Acquisition of Rickettsia rickettsii (Rickettsiales: Rickettsiaceae) by Haemaphysalis longicornis (Acari: Ixodidae) through co-feeding with infected Dermacentor variabilis (Acari: Ixodidae) in the laboratory

Haemaphysalis longicornis (Neumann) is an invasive ixodid tick originating from eastern Asia which recently has become established in the United States. In its native range, this tick can transmit several pathogens to animals and humans, but little is known about its ability to acquire and transmit pathogens native to the United States. Geographic overlap with ticks such as Dermacentor variabilis (Say), a known vector of Rickettsia rickettsii, makes investigation into the interactions between H. longicornis and D. variabilis of interest to the public health community. Previous studies have shown that H. longicornis can serve as a competent vector of R. rickettsii under laboratory settings, but there is little information on its ability to acquire this pathogen via other biologically relevant routes, such as co-feeding. Here, we assess the ability of H. longicornis nymphs to acquire R. rickettsii through co-feeding with infected D. variabilis adults on a vertebrate animal model under laboratory conditions. The median infection prevalence in engorged H. longicornis nymphs across 8 cohorts was 0% with an interquartile range (IQR) of 4.13%. Following transstadial transmission, the median infection prevalence in flat females was 0.7% (IQR = 2.4%). Our results show that co-feeding transmission occurs at low levels in the laboratory between these 2 species. However, based on the relatively low transmission rates, this may not be a likely mechanism of R. rickettsii introduction to H. longicornis.

Incompetence of the Asian Longhorned Tick (Acari: Ixodidae) in Transmitting the Agent of Human Granulocytic Anaplasmosis in the United States

The Asian longhorned tick, Haemaphysalis longicornis Neumann (Acari: Ixodidae), was recently introduced into the United States and is now established in at least 15 states. Considering its ability for parthenogenetic propagation and propensity for creating high-density populations, there is concern that this tick may become involved in transmission cycles of endemic tick-borne human pathogens. Human granulocytic anaplasmosis (HGA) caused by Anaplasma phagocytophilum is one of the more common tick-borne diseases in the United States, especially in the northeastern and midwestern states. There is considerable geographical overlap between HGA cases and the currently known distribution of H. longicornis, which creates a potential for this tick to encounter A. phagocytophilum while feeding on naturally infected vertebrate hosts. Therefore, we evaluated the ability of H. longicornis to acquire and transmit the agent of HGA under laboratory conditions and compared it to the vector competence of I. scapularis. Haemaphysalis longicornis nymphs acquired the pathogen with the bloodmeal while feeding on infected domestic goats, but transstadial transmission was inefficient and PCR-positive adult ticks were unable to transmit the pathogen to naïve goats. Results of this study indicate that the Asian longhorned tick is not likely to play a significant role in the epidemiology of HGA in the United States.

Reproductive incompatibility between Amblyomma maculatum (Acari: Ixodidae) group ticks from two disjunct geographical regions within the USA

The Amblyomma maculatum Koch group of ixodid ticks consists of three species: A. maculatum, A. triste, and A. tigrinum. However, since Koch described this group in 1844, the systematics of its members has been the subject of ongoing debate. This is especially true of A. maculatum and A. triste; recent molecular analyses reveal insufficient genetic divergence to separate these as distinct species. Further confounding this issue is the discovery in 2014 of A. maculatum group ticks in southern Arizona (AZ), USA, that share morphological characteristics with both A. triste and A. maculatum. To biologically evaluate the identity of A. maculatum group ticks from southern Arizona, we analyzed the reproductive compatibility between specimens of A. maculatum group ticks collected from Georgia (GA), USA, and southern Arizona. Female ticks from both Arizona and Georgia were mated with males from both the Georgia and Arizona Amblyomma populations, creating two homologous and two heterologous F1 cohorts of ticks: GA ♀/GA ♂, AZ ♀/AZ ♂, GA ♀/AZ ♂, and AZ ♀/GA ♂. Each cohort was maintained separately into the F2 generation with F1 females mating only with F1 males from their same cohort. Survival and fecundity parameters were measured for all developmental stages. The observed survival parameters for heterologous cohorts were comparable to those of the homologous cohorts through the F1 generation. However, the F1 heterologous females produced F2 egg clutches that did not hatch, thus indicating that the Arizona and Georgia populations of A. maculatum group ticks tested here represent different biological species.

The Ability of the Invasive Asian Longhorned Tick Haemaphysalis longicornis (Acari: Ixodidae) to Acquire and Transmit Rickettsia rickettsii (Rickettsiales: Rickettsiaceae), the Agent of Rocky Mountain Spotted Fever, Under Laboratory Conditions

The invasive Asian longhorned tick, Haemaphysalis longicornis Neumann, was first detected in the United States in 2017. It has since been found in 12 states, and there is concern that the tick's parthenogenetic ability and wide variety of host species may allow for broader dissemination. Of the tick-borne diseases endemic to the United States, Rocky Mountain spotted fever (RMSF), a rapidly progressive and potentially fatal disease caused by Rickettsia rickettsii, is the most severe. There is considerable geographical overlap between spotted fever rickettsioses cases, which include RMSF, and the currently known distribution of H. longicornis, providing the potential for this tick to encounter this pathogen. We have evaluated the ability of H. longicornis to acquire and transmit R. rickettsii under laboratory conditions. Haemaphysalis longicornis as larvae and nymphs acquired the pathogen while feeding on infected guinea pigs. The infection persisted through every life stage, all of which were able to transmit R. rickettsii to naïve hosts. The pathogen was also transmitted at a low frequency between generations of H. longicornis through the ova. While H. longicornis was demonstrated to be a competent vector for R. rickettsii under laboratory conditions, the probability of its involvement in the maintenance and transmission of this pathogen in nature, as well as its potential impact on human health, requires further study.

Minimal Duration of Tick Attachment Sufficient for Transmission of Infectious Rickettsia rickettsii (Rickettsiales: Rickettsiaceae) by Its Primary Vector Dermacentor variabilis (Acari: Ixodidae): Duration of Rickettsial Reactivation in the Vector Revisited

It has been reported that starving ticks do not transmit spotted fever group Rickettsia immediately upon attachment because pathogenic bacteria exist in a dormant, uninfectious state and require time for 'reactivation' before transmission to a susceptible host. To clarify the length of reactivation period, we exposed guinea pigs to bites of Rickettsia rickettsii-infected Dermacentor variabilis (Say) and allowed ticks to remain attached for predetermined time periods from 0 to 48 h. Following removal of attached ticks, salivary glands were immediately tested by PCR, while guinea pigs were observed for 10-12 d post-exposure. Guinea pigs in a control group were subcutaneously inoculated with salivary glands from unfed D. variabilis from the same cohort. In a parallel experiment, skin at the location of tick bite was also excised at the time of tick removal to ascertain dissemination of pathogen from the inoculation site. Animals in every exposure group developed clinical and pathological signs of infection. The severity of rickettsial infection in animals increased with the length of tick attachment, but even attachments for less than 8 h resulted in clinically identifiable infection in some guinea pigs. Guinea pigs inoculated with salivary glands from unfed ticks also became severely ill. Results of our study indicate that R. rickettsii residing in salivary glands of unfed questing ticks does not necessarily require a period of reactivation to precede the salivary transmission and ticks can transmit infectious Rickettsia virtually as soon as they attach to the host.

Evaluation of reference genes for quantitative polymerase chain reaction across life cycle stages and tissue types of Tribolium castaneum

The genome of the genetic model for coleopteran insects, Tribolium castaneum, is now available for downstream applications. To facilitate gene expression studies in T. castaneum, genes were evaluated for suitability as normalizers in comparisons across tissues and/or developmental stages. In less diverse samples, such as comparisons within developmental stages or tissue only, normalizers for mRNA were more stable and consistent. Overall, the genes for ribosomal proteins rps6, rpl13a, rps3, and rps18 were the most stable normalizers for broad scale gene expression analysis in T. castaneum. However, their stability ranking was dependent upon the instrument as well as the analysis program. These data emphasize the need to optimize normalizers used in all real-time polymerase chain reaction experiments specifically for the experimental conditions and thermocycler and to carefully evaluate data generated by computational algorithms.

Evaluation of quantitative PCR reference genes for gene expression studies in Tribolium castaneum after fungal challenge

To investigate gene expression in Tribolium castaneum exposed to Beauveria bassiana, reference genes for qPCR were evaluated. Of these, beta-actin, alpha-tubulin, and RPS6 were not stable. The most stable were ribosomal protein genes, RPS3, RPS18, and RPL13a. Syntaxin1, syntaxin6, and E-cadherin may be appropriate for some experimental systems.

Compensatory proteolytic responses to dietary proteinase inhibitors in the red flour beetle, Tribolium castaneum (Coleoptera: Tenebrionidae)

Increasing levels of inhibitors that target cysteine and/or serine proteinases were fed to Tribolium castaneum larvae, and the properties of digestive proteinases were compared in vitro. Cysteine proteinases were the major digestive proteinase class in control larvae, and serine proteinase activity was minor. Dietary serine proteinase inhibitors had minimal effects on either the developmental time or proteolytic activity of T. castaneum larvae. However, when larvae ingested cysteine proteinase inhibitors, there was a dramatic shift from primarily cysteine proteinases to serine proteinases in the proteinase profile of the midgut. Moreover, a combination of cysteine and serine proteinase inhibitors in the diet prevented this shift from cysteine proteinase-based digestion to serine proteinase-based digestion, and there was a corresponding substantial retardation in growth. These data suggest that the synergistic inhibitory effect of a combination of cysteine and serine proteinase inhibitors in the diet of T. castaneum larvae on midgut proteolytic activity and beetle developmental time is achieved through the prevention of the adaptive proteolytic response to overcome the activity of either type of inhibitor.

Effects of proteinase inhibitors on digestive proteinases and growth of the red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae)

The physiology of the gut lumen of the red flour beetle, T. castaneum, was studied to determine the conditions for optimal protein hydrolysis. Although the pH of gut lumen extracts from T. castaneum was 6.5, maximum hydrolysis of casein by gut proteinases occurred at pH 4.2. The synthetic substrate N-alpha-benzoyl-DL-arginine-rho-nitroanilide was hydrolyzed by T. castaneum gut proteinases in both acidic and alkaline buffers, whereas hydrolysis of N-succinyl-ala-ala-pro-phe rho-nitroanilide occurred in alkaline buffer. Inhibitors of T. castaneum digestive proteinases were examined to identify potential biopesticides for incorporation in transgenic seed. Cysteine proteinase inhibitors from potato, Job's tears, and sea anemone (equistatin) were effective inhibitors of in vitro casein hydrolysis by T. castaneum proteinases. Other inhibitors of T. castaneum proteinases included leupeptin, L-trans-epoxysuccinylleucylamido [4-guanidino] butane (E-64), tosyl-L-lysine chloromethyl ketone, and antipain. Casein hydrolysis was inhibited weakly by chymostatin, N-tosyl-L-phenylalanine chloromethyl ketone, and soybean trypsin inhibitor (Kunitz). The soybean trypsin inhibitor had no significant effect on growth when it was bioassayed alone, but it was effective when used in combination with potato cysteine proteinase inhibitor. In other bioassays with single inhibitors, larval growth was suppressed by the cysteine proteinase inhibitors from potato, Job's tears, or sea anemone. Levels of inhibition were similar to that observed with E-64, although the moles of proteinaceous inhibitor tested were approximately 1000-fold less. These proteinaceous inhibitors are promising candidates for transgenic seed technology to reduce seed damage by T. castaneum.

Digestive proteinases in Lasioderma serricorne (Coleoptera: Anobiidae)

The cigarette beetle, Lasioderma serricorne (Fabricius), is a common pest of stored foods. A study of digestive proteinases in L. serricorne was performed to identify potential targets for proteinaceous biopesticides, such as proteinase inhibitors. Optimal casein hydrolysis by luminal proteinases of L. serricorne was in pH 8.5-9.0 buffers, although the pH of luminal contents was slightly acidic. Results from substrate and inhibitor analyses indicated that the primary digestive proteinases were serine proteinases. The most effective inhibitors of caseinolytic hydrolysis were from soybean (both Bowman Birk and Kunitz), with some inhibition by chymostatin, N-tosyl-L-phenylalanine chloromethyl ketone, and leupeptin. Casein zymogram analysis identified at least eight proteolytic activities. Activity blot analyses indicated one major proteinase activity that hydrolysed the trypsin substrate N-alpha-benzoyl-L-arginine rho-nitroanilide, and three major proteinase activities that hydrolysed the chymotrypsin substrate N-succinyl ala-ala-pro-phe rho-nitroanilide. The absence of cysteine, aspartic, and metallo proteinases in L. serricorne digestion was evidenced by the lack of activation by thiol reagents, alkaline pH optima, and the results from class-specific proteinase inhibitors. The data suggest that protein digestion in L. serricorne is primarily dependent on trypsin- and chymotrypsin-like proteinases.