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Dermacentor variabilis, a common human-biting tick found throughout the eastern half and along the west coast of the United States, is a vector of multiple bacterial pathogens. Historically, D. variabilis has been considered a primary vector of Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever. A total of 883 adult D. variabilis, collected between 2012 and 2017 from various locations in 12 states across the United States, were screened for rickettsial DNA. Tick extracts were evaluated using three real-time PCR assays; an R. rickettsii-specific assay, a Rickettsia bellii-specific assay, and a Rickettsia genus-specific assay. Sequencing of ompA gene amplicons generated using a seminested PCR assay was used to determine the rickettsial species present in positive samples not already identified by species-specific real-time assays. A total of 87 (9.9%) tick extracts contained R. bellii DNA and 203 (23%) contained DNA of other rickettsial species, including 47 (5.3%) with Rickettsia montanensis, 11 (1.2%) with Rickettsia amblyommatis, 2 (0.2%) with Rickettsia rhipicephali, and 3 (0.3%) with Rickettsia parkeri. Only 1 (0.1%) tick extract contained DNA of R. rickettsii. These data support multiple other contemporary studies that indicate infrequent detection of R. rickettsii in D. variabilis in North America.

Based on limited serological studies, at least 10% of the US population has been exposed to spotted fever group Rickettsia (SFGR) species. The immunofluorescence antibody assay (IFA) has been the gold standard for the serodiagnosis of rickettsial infections such as spotted fever rickettsiosis (SFR). However, the IFA is semi-quantitative and subjective, requiring a high level of expertise to interpret it correctly. Here, we developed an enzyme-linked immunosorbent assay (ELISA) for the serodiagnosis of Rickettsia parkeri infection in the guinea pig. Our ELISA is an objective, quantitative, and high-throughput assay that shows greater sensitivity and resolution in observed titers than the IFA. We methodically optimized relevant parameters in sequence for optimal signal-to-noise ratio and low coefficient of variation% values. We used a guinea pig model as it is a part of our overall research efforts to understand the immunological and clinical response to SFGR species after tick transmission. Guinea pigs are a useful model to study SFR and show clinical signs of SFR, such as fever and eschars. We anticipate that this assay will be easily adapted to other hosts, including humans and other SFGR species.

Understanding diagnosis of heartworm disease in the context of animal shelters' needs and expectations is crucial for developing guidelines that specifically address the unique shelter population. Accurate and economical heartworm testing strategies are essential. However, current comprehensive guidelines for the management of heartworm disease in dogs are directed toward client-owned animals and do not address needs of animal shelters and other resource-scarce facilities. Additionally, testing recommendations do not take into account regional and local differences in heartworm prevalence across the United States that occur due to abiotic and biotic factors. The objective of this study was to determine the apparent prevalence of Dirofilaria immitis microfilaremia and antigenemia in dogs from Mississippi animal shelters. Further, we compare agreement between microfilaria and antigen testing in this population. We performed a cross-sectional study using canine serum and blood bank samples representative of the Mississippi shelter population. Microfilaria testing of whole blood included a blood smear and modified Knott test. Antigen testing of serum was performed using the DiroCHEK® antigen ELISA test. Analyses included descriptive and analytic statistics as well as Cohen's kappa for test agreement. A total of 283 whole blood samples and 363 serum samples, representing 363 dogs from 18 shelters in 17 Mississippi counties, were utilized in this study. Sixty-four (22.6%) whole blood samples demonstrated D. immitis microfilariae on the modified Knott test and 125 (34.4%) serum samples had detectable D. immitis antigen. Increasing age and low body condition were associated with antigen-positive test results. Only age was associated with microfilaria-positive test results. There was moderate agreement between the antigen ELISA test and the modified Knott microfilaria test and poor agreement between the antigen ELISA and the blood smear. This study provides the first known report of the prevalence of D. immitis microfilaremia and antigenemia in Mississippi shelter dogs. We observed that prevalence of both microfilaremia and antigenemia was significantly higher in these sampled dogs compared to previous reports for the owned canine population in Mississippi. Heartworm infection presents unique management challenges for animal shelters. Knowledge of the expected prevalence in the area can be utilized for management decisions related to prevention, testing, and treatment of dogs in shelter populations.

The guinea pig (Cavia porcellus) has an established track record as an animal model, with its utility in rickettsial research documented as early as the turn of the 20(th) century. From identifying Rickettsia rickettsii as the agent of Rocky Mountain spotted fever and ticks as the natural transmission route to evaluating protective immunity and treatment for tick-borne rickettsiae, guinea pigs have been essential for advances in our understanding of spotted fever rickettsioses (SFR). Tick feeding on guinea pigs is feasible and results in transmission of tick-borne rickettsiae. The resulting infection leads to the recapitulation of SFR as defined by clinical signs that include fever, unthrift, and in the case of transmission by a Rickettsia parkeri-infected Amblyomma maculatum tick, a characteristic eschar at the site of the bite. No other small animal model recapitulates SFR, is large enough to collect multiple blood and skin samples for longitudinal studies, and has an immune system as similar to the human immune system. In the 1980s, the use of the guinea pig was significantly reduced due to advances made to the more reproductively prolific and inexpensive murine model. These advances included the development of genetically modified murine strains, which resulted in the expansion of murine-specific reagents and assays. Still, the advantages of the guinea pig as a model for SFR persist, novel assays are being developed to better monitor guinea pig immune responses, and tools, like CRISPR/Cas9, are now available. These technical advances allow guinea pigs to again contribute to our understanding of SFR. Importantly, returning to the guinea pig model with enhanced tools will enable rickettsial researchers to corroborate and potentially refine results acquired using mice. This minireview summarizes Cavia porcellus as an animal model for human tick-borne rickettsial diseases.

Guinea pigs are an ideal animal model for the study of several infectious diseases, including tuberculosis, legionellosis, brucellosis, and spotted fever rickettsiosis. In comparison to the murine model, clinical signs in guinea pigs are more representative of disease in humans, the guinea pig immune system is more similar to that of the human, and their large size offers logistic advantages for sample collection while following disease progression. Unfortunately, the advantage of using guinea pigs in biomedical research, particularly in understanding the immune response to infectious agents, is limited in large part by the paucity of available reagents and lack of genetically manipulated strains. Here, we expand the utility of guinea pigs in biomedical research by establishing an optimized five-color/seven-parameter polychromatic flow cytometric assay for immunophenotyping lymphocytes. This assay fills a need for immunophenotyping peripheral blood lymphocytes and is an improvement over current published flow cytometry assays for guinea pigs. We anticipate that our approach will be an important starting point for developing new assays to evaluate the cellular immune response to infectious diseases in the guinea pig model. Importantly, we are currently using this assay for evaluating immunity to spotted fever rickettsiosis in a guinea pig-tick-Rickettsia system, where CD8+ T cells are a critical contributor to the immune response. Developing resources to utilize the guinea pig more effectively will enhance our ability to understand infectious diseases where the guinea pig would otherwise be the ideal model.

Amblyomma maculatum Koch, 1844 (also known as the Gulf Coast tick) is found in parts of the Americas, including the central and southern United States. Its primary importance is as the vector of Rickettsia parkeri, a spotted fever group rickettsia that causes an illness similar to, but milder than, Rocky Mountain spotted fever. A second spotted fever group rickettsia, "Candidatus Rickettsia andeanae," was detected in Gulf Coast ticks approximately 10 yr ago. However, the significance of this organism, including pathogenicity, has not yet been well-characterized. Here, we use transmission electron microscopy to describe bacteria within the tissues of A. maculatum ticks that were positive by polymerase chain reaction assay for "Ca. R. andeanae." In ultrathin sections of unfed A. maculatum adult females, we found evidence of bacteria with morphological features consistent with spotted fever group rickettsiae, including small size (≈0.3 by 0.9 μm), a halo zone (electron-lucent layer around the bacterium), and a trilaminar cell wall. In female ticks, bacteria were present in granular salivary glands and ducts, foregut, Malpighian tubules, nerve trunks, and reproductive tissue. These findings demonstrate evidence of "Ca. R. andeanae" in situ and contribute to our understanding of this novel rickettsia in A. maculatum.

Tick-borne borreliae include Lyme disease and relapsing fever agents, and they are transmitted primarily by ixodid (hard) and argasid (soft) tick vectors, respectively. Tick–host interactions during feeding are complex, with host immune responses influenced by biological differences in tick feeding and individual differences within and between host species. One of the first encounters for spirochetes entering vertebrate host skin is with local antigen-presenting cells, regardless of whether the tick-associated Borrelia sp. is pathogenic. In this study, we performed a basic comparison of cytokine responses in THP-1-derived macrophages after exposure to selected borreliae, including a nonpathogen. By using THP-1 cells, differentiated to macrophages, we eliminated variations in host response and reduced the system to an in vitro model to evaluate the extent to which the Borrelia spp. influence cytokine production. Differentiated THP-1 cells were exposed to four Borrelia spp., Borrelia hermsii (DAH), Borrelia burgdorferi (B31), B. burgdorferi (NC-2), or Borrelia lonestari (LS-1), or lipopolysaccharides (LPS) (activated) or media (no treatment) controls. Intracellular and secreted interferon (IFN)-γ, interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α were measured using flow cytometric and Luminex-based assays, respectively, at 6, 24, and 48 h postexposure time points. Using a general linear model ANOVA for each cytokine, treatment (all Borrelia spp. and LPS compared to no treatment) had a significant effect on secreted TNF-α only. Time point had a significant effect on intracellular IFN-γ, TNF-α and IL-6. However, we did not see significant differences in selected cytokines among Borrelia spp. treatments. Thus, in this model, we were unable to distinguish pathogenic from nonpathogenic borreliae using the limited array of selected cytokines. While unique immune profiles may be detectable in an in vitro model and may reveal predictors for pathogenicity in borreliae of unknown pathogenicity, a larger panel of cytokines would be desirable to test.

Although a variety of disease agents have been reported from bed bugs, the mechanical and biological disease transmission potential of bed bugs remains unelucidated. In this study we assayed survivability of the mildly pathogenic spotted fever group rickettsia, Rickettsia parkeri, in bed bugs after feeding on R. parkeri-infected chicken blood. Two groups of 15 adult bed bugs each were fed on infected or noninfected blood, and two groups of fourth-instar bed bugs also were fed on either infected or noninfected blood. One group of 15 adult bed bugs received no bloodmeal and was included as an additional control. Two weeks postfeeding, two pools of five live bed bugs from each group were surface sterilized, macerated, and placed in Vero cell cultures in an attempt to grow live organism. The remaining five individual bed bugs from each group were dissected, their salivary glands were removed for immunofluorescence assay (IFA) staining, and the remaining body parts were processed for polymerase chain reaction (PCR) analysis. Results indicated that no immature (now molted to fifth instar) bed bugs were positive for R. parkeri by IFA or PCR, indicating that organisms did not survive the molting process. After 4 wk of cell culture, no organisms were seen in cultures from any of the treatment or control groups, nor were any cultures PCR positive. However, two of the adult bed bugs were IFA positive for rickettsia-like organisms, and these two specimens were also PCR positive using R. parkeri-specific primers. These IFA and PCR results indicate that remnants of Rickettsia parkeri (possibly whole organisms) survived in the bugs for 2 wk, but the viability of the organisms in these two specimens could not be determined.

We detected Rickettsia parkeri in 20%-33% of Amblyomma maculatum ticks sampled in North Carolina. Results highlight the high frequencies of R. parkeri-infected ticks in the state with the highest annual incidence of Rocky Mountain spotted fever. Epidemiologic studies are needed to definitively link R. parkeri to cases of spotted fever rickettsiosis.

Rickettsia parkeri, a recently identified cause of spotted fever rickettsiosis in the United States, has been found in Amblyomma triste ticks in several countries of South America, including Argentina, where it is believed to cause disease in humans. We describe the clinical and epidemiologic characteristics of 2 patients in Argentina with confirmed R. parkeri infection and 7 additional patients with suspected R. parkeri rickettsiosis identified at 1 hospital during 2004-2009. The frequency and character of clinical signs and symptoms among these 9 patients closely resembled those described for patients in the United States (presence of an inoculation eschar, maculopapular rash often associated with pustules or vesicles, infrequent gastrointestinal manifestations, and relatively benign clinical course). Many R. parkeri infections in South America are likely to be misdiagnosed as other infectious diseases, including Rocky Mountain spotted fever, dengue, or leptospirosis.

Because tick-borne diseases are becoming increasingly important throughout the world, monitoring their causative agents in wildlife may serve as a useful indicator of potential human exposure. We assessed the presence of known and putative zoonotic, tick-borne agents in four wildlife species in Mississippi. Animals were tested for exposure to or infection with Ehrlichia chaffeensis, Ehrlichia ewingii, Borrelia lonestari, Rickettsia spp., Anaplasma phagocytophilum, and Francisella tularensis. Whole blood and serum were tested from white-tailed deer (WTD; Odocoileus virginianus) and feral swine (Sus scrofa); serum was tested from raccoons (Procyon lotor) and opossums (Didelphis virginiana). We used polymerase chain reaction to detect all agents in blood, whereas an indirect fluorescent antibody assay was used to detect antibodies to E. chaffeensis, B. lonestari, and Rickettsia parkeri (spotted fever group rickettsiae) antigens in serum. Molecular evidence of infection with E. chaffeensis, B. lonestari, and An. phagocytophilum was detected only in WTD. Antibodies to E. chaffeensis antigen were detected in 43.9% of WTD, 32.8% of swine, 42.1% of raccoons, and 15.8% of opossums. Serologic evidence of exposure to B. lonestari antigen was found in 19.3% of WTD, 6.9% of swine, and 5.3% of raccoons, but not in opossums. Interestingly, the percent of animals with antibodies reactive to spotted fever group rickettsiae (R. parkeri antigen) was highest in raccoons (73.7%) and opossums (57.9%). These results support the role of WTD as reservoirs for E. chaffeensis, B. lonestari, and An. phagocytophilum, as well as provide additional evidence for exposure of raccoons and opossums to E. chaffeensis. Finally, we provide new data that feral swine may have antibodies to these agents. Thus, in general, these four wildlife species are exposed to tick-borne disease agents in Mississippi, suggesting that ticks carry and have the potential to transmit the agents to humans in the state.

Until recently,Amblyomma maculatum(the Gulf Coast tick) had garnered little attention compared to other species of human-biting ticks in the United States.A. maculatumis now recognized as the principal vector ofRickettsia parkeri, a pathogenic spotted fever group rickettsia (SFGR) that causes an eschar-associated illness in humans that resembles Rocky Mountain spotted fever. A novel SFGR, distinct from other recognizedRickettsiaspp., has also been detected recently inA. maculatumspecimens collected in several regions of the southeastern United States. In this study, 198 questing adult Gulf Coast ticks were collected at 4 locations in Florida and Mississippi; 28% of these ticks were infected withR. parkeri, and 2% of these were infected with a novel SFGR. Seventeen isolates ofR. parkerifrom individual specimens ofA. maculatumwere cultivated in Vero E6 cells; however, all attempts to isolate the novel SFGR were unsuccessful. Partial genetic characterization of the novel SFGR revealed identity with several recently described, incompletely characterized, and noncultivated SFGR, including “CandidatusRickettsia andeanae” andRickettsiasp. Argentina detected in several species of Neotropical ticks from Argentina and Peru. These findings suggest that each of these “novel” rickettsiae represent the same species. This study considerably expanded the number of low-passage,A. maculatum-derived isolates ofR. parkeriand characterized a second, sympatricRickettsiasp. found in Gulf Coast ticks.

In this study, we evaluated Amblyomma americanum (lone star tick) in Mississippi for the presence of Ehrlichia chaffeensis, causative agent of human monocytic ehrlichiosis; Ehrlichia ewingii, causative agent of human and canine granulocytic ehrlichiosis; Borrelia lonestari, putative agent of southern tick-associated rash illness; Francisella tularensis, the agent of tularemia; and Rickettsia spp., particularly R. amblyommii, a suspected pathogen, We collected adult A. americanum from four regions of Mississippi: Northeast, Northwest, Southeast, and East. Of the ticks collected, 192 were dissected and DNA was extracted for nested polymerase chain reaction (PCR) assays to detect the above bacteria. In all, 3% of tick extracts had evidence of Borrelia sp., 4% for E. chaffeensis, 6% for E. ewingii, and 44% for a Rickettsia species. As determined by sequencing, most Rickettsia spp, were R. amblyommii. In addition, extracts from 42 pools (total of 950) of larval A. americanum collected in Southwest Mis...

Amblyomma americanum is an aggressive ixodid tick that has been implicated as a vector for several bacterial agents. Among these is Ehrlichia chaffeensis, which causes human monocytic (or monocytotropic) ehrlichiosis. In this study, experimental tick transmission of E. chaffeensis from infected lone star ticks to deer was revisited, and the question of whether it would be possible to re-isolate the organism from deer was asked, because this had not been done previously. Here, we were able to transmit a wild strain of E. chaffeensis from acquisition-fed lone star ticks to white-tailed deer. Ehrlichia chaffeensis was re-isolated from one white-tailed deer on multiple days during the infection and from another deer on one day during the infection. Peak rickettsemias for E. chaffeensis-infected deer were 17 DPI with acquisition-fed ticks and 14 DPI with needle-inoculated deer. This study supports the role of the lone star tick and white-tailed deer as vector and reservoir host for E. chaffeensis, demonstrating culture re-isolation of E. chaffeensis in deer infected by experimental tick transmission for the first time.

In late spring of 2009 and 2010, there were reports of severe black fly (Simulium spp., shown in Fig. 1) outbreaks in various counties in Mississippi, especially those in and around the Mississippi River Delta. Complaints were of black flies attacking multiple species of backyard poultry and causing high morbidity and mortality in affected flocks. At several affected locations, black flies were readily observed swarming around and feeding on birds. A large number of these parasites were easily trapped on fly strips (Fig. 2). Multifocal to coalescing cutaneous hemorrhagic lesions, consistent with fly bites, were seen on the birds. Upon necropsy examination, a large number of black flies were also observed in the digestive tract (Fig. 3). Although black flies may cause disease directly, such as cardiopulmonary collapse and anaphylactoid reactions, detection of Leucocytozoon in blood smears (Fig. 4) of affected birds prompted further investigations of this protozoan as a cause of disease. Leucocytozoon spp. are known to be transmitted by black flies and may be associated with morbidity and mortality in birds such as poultry. From June 2009 through July 2012, the investigation included a total collection of 1068 individual blood samples, representing 371 individual premises in 89 counties/parishes across Mississippi (59), Alabama (10), Louisiana (4), and Tennessee (16). Of the 371 premises where blood samples were collected, 96 (26%) were either positive or highly suspected to be positive for Leucocytozoon spp. by blood smear analysis, and 5 (1.2%) were positive for Haemoproteus spp. by blood smear analysis. Attempts to diagnose Leucocytozoon spp. by PCR analysis and sequencing were complicated by coinfections with two closely related haemosporidians (Haemoproteus spp. and Plasmodium spp.). A novel technique involving flow cytometry was also explored. This study discusses the black fly field outbreak, the involvement of haemosporidians, molecular methods for detection of both the black flies and blood parasites, and initial attempts at flow cytometry.

Latent infection with Toxoplasma gondii is common in humans (approximately 30% of the global population) and is a significant risk factor for schizophrenia. Since prevalence of T. gondii infection is far greater than prevalence of schizophrenia (0.5-1%), genetic risk factors are likely also necessary to contribute to schizophrenia. To test this concept in an animal model, Nurr1-null heterozygous (+/-) mice and wild-type (+/+) mice were evaluate using an emergence test, activity in an open field and with a novel object, response to bobcat urine and prepulse inhibition of the acoustic startle response (PPI) prior to and 6 weeks after infection with T. gondii. In the emergence test, T. gondii infection significantly decreased the amount of time spent in the cylinder. Toxoplasma gondii infection significantly elevated open field activity in both +/+ and +/- mice but this increase was significantly exacerbated in +/- mice. T. gondii infection reduced PPI in male +/- mice but this was not statistically significant. Aversion to bobcat urine was abolished by T. gondii infection in +/+ mice. In female +/- mice, aversion to bobcat urine remained after T. gondii infection while the male +/- mice showed no aversion to bobcat urine. Antibody titers of infected mice were a critical variable associated with changes in open field activity, such that an inverted U shaped relationship existed between antibody titers and the percent change in open field activity with a significant increase in activity at low and medium antibody titers but no effect at high antibody titers. These data demonstrate that the Nurr1 +/- genotype predisposes mice to T. gondii-induced alterations in behaviors that involve dopamine neurotransmission and are associated with symptoms of schizophrenia. We propose that these alterations in murine behavior were due to further exacerbation of the altered dopamine neurotransmission in Nurr1 +/- mice.

To the Editor: Rickettsia parkeri, a spotted fever group Rickettsia (SFGR) bacterium, is transmitted by Amblyomma maculatum, the Gulf Coast tick (1). The prevalence of R. parkeri in Gulf Coast ticks has been reported as

A 1-year-old castrated male Saanen goat was observed to have drooping and edema of the left ear consistent with published accounts of gotch ear in cattle associated with a tick bite. The goat's left ear was edematous from the tip of the pinna to the base of the ear. No signs of trauma or infectious processes were observed. Three engorged Gulf Coast ticks (Amblyomma maculatum) were observed attached inside the ear. Ticks were removed and the ear biopsied at tick attachment sites. The affected ear was treated topically with betadine after removal of the ticks. No other treatment was administered. The goat remained free of clinical signs and the edema of the ear resolved within 3 days after tick removal. No clinical adverse effects of the condition were evident. All three ticks were positive for spotted fever group rickettsia by polymerase chain reaction analysis and showed 100% similarity with the homologous sequence of Rickettsia parkeri. There was no immunohistochemical evidence of spotted fever group rickettsia in the ear samples, supporting the hypothesis that gotch ear is not due to rickettsial infection. This report represents the first apparent case of gotch ear in a goat.

The Companion Animal Parasite Council hosted a meeting to identify quantifiable factors that can influence the prevalence of tick-borne disease agents among dogs in North America. This report summarizes the approach used and the factors identified for further analysis with mathematical models of canine exposure to tick-borne pathogens.

Background Relapsing fever spirochetes are global yet neglected pathogens causing recurrent febrile episodes, chills, nausea, vomiting, and pregnancy complications. Given these nonspecific clinical manifestations, improving diagnostic assays for relapsing fever spirochetes will allow for identification of endemic foci and expedite proper treatment. Previously, an antigen designated the Borrelia immunogenic protein A (BipA) was identified in the North American species Borrelia hermsii. Thus far, BipA appears unique to relapsing fever spirochetes. The antigen remains unidentified outside of these pathogens, while interspecies amino acid identity for BipA in relapsing fever spirochetes is only 24–36%. The current study investigated the immunogenicity of BipA in Borrelia turicatae, a species distributed in the southern United States and Latin America. Methodology/Principal Findings bipA was amplified from six isolates of Borrelia turicatae, and sequence analysis demonstrated that the gene is conserved among isolates. A tick transmission system was developed for B. turicatae in mice and a canine, two likely vertebrate hosts, which enabled the evaluation of serological responses against recombinant BipA (rBipA). These studies indicated that BipA is antigenic in both animal systems after infection by tick bite, yet serum antibodies failed to bind to B. hermsii rBipA at a detectable level. Moreover, mice continued to generate an antibody response against BipA one year after the initial infection, further demonstrating the protein's potential toward identifying endemic foci for B. turicatae. Conclusions/Significance These initial studies support the hypothesis that BipA is a spirochete antigen unique to a relapsing fever Borrelia species, and could be used to improve efforts for identifying B. turicatae endemic regions., Author Summary Undiagnosed febrile illnesses continue to afflict those in resource poor countries. Relapsing fever spirochetes are one such pathogen causing a significant health burden, yet the pathogenesis, ecology, and distribution of B. turicatae is understudied. To address these shortcomings, we analyzed the amino acid sequence of the Borrelia immunogenic protein A (BipA) in isolates of B. turicatae. Mice and a canine were also infected by tick bite and transmission and serological responses were evaluated in these two likely mammalian hosts. B. turicatae was visualized within the blood of both animals and antibody responses generated against recombinant BipA indicated that the antigen that may be unique to infections caused by B. turicatae. Moreover, mice continued to generate antibodies a year after tick bite, suggesting a persistent infection. Our results indicate that the immune responses generated against BipA could identify additional vertebrate hosts, define endemic foci for B. turicatae, and increase the awareness of the disease to improve healthcare.

We reviewed scientific literature pertaining to known and putative disease agents associated with the lone star tick, Amblyomma americanum. Reports in the literature concerning the role of the lone star tick in the transmission of pathogens of human and animal diseases have sometimes been unclear and even contradictory. This overview has indicated that A. americanum is involved in the ecology of several disease agents of humans and other animals, and the role of this tick as a vector of these diseases ranges from incidental to significant. Probably the clearest relationship is that of Ehrlichia chaffeensis and A. americanum. Also, there is a definite association between A. americanum and tularemia, as well as between the lone star tick and Theileria cervi to white-tailed deer. Evidence of Babesia cervi (= odocoilei) being transmitted to deer by A. americanum is largely circumstantial at this time. The role of A. americanum in cases of southern tick-associated rash illness (STARI) is currently a subject of intensive investigations with important implications. The lone star tick has been historically reported to be a vector of Rocky Mountain spotted fever rickettsiae, but current opinions are to the contrary. Evidence incriminated A. americanum as the vector of Bullis fever in the 1940s, but the disease apparently has disappeared. Q fever virus has been found in unfed A. americanum, but the vector potential, if any, is poorly understood at this time. Typhus fever and toxoplasmosis have been studied in the lone star tick, and several non-pathogenic organisms have been recovered. Implications of these tick-disease relationships are discussed.