Identification, Baculoviral Expression, and Biochemical Characterization of a Novel Cholinesterase of Amblyomma americanum (Acari: Ixodidae)

A cDNA encoding a novel cholinesterase (ChE, EC 3.1.1.8) from the larvae of Amblyomma americanum (Linnaeus) was identified, sequenced, and expressed in Sf21 insect cell culture using the baculoviral expression vector pBlueBac4.5/V5-His. The open reading frame (1746 nucleotides) of the cDNA encoded 581 amino acids beginning with the initiation codon. Identical cDNA sequences were amplified from the total RNA of adult tick synganglion and salivary gland, strongly suggesting expression in both tick synganglion and saliva. The recombinant enzyme (rAaChE1) was highly sensitive to eserine and BW284c51, relatively insensitive to tetraisopropyl pyrophosphoramide (iso-OMPA) and ethopropazine, and hydrolyzed butyrylthiocholine (BuTCh) 5.7 times as fast as acetylthiocholine (ATCh) at 120 µM, with calculated K_M values for acetylthiocholine (ATCh) and butyrylthiocholine of 6.39 µM and 14.18 µM, respectively. The recombinant enzyme was highly sensitive to inhibition by malaoxon, paraoxon, and coroxon in either substrate. Western blots using polyclonal rabbit antibody produced by immunization with a peptide specific for rAaChE1 exhibited reactivity in salivary and synganglial extract blots, indicating the presence of AaChE1 antigenic protein. Total cholinesterase activities of synganglial or salivary gland extracts from adult ticks exhibited biochemical properties very different from the expressed rAaACh1 enzyme, evidencing the substantial presence of additional cholinesterase activities in tick synganglion and saliva. The biological function of AaChE1 remains to be elucidated, but its presence in tick saliva is suggestive of functions in hydrolysis of cholinergic substrates present in the large blood mean and potential involvement in the modulation of host immune responses to tick feeding and introduced pathogens.

Periviscerokinin (Cap2b; CAPA) receptor silencing in females of Rhipicephalus microplus reduces survival, weight and reproductive output

BACKGROUND

The cattle fever tick, Rhipicephalus (Boophilus) microplus, is a vector of pathogens causative of babesiosis and anaplasmosis, both highly lethal bovine diseases that affect cattle worldwide. In Ecdysozoa, neuropeptides and their G-protein-coupled receptors play a critical integrative role in the regulation of all physiological processes. However, the physiological activity of many neuropeptides is still unknown in ticks. Periviscerokinins (CAP2b/PVKs) are neuropeptides associated with myotropic and diuretic activities in insects. These peptides have been identified only in a few tick species, such as Ixodes ricinus, Ixodes scapularis and R. microplus, and their cognate receptor only characterized for the last two.

METHODS

Expression of the periviscerokinin receptor (Rhimi-CAP2bR) was investigated throughout the developmental stages of R. microplus and silenced by RNA interference (RNAi) in the females. In a first experiment, three double-stranded (ds) RNAs, named ds680-805, ds956-1109 and ds1102-1200, respectively, were tested in vivo. All three caused phenotypic effects, but only the last one was chosen for subsequent experiments. Resulting RNAi phenotypic variables were compared to those of negative controls, both non-injected and dsRNA beta-lactamase-injected ticks, and to positive controls injected with beta-actin dsRNA. Rhimi-CAP2bR silencing was verified by quantitative reverse-transcriptase PCR in whole females and dissected tissues.

RESULTS

Rhimi-CAP2bR transcript expression was detected throughout all developmental stages. Rhimi-CAP2bR silencing was associated with increased female mortality, decreased weight of surviving females and of egg masses, a delayed egg incubation period and decreased egg hatching (P < 0.05).

CONCLUSIONS

CAP2b/PVKs appear to be associated with the regulation of female feeding, reproduction and survival. Since the Rhimi-CAP2bR loss of function was detrimental to females, the discovery of antagonistic molecules of the CAP2b/PVK signaling system should cause similar effects. Our results point to this signaling system as a promising target for tick control.

Pyrokinin receptor silencing in females of the southern cattle tick Rhipicephalus (Boophilus) microplus is associated with a reproductive fitness cost

Background

Rhipicephalusmicroplus is the vector of deadly cattle pathogens, especially Babesia spp., for which a recombinant vaccine is not available. Therefore, disease control depends on tick vector control. However, R.microplus populations worldwide have developed resistance to available acaricides, prompting the search for novel acaricide targets. G protein-coupled receptors (GPCRs) are involved in the regulation of many physiological processes and have been suggested as druggable targets for the control of arthropod vectors. Arthropod-specific signaling systems of small neuropeptides are being investigated for this purpose. The pyrokinin receptor (PKR) is a GPCR previously characterized in ticks. Myotropic activity of pyrokinins in feeding-related tissues of Rhipicephalussanguineus and Ixodesscapularis was recently reported.

Methods

The R.microplus pyrokinin receptor (Rhimi-PKR) was silenced through RNA interference (RNAi) in female ticks. To optimize RNAi, a dual-luciferase assay was applied to determine the silencing efficiency of two Rhimi-PKR double-stranded RNAs (dsRNA) prior to injecting dsRNA in ticks to be placed on cattle. Phenotypic variables of female ticks obtained at the endpoint of the RNAi experiment were compared to those of control female ticks (non-injected and beta-lactamase dsRNA-injected). Rhimi-PKR silencing was verified by quantitative reverse-transcriptase PCR in whole females and dissected tissues.

Results

The Rhimi-PKR transcript was expressed in all developmental stages. Rhimi-PKR silencing was confirmed in whole ticks 4 days after injection, and in the tick carcass, ovary and synganglion 6 days after injection. Rhimi-PKR silencing was associated with an increased mortality and decreased weight of both surviving females and egg masses (P < 0.05). Delays in repletion, pre-oviposition and incubation periods were observed (P < 0.05).

Conclusions

Rhimi-PKR silencing negatively affected female reproductive fitness. The PKR appears to be directly or indirectly associated with the regulation of female feeding and/or reproductive output in R.microplus. Antagonists of the pyrokinin signaling system could be explored for tick control.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05349-w.

Effects of essential oils on native and recombinant acetylcholinesterases of Rhipicephalus microplus

This study reports the action of essential oils (EO) from five plants on the activity of native and recombinant acetylcholinesterases (AChE) from Rhipicephalus microplus. Enzyme activity of native susceptible AChE extract (S.AChE), native resistant AChE extract (R.AChE), and recombinant enzyme (rBmAChE1) was determined. An acetylcholinesterase inhibition test was used to verify the effect of the EO on enzyme activity. EO from Eucalyptus globulus, Citrus aurantifolia, Citrus aurantium var.dulcis inhibited the activity of S.AChE and R.AChE. Oils from the two Citrus species inhibited S.AChE and R.AChE in a similar way while showing greater inhibition on R.AChE. The oil from E. globulus inhibited native AChE, but no difference was observed between the S.AChE and R.AChE; however, 71% inhibition for the rBmAChE1 was recorded. Mentha piperita oil also inhibited S.AChE and R.AChE, but there was significant inhibition at the highest concentration tested. Cymbopogon winterianus oil did not inhibit AChE. Further studies are warranted with the oils from the two Citrus species that inhibited R.AChE because of the problem with R. microplus resistant to organophosphates, which target AChE. C. winterianus oil can be used against R. microplus populations that are resistant to organophosphates because its acaricidal properties act by mechanism(s) other than AChE inhibition.

Association of Salivary Cholinesterase With Arthropod Vectors of Disease

Acetylcholinesterase (AChE) was previously reported to be present in saliva of the southern cattle tick, Rhipicephalus (Boophilus) microplus (Canestrini), with proposed potential functions to 1) reduce acetylcholine toxicity during rapid engorgement, 2) modulate host immune responses, and 3) to influence pathogen transmission and establishment in the host. Potential modulation of host immune responses might include participation in salivary-assisted transmission and establishment of pathogens in the host as has been reported for a number of arthropod vector-borne diseases. If the hypothesis that tick salivary AChE may alter host immune responses is correct, we reasoned that similar cholinesterase activities might be present in saliva of additional arthropod vectors. Here, we report the presence of AChE-like activity in the saliva of southern cattle ticks, Rhipicephalus (Boophilus) microplus; the lone star tick, Amblyomma americanum (Linnaeus); Asian tiger mosquitoes, Aedes albopictus (Skuse); sand flies, Phlebotomus papatasi (Scopoli); and biting midges, Culicoides sonorensis Wirth and Jones. Salivary AChE-like activity was not detected for horn flies Haematobia irritans (L.), stable flies Stomoxys calcitrans (L.), and house flies Musca domestica L. Salivary cholinesterase (ChE) activities of arthropod vectors of disease-causing agents exhibited various Michaelis-Menten KM values that were each lower than the KM value of bovine serum AChE. A lower KM value is indicative of higher affinity for substrate and is consistent with a hypothesized role in localized depletion of host tissue acetylcholine potentially modulating host immune responses at the arthropod bite site that may favor ectoparasite blood-feeding and alter host defensive responses against pathogen transmission and establishment.

Sand Fly Colony Crash Tentatively Attributed to Nematode Infestation

Maintenance of laboratory colonies of insects and other arthropod pests offers significant research advantages. The availability, age, sex, housing conditions, nutrition, and relative uniformity over time of biological material for research facilitate comparison of results between experiments that would otherwise be difficult or impossible. A laboratory research colony of Phlebotomus papatasi (Scopoli), old world sand flies, was maintained with high-colony productivity for a number of years, but within a relatively short (4-6 mo) time period, colony productivity declined from over 10,000 flies per week to less than 100 per week. Mites and nematodes were both visible in the larval medium; however, the mites had been present throughout high productivity periods; therefore, it seemed reasonable to investigate the nematodes. PCR amplification of 18S rRNA yielded a clean cDNA sequence identified by BLAST search as Procephalobus sp. 1 WB-2008 (Rhabditida: Panagrolaimidae) small subunit ribosomal RNA gene, GenBank EU543179.1, with 475/477 nucleotide identities. Nematode samples were collected and identified as Tricephalobus steineri, (Andrássy, 1952) Rühm, 1956 (Rhabditida: Panagrolaimidae) based on morphological characteristics of the esophagus and the male copulatory apparatus. Mites (Tyrophagus putrescentiae [Acariformes: Acaridae]) may have played an additional predatory role in the loss of sand fly colony productivity. We hypothesized that the origin of the nematode infestation was rabbit dung from a local rabbitry used in preparation of the larval medium. Colony productivity was fully restored within 3 mo (two sand fly generational periods) by replacement of the rabbit dung from a clean source for use to prepare sand fly larval medium.

Terpenes on Rhipicephalus (Boophilus) microplus: Acaricidal activity and acetylcholinesterase inhibition

The Rhipicephalus (Boophilus) microplus tick is the main ectoparasite of cattle in tropical and subtropical regions worldwide. Resistance to chemical acaricides has become widespread affirming the need for new drugs to tick control. Terpenes have become a promising alternative for cattle tick control, however the mechanism of action of these compounds is still controversial. Inhibition of acetylcholinesterase (AChE) is a well established mechanism of action of organophosphate and carbamate acaricides, but the possible action of terpenes on tick AChEs has seldom been studied in resistant and sensitive strains of R. (B.) microplus. The aim of the present study was to evaluate terpene inhibition of AChE from resistant and sensitive strains of R. (B.) microplus in correlation with their acaricidal activity. Among the terpenes used in the present study, p-cymene, thymol, carvacrol, and citral displayed acaricidal activity with LC₅₀ of 1.75, 1.54, 1.41, and 0.38 mg.mL^-1 for the susceptible strain, and LC₅₀ of 1.40, 1.81, 1.10, and 1.13 mg.mL^-1 for the resistant strain. Thymol and carvacrol inhibited the AChE of the susceptible strain larvae with IC₅₀ of 0.93 and 0.04 mg.mL^-1, respectively. The IC₅₀ exhibited by eucalyptol, carvacrol and thymol for AChE of the resistant strain larvae were 0.36, 0.28, and 0.13 mg.mL^-1, respectively. This was the first study to investigate the action of terpenes on AChE from susceptible and resistant R. (B.) microplus. As not all terpenes with acaridical activity showed AChE inhibition, the participation of AChE in the acaricidal activity of terpenes needs further investigation.

Baculoviral Expression of Presumptive OP-Resistance Mutations in BmAChE1 of Rhipicephalus (Boophilus) microplus (Ixodida: Ixodidae) and Biochemical Resistance to OP Inhibition

The southern cattle tick, Rhipicephalus (Boophilus) microplus (Canestrini), transmits bovine babesiosis and anaplasmosis, and is endemic to Mexico, Latin and South America. Rhipicephalus (B.) microplus infestations within the United States are a continuing threat to U.S. cattle producers. An importation barrier between Texas and Mexico keeps the ticks from re-entering the United States. All cattle imported into the United States are dipped in an organophosphate (OP) acaricide and hand inspected for presence of ticks. Tick resistance has developed to most available acaricides, including coumaphos, the OP used in the cattle dip vats. OP-resistance can result from one or more mutations in the gene encoding the enzyme, acetylcholinesterase (AChE), resulting in production of an altered AChE resistant to OP inhibition. Previous research reported a large number of BmAChE1 mutations associated with OP resistance. We report baculovirus expression of recombinant tick BmAChE1 (rBmAChE) enzymes containing a single resistance-associated mutation, to assess their contribution to OP inhibition resistance. Surprisingly, of the naturally occurring BmAChE1 resistance-associated mutations, only D188G resulted in markedly reduced sensitivity to OP-inhibition suggesting that OP-insensitivity in BmAChE1 may result from the D188G mutation, or may possibly result from multiple mutations, each contributing a small decrease in OP sensitivity. Furthermore, an OP-insensitivity mutation (G119S) found in mosquitoes was expressed in rBmAChE1, resulting in 500-2000-fold decreased sensitivity to OP inhibition. Recombinant BmAChE1 with the G119S mutation demonstrated the lack of any structural prohibition to broad and high-level OP-insensitivity, suggesting potential increases in tick OP-resistance that would threaten the U.S. importation barrier to ticks.

The leucokinin-like peptide receptor from the cattle fever tick, Rhipicephalus microplus, is localized in the midgut periphery and receptor silencing with validated double-stranded RNAs causes a reproductive fitness cost

The cattle fever tick, Rhipicephalus microplus (Canestrini) (Acari: Ixodidae), is a one-host tick that infests primarily cattle in tropical and sub-tropical regions of the world. This species transmits deadly cattle pathogens, especially Babesia spp., for which a recombinant vaccine is not available. Therefore, disease control depends on tick vector control. Although R. microplus was eradicated in the USA, tick populations in Mexico and South America have acquired resistance to many of the applied acaricides. Recent acaricide-resistant tick reintroductions detected in the U.S. underscore the need for novel tick control methods. The octopamine and tyramine/octopamine receptors, both G protein-coupled receptors (GPCR), are believed to be the main molecular targets of the acaricide amitraz. This provides the proof of principle that investigating tick GPCRs, especially those that are invertebrate-specific, may be a feasible strategy for discovering novel targets and subsequently new anti-tick compounds. The R. microplus leucokinin-like peptide receptor (LKR), also known as the myokinin- or kinin receptor, is such a GPCR. While the receptor was previously characterized in vitro, the function of the leucokinin signaling system in ticks remains unknown. In this work, the LKR was immunolocalized to the periphery of the female midgut and silenced through RNA interference (RNAi) in females. To optimize RNAi experiments, a dual-luciferase system was developed to determine the silencing efficiency of LKR-double stranded RNA (dsRNA) constructs prior to testing those in ticks placed on cattle. This assay identified two effective dsRNAs. Silencing of the LKR with these two validated dsRNA constructs was verified by quantitative real time PCR (qRT-PCR) of female tick dissected tissues. Silencing was significant in midguts and carcasses. Silencing caused decreases in weights of egg masses and in the percentages of eggs hatched per egg mass, as well as delays in time to oviposition and egg hatching. A role of the kinin receptor in tick reproduction is apparent.

Interaction of plant essential oil terpenoids with the southern cattle tick tyramine receptor: A potential biopesticide target

An outbreak of the southern cattle tick, Rhipicephalus (Boophilus) microplus, (Canestrini), in the United States would have devastating consequences on the cattle industry. Tick populations have developed resistance to current acaricides, highlighting the need to identify new biochemical targets along with new chemistry. Furthermore, acaricide resistance could further hamper control of tick populations during an outbreak. Botanically-based compounds may provide a safe alternative for efficacious control of the southern cattle tick. We have developed a heterologous expression system that stably expresses the cattle tick's tyramine receptor with a G-protein chimera, producing a system that is amenable to high-throughput screening. Screening an in-house terpenoid library, at two screening concentrations (10 μM and 100 μM), has identified four terpenoids (piperonyl alcohol, 1,4-cineole, carvacrol and isoeugenol) that we believe are positive modulators of the southern cattle tick's tyramine receptor.

Tick Salivary Cholinesterase: A Probable Immunomodulator of Host-parasite Interactions

The southern cattle tick, Rhipicephalus (Boophilus) microplus (Canestrini), is the most economically important cattle ectoparasite in the world. Rhipicephalus microplus and Rhipicephalus annulatus (Say) continue to threaten U.S. cattle producers despite eradication and an importation barrier based on inspection, dipping of imported cattle in organophosphate (OP) acaricide, and quarantine of infested premises. OP acaricides inhibit acetylcholinesterase (AChE), essential to tick central nervous system function. Unlike vertebrates, ticks possess at least three genes encoding AChEs, differing in amino acid sequence and biochemical properties. Genomic analyses of R. microplus and the related tick, Ixodes scapularis, suggest that ticks contain many genes encoding different AChEs. This work is the first report of a salivary cholinesterase (ChE) activity in R. microplus, and discusses complexity of the cholinergic system in ticks and significance of tick salivary ChE at the tick-host interface. It further provides three hypotheses that the salivary ChE plausibly functions 1) to reduce presence of potentially toxic acetylcholine present in the large bloodmeal imbibed during rapid engorgement, 2) to modulate the immune response (innate and/or acquired) of the host to tick antigens, and 3) to influence transmission and establishment of pathogens within the host animal. Ticks are vectors for a greater number and variety of pathogens than any other parasite, and are second only to mosquitoes (owing to malaria) as vectors of serious human disease. Saliva-assisted transmission (SAT) of pathogens is well-known; however, the salivary components participating in the SAT process remain to be elucidated.

Pharmacological characterization of a tyramine receptor from the southern cattle tick, Rhipicephalus (Boophilus) microplus

The southern cattle tick (Rhipicephalus (Boophilus) microplus) is a hematophagous external parasite that vectors the causative agents of bovine babesiosis or cattle tick fever, Babesia bovis and B. bigemina, and anaplasmosis, Anaplasma marginale. The southern cattle tick is a threat to the livestock industry in many locations throughout the world. Control methods include the use of chemical acaricides including amitraz, a formamidine insecticide, which is proposed to activate octopamine receptors. Previous studies have identified a putative octopamine receptor from the southern cattle tick in Australia and the Americas. Furthermore, this putative octopamine receptor could play a role in acaricide resistance to amitraz. Recently, sequence data indicated that this putative octopamine receptor is probably a type-1 tyramine receptor (TAR1). In this study, the putative TAR1 was heterologously expressed in Chinese hamster ovary (CHO-K1) cells, and the expressed receptor resulted in a 39-fold higher potency for tyramine compared to octopamine. Furthermore, the expressed receptor was strongly antagonized by yohimbine and cyproheptadine, and mildly antagonized by mianserin and phentolamine. Tolazoline and naphazoline had agonistic or modulatory activity against the expressed receptor, as did the amitraz metabolite, BTS-27271; however, this was only observed in the presence of tyramine. The southern cattle tick's tyramine receptor may serve as a target for the development of anti-parasitic compounds, in addition to being a likely target of formamidine insecticides.

Discovery, adaptation and transcriptional activity of two tick promoters: Construction of a dual luciferase reporter system for optimization of RNA interference in rhipicephalus (boophilus) microplus cell lines

Dual luciferase reporter systems are valuable tools for functional genomic studies, but have not previously been developed for use in tick cell culture. We evaluated expression of available luciferase constructs in tick cell cultures derived from Rhipicephalus (Boophilus) microplus, an important vector of bovine babesiosis and anaplasmosis. Commercial promoters were evaluated for transcriptional activity driving luciferase expression in the tick cell lines. The human phosphoglycerate kinase (PGK) promoter resulted in detectable firefly luciferase activity within 2 days post-transfection of the R. microplus cell line BME26, with maximal activity at 5 days post-transfection. Several other promoters were weaker or inactive in the tick cells, prompting identification and assessment of transcriptional activity of the homologous ribosomal protein L4 (rpL4, GenBank accession no.: KM516205) and elongation factor 1α (EF-1α, GenBank accession no.: KM516204) promoters cloned from R. microplus. Evaluation of luciferase expression driven by various promoters in tick cell culture resulted in selection of the R. microplus rpL4 promoter and the human PGK promoter driving transcription of sequences encoding modified firefly and NanoLuc® luciferases for construction of a dual luciferase reporter system for use in tick cell culture.

Acetylcholinesterase of the sand fly, Phlebotomus papatasi (Scopoli): construction, expression and biochemical properties of the G119S orthologous mutant

Background

Phlebotomus papatasi vectors zoonotic cutaneous leishmaniasis. Previous expression of recombinant P. papatasi acetylcholinesterase (PpAChE1) revealed 85% amino acid sequence identity to mosquito AChE and identified synthetic carbamates that effectively inhibited PpAChE1 with improved specificity for arthropod AChEs compared to mammalian AChEs. We hypothesized that the G119S mutation causing high level resistance to organophosphate insecticides in mosquitoes may occur in PpAChE1 and may reduce sensitivity to inhibition. We report construction, expression, and biochemical properties of rPpAChE1 containing the G119S orthologous mutation.

Methods

Targeted mutagenesis introduced the G119S orthologous substitution in PpAChE1 cDNA. Recombinant PpAChE1 enzymes containing or lacking the G119S mutation were expressed in the baculoviral system. Biochemical assays were conducted to determine altered catalytic properties and inhibitor sensitivity resulting from the G119S substitution. A molecular homology model was constructed to examine the modeled structural interference with docking of inhibitors of different classes. Genetic tests were conducted to determine if the G119S orthologous codon existed in polymorphic form in a laboratory colony of P. papatasi.

Results

Recombinant PpAChE1 containing the G119S substitution exhibited altered biochemical properties, and reduced inhibition by compounds that bind to the acylation site on the enzyme (with the exception of eserine). Less resistance was directed against bivalent or peripheral site inhibitors, in good agreement with modeled inhibitor docking. Eserine appeared to be a special case capable of inhibition in the absence of covalent binding at the acylation site. Genetic tests did not detect the G119S mutation in a laboratory colony of P. papatasi but did reveal that the G119S codon existed in polymorphic form (GGA + GGC).

Conclusions

The finding of G119S codon polymorphism in a laboratory colony of P. papatasi suggests that a single nucleotide transversion (GGC → AGC) may readily occur, causing rapid development of resistance to organophosphate and phenyl-substituted carbamate insecticides under strong selection. Careful management of pesticide use in IPM programs is important to prevent or mitigate development and fixation of the G119S mutation in susceptible pest populations. Availability of recombinant AChEs enables identification of novel inhibitory ligands with improved efficacy and specificity for AChEs of arthropod pests.

Discovery of microRNAs of the stable fly (Diptera: Muscidae) by High-throughput sequencing

The stable fly, Stomoxys calcitrans (L.), is a serious ectoparasite affecting animal production and health of both animals and humans. Stable fly control relies largely on chemical insecticides; however, the development of insecticide resistance as well as environmental considerations requires continued discovery research to develop novel control technologies. MicroRNAs (miRNAs) are a class of short noncoding RNAs that have been shown to be important regulators of gene expression across a wide variety of organisms, and may provide an innovative approach with regard to development of safer more targeted control technologies. The current study reports discovery ad initial comparative analysis of 88 presumptive miRNA sequences from the stable fly, obtained using high-throughput sequencing of small RNAs. The majority of stable fly miRNAs were 22-23 nt in length. Many miRNAs were arthropod specific, and several mature miRNA sequences showed greater sequence identity to miRNAs from other blood-feeding dipterans such as mosquitoes rather than to Drosophilids. This initial step in characterizing the stable fly microRNAome provides a basis for further analyses of life stage-specific and tissue-specific expression to elucidate their functional roles in stable fly biology.

Inhibitor profile of bis(n)-tacrines and N-methylcarbamates on acetylcholinesterase from Rhipicephalus (Boophilus) microplus and Phlebotomus papatasi

The cattle tick, Rhipicephalus (Boophilus) microplus (Bm), and the sand fly, Phlebotomus papatasi (Pp), are disease vectors to cattle and humans, respectively. The purpose of this study was to characterize the inhibitor profile of acetylcholinesterases from Bm (BmAChE1) and Pp (PpAChE) compared to human and bovine AChE, in order to identify divergent pharmacology that might lead to selective inhibitors. Results indicate that BmAChE has low sensitivity (IC₅₀ = 200 μM) toward tacrine, a monovalent catalytic site inhibitor with sub micromolar blocking potency in all previous species tested. Similarly, a series of bis(n)-tacrine dimer series, bivalent inhibitors and peripheral site AChE inhibitors possess poor potency toward BmAChE. Molecular homology models suggest the rBmAChE enzyme possesses a W384F orthologous substitution near the catalytic site, where the larger tryptophan side chain obstructs the access of larger ligands to the active site, but functional analysis of this mutation suggests it only partially explains the low sensitivity to tacrine. In addition, BmAChE1 and PpAChE have low nanomolar sensitivity to some experimental carbamate anticholinesterases originally designed for control of the malaria mosquito, Anopheles gambiae. One experimental compound, 2-((2-ethylbutyl)thio)phenyl methylcarbamate, possesses >300-fold selectivity for BmAChE1 and PpAChE over human AChE, and a mouse oral LD₅₀ of >1500 mg/kg, thus providing an excellent new lead for vector control.

Acetylcholinesterase of the sand fly, Phlebotomus papatasi (Scopoli): cDNA sequence, baculovirus expression, and biochemical properties

Background

Millions of people and domestic animals around the world are affected by leishmaniasis, a disease caused by various species of flagellated protozoans in the genus Leishmania that are transmitted by several sand fly species. Insecticides are widely used for sand fly population control to try to reduce or interrupt Leishmania transmission. Zoonotic cutaneous leishmaniasis caused by L. major is vectored mainly by Phlebotomus papatasi (Scopoli) in Asia and Africa. Organophosphates comprise a class of insecticides used for sand fly control, which act through the inhibition of acetylcholinesterase (AChE) in the central nervous system. Point mutations producing an altered, insensitive AChE are a major mechanism of organophosphate resistance in insects and preliminary evidence for organophosphate-insensitive AChE has been reported in sand flies. This report describes the identification of complementary DNA for an AChE in P. papatasi and the biochemical characterization of recombinant P. papatasi AChE.

Methods

A P. papatasi Israeli strain laboratory colony was utilized to prepare total RNA utilized as template for RT-PCR amplification and sequencing of cDNA encoding acetylcholinesterase 1 using gene specific primers and 3’-5’-RACE. The cDNA was cloned into pBlueBac4.5/V5-His TOPO, and expressed by baculovirus in Sf21 insect cells in serum-free medium. Recombinant P. papatasi acetylcholinesterase was biochemically characterized using a modified Ellman’s assay in microplates.

Results

A 2309 nucleotide sequence of PpAChE1 cDNA [GenBank: JQ922267] of P. papatasi from a laboratory colony susceptible to insecticides is reported with 73-83% nucleotide identity to acetylcholinesterase mRNA sequences of Culex tritaeniorhynchus and Lutzomyia longipalpis, respectively. The P. papatasi cDNA ORF encoded a 710-amino acid protein [GenBank: AFP20868] exhibiting 85% amino acid identity with acetylcholinesterases of Cx. pipiens, Aedes aegypti, and 92% amino acid identity for L. longipalpis. Recombinant P. papatasi AChE1 was expressed in the baculovirus system and characterized as an insect acetylcholinesterase with substrate preference for acetylthiocholine and inhibition at high substrate concentration. Enzyme activity was strongly inhibited by eserine, BW284c51, malaoxon, and paraoxon, and was insensitive to the butyrylcholinesterase inhibitors ethopropazine and iso-OMPA.

Conclusions

Results presented here enable the screening and identification of PpAChE mutations resulting in the genotype for insensitive PpAChE. Use of the recombinant P. papatasi AChE1 will facilitate rapid in vitro screening to identify novel PpAChE inhibitors, and comparative studies on biochemical kinetics of inhibition.

Acetylcholinesterase of Haematobia irritans (Diptera: Muscidae): baculovirus expression, biochemical properties, and organophosphate insensitivity of the G262A mutant

This study reports the baculovirus expression and biochemical characterization of recombinant acetylcholinesterase from Haematobia irritans (L.) (rHiAChE) and the effect of the previously described G262A mutation on enzyme activity and sensitivity to selected organophosphates. The rHiAChE was confirmed to be an insect AChE2-type enzyme with substrate preference for acetylthiocholine (Km 31.3 microM) over butyrylthiocholine (Km 63.4 microM) and inhibition at high substrate concentration. Enzyme activity was strongly inhibited by eserine (2.3 x 10(-10) M), BW284c51 (3.4 x 10(-8) M), malaoxon (3.6 x 10(-9) M), and paraoxon (1.8 x 10(-7) M), and was less sensitive to the butyrylcholinesterase inhibitors ethopropazine (1.1 x 10(-6) M) and iso-OMPA (4.1 x 10(-4) M). rHiAChE containing the G262A substitution exhibited decreased substrate affinity for both acetylthiocholine (Km 40.9 microM) and butyrylthiocholine (Km 96.3 microM), and exhibited eight-fold decreased sensitivity to paraoxon, and approximately 1.5- to 3-fold decreased sensitivity to other inhibitors. The biochemical kinetics are consistent with previously reported bioassay analysis, suggesting that the G262A mutation contributes to, but is not solely responsible for observed phenotypic resistance to diazinon or other organophosphates.

Sequence polymorphism in acetylcholinesterase transcripts and genotyping survey of BmAchE1 in laboratory and Mexican strains of Rhipicephalus (Boophilus) microplus

ABSTRACT Acetylcholinesterase cDNAs, BmAChE1, BmAChE2, and BmAChE3 of Rhipicephalus (Boophilus) microplus (Canestrini) were sequenced and found to exhibit significant polymorphism. A portion of the predicted amino acid substitutions in BmAChE1, BmAChE2, and BmAChE3 were found predominantly in organophosphate-resistant strains, but most did not correlate with resistant status. Multiple transcripts were observed from individual ticks, suggesting possible gene duplication or alternative splicing to produce more than two transcripts per individual. BmAChE1 transcript polymorphisms associating with organophosphate-resistant status in laboratory strains were surveyed in laboratory and Mexican strains of R. microplus by sequencing BmAChE1 genomic DNA. Quantitative real-time polymerase chain reaction was used to determine copy numbers of BmAChE1 (eight copies/haploid genome), BmAChE2 (16 copies/haploid genome), and BmAChE3 (four copies/haploid genome). Presence of at least three highly polymorphic amplified genes expressing AChE in tick synganglion suggested that ticks maintain a large and diverse assortment of AChE alleles available for rapid recombination and selection, which potentially reduces fitness costs associated with individual mutations. Elevated copy numbers for each of the BmAChEs may also explain previous failures to identify mutations resulting in insensitivity to organophosphates. It is clear that development of phenotypic resistance to organophosphates is highly complex and may be multigenic in character.

Acetylcholinesterase of Stomoxys calcitrans (L.) (Diptera: Muscidae): cDNA sequence, baculovirus expression, and biochemical properties

A 2193-nucleotide cDNA encoding acetylcholinesterase (AChE) of the stable fly, Stomoxys calcitrans (L.) was sequenced and expressed in the baculovirus system. The open reading frame encoded a 91 amino acid secretion signal peptide and a 613 amino acid mature protein with 96% and 94% identity to the AChEs of Haematobia irritans (L.) and Musca domestica (L.), respectively. Structural characteristics of M. domestica, H. irritans, and Drosophila melanogaster AChEs were conserved in the S. calcitrans AChE. The recombinant enzyme was inhibited by eserine, coroxon, and paraoxon and exhibited K(m) values of 63.9μM for acetylthiocholine and 96.7μM for butyrylthiocholine, confirming its biochemical identity as an acetylcholinesterase (EC 3.1.1.7). These data will enable rapid identification and assay for mutations that reduce AChE sensitivity to organophosphate (OP) pesticides, potentially aiding resistance management efforts to prevent fixation of the mutations in pest populations.

Multiple transcripts encode glucose 6-phosphate dehydrogenase in the southern cattle tick, Rhipicephalus (Boophilus) microplus

Glucose 6-phosphate dehydrogenase (G6PDH) is an enzyme that plays a critical role in the production of NADPH. Here we describe the identification of four transcripts (G6PDH-A, -B, -C, and -D) that putatively encode the enzyme in the southern cattle tick, Rhipicephalus (Boophilus) microplus. The genomic DNA that is spliced to produce G6PDH-A and -B is 8,600-9,000 bases in length and comprises 12 exons. Comparison of the R. microplus G6PDH gene structure with those available from insects and mammals revealed that the tick gene is most like that of humans. Detection of the four transcripts was evaluated by quantitative RT-PCR using template from larvae, unfed adult females and males, salivary gland tissues from 2- to 3-day-fed adult females and males, and salivary gland tissue of 4- to 5-day-fed adult females. The G6PDH-A and -C transcripts were present in all templates, and both displayed induced expression in salivary gland tissue of fed, adult females but not matched males. The G6PDH-D transcript was detected only in unfed adults and in larvae, a stage in which it was most abundant relative to the other three transcripts. The G6PDH-B transcript, while detectable in all templates, was of low copy number suggesting it is a rare transcript. Induced expression of G6PDH-A and G6PDH-C in fed females may play a role in the tolerance of oxidative stress that is induced upon feeding, and the transcript abundance in fed females may be a function of bloodmeal volume and the time adult females spend on the host relative to adult males.

Genotyping mutations in BmAChE3: A survey of organophosphate-resistant and -susceptible strains of Rhipicephalus (Boophilus) microplus

Mutations I48L, I54V, R86Q, V137I, I492M, and T548A were identified previously in BmAChE3, a gene encoding acetylcholinesterase, from the organophosphate (OP) acaricide-resistant San Rommn strain of Rhipicephalus (Boophilus) microplus. Recombinant BmAChE3 acetylcholinesterase containing the R86Q mutation was shown to exhibit nearly 20-fold reduction in the rate of phosphorylation by paraoxon relative to the wild-type sequence. In addition, the R86Q mutation was present in resistant laboratory strains at elevated frequency compared with OP-susceptible strains but was insufficient to alone generate the OP-resistant phenotype (J. Med. Entomol. 44: 1013-1018). Here, we developed assays to genotype the remaining five mutations and evaluated frequency of all six BmAChE3 mutations in individual R. microplus ticks from laboratory and Mexican field-collected strains. We found a substantial number of individuals in known OP-susceptible strains that seemed to be homozygous for each of the mutations surveyed, the exception being I48L, which was infrequent in all strains, leading us to conclude that none of the mutations alone were responsible for generation of phenotypic resistance to OP acaricide.

Identification and characterization of a cDNA encoding the acetylcholinesterase ofHaematobia irritans(L.) (Diptera: Muscidae)

A 2217-nucleotide cDNA presumptively encoding acetylcholinesterase (AChE) of the horn fly, Haematobia irritans (L.) was sequenced. The open reading frame (ORF) encoded a 91 amino acid secretion signal peptide and a 613 amino acid mature protein with 95% identity and 98% similarity to the AChE of Musca domestica (L.). Structural features characteristic of the M. domestica and Drosophila melanogaster AChEs are conserved in the H. irritans AChE. The M. domestica and D. melanogaster AChEs are target sites for organophosphate inhibition as previously shown (Walsh et al. 2001. Biochem. J. 359: 175-181, Kozaki et al. 2002. Appl. Entomol. Zool. 37: 213-218), suggesting that this H. irritans AChE2 may be the target site for organophosphate.

A survey of Rhipicephalus microplus populations for mutations associated with pyrethroid resistance

Mutations associated with pyrethroid resistance were found in Mexican strains of Rhipicephalus microplus (Canestrini). A mutation in the sodium channel gene was reported in strains highly resistant to permethrin and another mutation in an esterase gene in a strain that shows moderate resistance to the same pesticide. Methods based on the melting temperature difference of amplified allele-specific DNA fragments were developed that can detect these mutations rapidly in individual larvae. When these methods were applied to ticks from various strains of R. microplus from Australia, neither of these mutations could be demonstrated. Different resistance mechanisms have apparently developed independently between Australian and Mexican strains of R. microplus.

R86Q, a mutation in BmAChE3 yielding a Rhipicephalus microplus organophosphate-insensitive acetylcholinesterase

Mutations were identified in the cDNA sequence encoding the acetylcholinesterase BmAChE3 in strains of Rhipicephalus (Boophilus) microplus (Canestrini) resistant or susceptible to organophosphate (OP) acaricide. The mutation that occurred most frequently in the OP-resistant San Román strain resulted in a substitution of glutamine (Q) for arginine (R) at position 86 in BmAChE3 (position 66 in mature BmAChE). Clones containing the mutant and wild-type cDNA sequences were expressed in the baculovirus system. Enzyme kinetics of recombinant BmAChE3 containing or lacking the R86Q mutation demonstrated that the R86Q mutation increased substrate affinity and conferred insensitivity to paraoxon inhibition. This is the first demonstration of a mutation in a gene encoding an ixodid acetylcholinesterase resulting in OP insensitivity. A restriction fragment length polymorphism assay was developed and used to diagnose the frequency of the R86Q mutation in BmAChE3 genomic DNA from seven laboratory-colonized strains. Use of the R86Q diagnostic assay detected an increased frequency of the R86Q mutation in OP-resistant tick strains compared with that of OP-susceptible strains; however, the R86Q mutation was also present in OP-susceptible strains at unexpectedly high frequency. Because the R86Q mutation generates an OP-resistant enzyme in vitro and it is present at an elevated frequency in laboratory strains selected for OP resistance, we conclude that the data are consistent with a potential role for BmAChE3 in development of OP resistance; however, because the R86Q mutation has a high frequency in susceptible strains, the R86Q mutation alone is insufficient to generate the OP-resistant phenotype at the organismal level. There are likely to be additional mutations in BmAChE3, mutations in additional acetylcholinesterase genes, or additional resistance mechanisms (e.g., oxidative metabolism) that contribute to expression of the OP-resistant phenotype.

Baculovirus expression of BmAChE3, a cDNA encoding an acetylcholinesterase of Boophilus microplus (Acari: Ixodidae)

The complete cDNA sequence encoding a Boophilus microplus (Canestrini) (Acari: Ixodidae) acetylcholinesterase (AChE3) was expressed in the baculovirus system. The recombinant AChE3 protein (rBmAChE3) was secreted as a soluble form into the cell culture medium and was identified as a functional AChE by substrate specificity and by inhibition with the AChE-specific inhibitors eserine sulfate and BW284c51. Inhibition kinetics of rBmAChE3, in the presence of the organophosphate paraoxon, revealed sensitivity comparable with that of adult, organophosphate-susceptible neural AChE. To our knowledge, this is the first report of the cloning and successful expression of a functional ixodid AChE.

Identification of a third Boophilus microplus (Acari: Ixodidae) cDNA presumptively encoding an acetylcholinesterase

Oligodeoxynucleotide primers, based on amino acid sequences conserved in known acetylcholinesterases (AChEs), were used in reverse-transcription polymerase chain reaction (RT-PCR) with mRNA from Boophilus microplus (Canestrini) as the template. Primer walking and rapid amplification of cDNA ends (RACE) techniques were used to complete the cDNA sequence identified by RT-PCR. The complete B. microplus cDNA sequence contained an open reading frame encoding a 620 amino acid protein with a 20 amino acid signal peptide at the N-terminus targeting the protein for the secretion pathway. BLAST searches of GenBank using the presumptively encoded protein revealed highest sequence similarity to AChEs. The presumptively encoded protein was of similar size and structural properties to other identified AChEs, including the presence of the catalytic triad (Ser, Glu, His) and appropriate placement of internal cysteines to yield three internal disulfide bonds corresponding to those of known AChEs. Putative conserved domains identified the sequence as a member of the carboxylesterase family, pfam00135.8, of which AChE is a member. This cDNA therefore presumptively encodes a third transcribed AChE (AChE3) cDNA of B. microplus. Comparison of the three AChE eDNA sequences expressed in B. microplus demonstrated no discernible nucleotide sequence homology and relatively low amino acid sequence homology, strongly suggesting that they are not alleles of one another. The potential presence of multiple expressed AChEs in B. microplus suggests alternative mechanisms for development of resistance to pesticides that target AChE. The homology-based identification of a third expressed AChE in B. microplus is a surprising result and strongly implies the need for confirmation of gene identity for presumptive AChEs.

Cloning and sequence analysis of a cDNA encoding Pso o II, a mite group II allergen of the sheep scab mite (Acari: Psoroptidae)

Psoroptes ovis (Hering), the sheep scab mite, is responsible for psoroptic scabies of cattle and sheep. Reverse translation of 30 N-terminal amino acids of the major P. ovis allergen, previously chosen as a candidate immunogen and identified as a 16 kDa protein yielded a degenerate sequence used to design oligodeoxynucleotide polymerase chain reaction (PCR) primers. Use of the PCR primers with a P. ovis cDNA library succeeded in amplification of a 90 bp cDNA gene fragment that was cloned, sequenced, and used to select unique sequencing/PCR primers. Primer walking generated overlapping subclones which yielded the 588 nucleotide consensus sequence of the cDNA encoding the 143 amino acid P. ovis allergen precursor. Nucleotide and translated sequences of the cDNA were compared with sequences in GenBank and found to be homologous to mite group II allergens Lep d II (formerly Lep d I) of Lepidoglyphus destructor Schrank, DerfII of Dermatophagoides farinae Hughes, Der p II of Derrmatophagoides pteronyssinus (Trouessart), Tyr p II of Tyrophagus putrescentiae (Schrank), Eur m II of Euroglyphus maynei (Cooreman) and Gly d II of Glycophagus domesticus (De Geer). The mature P. ovis allergen is composed of 126 amino acids with a calculated molecular mass of 13,468 Da, three disulfide bonds, and pI of 6.06 with one potential o-glycosylation site at Thr116. We designate the P. ovis 16 kDa protein as Pso o II in conformity with nomenclature for mite group II allergens.

Evaluation of natural Psoroptes ovis (Acarina: Psoroptidae) soluble proteins as candidate vaccine immunogens

In this study potential vaccine candidate immunogens were identified and evaluated in a vaccine challenge trial. Calves vaccinated with a partially purified fraction of Psoroptes ovis-soluble proteins had 8 of 14 calves free of palpable lesions 8 wk after a challenge infestation. A self-grooming behavioral response elicited by a pruritic immediate-type allergic reaction was believed to be an effector in protecting the vaccinated calves from a clinical P. ovis infestation.

A molecular model for illegitimate recombination in Bacillus subtilis

The recombinant DNA junctions at which pUB110 and Bacillus subtilis chromosomal DNA were joined to form the plasmid pKBT1 were cloned and sequenced. From the sequencing data we conclude that the pUB110 sequence is intact in the pair of cloned pKBT1 fragments and pTL12 sequences are not present. A molecular model for the formation of pKBT1 based on structural motifs characteristic of the joint sites is presented.

Shared epitopes between the soluble proteins of Hypoderma lineatum and Hypoderma bovis first instars

Cattle are known to acquire immunological resistance to hypodermyiasis by repeated exposure to both species of cattle grubs, Hypoderma lineatum (Villers) and Hypoderma bovis (L.). Vaccination of cattle with purified proteins of H. lineatum, particularly hypodermin A, is known to protect cattle against hypodermyiasis by this species. The development of a protective recombinant vaccine against both species using hypodermin A isolated from H. lineatum would require that immunological epitopes be shared by complementary proteins in H. bovis. The purpose of this study was to investigate the soluble proteins of H. bovis first-instars for shared epitopes with H. lineatum. Soluble H. lineatum and H. bovis first-instar larval proteins were resolved by nondenaturing polyacrylamide electrophoresis, blotted onto nitrocellulose paper, and probed with selected polyclonal cow, polyclonal rabbit, and mouse monoclonal antisera. Considerable cross-reactivity was demonstrated by antibodies in the serum of an H. lineatum-infested cow as 6 of 10 resolved H. bovis proteins were bound by the antibodies. The most common shared epitope(s) was associated with hypodermin C, a collagenolytic protease. Hypodermin A shared epitope(s) were noted on 1 prominent H. bovis band (HB1-2). Hypodermin B, a prominent protein in H. lineatum, did not appear to share epitopes with H. bovis proteins. Shared epitopes between H. bovis proteins and hypodermins A and C of H. lineatum would suggest that cross-protection of cattle against H. bovis can be expected by vaccination with recombinant proteins of H. lineatum.

Illegitimate recombination in Bacillus subtilis: a site-specific mechanism in the formation of plasmid pKBT1

The Bacillus subtilis plasmid pKBT1, the product of in vivo recE4-independent recombinal events, contains segments derived from pUB110 and the B. subtilis chromosome. To determine whether the pUB110 sequence is intact in PKBT1, two 1 kb fragments, each containing a site at which chromosomal and pUB110 sequences are joined, were cloned and sequenced. Sequencing data revealed that: 1). An intact copy of pUB110 is present in pKBT1; 2) The apparent recombination sites were adjacent to the Bam HI-generated ends of pUB110 sequences; 3) pTL12-derived sequences from the original transforming DNA were limited to no more than 1 bp outside the Bgl II recognition sequence; 4) Recombination sites at both ends of pUB110 contain a 19 bp inverted repeat with 15 homologous nucleotides. These findings suggest a site-specific mechanism acting during in vivo formation of pKBT1.

Colostral transfer of antibodies specific for Hypoderma lineatum proteins

Antibodies to Hypoderma lineatum were transferred to calves via colostrum. The antibodies transferred in the colostrum demonstrated specificity to all the H. lineatum first-instar proteins which were resolved by non-denaturing polyacrylamide gel electrophoresis and were capable of mediating a Type I hypersensitivity reaction. The kinetic decline of colostrum-acquired antibodies, the effect upon development of an autologous humoral response to H. lineatum and the host or parasite protective role of these antibodies are discussed.

Antigenicity and immunogenicity of Hypoderma lineatum soluble proteins in the bovine host

Protein species found in soluble crude extracts of Hypoderma lineatum (common cattle grub) 1st-instar larvae (HL1) were separated by non-denaturing and denaturing polyacrylamide gel electrophoresis (PAGE) and analyzed for antigenicity by Western blotting using serum from H. lineatum-infested and vaccinated cattle. All HL1 proteins resolved by non-denaturing PAGE were found to be antigenic in the infested bovine host. Treatment of the proteins with sodium dodecyl sulfate and 2-mercaptoethanol destroyed the ability of hypodermin B and the Peak 2 proteins from DEAE-ion exchange HPLC to be bound by antibody. The principal proteins, hypodermin A and hypodermin C (collagenase), appear to be the most immunogenic of the larval proteins. Although having similar amino acid composition, hypodermin A did not appear to share an antigenic epitope with the most prevalent protein, hypodermin C. These results may allow for the selection of proteins to be used in vaccine trials and studies of protective immunological mechanisms associated with acquired resistance to H. lineatum infestation in the bovine host.

Intermolecular recE4-independent recombination in Bacillus subtilis: formation of plasmid pKBT1

The plasmid pKBT1 was derived by in vivo recE4-independent recombinational event(s) yielding a structure containing regions of plasmid and chromosomal origin. BamHI digests of plasmid pUB110 (Kanr/Neor) and Bg/II digests of pTL12 (Tmpr, leuA) were mixed, ligated and used to transform competent cells of a recE4 strain of Bacillus subtilis. Kanamycin-resistant transformants were electrophoretically screened for hybrid plasmids. Plasmid pKBT1 (8.0 kb) was smaller than pTL12 (10.4 kb) but larger than monomeric pUB110 (4.5 kb). Plasmid pKBT1 was stably maintained in recE4 strains of B. subtilis and conferred kanamycin resistance but did not specify trimethoprim resistance or leucine prototrophy. At least 86% of the pUB110 monomer length was present in pKBT1 and was completely contained within a single 5.58 kb HindIII fragment. The other segment of pKBT1 was of chromosomal origin as evidenced by lack of homology to pTL12 and strong hybridization to B. subtilis chromosomal DNA. At least one of the in vivo recE4-independent event(s) which produced pKBT1 must have involved intermolecular recombination between transforming and chromosomal DNA. This finding differs from previous reports in which recE4-independent recombination involving pUB110 sequences was a strictly intramolecular event.

Larvicidal activity of Bacillus thuringiensis subsp. israelensis in the dipteran Haematobia irritans

A strain of Bacillus thuringiensis subsp. israelensis was found to be larvicidal to horn flies, Haematobia irritans (L. [Diptera:Muscidae]). The toxic activity was particulate, appeared during sporulation, and could be prevented by the addition of streptomycin before sporulation. Density gradient centrifugation in Renografin was used to separate endospores, crystals, and low-density particulate matter (fraction 3) from sporulated preparations. Larvicidal activity was restricted to purified crystals and fraction 3, indicating that delta-endotoxin of B. thuringiensis subsp. israelensis was active against horn fly larvae. Purified crystals produced mortality during larval feeding stages, but not pupal stages. Fraction 3 produced significant mortality during both larval and pupal stages. The mortality data indicated the presence of at least two dipteran-active toxins.