Synchronous Langat Virus Infection of Haemaphysalis longicornis Using Anal Pore Microinjection

The Vector Competence of Asian Longhorned Ticks in Langat Virus Transmission

Haemaphysalis longicornis (the longhorned tick), the predominant tick species in China, serves as a vector for a variety of pathogens, and is capable of transmitting the tick-borne encephalitis virus (TBEV), the causative agent of tick-borne encephalitis. However, it is unclear how these ticks transmit TBEV. Langat virus (LGTV), which has a reduced pathogenicity in humans, has been used as a surrogate for TBEV. In this study, we aimed to investigate the vector competence of H. longicornis to transmit LGTV and demonstrate the efficient acquisition and transmission of LGTV between this tick species and mice. LGTV localization was detected in several tick tissues, such as the midgut, salivary glands, and synganglion, using quantitative PCR and immunohistochemical staining with a polyclonal antibody targeting the LGTV envelope protein. We demonstrated the horizontal transmission of LGTV to different developmental stages within the same generation but did not see evidence of vertical transmission. It was interesting to note that we observed mice acting as a bridge, facilitating the transmission of LGTV to neighboring naïve ticks during blood feeding. In conclusion, the virus-vector-host model employed in this study provides valuable insights into the replication and transmission of LGTV throughout its life cycle.

Targeting Haemaphysalis longicornis serpin to prevent tick feeding and pathogen transmission

Serine proteinase inhibitors (serpins), identified from the hard tick Haemaphysalis longicornis of China, play significant roles in various animal physiological processes. In this study, we showed that H. longicornis serpins (Hlserpin-a and Hlserpin-b) were induced during blood-feeding in nymph ticks and exhibited anticoagulation activity in vitro. Silencing Hlserpins through RNA interference (RNAi) significantly impaired tick feeding. Immunization of mice with recombinant Hlserpins or passive transfer of Hlserpin antiserum significantly curtails the efficacy of tick feeding. Concurrently, the transmission of the Langat virus (LGTV) from ticks to mice witnessed a substantial decrease when Hlserpins were silenced. Our findings suggest that inhibiting Hlserpins can hamper tick engorgement and pathogen transmission, indicating the potential of Hlserpins as a vaccine to counter tick-borne diseases.

Dermacentor silvarum, a Medically Important Tick, May Not Be a Competent Vector to Transmit Jingmen Tick Virus

Background: Jingmen tick virus (JMTV) has attracted great attention due to its potential pathogenicity in humans and its transmission by ticks. Dermacentor silvarum (D. silvarum) is one of the dominant tick species in northeastern China, and can transmit many pathogens to humans and animals. However, there have been no report of transmission of JMTV by D. silvarum. Materials and Methods: Ticks were collected from vegetation at the Aershan Port in Inner Mongolia in April 2019. And we do attempt to infect D. silvarum with JMTV by the immersion technique in laboratory conditions. The transmission of JMTV was examined by reverse transcriptase PCR, fluorescence in situ hybridization, and indirect immunofluorescence assay. Statistical analysis was performed using SPSS 24.0. Results: We found that JMTV may only be maintained in the tick without replication, and could not be transmitted to a host following transstadial transmission. Moreover, no virus colonization was found in the midgut or salivary glands of unfed D. silvarum; therefore, D. silvarum may not be susceptible to JMTV infection and therefore unlikely to carry and transmit JMTV. Conclusion: Our study has to some extent filled the knowledge gap regarding the possibility of JMTV transmission by a medically important tick vector, D. silvarum.

Horizontal and Vertical Transmission of Powassan Virus by the Invasive Asian Longhorned Tick, Haemaphysalis longicornis, Under Laboratory Conditions

The Asian longhorned tick, Haemaphysalis longicornis, is an ixodid tick native to East Asia that was first detected in North America outside a port of entry in 2017. This invasive species has since been detected in 17 states. As the invasive range of the tick continues to expand, the vector competence of H. longicornis for pathogens native to North America must be assessed. Here, we evaluate the vector competence of H. longicornis for Powassan virus (POWV) under laboratory conditions. POWV is a North American tick-borne flavivirus that is typically transmitted through the bite of Ixodes species ticks. The invasive range of H. longicornis is expected to overlap heavily with the geographic range of Ixodes scapularis and POWV cases, highlighting the potential for this invasive tick species to amplify POWV transmission in natural foci should the native tick vectors and H. longicornis share similar hosts. In these studies, adult female H. longicornis ticks were infected with POWV via anal pore microinjection. Viral RNA and infectious virions were detected in tick tissues via q-RT-PCR and focus-forming assay, respectively. POWV-injected female ticks were infested on mice, and virus was transmitted to mice during tick feeding, as shown by clinical signs of disease and seroconversion in the tick-exposed mice, as well as the detection of viral RNA in various mouse tissues. A POWV-injected female tick transmitted virus to her larval progeny, indicating that H. longicornis can vertically transmit POWV. These naturally-infected larval ticks were also able to transmit POWV to the mouse on which they fed and to the nymphal stage after molting, further demonstrating that H. longicornis can transmit POWV in the horizontal and transstadial modes. Larval and nymphal ticks were also orally infected with POWV while feeding on viremic mice. Additionally, this study provides the first report of POWV neuropathology based on a natural tick transmission model of POWV. Together, our results suggest that the invasive H. longicornis tick is a competent vector of POWV. These findings underline the growing danger this tick may pose to human health in the United States. Additional scholarship on the tick's biology, ecology, and pathogen transmission dynamics in nature will be important towards understanding the full public health impact of this invasive species.

An Experimental Murine Model to Study Acquisition Dynamics of Tick-Borne Langat Virus in Ixodes scapularis

Ixodes scapularis ticks acquire several pathogens from reservoir animals and transmit them to humans. Development of an animal model to study acquisition/transmission dynamics of these pathogens into and from ticks, respectively, is challenging due to the fact that in nature ticks feed for a longer duration and on multiple vertebrate hosts. To understand the complex nature of pathogen acquisition/transmission, it is essential to set up a successful tick blood feeding method on a suitable vertebrate host. In this study, we provide evidence that murine model can be successfully used to study acquisition dynamics of Langat virus (LGTV), a member of tick-borne flaviviruses. Mice were inoculated intraperitoneally with LGTV that showed detectable viral loads in blood, skin, and other tissues including the brain. Both larval and nymphal ticks that were allowed to feed on the murine host successfully acquired LGTV loads. Also, we found that after molting, LGTV was transstadially transmitted from larval to nymphal stage. In addition, we noted that LGTV down-regulated IsSMase expression in all groups of ticks possibly for its survival in its vector host. Taken together, we provide evidence for the use of murine model to not only study acquisition dynamics of LGTV but also to study changes in tick gene expression during acquisition of arboviruses into ticks.

An Ixodes scapularis glutathione S-transferase plays a role in cell survival and viability during Langat virus infection of a tick cell line

Ticks are important vectors of diseases affecting both humans and animals. To be an efficient vector, ticks have to survive infection by pathogens such as the Langat virus (LGTV). One method utilized by ticks is their complex antioxidant mechanism. Included in the vast antioxidant processes are several enzymes involved in redox homeostasis. The ubiquitous glutathione S-transferases (GSTs) belong to the antioxidant family of enzymes. In this study, we evaluated the role of a GST during LGTV infection. ISE6 cells were infected with LGTV with a multiplicity of infection (MOI) of 0.01 and observed daily. The infection success was monitored via indirect immunofluorescent antibody test (IFAT) for LGTV for up to 4 days. The gene expression of IsGST1 was determined by real-time polymerase chain reaction (PCR) using IsGST1 gene-specific primers. Knockdown of the IsGST1 gene with subsequent LGTV infection was also performed. Afterward, ISE6 cell mortality and viability were checked daily until the fourth day. The virus titer from supernatants of IsGST1-knockdown cells was quantified using a focus-formation assay. IFAT data showed that LGTV infects ISE6 cells in a time-dependent manner with increasing infection from day 0 to day 4. The IsGST1 genes showed an increasing expression until day 2 of infection, while decreased expression was observed from day 3 to day 4 post-infection. Knockdown of the IsGST1 resulted in increased mortality on the third day of infection, while the cell viability was also negatively affected by the knockdown of the IsGST1 genes from day 0 to day 4 post-infection. Knockdown of the IsGST1 genes also resulted in a decreased viral titer from the supernatants of the ISE6 cells infected with LGTV. Based on the results, GSTs are possibly utilized both by cells and the virus for mutual survival and proliferation.

An efficient microinjection method to generate human anaplasmosis agent Anaplasma phagocytophilum-infected ticks

Ticks are important vectors that transmit several pathogens including human anaplasmosis agent, Anaplasma phagocytophilum. This bacterium is an obligate intracellular rickettsial pathogen. An infected reservoir animal host is often required for maintenance of this bacterial colony and as a source for blood to perform needle inoculations in naïve animals for tick feeding studies. In this study, we report an efficient microinjection method to generate A. phagocytophilum-infected ticks in laboratory conditions. The dense-core (DC) form of A. phagocytophilum was isolated from in vitro cultures and injected into the anal pore of unfed uninfected Ixodes scapularis nymphal ticks. These ticks successfully transmitted A. phagocytophilum to the murine host. The bacterial loads were detected in murine blood, spleen, and liver tissues. In addition, larval ticks successfully acquired A. phagocytophilum from mice that were previously infected by feeding with DC-microinjected nymphal ticks. Transstadial transmission of A. phagocytophilum from larvae to nymphal stage was also evident in these ticks. Taken together, our study provides a timely, rapid, and an efficient method not only to generate A. phagocytophilum-infected ticks but also provides a tool to understand acquisition and transmission dynamics of this bacterium and perhaps other rickettsial pathogens from medically important vectors.

Effects of the essential oil from Cymbopogon citratus on mortality and morphology of the tick Haemaphysalis longicornis (Acari: Ixodidae)

Haemaphysalis longicornis is one of the most prevalent tick species across eastern Asia, Australia, and New Zealand, and has been implicated as a vector of several pathogenic agents. This study evaluated the in vitro acaricidal efficacy of Cymbopogon citratus (lemongrass) essential oil on unfed H. longicornis using the adult and nymph immersion test, and the larval packet test. Six concentrations with three replications each of 10, 20, 30, 40, 50 and 60 mg/mL (adults and nymphs) were used, and 2.5, 5, 10, 20, 40 and 80 mg/mL (larvae), with control group (50% ethanol). The adult and nymph mortality rates were 98 and 100% at 50 mg/mL, and 95 and 100% at 60 mg/mL, respectively, whereas the larval mortality rate was 94 and 96% at 40 and 80 mg/mL, respectively. Mortality of adult, nymph and larva increased significantly in a dose-dependent manner. The LC₅₀ for adult, nymph, and larva, were 29.21 (95% confidence interval 25.90-32.58), 28.18 (23.78-32.25), and 28.06 (25.57-30.90) mg/mL, respectively. Scanning electron microscopy and light microscopy revealed a disjointed sensilla base from the sockets, cuticular cracks, blocked aeropyles, and shrinking of the midgut. These results showed that C. citratus essential oil could be a good eco-friendly alternative control strategy against ectoparasites like ticks due to its high acaricidal efficacy.

A Continuing Exploration of Tick-Virus Interactions Using Various Experimental Viral Infections of Hard Ticks

To fully unravel the ixodid ticks’ role as vectors of viral pathogens, their susceptibility to new control measures, and their ability to develop acaricide resistance, acclimatization of ticks under laboratory conditions is greatly needed. However, the unique and complicated feeding behavior of these ticks compared to that of other hematophagous arthropods requires efficient and effective techniques to infect them with tick-borne viruses (TBVs). In addition, relatively expensive maintenance of animals for blood feeding and associated concerns about animal welfare critically limit our understanding of TBVs. This mini review aims to summarize the current knowledge about the artificial infection of hard ticks with viral pathogens, which is currently used to elucidate virus transmission and vector competence and to discover immune modulators related to tick–virus interactions. This review will also present the advantages and limitations of the current techniques for tick infection. Fortunately, new artificial techniques arise, and the limitations of current protocols are greatly reduced as researchers continuously improve, streamline, and standardize the laboratory procedures to lower cost and produce better adoptability. In summary, convenient and low-cost techniques to study the interactions between ticks and TBVs provide a great opportunity to identify new targets for the future control of TBVs.

Vector competence of Haemaphysalis longicornis ticks for a Japanese isolate of the Thogoto virus

Thogoto virus (THOV), a tick-borne arbovirus not previously reported in East Asia, was recently isolated from Haemaphysalis longicornis in Kyoto, Japan. In this study, we investigated the vector competence of H. longicornis ticks for a Japanese isolate of the Thogoto virus using anal pore microinjection and experimental virus acquisition. Our results showed that anal pore microinjection can readily infect adult ticks, and THOV-infected ticks can successfully transmit the virus to mice. Blood feeding was also critical in the distribution of the virus in tick organs, most especially in the salivary glands. Furthermore, co-feeding between an infected adult and naïve nymphs can also produce infected molted adults that can horizontally transmit THOV to mice. Altogether, our results suggest that H. longicornis is a competent vector for the Japanese THOV isolate and could be the primary tick vector of the virus in Japan.