphs. We compared the results using nymphs from two different tick colonies that were derived from I. scapularis collected in the US states of Connecticut and Rhode Island (northern origin) and Oklahoma (southern origin), expecting that ticks of different origin would display differences in host-seeking behavior. RESULTS: The results between bioassay methods did not differ significantly, even when comparing those that provide the stimulus of human skin with those that do not. We also found that tick colony source can impact the outcome of repellency bioassays due to differences in movement speed; behavioral differences were incorporated into the assay screening. DEET effectively repelled nymphs for the full 6-h duration of the study. Peppermint oil showed a similar repellent efficacy to DEET during the first hour, but it decreased sharply afterwards. Rosemary oil did not effectively repel nymphs across any of the time points. CONCLUSIONS: The repellency results did not differ significantly between the four bioassay methods tested. The results also highlight the need to consider the geographic origin of ticks used in repellency bioassays in addition to species and life stage. Finally, our results indicate a limited repellent efficacy of the two essential oils tested, which highlights the need for further studies on the duration of repellency for similar botanically derived active ingredients and for evaluation of formulated products.

Deer management generally reduces densities of nymphal Ixodes scapularis, but not prevalence of infection with Borrelia burgdorferi sensu stricto.

Human Lyme disease-primarily caused by the bacterium Borrelia burgdorferi sensu stricto (s.s.) in North America-is the most common vector-borne disease in the United States. Research on risk mitigation strategies during the last three decades has emphasized methods to reduce densities of the primary vector in eastern North America, the blacklegged tick (Ixodes scapularis). Controlling white-tailed deer populations has been considered a potential method for reducing tick densities, as white-tailed deer are important hosts for blacklegged tick reproduction. However, the feasibility and efficacy of white-tailed deer management to impact acarological risk of encountering infected ticks (namely, density of host-seeking infected nymphs; DIN) is unclear. We investigated the effect of white-tailed deer density and management on the density of host-seeking nymphs and B. burgdorferi s.s. infection prevalence using surveillance data from eight national parks and park regions in the eastern United States from 2014-2022. We found that deer density was significantly positively correlated with the density of nymphs (nymph density increased by 49% with a 1 standard deviation increase in deer density) but was not strongly correlated with the prevalence of B. burgdorferi s.s. infection in nymphal ticks. Further, while white-tailed deer reduction efforts were followed by a decrease in the density of I. scapularis nymphs in parks, deer removal had variable effects on B. burgdorferi s.s. infection prevalence, with some parks experiencing slight declines and others slight increases in prevalence. Our findings suggest that managing white-tailed deer densities alone may not be effective in reducing DIN in all situations but may be a useful tool when implemented in integrated management regimes.

Identifying suitable habitat for Ixodes scapularis (Acari: Ixodidae) infected with Anaplasma phagocytophilum (Rickettsiales: Anaplasmataceae), Babesia microti (Piroplasmida: Babesiidae), and Borrelia miyamotoi (Spirochaetales: Spirochaetaceae) to guide surveillance efforts in the eastern United States.

Understanding the distribution of infected ticks is informative for the estimation of risk for tickborne diseases. The blacklegged tick, Ixodes scapularis (Acari: Ixodidae), is the primary vector for 7 medically significant pathogens in United States. However, knowledge of the ranges of these pathogens in host-seeking ticks is incomplete, particularly for those occurring at low prevalence. To aid in prioritizing costly field sampling efforts, we estimated ranges of suitable habitat for Anaplasma phagocytophilum, Babesia microti, and Borrelia miyamotoi in the eastern United States based on existing county-level surveillance records. The resulting suitability maps were compared against those developed previously for Bo. burgdorferi s.s., which shares similar ecology but has been detected in a greater number of counties. The overall accuracy of the habitat suitability models was high (AUC ≥ 0.92) for all 4 pathogens. The most important predictors were related to temperature and moisture. The upper midwestern and northeastern states were predicted to be highly suitable for all 4 pathogens. Based on our models, we prioritized sampling in 431, 275, and 539 counties currently lacking pathogen records that our models classified as suitable for A. phagocytophilum, Ba. microti, and Bo. miyamotoi, respectively. As a second-tier priority, we identified 311 (A. phagocytophilum), 590 (Ba. microti), and 252 (Bo. miyamotoi) counties, based on high suitability scores for Bo. burgdorferi. Our models can be used to improve cost-effectiveness of field sampling efforts aimed at improving accuracy and completeness of pathogen distribution maps.

A serological assay to detect and differentiate rodent exposure to soft tick and hard tick relapsing fever infections in the United States.

Human cases of relapsing fever (RF) in North America are caused primarily by Borrelia hermsii and Borrelia turicatae, which are spread by argasid (soft) ticks, and by Borrelia miyamotoi, which is transmitted by ixodid (hard) ticks. In some regions of the United States, the ranges of the hard and soft tick RF species are known to overlap; in many areas, recorded ranges of RF spirochetes overlap with Lyme disease (LD) group Borrelia spirochetes. Identification of RF clusters or cases detected in unusual geographic localities might prompt public health agencies to investigate environmental exposures, enabling prevention of additional cases through locally targeted mitigation. However, exposure risks and mitigation strategies differ among hard and soft tick RF, prompting a need for additional diagnostic strategies that differentiate hard tick from soft tick RF. We evaluated the ability of new and previously described recombinant antigens in serological assays to differentiate among prior exposures in mice to LD, soft or hard tick RF spirochetes. We extracted whole-cell protein lysates from RF Borrelia cultures and synthesized six recombinant RF antigens (Borrelia immunogenic protein A (BipA) derived from four species of RF Borrelia, glycerophosphodiester phosphodiesterase (GlpQ), and Borrelia miyamotoi membrane antigen A (BmaA)) to detect reactivity in laboratory derived (Peromyscus sp. and Mus sp.) mouse serum infected with RF and LD Borrelia species. Among 44 Borrelia exposed mouse samples tested, all five mice exposed to LD spirochetes were correctly differentiated from the 39 mice exposed to RF Borrelia using the recombinant targets. Of the 39 mice exposed to RF spirochetes, 28 were accurately categorized to species of exposure (71%). Segregation among soft tick RF species (Borrelia hermsii, Borrelia parkeri and Borrelia turicatae) was inadequate (58%) owing to observed cross-reactivity among recombinant BipA protein targets. However, among the 28 samples accurately separated to species, all were accurately assigned to soft tick or hard tick RF type. Although not adequately specific to accurately categorize exposure to soft tick RF species, the recombinant BipA protein targets from soft and hard tick RF species show utility in accurately discriminating mouse exposures to LD or RF Borrelia, and accurately segregate hard tick from soft tick RF Borrelia exposure.

Experimental Demonstration of Reservoir Competence of the White-Footed Mouse, Peromyscus leucopus (Rodentia: Cricetidae), for the Lyme Disease Spirochete, Borrelia mayonii (Spirochaetales: Spirochaetaceae).

The white-footed mouse, Peromyscus leucopus (Rafinesque), is a reservoir for the Lyme disease spirochete Borrelia burgdorferi sensu stricto in the eastern half of the United States, where the blacklegged tick, Ixodes scapularis Say (Acari: Ixodidae), is the primary vector. In the Midwest, an additional Lyme disease spirochete, Borrelia mayonii, was recorded from naturally infected I. scapularis and P. leucopus. However, an experimental demonstration of reservoir competence was lacking for a natural tick host. We therefore experimentally infected P. leucopus with B. mayonii via I. scapularis nymphal bites and then fed uninfected larvae on the mice to demonstrate spirochete acquisition and passage to resulting nymphs. Of 23 mice fed on by B. mayonii-infected nymphs, 21 (91%) developed active infections. The infection prevalence for nymphs fed as larvae on these infected mice 4 wk post-infection ranged from 56 to 98%, and the overall infection prevalence for 842 nymphs across all 21 P. leucopus was 75% (95% confidence interval, 72-77%). To assess duration of infectivity, 10 of the P. leucopus were reinfested with uninfected larval ticks 12 wk after the mice were infected. The overall infection prevalence for 480 nymphs across all 10 P. leucopus at the 12-wk time point was 26% (95% confidence interval, 23-31%), when compared with 76% (95% confidence interval, 71-79%) for 474 nymphs from the same subset of 10 mice at the 4-wk time point. We conclude that P. leucopus is susceptible to infection with B. mayonii via bite by I. scapularis nymphs and an efficient reservoir for this Lyme disease spirochete.

Failure of the Asian longhorned tick, Haemaphysalis longicornis, to serve as an experimental vector of the Lyme disease spirochete, Borrelia burgdorferi sensu stricto.

The invasive, human-biting Asian longhorned tick, Haemaphysalis longicornis, was detected in New Jersey in the eastern United States in August of 2017 and by November of 2018 this tick had been recorded from 45 counties across 9 states, primarily along the Eastern Seaboard. The establishment of H. longicornis in the United States has raised the questions of how commonly it will bite humans and which native pathogens may naturally infect this tick. There also is a need for experimental vector competence studies with native pathogens to determine if H. longicornis can acquire a given pathogen while feeding, pass it transstadially, and then transmit the pathogen in the next life stage. In this experimental study, we evaluated the vector competence of a population of H. longicornis originating from the United States (New York) for a native isolate (B31) of the Lyme disease spirochete, Borrelia burgdorferi sensu stricto (s.s.). In agreement with a previous experimental study on the vector competence of H. longicornis for Borrelia garinii, we found that uninfected H. longicornis larvae could acquire B. burgdorferi s.s. while feeding on infected Mus musculus mice (infection prevalence >50% in freshly fed larvae) but that the infection was lost during the molt to the nymphal stage. None of 520 tested molted nymphs were found to be infected, indicating that transstadial passage of B. burgdorferi s.s. is absent or rare in H. longicornis; and based on the potential error associated with the number of nymphs testing negative in this study, we estimate that the upper 95% limit for infection prevalence was 0.73%. An Ixodes scapularis process control showed both effective acquisition of B. burgdorferi s.s. from infected mice by uninfected larvae and transstadial passage to the nymphal stage (infection prevalence of 80-82% for both freshly fed larvae and molted nymphs). We also observed that although H. longicornis larvae could be compelled to feed on mice by placing the ticks within feeding capsules, attachment and feeding success was minimal (<0.5%) when larvae were placed freely on the fur of the mice. We conclude that H. longicornis is unlikely to contribute more than minimally, if at all, to transmission of Lyme disease spirochetes in the United States.