History of the geographic distribution of the western blacklegged tick, Ixodes pacificus, in the United States

Ixodes pacificus (the western blacklegged tick) occurs in the far western United States (US), where it commonly bites humans. This tick was not considered a species of medical concern until it was implicated in the 1980s as a vector of Lyme disease spirochetes. Later, it was discovered to also be the primary vector to humans in the far western US of agents causing anaplasmosis and hard tick relapsing fever. The core distribution of I. pacificus in the US includes California, western Oregon, and western Washington, with outlier populations reported in Utah and Arizona. In this review, we provide a history of the documented occurrence of I. pacificus in the US from the 1890s to present, and discuss associations of its geographic range with landscape, hosts, and climate. In contrast to Ixodes scapularis (the blacklegged tick) in the eastern US, there is no evidence for a dramatic change in the geographic distribution of I. pacificus over the last half-century. Field surveys in the 1930s and 1940s documented I. pacificus along the Pacific Coast from southern California to northern Washington, in the Sierra Nevada foothills, and in western Utah. County level collection records often included both immatures and adults of I. pacificus, recovered by drag sampling or from humans, domestic animals, and wildlife. The estimated geographic distribution presented for I. pacificus in 1945 by Bishopp and Trembley is similar to that presented in 2022 by the Centers for Disease Control and Prevention. There is no clear evidence of range expansion for I. pacificus, separate from tick records in new areas that could have resulted from newly initiated or intensified surveillance efforts. Moreover, there is no evidence from long-term studies that the density of questing I. pacificus ticks has increased over time in specific areas. It therefore is not surprising that the incidence of Lyme disease has remained stable in the Pacific Coast states from the early 1990s, when it became a notifiable condition, to present. We note that deforestation and deer depredation were less severe in the far western US during the 1800s and early 1900s compared to the eastern US. This likely contributed to I. pacificus maintaining stable, widespread populations across its geographic range in the far western US in the early 1900s, while I. scapularis during the same time period appears to have been restricted to a small number of geographically isolated refugia sites within its present range in the eastern US. The impact that a warming climate may have had on the geographic distribution and local abundance of I. pacificus in recent decades remains unclear.

It ain’t easy being orange: lizard colour morphs occupying highly vegetated microhabitats suffer greater ectoparasitism

Intraspecific colour morphs usually differ in more traits than just colour. These traits can manifest as differences in morph physiology, behaviour, and ecology. Ecological differences among colour morphs, such as the degree of parasitism, can influence the evolution, maintenance, and loss of morphs from populations. High ectoparasite load can directly and deleteriously impact host fitness, and thus could influence colour morph persistence in populations if certain morphs are more frequently exposed to parasites or are more susceptible to parasitism. The Aegean wall lizard, Podarcis erhardii, is a colour polymorphic island lizard that is parasitized externally by ticks and mites. These ectoparasites can affect aspects of host lizard behaviour and physiology – including thermoregulation and body mass – and therefore are an important factor influencing the ecology and fitness of P. erhardii. We find that among sympatric colour morphs, ectoparasite loads differ; namely, monochromatic orange morphs have the highest numbers of ectoparasites, and in general, morphs with orange alleles (orange, orange-white, and orange-yellow) are more heavily parasitized by ticks and mites than the other morphs. Our results indicate that morphs with orange alleles tend to occupy microhabitats with significantly more vegetation cover and thus may increase their exposure to ticks and mites. Ecological differences between morphs could be an important factor contributing to demonstrated patterns of orange morph rarity and loss in P. erhardii populations.

Sex-biased parasitism and expression of a sexual signal

Given that sexual signals are often expressed more highly in one sex than the other, they can impose a sex-specific cost of reproduction through parasitism. The two primary paradigms regarding the relationship of parasites to sexual signals are the good genes hypothesis and the immunocompetence handicap hypothesis; however, there are other ecological, morphological and energetic factors that might influence parasite infections in a sex-specific fashion. We tested the relationship between expression of a sexual signal (the dewlap) and ecological, morphological and energetic factors mediating ectoparasite (mite) load between male and female Panamanian slender anoles (Anolis apletophallus). We found that males were more highly parasitized than females because of the preponderance of ectoparasites on the larger dewlap of males. Indeed, ectoparasite infection increased with both body size and dewlap size in males but not in females, and parasite infection was related to energy storage in a sex-specific fashion for the fat bodies, liver and gonads. Our work and previous work on testosterone in anoles suggests that this pattern did not arise solely from immunosuppression by testosterone, but that mites prefer the dewlap as an attachment site. Thus, the expression of this sexual signal could incur a fitness cost that might structure life-history trade-offs.

Body size, not age, predicts parasite load in Clark’s Spiny Lizards (Sceloporus clarkii)

Determining the factors that influence parasite load is a fundamental goal of parasitology. Body size often influences parasite load in reptiles, but it is unclear whether higher levels of parasitism are a result of greater surface area of individuals (a function of size) or of longer periods of exposure to parasites (a function of age). Using skeletochronology in a wild population of Clark’s Spiny Lizards (Sceloporus clarkii Baird and Girard, 1852), we tested the hypotheses that (i) larger individuals have higher parasite loads due to increased surface area available for colonization by parasites and their vectors and that (ii) older individuals have higher parasite loads because they have had longer exposure to parasites and their vectors. Males harboured more ectoparasites than females. Males and females differed in how body size influenced chigger (Acari: Trombiculidae) load; larger males harboured more chiggers than smaller males, but this was not the case in females. Age did not affect ectoparasite load in either sex. These results emphasize the importance of disentangling the effects of size and age in models of parasitism to gain a clearer understanding of intraspecific variation in parasite load.

Seasonal variation of bat-flies (Diptera: Streblidae) in four bat species from a tropical dry forest

Seasonality of climate promotes differences in abundance and species composition of parasites, affecting host-parasite interactions. Studies have reported seasonal variation in bat-flies, which are obligate bat ectoparasites. We characterized the bat-fly load of three insectivores [Pteronotus davyi (Gray), Pteronotus parnellii (Gray) and Pteronotus personatus (Wagner)] and one nectarivorous [Leptonycteris yerbabuenae (Martínez and Villa-R.)] bat species in a tropical dry forest to test the existence of seasonality in response to the availability of resources during the wet and dry seasons. We collected 3710 bat-fly specimens belonging to six species and two genera from 497 bats. Most of the ectoparasite load parameters examined (mean abundance, mean intensity, richness, etc.), including comparisons among reproductive conditions and sex of the host, were similar in both seasons. Prevalence was the parameter that varied the most between seasons. The six bat-fly species were found in all bat species except P. personatus. The latter species and L. yerbabuenae had four and five bat-fly species in the wet and dry seasons, respectively. This study provides significant information of ectoparasites ecology in relation to seasonality, contributes to the understanding of host-parasite relationships in tropical dry forests and discusses the relevance of the abiotic and biotic factors that could impact host-parasite interactions.

The effects of seasonality on host-bat fly ecological networks in a temperate mountain cave

Changes in the specialization of parasite-host interactions will be influenced by variations in host species composition. We evaluated this hypothesis by comparing the composition of bats and bat flies within a roost cave over one annual. Five bat and five bat fly species occupied the cave over the course of the study. Bat species composition was 40% different in the rainy season compared with the dry-cold and dry-warm seasons. Despite the incorporation of three new bat species into the cave during the rainy season, bat fly species composition was not affected by seasonality, since the bats that arrived in the rainy season only contributed one new bat fly species at a low prevalence. Bat-bat fly ecological networks were less specialized in the rainy season compared with the dry-cold and dry-warm seasons because of the increase of host overlap among bat fly species during this season. This study suggests that seasonality promote: (1) differences in host species composition, and (2) a reduction in the specialization of host-parasite ecological networks.

Sylvatic Infestation of Oklahoma Reptiles with Immature Ixodes scapularis (Acari: Ixodidae)

Reptiles were collected in nine counties in Oklahoma from September 2002 to May 2004 and examined for Ixodes scapularis (Say) larvae and nymphs to determine seasonal incidence and prevalence of these ticks. In total, 209 reptile specimens consisting of nine species of lizards and seven species of snakes were collected. Plestiodon fasciatus (L.) was the most numerous species collected (55%) followed by Sceloporus undulatus (Latreille) (17%) and Scincella lateralis (Say) (11%). Less than 10 individuals were collected for all remaining reptile species. The infestation prevalence of I. scapularis on all reptile specimens collected was 14% for larvae and 25% for nymphs. Larvae were found on lizards from April until September and peaked in May, while nymphs were found from March until September and peaked in April. I. scapularis larvae (84%) and nymphs (73%) preferentially attached to the axillae/front leg of P. fasciatus. Two chigger species, Eutrombicula splendens (Ewing) and Eutrombicula cinnabaris (Ewing), were found on 2% of the reptiles collected. No ectoparasites, including ticks, were obtained from the seven species of snakes collected.

Effect of host lizard anemia on host choice and feeding rate of larval western black-legged ticks (Ixodes pacificus)

Although ticks are known to exhibit preferences among host species, there is little evidence that ticks select hosts within a species based on physiological condition. It may be beneficial for ticks to choose hosts that are easier to feed upon if the ticks can perceive indicative chemical or other signals from the host. For example, if ticks can detect host hematocrit they may choose hosts with high hematocrit, facilitating a faster blood meal. It may similarly be adaptive for ticks to avoid anemic hosts because it may be difficult for them to obtain an adequate meal and feeding duration may be extended. We tested the hypothesis that larval western black-legged ticks (Ixodes pacificus) detect host hematocrit using external cues and choose healthy over anemic hosts, allowing them to feed more quickly. We presented groups of larval ticks with pairs of healthy and anemic male western fence lizards (Sceloporus occidentalis), allowed them to select a host, and measured the feeding duration of the ticks. We found that the ticks did not exhibit a statistically significant preference for healthy over anemic lizards, but that the ticks fed to repletion significantly faster on healthy hosts than on anemic hosts. Larval ticks may not be able to detect external cues indicating the health of the host, at least not in terms of their hematocrit. The extended feeding duration likely reflects the extra time needed for the ticks to concentrate the blood meal of their anemic hosts.