The Attractivity of the Head Louse, Pediculus humanus capitis (Pediculidae: Anoplura) to Isolated Compounds and Blends of Human Skin.

Human head lice Pediculus humanus capitis De Geer (Phthiraptera: Pediculidae) are ectoparasites that cause pediculosis, a global scale disease mainly found in school-age children. Previous works from our laboratory found nonanal, sulcatone, and geranylacetone as the main human scalp volatile components, and individually evaluated their attraction to head lice using an olfactometer. In this work, we compared how their blends at different concentrations attract head lice, and how their blended effect compares to the effect of isolated compounds. At the concentrations evaluated, individual components did not show attraction towards head lice, but a ternary mixture of them was attractive. Moreover, a solvent extract from the human head scalp was analyzed by GC-MS, finding that tetradecanoic acid, palmitic acid, oleic acid, palmitoleic acid, and squalene are the most abundant components. Attraction to these individual compounds at natural concentrations was tested by bioassays in a circular experimental arena. No attraction was observed when the components were tested individually, but when they were evaluated as a blend they attracted head lice. This work presents new information about how chemical signals are attractive at certain concentrations and proportions. This information could be used to better understand communication mechanisms in head lice and for the development of louse repellents.

Características clínicas y evolutivas de niños con infección por SARS-CoV-2 en un hospital de alta complejidad: estudio de cohorte / Clinical and outcome characteristics of children with SARS CoV-2 infection in a tertiary-care hospital: a cohort study

Introducción: Las infecciones por SARS-CoV-2 representan un problema de salud pública a nivel mundial. En los niños se reporta menor incidencia y cuadros clínicos más leves. Se realizó el presente estudio con el objetivo de describir las características clínicas y evolutivas de los niños con diagnóstico de infección por SARS CoV-2 en el Hospital Juan P. Garrahan. Material y métodos: estudio de cohorte prospectivo. Se incluyeron todos los pacientes con diagnóstico confirmado por PCR de COVID-19 desde 20.4.20 hasta el 3.07.21 y con seguimiento en el hospital de Pediatría Juan P. Garrahan. Resultados: n: 1644. Eran varones 836 (51%). La mediana de edad fue 75 meses (RIC 22- 143). Tenían alguna enfermedad de base previa al diagnóstico de COVID-19: 884 pacientes (53,7%), la más frecuente fue la enfermedad oncohematológica. Estaban asintomáticos 423 pacientes (25,7%). De los pacientes sintomáticos, 1071 (65,1%) presentaron cuadro leve, 5 (0,3%) moderado, 69 (4,2%) grave y 76 (4,6%) crítico. La fiebre fue el hallazgo más frecuente n: 782; (47,5%). Se internaron 900 pacientes (54,7%), 33 en UCI (2%). Fallecieron 7 pacientes (0,4%), todos ellos con comorbilidades graves. Conclusiones: En este estudio de cohorte de niños con infección por SARS-CoV-2 confirmada, predominaron los pacientes con enfermedad de base y las formas leves de COVID-19. El ingreso a UCI fue menor al 2%. Fallecieron 7 pacientes (0.4%) todos ellos con comorbilidades y coinfecciones (AU)

Introduction: SARS-CoV-2 infections represent a worldwide public health problem. A lower incidence and milder clinical pictures are reported in children. The aim of this study was to describe clinical and outcome characteristics of children diagnosed with SARS-CoV-2 infection at Hospital de Pediatría Juan P. Garrahan. Methods: A prospective cohort study was conducted. All patients with a PCR-confirmed diagnosis of COVID-19 seen between 20.4.20 and 3.07.21 and followed-up at Hospital de Pediatría Juan P. Garrahan were included. Results: n: 1644; 836 males (51%) were male. Median age was 75 months (IQR, 22-143). Overall, 884 patients (53.7%) had an underlying disease prior to COVID-19 diagnosis, most frequently hematologic/ oncologic disease. 423 patients (25.7%) were asymptomatic. Of the symptomatic patients, 1071 (65.1%) had mild, 5 (0.3%) moderate, 69 (4.2%) severe, and 76 (4.6%) critical disease. Fever was the most frequent finding (n: 782; 47.5%). A total of 900 patients (54.7%) were admitted, 33 of whom to the ICU (2%). Seven patients (0.4%) died, all with severe comorbidities. Conclusions: In this cohort study of children with confirmed SARSCoV-2 infection, patients with underlying disease and mild forms of COVID-19 predominated. ICU admission occurred in less than 2%. Seven patients (0.4%) died, all of them with comorbidities and coinfections. (AU)

Síndrome inflamatorio multisistémico pediátrico (SIMS) asociado con el SARS-CoV-2: Un estudio de cohorte retrospectivo / SARS-CoV-2-associated pediatric multisystemic inflammatory syndrome: A retrospective cohort study

Introduccion: El Síndrome inflamatorio multisistémico pediátrico (SIMS) asociado con el SARS-CoV-2 es una enfermedad aguda acompañada de un síndrome hiperinflamatorio, con falla multiorgánica y shock, asociada a la infección por SARS CoV2, que produce alta morbilidad en la población pediátrica, que hasta el momento es la afectada por este síndrome. Objetivo: Evaluar las características diferenciales del síndrome multisistémico inflamatorio asociado al SARS-COV-2 (SIMS) en niños. Métodos: se realizó un estudio de cohorte retrospectivo. La definición de SIMS se basó en los criterios de la OMS. Los pacientes con COVID-19 relacionados temporalmente se incluyeron como controles. Resultados: se incluyeron 25 pacientes con SIMS y 75 controles. El modelo de regresión logística múltiple de las variables que mostraron ser significativas en el análisis univariado reveló que la edad ≥ 2 años (OR 24,7; IC del 95%: 1,03 -592,4; P = 0,048), la linfopenia (OR 9,03; IC del 95%: 2,05-39,7; P = 0,004), y el recuento de plaquetas <150x109 / L (OR 11,7; IC del 95%: 1,88-75,22; P = 0,009) se asociaron significativamente con SIMS. La presencia de una enfermedad subyacente pareció reducir el riesgo de SIMS (OR 0,06; IC del 95%: 0,01-0,3). Conclusión: El SIMS fue más común en pacientes mayores de 2 años y en aquellos con linfopenia o trombocitopenia. La enfermedad subyacente parece reducir el riesgo del mismo. (AU)

Introduction: SARS-CoV-2-associated pediatric multisystemic inflammatory syndrome (PMIS) is an acute disease accompanied by a hyperinflammatory syndrome, with multiorgan failure and shock associated with SARS CoV2 infection, producing high morbidity in the pediatric population, which so far is affected by this syndrome. Objective: To evaluate the differential characteristics of SARS-COV-2-associated PMIS in children. Methods: A retrospective cohort study was conducted. The definition of PMIS was based on WHO criteria. Patients with temporally related COVID-19 were included as controls. Results: 25 patients with PMIS and 75 controls were included. A multiple logistic regression model of the variables shown to be significant in univariate analysis revealed that age ≥ 2 years (OR 24.7; 95% CI: 1.03 -592.4; P = 0.048), lymphopenia (OR 9.03; 95% CI 2.05-39.7; P = 0.004), and platelet count < 150x109/L (OR 11.7; 95% CI: 1.88-75.22; P = 0.009) were significantly associated with PMIS. The presence of an underlying disease appeared to reduce the risk of PMIS (OR 0.06; 95% CI: 0.01-0.3). Conclusion: PMIS was more common in patients older than 2 years and in those with lymphopenia or thrombocytopenia. Underlying disease appears to reduce the risk of SMIS.(AU)

Sensory features of the human louse antenna: new contributions and comparisons between ecotypes.

Body and head lice are known to be exclusive ectoparasites of human hosts. Current genomic and transcriptomic data suggest that both louse types represent ecotypes of the same species. They exhibit morphological and physiological differences that probably arose from living in different biotopes. Sensory traits represent suitable candidates to be affected by environmental heterogeneity. Therefore, through scanning electron microscope studies, this study analysed and compared the sensory structures of both ecotypes' antennae. Seven types of sensory structures were identified in both ecotypes: bristles, tuft organs, pore organs, single pore and three morphotypes of sensilla basiconica. Notably, the single pore and the morphotypes of sensilla basiconica were described for the first time in the body louse antenna. This study's comparative analysis mainly revealed size differences across the sensory structures of the ecotypes. Bristles of the flagellomere 2 of the body louse antenna were longer than the head louse bristles. In addition, the pore organs of the head louse antenna presented a higher diameter than those of the body louse. The possible relevance of size differences regarding the biotopes exploited by the body louse and the head louse is discussed. Yet, physiological studies may help to fully understand the phenotypical differences of both ecotypes.

Sifilides elegantes de BROCQ en una paciente con SIDA y sífilis secundaria. Presentación de un caso y revisión de la literatura / 5 BROCQ ELEGANTS SYPHILIDES IN A PATIENT WITH AIDS AND SECONDARY SYPHILLIS

La coinfección entre el Treponema pallidum y el virus de la inmunodeiciencia humana (VIH) altera el curso clínico clásico de la sífilis aumentando la probabilidad de aparición de formas atípicas del secundarismo sifilítico. Entre estas formas se ha descripto a la sífilis elegante, entidad caracterizada por un exantema maculopapuloso descamativo, de aspecto anular, por lo general, con indemnidad de las regiones palmo plantar y de las mucosas. Se presenta un caso de sífilis secundaria, con lesiones típicas por su aspecto y localización, de sifílides elegantes en una paciente con diagnóstico de sida

Co-infection between Treponema pallidum and HIV alters the classic clinical course of syphilis, increasing the likelihood of atypical forms of syphilitic secondaryism. Among these forms, elegant syphilis has been described, an entity characterized by a desquamating maculopapular rash of annular appearance, with indemnity of the palmoplantar surface and mucous regions. Here, we present a case of secondary syphilis with typical lesions of elegant syphillides, in a patient diagnosed with AIDS

Head Louse Feces: Chemical Analysis and Behavioral Activity.

Human head lice Pediculus humanus capitis (De Geer) (Phthiraptera: Pediculidae) are insect parasites closely associated with humans, feeding on the blood of their hosts and causing them skin irritation and probable secondary infections. Despite being a severe nuisance, very few studies have reported on intraspecific chemical communication in head lice. Here, we evaluated the attractive response of head lice to the volatile compounds and solvent extracts from their feces. We also chemically analyzed the main volatile components of these feces and those of the feces' extracts. Head lice were attracted to the methanol extract of their feces but not to the hexane or dichloromethane extracts, suggesting the polar nature of bioactive chemicals present in head louse feces. Follow-up chemical identifications, in fact, showed the presence of hypoxanthine, uric acid, and another purine tentatively identified as either guanine or iso-guanine. Additionally, head lice were significantly attracted by volatiles emitted from samples containing feces. The volatiles emanated from feces alone contained 19 identified substances: 2-pentanone, hexanal, heptanal, 3-methyl-3-buten-1-ol, octanal, sulcatone, nonanal, acetic acid, 2-ethyl-1-hexanol, decanal, 1-octanol, butyric acid, 1-nonanol, hexanoic acid, octanoic acid, 2,6-dimethyl-7-octen-2-ol, 2-undecanone, geranylacetone, and hexadecane. The major compounds found were decanal, nonanal, hexanal, and acetic acid, together representing approximately 60% of the identified compounds. This work represents the first chemical evidence of intraspecies communication among head lice. The results support the existence of active substances present in the feces of P. humanus capitis that may be involved in its aggregation behavior.

Modelling the association between deltamethrin resistance in Triatoma infestans populations of the Argentinian Gran Chaco region with environmental factors.

In Latin America, Triatoma infestans is the main vector of the protozoan Trypanosoma cruzi, causal agent of Chagas disease. This blood-sucking triatomine is widely distributed in the Gran Chaco ecoregion, where chemical control has failed because of the evolution of resistance to pyrethroid insecticides. Recently, we described a deltamethrin high resistant focus in Güemes Department (Chaco province) characterized by susceptible populations, populations with low resistance (without field control failures) and some of the populations with the highest resistance level detected. This toxicological heterogeneity could be a result of non-homogenous insecticide pressure and be influenced by environmental factors. The present study evaluated the association of deltamethrin resistance ratios (RR50s) of T. infestans populations with explanatory variables extracted from the WorldClim dataset and constructed from information of National Chagas Program actions during 2005-2015. Control actions were distributed throughout the analyzed period, representing a homogeneous selective pressure. The average percentage of total positive houses was 33.66%. Models that included temperature and precipitation indicators presented 65% explanation. When village size variables where added, the explanatory power reached 70%. This observational result suggests that the climate may favor directly or indirectly the development/selection for resistance, representing a valuable tool to understand the occurrence of resistance that could increase the Chagas disease in the Gran Chaco.

Response of Pediculus humanus capitis (Phthiraptera: Pediculidae) to Volatiles of Whole and Individual Components of the Human Scalp.

The head louse Pediculus humanus capitis (De Geer) (Phthiraptera: Pediculidae) is a cosmopolitan human ectoparasite causing pediculosis, one of the most common arthropod parasitic conditions of humans. The mechanisms and/or chemicals involved in host environment recognition by head lice are still unknown. In this study, we evaluated the response of head lice to volatiles that emanate from the human scalp. In addition, we identified the volatile components of the odor and evaluated the attractive or repellent activity of their pure main components. The volatiles were collected by means of Solid Phase microextraction and the extract obtained was chemically analyzed by gas chromatograph-mass spectrometer. Twenty-four volatile were identified in the human scalp odor, with the main compounds being the following: nonanal, sulcatone, geranylacetone, and palmitic acid. Head lice were highly attracted by the blend human scalp volatiles, as well as by the individual major components. A significant finding of our study was to demonstrate that nonanal activity depends on the mass of the compound as it is repellent at high concentrations and an attractant at low concentrations. The results of this study indicate that head lice may use chemical signals in addition to other mechanisms to remain on the host.

Demographic effects of deltamethrin resistance in the Chagas disease vector Triatoma infestans.

Triatoma infestans (Heteroptera: Reduviidae) Klug is the main vector of Chagas disease in Latin America. Resistance to deltamethrin was reported in Argentina and recently associated with reproductive and longevity trade-offs. The objectives of the present study were to describe the demographic consequences of deltamethrin resistance in T. infestans and to establish possible target stages for chemical control in susceptible and resistant colonies. A stage-classified matrix model was constructed based on the average stage length for susceptible, resistant and reciprocal matings' progeny. The differences between colonies were analysed by prospective and retrospective analysis. The life table parameters indicated reduced fecundity, fertility and population growth in resistant insects. The retrospective analysis suggested the latter was associated with lower reproductive output and increased fifth-instar nymph stage length. The prospective analysis suggested that the adult stage should be the main target for insecticide control. Although, fifth-instar nymphs should also be targeted when resistance has been detected. The presented results show demographic effects of deltamethrin resistance in T. infestans. While the older stages could be the main targets for chemical control, this approach is impeded by their higher tolerance to insecticides. It is concluded that the different mode of action insecticides would be more effective than a dose increase for the control of deltamethrin-resistant T. infestans.

Geographical Variation of Deltamethrin Susceptibility of Triatoma infestans (Hemiptera: Reduviidae) in Argentina With Emphasis on a Resistant Focus in the Gran Chaco.

Chagas disease is one of the most important parasitic infections in Latin America. The main vector of the protozoan Trypanosoma cruzi in America is Triatoma infestans, a blood-sucking triatomine bug who is widely distributed in the Gran Chaco ecoregion. Control programs in endemic countries are focused in the elimination of triatomine vectors with pyrethroid insecticides. However, chemical control has failed in the Gran Chaco over the last two decades because of several factors. Previous studies have reported the evolution of different levels of resistance to deltamethrin in Tri. infestans Recently, very high resistance has been found in the central area of the Argentine Gran Chaco. However, the origin and the extension of this remarkably resistant focus remain unknown. The aim of this study was to evaluate the geographical variation of deltamethrin susceptibility of Tri. infestans in different endemic provinces of Argentina, with emphasis in the center of the Argentine Gran Chaco ecoregion where this main vector has not been reduced. Populations of Mendoza, San Juan, Santiago del Estero, and Tucumán provinces were all susceptible. Resistant populations were only detected in the province of Chaco, where a mosaic resistant focus was described at the Güemes Department. It was characterized into three pyrethroid resistance categories: susceptible, low, and highly resistant populations. We found the populations with the highest resistance levels to deltamethrin, with resistant ratios over 1000.

Examining Mechanisms of Pyrethroid Resistance in Eggs of Two Populations of the Chagas' Disease Vector Triatoma infestans (Hemiptera: Reduviidae).

Chagas disease is a zoonosis transmitted to man by blood-sucking triatomine bugs found in the Americas. Triatoma infestans (Klug, 1834) is the main vector of Chagas' disease in Argentina. The control of this illness relies heavily on vector control through the use of insecticide. However, resistance to pyrethroid insecticides associated with ineffective field treatments has been increasingly reported in T. infestans from Argentina and Bolivia. There are few reports on the expression and causes of resistance in eggs of resistant populations, and even fewer studies on insecticide resistance throughout embryonic development. In this study, we explore the biochemical and molecular mechanisms potentially associated with the deltamethrin resistance assessed in the developing eggs of the Argentinean (Campo Largo) and Bolivian (Entre Ríos) T. infestans populations.We found measurable activity of monooxigenases and pyrethroid esterases throughout embryonic development. The pyrethroid esterase activity grew steadily throughout development in all the studied populations and was highest in eggs 12 d old. Mean enzyme activity increased from 13.6 to 16.3 and 22.2 picomol 7-hydroxycoumarin/min (7-OHC) in eggs of 4-, 7-, and 12 d old from the susceptible reference bug colony. Mean activity of resistant populations increased from 16.0 to 25.9 picomol 7-OHC/min in eggs of 4- to 12 d old in Entre Ríos population, and from 15.9 to 28.9 picomol 7-OHC/min in Campo Largo population. Molecular analysis of susceptible and resistant developing eggs detected L1014F mutation in both resistant populations, but no L925I mutation was found in any of the studied populations.Higher esterase activity and L1014F presence justify the resistance to pyrethroid throughout developing eggs of both studied T. infestans populations. The description of resistance profiles including resistance mechanisms involved will allow a rational design of campaigns for the control of Chagas disease transmission.

Effect of Environmental Conditions and Toxic Compounds on the Locomotor Activity of Pediculus humanus capitis (Phthiraptera: Pediculidae).

In this work, we evaluated the effect of environmental variables such as temperature, humidity, and light on the locomotor activity of Pediculus humanus capitis. In addition, we used selected conditions of temperature, humidity, and light to study the effects of cypermethrin and N,N-diethyl-3-methylbenzamide (DEET) on the locomotor activity of head lice. Head lice increased their locomotor activity in an arena at 30°C compared with activity at 20°C. When we tested the influence of the humidity level, the locomotor activity of head lice showed no significant differences related to humidity level, both at 30°C and 20°C. Concerning light influence, we observed that the higher the intensity of light, the slower the movement of head lice. We also demonstrated that sublethal doses of toxics may alter locomotor activity in adults of head lice. Sublethal doses of cypermethrin induced hyperactivated responses in adult head lice. Sublethal doses of DEET evocated hypoactivated responses in head lice. The observation of stereotyped behavior in head lice elicited by toxic compounds proved that measuring locomotor activity in an experimental set-up where environmental conditions are controlled would be appropriate to evaluate compounds of biological importance, such as molecules involved in the host-parasite interaction and intraspecific relationships.

Fenitrothion: an alternative insecticide for the control of deltamethrin-resistant populations of Triatoma infestans in northern Argentina.

Deltamethrin-based campaigns to control Triatoma infestans (Klug) (Hemiptera: Reduviidae) have decreased in success as a result of the development of insecticide resistance. We compared the in vitro effects of the pyrethroid deltamethrin and two doses of the organophosphate fenitrothion, presented on different materials, on T. infestans from La Esperanza, Argentina. Laboratory tests demonstrated a decrease in susceptibility to deltamethrin in the field population [LD50 : 30.32 nanograms per insect (ng/i)] compared with the reference population (LD50 : 0.13 ng/i), giving a high resistance ratio of 233.42. By contrast, similar susceptibility to fenitrothion was assessed in both the field and reference populations (LD50 : 21.65 ng/i and 21.38 ng/i, respectively). The effectiveness of the formulated insecticides varied according to the surfaces to which they were applied. The application of fenitrothion formulations to glass or brick resulted in mortality of 90-100%. The application of fenitrothion formulations to wood or mud caused mortality in the range of 6.7-56.7%. Resistant insects presented low mortality when exposed to the deltamethrin formulation and high mortality when exposed to fenitrothion formulations. Moreover, the insecticides demonstrated residual activity only when applied to glass. The present work demonstrates that fenitrothion is an alternative to pyrethroids for the management of deltamethrin-resistant insects in La Esperanza. However, this effectiveness is not sustained over time.

Chemical curing in alkyd paints: an evaluation via FT-IR and NMR spectroscopies.

A study aimed at determining the time necessary for an alkyd paint to attain chemical curing is presented. In particular, the object of our investigation was an oil paint made by Winsor & Newton, namely French ultramarine (PB29) in the Griffin Alkyd "fast drying oil colour" series. Using this paint, we prepared several mock-ups on glass. These were left in the laboratory at room temperature in a piece of furniture with glass doors for a total of 70 days. Samples were taken at different times, and the changes in their composition were monitored by means of FT-IR and multinuclear NMR spectroscopic analyses. Since the cross-linking reactions involved in the formation of the pictorial film mainly affect the amount of carbon-carbon double bonds, we monitored the decrease in allyl, diallyl and vinyl protons and carbons. The data obtained from the use of both techniques led us to conclude that, in our particular experimental conditions, the chemical curing of the paint layer is reached within the first 70 days, thus establishing the beginning of the ageing phenomena.

Scientific evidence of three different insecticide-resistant profiles in Triatoma infestans (Hemiptera: Reduviidae) populations from Argentina and Bolivia.

Triatoma infestans (Klug, 1834) (Hemiptera, Reduviidae) is the main vector of Chagas disease in the southern cone South America. Chemical control to the vectors appears to be the best option to reduce the incidence of the disease. However, since 2002, high resistance to insecticides that correlated with field control failures was detected in T. infestans from Argentina and Bolivia. In this paper, we analyzed three T. infestans populations whose pyrethroid-resistance had been recently detected, and we defined at least three resistant profiles according to the toxicological and biochemical characteristics of the studied resistant populations. The resistance profiles were identified as Ti-R1, Ti-R2, and Ti-R3, corresponding to the Argentinean Acambuco, and the Bolivians Entre Ríos and Mataral. Ti-R1 exhibited nymphs and eggs with medium resistance level to deltamethrin (RR = 32.5 and 28.6; respectively). Pyrethroid-esterases played a relevant role in deltamethrin resistance. Ti-R2 exhibited nymphs with high resistance to deltamethrin (RR = 173.8) and low resistance to fipronil (RR = 12.4). Pyrethroid-esterases were involved in resistance. Moreover, eggs showed medium resistance level to deltamethrin (RR = 39.1). Ti-R3 had nymphs with low resistance to deltamethrin (RR = 17.4), and medium resistance to fipronil (RR = 66.8). Pyrethroid-esterases showed increased activity, and eggs possessed low resistance to deltamethrin (RR = 8.4). The characterization of the resistance to pyrethroid in these T. infestans populations from Argentina and Bolivia do not permit the generalization of three forms of resistance profile. So far as we appear to know, the forms of mechanisms and their frequencies reported here are selected independently, so additional sites might well show additional combinations of resistance mechanisms and their frequencies.

Insecticidal activity of individual and mixed monoterpenoids of geranium essential oil against Pediculus humanus capitis (Phthiraptera: Pediculidae).

The major components of geranium (Geranium maculatum L.) oil and their mixtures were tested against female Pediculus humanus capitis De Geer (Phthiraptera: Pediculidae). Chemical analysis by gas chromatography coupled to mass spectrometry revealed four major constituents: citronellol (38%), geraniol (16%), citronellyl formate (10.4%), and linalool (6.45%) (concentration expressed as percentage of total). Topical application demonstrated that the most potent component was citronellol and geraniol, with LD50 values 9.7 and 12.7 microg/insect, respectively. Linalool and Citronellyl formate were less toxic with LD50 values 24.7 and 38.5 microg/insect, respectively. Toxicity of these four major constituents in the same proportion as the natural oil, was greater than whole oil and each individual component. Removal of any four constituents produced a decreased in effectiveness. The absence of citronellol caused the greatest decrease in toxicity (DL50 from 2.2 to 10.9 microg/insect), leading us to conclude that this constituent is the major contributor to oil toxicity. The knowledge of the role of each constituent in the toxicity of the whole oil gives the possibility to create artificial blends of different constituents for the development of more effective control agents.

Cholinesterase activity during embryonic development in the blood-feeding bug Triatoma patagonica.

Triatoma patagonica Del Ponte (Hemiptera: Reduviidae), a vector of Chagas' disease, is widely distributed in Argentina and is found in sylvatic and peridomiciliary ecotopes, as well as occasionally in human dwellings after the chemical control of Triatoma infestans. Anti-cholinesteratic products can be applied in peridomiciliary areas and thus knowledge of cholinesterase activity during embryonic development in this species might contribute further information relevant to effective chemical control. Cholinesterase activity was characterized by reactions to eserine 10(-5) m, to increasing concentrations of substrate and to varying centrifugal speeds. Acetylcholinesterase activity was detected on day 4 and was significant from day 5. A reduction in cholinesterase activity towards acetylthiocholine (ATC) was observed on days 9 and 10 of development. Cholinesterase activity towards ATC and butyrylthiocholine (BTC) in homogenates of eggs was inhibited by eserine 10(-5) m. The shape of the curve indicating levels of inhibition at different concentrations of ATC was typical of acetylcholinesterase. Activity towards BTC did not appear to be inhibited by excess substrate, which parallels the behaviour of butyrylcholinesterases. Cholinesterase activity towards ATC was reduced in supernatant centrifuged at 15 000 g compared with supernatant centrifuged at 1100 g. The cholinesterase system that hydrolyzes mainly ATC seems to belong to the nervous system, as indicated by its behaviour towards the substrates assayed, its greater insolubility and the fact that it evolves parallel to the development of the nervous system. Knowledge of biochemical changes associated with the development and maturation of the nervous system during embryonic development would contribute to the better understanding of anti-cholinesteratic compounds with ovicidal action that might be used in control campaigns against vectors of Chagas' disease.