Broad-spectrum pesticides can have immediate toxic effects both on target pest species and on non-target species. They may also have positive residual effects on mosquitoes after pesticide degradation, by altering the community structure, that is, by reducing abundances of mosquito competitors and predators, and via a trophic cascade, which may increase food resources for mosquito larvae. Alternatively, if a pesticidemediated trophic cascade results in toxic or inedible algae, the pesticide can act as an ecological trap for some taxa by attracting oviposition in sites where algae are abundant but unsuitable. The present study assessed mosquito oviposition habitat selection, mosquito larval performance, and community structure alterations after applications of various pesticides. The experiment was conducted in outdoor mesocosms assigned to one of four treatments: (1) control, no pesticides; (2) Bacillus thuringiensis var. israelensis (Bti), a narrow-spectrum bacterium well known for its larvicidal activity on mosquitoes and other dipterans; (3) temephos, an organophosphate mosquito larvicide with community-wide spectrum effects; and (4) pyriproxyfen, a pyridine-based insect growth regulator (IGR) class with wide-spectrum effects. Soon after pesticide application, Culex pipiens oviposition was highest in the control pools. Invertebrate species richness and abundance were strongly reduced in the broad-spectrum pesticide treatments (temephos and pyriproxyfen) when compared to control. One month after pesticide application, Cx. pipiens oviposition was highest in the pyriproxyfen-treated pools, although larval survival remained lowest in the pyriproxyfen-treated pools. Our results suggest that pyriproxyfen causes a chemically mediated trophic cascade and provides an ecological trap, that is, attracting mosquito oviposition due to an altered community structure, but causing high mosquito larval mortality.
Amblyomma maculatum Koch (Acari: Ixodidae), the primary vector for Rickettsia parkeri, may also be infected with a rickettsia of unknown pathogenicity, "Candidatus Rickettsia andeanae." Infection rates with these rickettsiae vary geographically, and coinfected ticks have been reported. In this study, infection rates of R. parkeri and "Ca. R. andeanae" were evaluated, and rickettsial DNA levels quantified, in 335 questing adult A. maculatum collected in 2013 (n = 95), 2014 (n = 139), and 2015 (n = 101) from Oktibbeha County, MS. Overall infection rates of R. parkeri and "Ca. R. andeanae" were 28.7% and 9.3%, respectively, with three additional A. maculatum (0.9%) coinfected. While R. parkeri-infected ticks were detected all three years (34.7% in 2013; 13.7% in 2014; 43.6% in 2015), "Ca. R. andeanae" was not detected in 2013, and was detected at rates of 10.8% in 2014, and 15.8% in 2015. Interestingly, rickettsial DNA levels in singly-infected ticks were significantly lower in "Ca. R. andeanae"-infected ticks compared to R. parkeri-infected ticks (P < 0.0001). Thus, both infection rates and rickettsial DNA levels were higher for R. parkeri than "Ca. R. andeanae." Infection rates of R. parkeri were also higher, and "Ca. R. andeanae" lower, here compared to A. maculatum reported previously in Kansas and Oklahoma. As we continue to monitor infection rates and levels, we anticipate that understanding temporal changes will improve our awareness of human risk for spotted fever rickettsioses. Further, these data may lead to additional studies to evaluate potential interactions among sympatric Rickettsia species in A. maculatum at the population level.
The Gulf Coast tick, Amblyomma maculatum Koch, has become increasingly important in public health for its role as a vector of the recently recognized human pathogen, Rickettsia parkeri. More recently, these ticks were also found to harbor a novel spotted fever group rickettsia, "Candidatus Rickettsia andeanae." First identified in Peru, and subsequently reported in ticks collected in the United States, Chile, and Argentina, "Ca. R. andeanae" remains largely uncharacterized, in part because of the lack of a stable isolate. Although the isolation of "Ca. R. andeanae" was recently described in DH82, Vero, and Drosophila S2 cells, its stability in these cell lines was not shown. To evaluate "Ca. R. andeanae" transmission and pathogenicity in vertebrates, as well as further describe biological characteristics of this candidate species to fulfill criteria for its establishment as a new species, availability of a stable isolate is essential. Here we describe the propagation of "Ca. R. andeanae" by using a primary culture derived from naturally infected A. maculatum embryos. Subsequent passage of the "Ca. R. andeanae" isolate to ISE6 (Ixodes scapularis embryonic) and Vero (African green monkey kidney epithelial) cell lines demonstrated limited propagation of the rickettsiae. Treatment of the infected primary cells with tetracycline resulted in cultures negative for "Ca. R. andeanae" by polymerase chain reaction and microscopy. Establishment of an isolate of "Ca. R. andeanae" will promote further investigation into the significance of this tick-associated rickettsia, including its role in spotted fever and interactions with the sympatric species, R. parkeri in A.
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Pulsed disturbances of larval mosquito sites are likely to have a direct negative effect on mosquitoes but may also have indirect effects due to the alteration of community structure. These altered communities may become attractive to gravid mosquitoes searching for oviposition sites when the disturbances decrease the abundance of mosquito antagonists such as competitors, which often results in an increase in mosquito food resources. However, flash flood disturbances in intermittent riverbeds can also remove mosquito food resources such as algae, so that the net effect of flash floods could be either to increase or decrease mosquito abundance. We conducted an outdoor mesocosm experiment to assess the effects of flash floods on mosquito oviposition habitat selection and larval abundance during the post-disturbance period of community recovery. Mesocosms were artificially flooded. Mosquito oviposition, immature abundance, invertebrate species diversity, chlorophyll a, and abiotic parameters were monitored. Our results showed that the flash flood negatively affected phytoplankton and zooplankton, leading to a decrease of mosquito oviposition in flooded mesocosms compared to non-flooded mesocosms. More broadly, this study indicates how disturbances influence mosquito oviposition habitat selection due to the loss of food resources in ephemeral pools, and it highlights the importance of considering the effects of disturbances in management, habitat restoration, and biodiversity conservation in temporary aquatic habitats. Journal of Vector Ecology 42 (2): 254-263. 2017.
Rickettsia parkeri is a recently recognized human pathogen primarily associated with the Gulf Coast tick Amblyomma maculatum, with immature stages of this tick reported from wild vertebrates. To better understand the role of vertebrates in the natural history of this bacterium, we evaluated small mammals and ground-dwelling birds for evidence of infection with R. parkeri or exposure to the organism. We sampled small mammals (n = 39) and passerines (n = 47) in both north-central and southeast Mississippi, while northern bobwhite (Colinus virginianus) samples (n = 31) were obtained from farms in central Mississippi. Blood from all sampled animals was tested using polymerase chain reaction (PCR) for spotted fever group rickettsiae (SFGR), and for antibodies to SFGR using R. parkeri antigen. Ectoparasite samples were removed from animals and included mites, lice, fleas, and immature ticks. Of 39 small mammal samples collected, 7 were positive for antibodies to SFGR; none tested positive by PCR for DNA of SFGR. Of 47 passerine blood samples collected, none were positive for DNA of SFGR by PCR, nor did any show serological evidence of exposure. Finally, none of 31 northern bobwhite samples tested were positive for SFGR DNA, while 7 were seropositive for rickettsial antibodies. Detection of seropositive rodents and quail suggests a role for these host species in the natural history of SFGR, possibly including R. parkeri, but the extent of their role has not yet been elucidated.
BackgroundAmblyomma maculatum is the primary vector for Rickettsia parkeri, a spotted fever group rickettsia (SFGR) and human pathogen. Cotton rats and quail are known hosts for larval and nymphal A. maculatum; however, the role of these hosts in the ecology of R. parkeri is unknown.MethodsCotton rats and quail were inoculated with low or high doses of R. parkeri (strain Portsmouth) grown in Vero cells to evaluate infection by R. parkeri in these two hosts species. Animals were euthanized 2, 4, 7, 10, and 14 days post-injection (dpi) and blood, skin, and spleen samples were collected to analyze by Vero cell culture and polymerase chain reaction (PCR). In a second trial, cotton rats and quail were inoculated with R. parkeri and nymphal A. maculatum ticks were allowed to feed on animals. Animals were euthanized on 14, 20, 28, 31, and 38 dpi and blood and tissues were collected for serology and PCR assays. Fed ticks were tested for R. parkeri by PCR and Vero cell culture.ResultsRickettsia parkeri was isolated in cell culture and detected by PCR in skin, blood, and spleen tissues of cotton rats in the initial trial 2, 4, and 7 dpi, but not in quail tissues. In the second trial, no ticks tested positive for R. parkeri by PCR or cell culture.ConclusionsThese studies demonstrate that viable R. parkeri rickettsiae can persist in the tissues of cotton rats for at least 7 days following subcutaneous inoculation of these bacteria; however, quail are apparently resistant to infection. Rickettsia parkeri was not detected in nymphal ticks that fed on R. parkeri-inoculated cotton rats or quail, suggesting an alternate route of transmission to naïve ticks.
Our objective of this study was to explore the bacterial microbiome in fresh or fresh-frozen adult Amblyomma maculatum (Gulf Coast ticks) using extracts enriched for microbial DNA. We collected 100 questing adult A. maculatum, surface disinfected them, and extracted DNA from individual ticks collected the same day or after storage at -80 °C. Because only extracts with microbial DNA concentrations above 2 ng/μL were considered suitable for individual analysis, we expected fewer samples to meet these requirements. Of individual ticks extracted, 48 extracts met this minimum concentration. We pooled 20 additional extracts that had lower concentrations to obtain seven additional pools that met the minimum DNA concentration. Libraries created from these 55 samples were sequenced using an Illumina MiSeq platform, and data sets were analyzed using QIIME to identify relative abundance of microorganisms by phylum down to genus levels. Proteobacteria were in greatest abundance, followed by Actinobacteria, Firmicutes, and Bacteroidetes, at levels between 1.9% and 6.4% average relative abundance. Consistent with the Francisella-like endosymbiont known to be present in A. maculatum, the genus Francisella was detected at highest relative abundance (72.9%; SE 0.02%) for all samples. Among the top ten genera identified (relative abundance ≥ 0.5%) were potential extraction kit contaminants, Sphingomonas and Methylobacterium, the soil bacterium Actinomycetospora, and the known A. maculatum-associated genus, Rickettsia. Four samples had Rickettsia at greater than 1% relative abundance, while nine additional samples had Rickettsia at low (0.01-0.04%) relative abundance. In this study, we used the entire microbe-enriched DNA extract for whole ticks for microbiome analysis. A direct comparison of the microbiome in microbe-enriched DNA and total genomic DNA extracts from halves of the same tick would be useful to determine the utility of this extraction method in this system. We anticipate that future tick microbiome studies will be valuable to explore the influence of microbial diversity on pathogen maintenance and transmission, and to evaluate niche-specific microbiomes within individual tick tissues.
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