The action of two ethyl carbamates on acetylcholinesterase and reproductive organs of Rhipicephalus microplus

Efficacy of ethyl-4-bromophenyl carbamate on different Rhipicephalus microplus stages implanted in cattle

The effect of ethyl-4-bromophenyl carbamate on different Rhipicephalus microplus stages implanted in cattle was evaluated using the pen test with infestation chambers. Twelve steers were distributed into four groups (n = 3), each with four chambers (12 chambers per group), where approximately 1,000 R. microplus larvae were placed in each chamber. The chambers of the first group were sprayed with a solution of ethyl-4-bromophenyl carbamate (0.668 mg/mL) on day 2 post-infestation (PI) (exposed larvae). The chambers of the second group were sprayed with the same solution on day 8 PI (exposed nymphs), and the chambers of the third group were sprayed on day 16 PI (exposed adults) with the same solution. The chambers of the fourth group were used as controls. The percentages of engorged females, egg laying, egg production and egg hatching were evaluated in all groups. The percentage of cumulative reduction of hatched larvae was 98.3, 96.1 and 94.4% when larvae, nymph and adult stages were treated, respectively. The average cumulative reduction of hatched larvae, considering the three treated stages, was 96.3%, whereby the reproductive potential of this tick was drastically reduced. In conclusion, ethyl-4-bromophenyl carbamate acted as an ixodicide (lethal effect) when larval stages were sprayed and as a growth regulator when nymphal and adult stages were sprayed. The sum of these effects had a direct impact on the efficacy of the product in the pen test, and future studies will indicate the potential use of this product for tick control.

A practical protocol to prepare paraffin-embedded whole tick histology sections

Ticks are important ectoparasites that are capable of transmitting multiple classes of pathogens and are currently linked with many emerging tick-borne diseases worldwide. With increasing occurrences of tick-borne diseases in both humans and veterinary species, there is a continuous need to further our understanding of ticks and the pathogens they transmit. Whole tick histology provides a full scope of the tick internal anatomy, allowing researchers to examine multiple organs of interest in a single section. This is in contrast to other techniques that are more commonly utilized in tick-borne disease research, such as electron microscopy and light microscopy of individual organs. There is a lack of literature describing a practical technique to process whole tick histologic sections. Therefore, the current study aims to provide researchers with a workable protocol to prepare high quality paraffin-embedded whole tick histology sections. Amblyomma americanum adults were used as an example species for this study. After a series of pilot experiments using a combination of various fixatives, softening agents and processing techniques, we elected to compare two common fixatives, 10% neutral-buffered formalin (NBF) and Bouin's solution for whole ticks. Equal numbers of A. americanum unfed adults (n = 10/fixative) were processed identically and their whole tick histology coronal sections were individually scored. Higher scores were assigned to whole tick sections that contained more internal organs that are crucial for tick-borne disease research (e.g. salivary glands and midgut), high integrity of tissues and exoskeleton on the section, and good fixation and staining quality of the tissues. The mean total scores for Bouin's-fixed ticks were significantly higher compared to NBF-fixed ticks (p = 0.001). To further assess our preferred technique, we also demonstrated the feasibility of producing high quality whole tick sections for three other common tick species of medical importance (Rhipicephalus sanguineus, Ixodes scapularis, and Dermacentor variabilis) using Bouin's solution. While this technique may require further optimization for other tick species, we described a feasible protocol that uses commonly available tools, reagents and standard histologic equipment. This should allow any investigator to easily make adjustments to this protocol as needed based on their experimental goals.

Molecular targets for the development of new acaricides against Rhipicephalus microplus: a review

The cattle tick Rhipicephalus microplus is an ectoparasite with high economic importance to bovine culture, mainly in tropical and subtropical regions. The resistance of the tick from the commercial acaricides has hindered its control, thus motivating the search for new strategies. The purpose of this study was to perform a critical review about the main molecular targets of R. microplus that are useful for the discovery of new acaricides. Bibliographic search was conducted in the databases PubMed, ScienceDirect and CAB Direct, using the following descriptors: ‘Rhipicephalus microplus’, ‘Boophilus microplus’, ‘molecular targets’ and ‘action’, published between 2010 and 2021. Out of the 212 publications identified, 17 articles were selected for study inclusion. This review described 14 molecular targets and among these 4 are targets from commercial acaricides. Most of them are enzymes to catalyse important reactions to tick survival, related to energetic metabolism, mechanisms of biotransformation and neurotransmission. The data will be helpful in the development of new more effective and selective acaricides.

Effect of the formulation with fipronil and fluazuron on the reproductive biology and ovaries histopathology of Rhipicephalus microplus engorged females

This study aimed to evaluate the effects of a commercial formulation containing fipronil and fluazuron on the reproductive biology and the morphology of ovaries from Rhipicephalus microplus engorged females. To carry out the study, three calves were artificially infested every 3 days with approximately 5000 larvae. On day 0, the animals were treated with a commercial formulation containing fipronil (1.25 mg/kg) + fluazuron (2.5 mg/kg). Before the application of the acaricide, engorged females of R. microplus were collected to constitute the control group (10 for biology analyses and 20 for histology analyses). After applying the commercial formulation, naturally detached engorged females were recovered on days + 5, + 10, and + 20 (10 engorged females/day) to evaluate their reproductive biology, and on days + 4, + 12, and + 20 (20 engorged females/day) for histological evaluation of the ovaries. Females from the treated groups produced smaller amounts of eggs, exhibiting lower viability when compared to eggs from the control group (p < 0.05). The ovaries of females from all treated groups (+ 4, + 12, and + 20) showed morphological changes, including: cytoplasmic disorganization, cytoplasmic degradation, irregular shape of the oocyte and germinal vesicle, reduction and vacuolization of yolk granules and oocyte disruption. Oocytes were observed in smaller numbers in all stages of development (I, II, III, IV, and V) and greater numbers of indeterminate oocytes were verified in the ovaries of the treated groups when compared to the control group. Therefore, results showed that the commercial formulation containing fipronil and fluazuron affected the reproductive biology, caused morphological changes in the ovaries, and reduced the number of oocytes in R. microplus engorged females.

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.

Structure-Activity Relationship (SAR) and in vitro Predictions of Mutagenic and Carcinogenic Activities of Ixodicidal Ethyl-Carbamates

Ethyl-4-bromophenyl-carbamate (LQM 919) and Ethyl-4-chlorophenyl-carbamate (LQM 996) are compounds that inhibit egg-laying and hatching of tick larvae that are resistant to conventional ixodicides. The structure-activity relationship (SAR) to get the endpoint predictions of mutagenicity and carcinogenicity of the LQM 919 and LQM 996 was performed and the absence of mutagenicity was confirmed by Ames test. SAR analysis show no structural alerts indicating the ability of ethyl-carbamates to bind biomolecules or estrogen receptors. Endpoint of mutagenicity with and without metabolic activation showed that the ethyl-carbamates were negative (p <0.05) for mutagenicity induction in strains TA97, TA98, TA102, TA1535, TA1537 and TA1538 of Salmonella typhimurium. Pre-incubation with different ethyl-carbamate concentrations did not increase the number of spontaneously reverting colonies; moreover, the compounds did not induce a concentration-dependent increase in the number of reverting colonies in any of the strains used. This confirmed the absence of mutagenic activity in this test system. Exogenous metabolic activation did not modify these observations; suggesting that no metabolites with mutagenic activity were present. The endpoint of carcinogenicity in rats were negative for LQM 919 (p <0.05,) and LQM 996 (p <0.001). The results of the present study strongly suggest that ethyl-carbamates do not represent a risk for cancer in mammals.

In vivo and in vitro apoptosis induced by new acaricidal ethyl-carbamates in Rhipicephalus microplus

The ability of ethyl-4-bromophenylcarbamate (LQM 919) and ethyl-4-chlorophenylcarbamate (LQM 996) to induce in vivo apoptosis of Rhipicephalus microplus ovarian cells and in vitro apoptosis of tick and mammalian cell culture was evaluated. The ovaries of engorged females treated with 1 mg mL^-1 LQM 919 or LQM 996 presented more (p < 0.001) peroxidase-TUNEL-positive labeled cells (apoptotic cells) in situ than their respective control groups, and this increase was time-dependent (p < 0.001). The majority of apoptotic cells were observed in the epithelium and ovarian pedicel. HepG2, Vero and Rm-sus cells, as well as cells from primary cultures of R. microplus salivary glands, intestine and ovaries were exposed to different concentrations of the ethyl-carbamates. Both ethyl-carbamates induced a concentration-dependent reduction in the viability of all cell types (p < 0.001). Exposure to the ethyl-carbamates increased caspase 3 activity (p < 0.01) in primary cultures and cell lines, except in HepG2 cells. Fluorescent TUNEL-positive cells were observed in all cell types treated with 600 μM LQM 919 or LQM 996. These results indicate that both ethyl-carbamates induce apoptosis of the ovarian, intestinal and salivary glands cells in R. microplus and strongly suggest that this is their main mechanism of acaricidal action.

1,8-cineole and castor oil in sodium lauryl ether sulphate disrupt reproduction and ovarian tissue of Rhipicephalus (Boophilus) microplus

Essential and fixed oils have been researched as alternatives to chemical acaricides. The activity of volatile compounds from essential oils (1,8-cineole, citral and eugenol) at 1.0% (w/v) and fixed oil (castor oil) at 0.3% (w/v) dissolved in 2.0% (v/v) dimethyl sulfoxide (DMSO) + 0.2% (w/v) Tween 80^® was assessed against Rhipicephalus microplus using immersion tests. 1,8-cineole (29.0%) and castor oil (30.2%) had the highest reproductive inhibition rate. A second experiment was performed to verify the effect of the 1,8-cineole (10.0% w/v) and, or castor oil (0.3% w/v) on tick reproduction using different solubilizing agents. The highest reproductive inhibition was observed for the combination of 1,8-cineole/castor oil (94.1%) and 1,8-cineole in 2.0% (w/v) sodium lauryl ether sulphate (SLES) (92.8%). A third experiment showed morphological changes in R. microplus oocytes at different stages of development, as well as in pedicel cells. The most intense effects were observed when ticks were immersed in the formulation containing 1,8-cineole (10.0% w/v) and castor oil (0.3% w/v) dissolved in 2% (w/v) SLES. These findings highlight the potential of this formulation as an alternative for managing cattle ticks as their cytotoxic effects can reduce R. microplus reproductive success.

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.

Acute oral and contact toxicity of new ethyl-carbamates on the mortality and acetylcholinesterase activity of honey bee (Apis mellifera)

The effects produced by the ethyl-carbamates: ethyl-4-bromophenyl carbamate (LQM 919) and ethyl-4-chlorophenyl carbamate (LQM 996) on the mortality and behavior of Apis mellifera were evaluated by the acute oral toxicity test and the acute contact toxicity test. The oral lethal dose, 50% of the ethyl-carbamates was >145.24 μg per bee, and the oral lethal dose, 50% of propoxur was 0.072 μg per bee. Therefore, according to the OECD criteria, the ethyl-carbamates were classified as relatively nontoxic orally; meanwhile, propoxur was classified as highly toxic orally. In the contact test, lethal concentrations 50% of the ethyl-carbamates were 4.83 and 2.23 μg/cm² for LQM 919 and LQM 996, respectively; therefore, they were at least 10-fold less lethal (p < 0.05) than propoxur (0.22 μg/cm²). The ethyl-carbamates reduced the activity of A. mellifera acetylcholinesterase by up to 30%. The k_i and k_d values of both ethyl-carbamates were lower (p < 0.05) than those of propoxur and indicated that they are weak inhibitors and with low affinity to A. mellifera acetylcholinesterase, which along with the absence of behavioral alterations suggests that the mortality caused by ethyl carbamates is not related to damage to the nervous system. According to these results, the evaluated ethyl-carbamates can be considered a low ecotoxic risk for A. mellifera.

Toxic effects of new ethyl-carbamates on the morphology, mortality and acetylcholinesterase activity of Eisenia foetida

The toxicity of the ixodicidal carbamates ethyl-4-bromophenyl carbamate (LQM 919), ethyl-4-chlorophenyl carbamate (LQM 996) and propoxur on Eisenia foetida adults was evaluated to estimate their ecotoxic potential. The earthworm mortality and weight loss produced by the three evaluated carbamates showed a concentration-dependent effect (p < 0.0001) in the contact filter paper test (CFPT). In the artificial soil test (AST), mortality increased in relation to the exposure time (p < 0.0001) and the concentration (p < 0.01) of the carbamates. Only the earthworms exposed in the CFPT showed morphological alterations. According to the LC₅₀ obtained in the CFPT, the three carbamates were classified as very toxic and, according to the LC₅₀ obtained in the AST, the three carbamates were classified as highly toxic for E. foetida. The values of k_i and k_d indicated that LQM 919 and LQM 996 are weak inhibitors with lower affinity for the acetylcholinesterase of E. foetida than that of propoxur. The concentrations in the CFPT and AST at which 100% mortality was observed in E. foetida were 64- and 4-fold higher, respectively, than the egg hatching inhibitory concentration 99% reported for ticks.

Polypyridylruthenium(II) complexes exert anti-schistosome activity and inhibit parasite acetylcholinesterases

BACKGROUND

Schistosomiasis affects over 200 million people and there are concerns whether the current chemotherapeutic control strategy (periodic mass drug administration with praziquantel (PZQ)-the only licenced anti-schistosome compound) is sustainable, necessitating the development of new drugs.

METHODOLOGY/PRINCIPAL FINDINGS

We investigated the anti-schistosome efficacy of polypyridylruthenium(II) complexes and showed they were active against all intra-mammalian stages of S. mansoni. Two compounds, Rubb12-tri and Rubb7-tnl, which were among the most potent in their ability to kill schistosomula and adult worms and inhibit egg hatching in vitro, were assessed for their efficacy in a mouse model of schistosomiasis using 5 consecutive daily i.v. doses of 2 mg/kg (Rubb12-tri) and 10 mg/kg (Rubb7-tnl). Mice treated with Rubb12-tri showed an average 42% reduction (P = 0.009), over two independent trials, in adult worm burden. Liver egg burdens were not significantly decreased in either drug-treated group but ova from both of these groups showed significant decreases in hatching ability (Rubb12-tri-68%, Rubb7-tnl-56%) and were significantly morphologically altered (Rubb12-tri-62% abnormal, Rubb7-tnl-35% abnormal). We hypothesize that the drugs exerted their activity, at least partially, through inhibition of both neuronal and tegumental acetylcholinesterases (AChEs), as worms treated in vitro showed significant decreases in activity of these enzymes. Further, treated parasites exhibited a significantly decreased ability to uptake glucose, significantly depleted glycogen stores and withered tubercules (a site of glycogen storage), implying drug-mediated interference in this nutrient acquisition pathway.

CONCLUSIONS/SIGNIFICANCE

Our data provide compelling evidence that ruthenium complexes are effective against all intra-mammalian stages of schistosomes, including schistosomula (refractory to PZQ) and eggs (agents of disease transmissibility). Further, the results of this study suggest that schistosome AChE is a target of ruthenium drugs, a finding that can inform modification of current compounds to identify analogues which are even more effective and selective against schistosomes.

Assessment of acute oral and dermal toxicity of 2 ethyl-carbamates with activity against Rhipicephalus microplus in rats

The acute oral and dermal toxicity of two new ethyl-carbamates (ethyl-4-bromophenyl-carbamate and ethyl-4-chlorophenyl-carbamate) with ixodicide activity was determined in rats. The oral LD50 of each carbamate was 300 to 2000 mg/kg, and the dermal LD50 of each carbamate was >5000 mg/kg. Clinically, the surviving rats that had received oral doses of each carbamate showed decreased weight gain (P < 0.05) and had slight nervous system manifestations. These clinical signs were evident from the 300 mg/kg dose and were reversible, whereas the 2000 mg/kg dose caused severe damage and either caused their death or was motive for euthanasia. At necropsy, these rats had dilated stomachs and cecums with diffuse congestion, as well as moderate congestion of the liver. Histologically, the liver showed slight degenerative lesions, binucleated hepatocytes, focal coagulative necrosis, and congestion areas; the severity of the lesions increased with dosage. Furthermore, an slight increase in gamma-glutamyltransferase, lactate dehydrogenase, and creatinine was observed in the plasma. The dermal application of the maximum dose (5000 mg/kg) of each carbamate did not cause clinical manifestations or liver and skin alterations. This finding demonstrates that the carbamates under study have a low oral hazard and low acute dermal toxicity.

Subchronic toxicity study in rats of two new ethyl-carbamates with ixodicidal activity

Female and male Wistar rats were used to determine the subchronic oral toxicities of two new ethyl-carbamates with ixodicidal activities (ethyl-4-bromphenyl-carbamate and ethyl-4-chlorphenyl-carbamate). The evaluated carbamates were administered in the drinking water (12.5, 25 and 50 mg/kg/day) for 90 days. Exposure to the evaluated carbamates did not cause mortality or clinical signs and did not affect food consumption or weight gain. However, exposure to these carbamates produced alterations in water consumption, hematocrit, percentages of reticulocytes, plasma proteins, some biochemical parameters (aspartate aminotransferase, gamma-glutamyl transpeptidase, cholinesterase, and creatinine activities), thiobarbituric acid reactive substances, and the relative weight of the spleen. Histologically, slight pathological alterations were found in the liver that were consistent with the observed biochemical alterations. The nonobserved adverse effect levels (NOAELs) of the evaluated carbamates were 12.5 mg/kg/day for both the female and male rats. The low severity and reversibility of the majority of the observed alterations suggest that the evaluated carbamates have low subchronic toxicity.