Association of Ixodes pacificus (Acari: ixodidae) with the spatial and temporal distribution of equine granulocytic ehrlichiosis in California

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.

Equine Granulocytic Anaplasmosis 28 years later

Equine granulocytic anaplasmosis (EGA) is an important tick borne disease of equines that is caused by Anaplasma phagocytophilum (A. phagocytophilum). The etiological agent has veterinary as well as public health importance because of its zoonotic nature. A. phagocytophilum causes an acute illness in equines with loss of appetite, lethargy, hemorrhages and lameness. Clinically, EGA is diagnosed upon examination of morulae within neutrophils especially granulocytes in the blood. The best diagnostic tool for the detection of EGA is Polymerase chain reaction (PCR). Previous studies suggested that EGA is a self-limiting disease and tetracycline therapy is considered as a best treatment regimen. There is no comprehensive summary on the occurrence and distribution of the infection at global level. Therefore, we intended to provide a comprehensive summary on the prevalence and epidemiology of EGA in different areas of the world. It includes mapping the global distribution of EGA in different areas of the world to identify the endemic regions which may be a source of potential disease outbreak. For this purpose, the published data from 1990 to 2018 on EGA was reviewed and collected by electronic literature search of five databases including Google, Google Scholar, Science Direct, PubMed and Web of Science.

Effects of temperature on feeding duration, success, and efficiency of larval western black-legged ticks (Acari: Ixodidae) on western fence lizards

The western black-legged tick (Ixodes pacificus) is a common tick species throughout the western USA and is the major vector for Borrelia burgdorferi, the Lyme disease causing bacterium. Western fence lizards (Sceloporus occidentalis) are a major host for juvenile I. pacificus, but are incompetent hosts for B. burgdorferi, which makes this host-parasite relationship of particular interest. In order to shed further light on this complex host-parasite relationship, we investigated the effects of temperature on feeding duration (number of days to repletion), success (number feeding to repletion), and efficiency (replete tick mass) of larval I. pacificus. Western fence lizards were experimentally infested with larval ticks and exposed to three constant temperatures (21, 27, 33 °C). Larvae feeding at 21 °C took approximately twice as long as larvae at 27 and 33 °C. Effects of temperature on feeding duration are likely mediated through effects on host blood circulation and functionality of tick salivary proteins. Our results here suggest temperature is another important factor influencing the feeding dynamics of I. pacificus, and likely other tick species. Future research is needed to clarify the exact mechanisms behind temperature effects on tick feeding.

Possible differential host tropism in Anaplasma phagocytophilum strains in the Western United States

Despite organization into a single species, Anaplasma phagocytophilum, of bacteria from diverse hosts, there is evidence that different hosts vary in susceptibility to local strains of this bacterial pathogen. In particular, there is evidence that clinical hosts (humans, horses, and dogs) in the western part of the United States may be infected with strains of bacteria that differ from those in the reported wildlife reservoir, the dusky-footed woodrat. There is a lack of spatial overlap between clinical cases with woodrat exposure, genetic studies show highly distinctive strains in woodrats compared with those from clinical hosts, and experimental studies failed to transfer infection from woodrats into horses. These data suggest that multiple distinct A. phagocytophilum strains could circulate in western U.S. ecosystems. Host barriers to different bacterial strains would suggest the need for careful discrimination of A. phagocytophilum strains and further research to understand the host-pathogen interactions that result in differential outcomes in infection.

Co-phylogenetic analysis of Anaplasma phagocytophilum and its vectors, Ixodes spp. ticks

The coevolutionary history of Ixodes spp. ticks, the obligately tick-transmitted bacterial pathogen Anaplasma phagocytophilum, and its various rodent reservoir hosts world-wide is not known. According to coevolution theory, the most recently evolved of tick-bacterial complexes could have difficulty maintaining A. phagocytophilum in nature, because transmissibility has not been efficiently maximized. This study was intended to examine the phylogeographic history of I. ricinus-subgroup ticks and A. phagocytophilum, provide an estimate for the date of the divergence of A. marginale and A. phagocytophilum, and evaluate whether there is correspondence between tick and Anaplasma spp. trees. Analysis of Ixodes spp. ticks showed a New World clade consisting of I. scapularis and I. pacificus, European I. ricinus as a sister group to this clade, and Asian I. persulcatus as basal. Of the three A. phagocytophilum genes evaluated, the most resolution was provided by the ankA gene. ankA sequences formed an Old World clade with eastern North America strains as a sister clade. California strains were highly diverse and did not form a clade. Base substitution rates were very comparable along both A. marginale and A. phagocytophilum lineages. Based on 16S rDNA analysis, maximum and minimum divergence times of A. phagocytophilum and A. marginale were calculated to be 78,296,703 and 43,415,708 years, respectively. If A. phagocytophilum did closely coevolve with specific I. ricinus-subgroup tick species, then A. phagocytophilum strains could have specialized on local tick species and optimized local infectivity in the Old World and eastern US. However, lack of absolute resolution of tick trees and conflicting prevalence data (with low prevalence in Asia and western North America) preclude us from inferring a tight coevolutionary relationship of tick species from this phylogeographic analysis.

Differences in the transmissibility of two Anaplasma phagocytophilum strains by the North American tick vector species, Ixodes pacificus and Ixodes scapularis (Acari: Ixodidae)

The etiologic agent of granulocytic anaplasmosis, Anaplasma phagocytophilum, has a circum-global distribution within the northern hemisphere and shows a host species predilection that varies by the geographic region in which the disease is found. Adaptation by the bacterium to a host species potentially contributes to the variation found worldwide but this is confounded by the bacterium's relationship with its tick vectors, all of which belong to the Ixodes ricinus group. We tested the hypothesis that tick vector species collected from geographic regions sympatric with particular A. phagocytophilum strains will show evidence of a higher degree of vector competence than will tick species and allopatric A. phagocytophilum strains. A reciprocal cross-transmission experiment was performed using an eastern and a western North American strain of A. phagocytophilum (Webster and MRK, respectively) and the two tick species, I. scapularis and I. pacificus, most commonly associated with human and animal transmission of the bacteria in the United States. The western tick, I. pacificus, showed a significantly higher vector competence for A. phagocytophilum than I. scapularis and the eastern isolate, Webster, was more transmissible than its western counterpart, MRK. These results indicate that geographic variation in host susceptibility to A. phagocytophilum strains may play a more important role in the epidemiology of granulocytic anaplasmosis than does the competence of its tick vectors to transmit the pathogen.

Ticks and tick-borne disease in Guatemalan cattle and horses

Blood samples and ticks were collected from 48 cattle and 74 horses from seven sites in the Peten region of Guatemala. Data on body condition, mucous membrane capillary refill time and tick infestation levels were recorded for each animal in the study. Horses had significantly higher levels of tick infestation than cattle, as well as poorer body condition scores. Seroprevalence of Babesia spp. was 95.8% for B. bovis in cattle, 89.6% for B. bigemina in cattle, and 92.7% for B. equi in horses. Seroprevalence of Anaplasma marginale in cattle was 87.5%, similar to reports in animals from other regions of Central America. This is the first time that A. phagocytophilum has been reported in animals from this region, with overall PCR-prevalence of 27.6% in cattle and horses, and seroprevalence of 28.4% (52% in cattle and 13% in horses). An agent was identified with serological cross-reactivity and close genetic relatedness to Ehrlichia ruminantium, but further testing confirmed that the agent in Guatemalan cows was not the agent of heartwater. Ticks were identified to species with the predominant species identified on cattle as Boophilus microplus and Amblyomma cajennense, while Anocentor nitens and A. cajennense were most commonly found on horses. Prevalence of infection, tick infestation levels, host factors and environmental data were analyzed for association; A. nitens was significantly associated with A. phagocytophilum prevalence by village.

GIS-facilitated spatial epidemiology of tick-borne diseases in coyotes (Canis latrans) in northern and coastal California

Ixodes pacificus is the main tick vector for transmission of Anaplasma phagocytophilum and Borrelia burgdorferi to large vertebrates in California. The present study was undertaken in I. pacificus-infested counties in California to examine spatial and temporal relationships among A. phagocytophilum and B. burgdorferi-exposed coyotes with vegetation type and climate. The overall A. phagocytophilum and B. burgdorferi seroprevalences were 39.5% (N=215) and 18.9% (N=148), respectively, with no association with sex. PCR for A. phagocytophilum and B. burgdorferi was negative in all blood and kidney samples. Increased seroprevalence was a positive function of rainfall. Ehrlichial seropositivity was increased in blue-oak foothill pine, montane hardwood, and redwood vegetation regions, and decreased in coastal sagebrush and cropland. Increased exposure to B. burgdorferi occurred in blue oak woodland.

Power evaluation of disease clustering tests

BACKGROUND: Many different test statistics have been proposed to test for spatial clustering. Some of these statistics have been widely used in various applications. In this paper, we use an existing collection of 1,220,000 simulated benchmark data, generated under 51 different clustering models, to compare the statistical power of several disease clustering tests. These tests are Besag-Newell's R, Cuzick-Edwards' k-Nearest Neighbors (k-NN), the spatial scan statistic, Tango's Maximized Excess Events Test (MEET), Swartz' entropy test, Whittemore's test, Moran's I and a modification of Moran's I. RESULTS: Except for Moran's I and Whittemore's test, all other tests have good power for detecting some kind of clustering. The spatial scan statistic is good at detecting localized clusters. Tango's MEET is good at detecting global clustering. With appropriate choice of parameter, Besag-Newell's R and Cuzick-Edwards' k-NN also perform well. CONCLUSION: The power varies greatly for different test statistics and alternative clustering models. Consideration of the power is important before we decide which test statistic to use.

Detection of Borrelia burgdorferi, Ehrlichia chaffeensis, and Anaplasma phagocytophilum in ticks (Acari: Ixodidae) from a coastal region of California

A study was conducted in Santa Cruz County to estimate the prevalence and distribution of the agents of Lyme disease, human granulocytic (HGE), and human monocytic (HME) ehrlichiosis in 1,187 adult ixodid ticks collected from eight public-use recreation areas over a 2-yr period. Borrelia burgdorferi, the causative agent of Lyme disease, was detected by a polymerase chain reaction (PCR) in 44 of 776 (5.67%) Ixodes pacificus ticks and in 3 of 58 (5.17%) Dermacentor variabilis ticks. Anaplasma phagocytophilum, the causative agent of HGE, was detected by PCR in 48 (6.19%) I. pacificus ticks and 5 (8.62%) D. variabilis ticks. Ehrlichia chaffeensis, the causative agent of HME, was detected by nested PCR in just five (0.64%) I. pacificus ticks and four (6.9%) D. variabilis ticks. Interestingly, eight (1.03%) I. pacificus ticks were co-infected with B. burgdorferi and A. phagocytophilum, and just one (0.12%) tick was co-infected with B. burgdorferi and E. chaffeensis. Less than 1% of 353 Dermacentor occidentalis ticks showed evidence of infection with any of the agents tested. To our knowledge, this is the first reported identification of A. phagocytophilum and E. chaffeensis in D. occidentalis ticks from California This study represents the first extensive survey of Lyme and the ehrlichial diseases across multiple areas of Santa Cruz County, and suggests that prevalence of B. burgdorferi in Santa Cruz County may be higher than other areas of the state.

Acarologic risk of exposure to emerging tick-borne bacterial pathogens in a semirural community in northern California

An acarologic study was conducted in a semirural community in northern California to determine the relative abundance of, and the prevalence of infection with, three emerging bacterial pathogens in the western black-legged tick (Ixodes pacificus). These included the agents causing Lyme disease (Borrelia burgdorferi), human granulocytic ehrlichiosis [Ehrlichia phagocytophila (formerly Ehrlichia equi)], and human monocytic ehrlichiosis (Ehrlichia chaffeensis). The study area in Sonoma County consisted of two properties each with four residents and an uninhabited adjacent comparison area. Six of the eight residents had been either physician-diagnosed or serodiagnosed previously with Lyme disease, and, of these, one also had been serodiagnosed with human monocytic ehrlichiosis. Direct immunofluorescent/culture assays and bacterial species-specific polymerase chain reaction assays were used to test whole ticks individually for presence of B. burgdorferi and Ehrlichia spp., respectively. Overall, 6.5% of the nymphal (n = 589) and 1.6% of the adult ticks (n = 318) from the same generational cohort were found to contain B. burgdorferi. In contrast, none of 465 nymphs and 9.9% of 202 adults were infected with E. phagocytophila. Excised tissues from another 95 adult ticks yielded a comparable E. phagocytophila infection prevalence of 13.7%. E. chaffeensis was not detected in either nymphal or adult ticks. Using a combination of culture and polymerase chain reaction assays, coinfection of I. pacificus adults with B. burgdorferi and E. phagocytophila was demonstrated for the first time. The marked disparity in the infection prevalence of these pathogens in nymphal and adult ticks suggests that their maintenance cycles are inherently different.

Urban zoonoses caused by Bartonella, Coxiella, Ehrlichia, and Rickettsia species

The last half of the 20th Century witnessed an increase in the occurrence and recognition of urban zoonoses caused by members of the genera Bartonella, Coxiella, Ehrlichia, and Rickettsia, all traditionally considered to be members of the family Rickettsiaceae. In recent years, new human pathogens (Bartonella elizabethae, Bartonella henselae, and Rickettsia felis) have been recognized in urban environments. Other newly recognized pathogens (Ehrlichia chaffeensis and Ehrlichia phagocytophila in the United States) have sylvan zoonotic cycles but are present in urban areas because their vertebrate hosts and associated ectoparasitic arthropod vectors are able to survive in cities. Still other agents, which were primarily of historical importance (Bartonella quintana) or have not traditionally been associated with urban environments (Rickettsia rickettsii), have been recognized as causes of human disease in urban areas. Some diseases that have traditionally been associated with urban environments, such as rickettsialpox (caused by Rickettsia akari) and murine typhus (caused by Rickettsia typhi), still occur in large cities at low or undetermined frequencies and often go undetected, despite the availability of effective measures to diagnose and control them. In addition, alternate transmission cycles have been discovered for Coxiella burnetii, Rickettsia prowazekii, and R. typhi that differ substantially from their established, classic cycles, indicating that the epidemiology of these agents is more complex than originally thought and may be changing. Factors leading to an increase in the incidence of illnesses caused by these bacteria in urban areas include societal changes as well as intrinsic components of the natural history of these organisms that favor their survival in cities. Transovarial and transstadial transmission of many of the agents in their arthropod hosts contributes to the highly focal nature of many of the diseases they cause by allowing the pathogens to persist in areas during adverse times when vertebrate amplifying hosts may be scarce or absent. Domesticated animals (primarily cats, dogs, and livestock) or commensal rodents [primarily Norway rats (Rattus norvegicus) and house mice (Mus musculus)] can serve as vertebrate amplifying hosts and bring these agents and their ectoparasitic arthropod vectors into direct association with humans and help maintain transmission cycles in densely populated urban areas. The reasons for the increase in these urban zoonoses are complex. Increasing population density worldwide, shifts in populations from rural areas to cities, increased domestic and international mobility, an increase in homelessness, the decline of inner-city neighborhoods, and an increase in the population of immunosuppressed individuals all contribute to the emergence and recognition of human diseases caused by these groups of agents. Due to the focal nature of infections in urban areas, control or prevention of these diseases is possible. Increased physician awareness and public health surveillance support will be required to detect and treat existing urban infections caused by these agents, to determine the disease burden caused by them, to design and implement control programs to combat and prevent their spread, and to recognize emerging or resurging infections caused by members of these genera as they occur.

Spatial distribution of seropositivity to the causative agent of granulocytic ehrlichiosis in dogs in California

OBJECTIVE

To assess spatial and temporal patterns of seroprevalence among dogs in California to the causative agent of granulocytic ehrlichiosis (GE).

SAMPLE POPULATION

Sera of 1,082 clinically normal dogs from 54 of 59 counties in California in 1997 to 1998.

PROCEDURES

Serum-specific IgG reactivity to Ehrlichia equi was assessed by use of an immunofluorescent antibody assay, using E. equi-infected horse neutrophils as substrate. Data were analyzed, using a geographic information system. Spatial analysis of seroprevalence included first order Bayesian analysis of seroprevalence and second order analysis of clustering by K-function and Cuzick-Edwards tests. Monthly seroprevalence among dogs was examined by use of regression on monthly densities of Ixodes pacificus adults and nymphs.

RESULTS

Seroprevalence among dogs to E. equi was 8.68%. Data were seasonally bimodal with highest prevalence in winter (when adult ticks were abundant) and a secondary peak in late spring (corresponding to nymphal ticks). Humboldt County had the highest seroprevalence (47.3%), and other northern coast range counties had seroprevalence from 15 to 30%.

CONCLUSIONS AND CLINICAL RELEVANCE

The patchy distribution of exposure to Ehrlichia organisms is a subset of the distribution of the tick vector. This may reflect enzootic cycles or climatic or historical factors that limited the range of the disease. Dogs, horses, and humans from north coast range counties in California are at increased risk of GE. These data provide a background for assessing risk of infection in horses and dogs, depending on geographic location. Dogs may be sentinels for assessing risk of GE in humans.

Ehrlichial diseases

Equine granulocytic and monocytic ehrlichiosis caused by Ehrlichia equi and E. risticii, respectively, are seasonal diseases in horses that occur throughout the United States E. equi is transmitted by lxodes ticks and causes high fever, depression, anorexia, limb edema, petechiation, icterus, ataxia, and stiffness in gait. E. risticii, also known as the agent of Potomac horse fever, causes a febrile illness with a colitis of variable severity. Its occurrence is associated with aquatic habitats. The natural route of transmission is oral, through the ingestion of E. risticii infected trematode stages either free in water or in an intermediate host, such as aquatic animals.