Hallazgo histopatológico de Strongyloides stercoralis en enfermedad de Crohn: reporte de caso / Histopathologic finding of Strongyloides stercoralis in a patient with Crohn's disease: Case report

Resumen Caso clínico: Se describe un caso clínico poco frecuente en un paciente inmunocomprometido con hallazgo histopatológico de infestación parasitaria. Es un paciente masculino de edad media que habita en zona subtropical con diagnóstico de enfermedad de Crohn tratado con corticoide e inmunomoduladores, presentaba dolor abdominal y anemia crónica de 1 año de evolución, analítica negativa para parásitos, reactantes de fase aguda normales, gastroscopia y colonoscopia previas (6 meses) sin hallazgos relevantes. Por la persistencia del cuadro clínico se repitieron los estudios endoscópicos en los que se visualizaron hemorragias subepiteliales con resultados histopatológicos de Strongyloides stercoralis. Conclusión: En el contexto de un paciente inmunocomprometido, en zona endémica y con evolución tórpida, debe obligar a realizar un diagnóstico diferencial en el que se debe sospechar siempre de infestación parasitaria. Aunque la endoscopia no se necesita para el diagnóstico de estrongiloidiasis, su intervención puede ser oportuna.

Abstract Clinical case: The following is a rare clinical case in an immunocompromised patient with histopathological findings of parasitic infestation. The patient is a middle-aged male who lives in a subtropical area and has a diagnosis of Crohn's disease treated with corticosteroids and immunomodulators. The patient presented with abdominal pain and chronic anemia for 1 year, with negative laboratory tests for parasites and normal acute phase reactants. Gastroscopy and colonoscopy were performed before the consultation (6 months) without relevant findings. Due to the persistence of the symptoms, endoscopic studies were repeated, finding subepithelial bleeding with histopathological results of Strongyloides stercoralis. Conclusion: In the context of an immunocompromised patient living in an endemic area and with a torpid evolution, a differential diagnosis should be made always suspecting a parasitic infestation. Although endoscopy is not necessary to diagnose strongyloidiasis, its use may be convenient.

Lagoquilascaríase humana: um diagnóstico a ser lembrado diante de tumoração de cabeça e pescoço: relato de caso / Human lagochilascariasis: a differential diagnosis to be remembered as it is related to head and neck tumor: a case report

A lagoquilascaríase é uma zoonose incomum decorrente do parasitismo por Lagochilascaris minor. Na Região Amazônica, o primeiro caso foi descrito em 1978, em um paciente do sexo masculino, e caracterizou-se pelo aparecimento de tumoração em região de cabeça e pescoço. O objetivo deste estudo foi relatar um caso de lagoquilascaríase. Paciente do sexo feminino, 18 anos, procedente de Itaituba, área de garimpo, em Belém (PA). Relatava surgimento de nodulação cervical durante gestação, com eliminação de vermes pelo ouvido e por escarro. Foi encaminhada, em 1987, à Fundação Centro de Controle de Oncologia do Estado do Amazonas, onde biópsia evidenciou resultado de processo inflamatório granulomatoso de especificidade não estabelecida. Evoluiu 1 ano depois com massa tumoral em face lateral cervical esquerda, região submandibular e retroauricular, associada à perda ponderal, queda do estado geral e intensa astenia. Apresentava cicatriz cirúrgica prévia, eliminando secreção cremosa, purulenta e inodora. Foi instituído tratamento com tiabendazol e levamisol, com remissão da lesão e parada da eliminação de vermes por escarro e ouvido esquerdo. A lagoquilascaríase humana é uma zoonose causada por L. minor, que cursa com massa cervical e deve ser listada como diagnóstico diferencial das tumorações da cabeça e pescoço.(AU)

Lagochilascariasis is an uncommon parasitic zoonosis caused by Lagochilascaris minor. In the Amazon region, the first case was described in 1978 in a male patient, and it was characterized by the appearance of tumors in the head and neck. The aim of this study was to report a case of lagochilascariasis. An 18-yearold female patient from the city of Itaituba, a mining area in Belém do Pará, reported the emergence of cervical nodulation during pregnancy with elimination of worms in the sputum and from the ears. In 1987 she was sent to the Foundation Oncology Control Center of the state of Amazonas, where biopsy results showed a nonspecific granulomatous inflammation. A year later, it progressed to a tumor mass on the left lateral cervical aspect, in the submandibular and retroauricular region, which was associated with weight loss, poor general condition, and severe asthenia. She had a previous surgical scar that discharged a creamy, purulent and odorless secretion. Treatment was initiated with thiabendazole and levamisole, with remission of the lesion and interruption of the elimination of worms in the sputum and from the left ear. Human lagochilascariasis is an infection caused by L. minor that leads to cervical mass, and should be listed as a differential diagnosis of head and neck tumors.(AU)

Gnathostomiasis: an emerging infectious disease relevant to all dermatologists

Abstract: Gnathostomiasis is a parasitic infection caused by the third larval stage of nematodes of the genus Gnathostoma. The disease is endemic in some countries around the world. In the American continent, the majority of cases is concentrated in Mexico, Ecuador, and Peru. However, due to increasing traveling either at the intercontinental or intracontinental level, the disease is seen each time more frequently in tourists. Furthermore, countries, such as Brazil, that have never been considered endemic are reporting autochthonous cases. The disease usually presents as a deep-seated or slightly superficial migratory nodule in patients with history of eating raw fish, in the form of ceviche, sushi, or sashimi. Along with the clinical presentation, diagnostic criteria include either blood or tissue eosinophilia. In most instances, these criteria are enough for the attending physician to institute therapy. Chances of finding the parasite are low, unless the biopsy is taken from a very specific area that develops after antiparasitic treatment is started. The potential of other organ involvement with more serious consequences should always be kept in mind.

Analysis of monitoring results of soil-borne nematode disease in Jianhe County of Guizhou in 2016 / 中华地方病学杂志

Objective To investigate the infection status of soil-borne nematode in rural areas of Jianhe County of Guizhou Province and to provide scientific basis for formulating prevention and control measures.Methods In 2016,according to "the Implementation Plan of Soil-Borne Nematode Surveillance in Jianhe County",five surveillance sites were set up in Jianhe rural areas,and 200 permanent residents over 3 years old at each monitoring site were selected to collect fecal samples.The eggs of intestinal nematodes (Ascaris,Hookworm,Trichuris,etc.) in samples were detected with the Kato thick smear method,and the enterobius vermicularis was looked for using the transparent adhesive paper anal swab method.Results A total of 1 000 human fecal samples were tested,and 169 persons were identified to be positive for Ascaris,Hookworm,and Trichuris,and the infection rate was 16.90％ (169/1 000),in which the infection rates of Ascaris,Hookworm and Trichuris were 13.80％ (138/1 000),2.00％ (20/1 000) and 1.10％ (11/1 000),respectively,and Tapeworm and Liver fluke were not detected.The infection rates of soil-borne nematode in male and female were 14.09％ (72/511) and 19.84％ (97/489),respectively,and the difference between genders was statistically significant (x2 =5.88,P ＜ 0.05).The infection rates of soilborne nematode in the ages group of 0-,18-,41-and ≥66 were 18.94％ (50/264),13.18％ (29/220),15.12％ (52/344) and 22.09％ (38/172),respectively,but the differences were not statistically significant (x2=7.03,P ＞ 0.05).Hookworm infection was not found in the group of 0 to 17 years old.The prevalence rates of soil-borne nematodes in primary school and below,junior high school,high school and secondary school education were 18.52％ (138/745),14.83％ (31/209),and 0 (0/46),respectively,and the differences in educational levels were statistically significant (x2 =11.39,P ＜ 0.05).A total of 174 children aged 3 to 9 years old were tested for enterobius vermicularis,18 were infected,and the infection rate was 10.34％ (18/174).Conclusions The situation of soilborne nematode infection is still severe in rural areas of Jianhe County.It should be strengthen health education and improve the knowledge of nematode diseases control among the population.

Nematodos con potencial zoonótico en parques públicos de la ciudad de Tunja, Colombia / Nematodes with zoonotic potential in parks of the city of Tunja, Colombia

Objetivo. Identificar la presencia de parásitos con potencial zoonótico en los principales parques de la ciudad de Tunja, Boyacá, Colombia. Material y métodos. Se seleccionaron 28 parques de la ciudad en los que se recogieron 124 muestras de materia fecal de perros y muestras de suelo con ayuda de una espátula; se reunieron aproximadamente 150 g por muestra. Cada una de las muestras fue procesada por el método de concentración de Ritchie modificado, con lo que se realizó la identificación de formas parasitarias en microscopía óptica. Resultados. 60.7% de los parques resultaron positivos a nematodos en muestras de materia fecal de canino y 100% en tierra. Los nematodos encontrados fueron huevos y larvas de Toxocara spp, Ancylostoma spp, Trichuris vulpis y Strongiloides spp. Conclusión. Este estudio demostró el riesgo potencial de la transmisión de zoonosis causadas por nematodos de caninos y la necesidad de reforzar las medidas de salud pública para disminuir el riesgo de la población expuesta a dichas zoonosis.

Objective. To identify the presence of parasites with zoonotic potential in major parks in the city of Tunja, Boyacá. Materials and methods. Twenty eight parks in the city were selected, where 124 samples of feces of dogs and soil were collected with the help of a spatula, gathering approximately 150 g per sample. They were processed by the method of concentration of Ritchie modified making the identification of parasitic forms in an optical microscope. Results. A 60.7% of the parks were positive to nematodes in samples of canine fecal material and 100% on soil. Found nematodes were eggs and larvae of Toxocara spp., Ancylostoma spp., Trichuris vulpis and Strongiloides spp. Conclusion. This study demonstrated the potential risk of transmission of zoonoses caused by nematodes in canines and for the need to strengthen public health measures to reduce the risk shows the population exposed to such zoonoses.

Estimación de los Valores de Hematocrito y Hemoglobina en Presencia de Haemonchus sp. en Ovinos de Oicatá, Colombia / Estimation of Hematocrit Packed Cell Volume and Hemoglobin with the Presence of Haemonchus sp. in Sheep of Oicatá, Colombia

La hemoncosis causada por el nemátodo del abomaso de rumiantes, Haemonchus contortus, representa uno de los mayores problemas en los sistemas ovinos debido a su impacto sobre la producción y el costo de los tratamientos. El objetivo del presente estudio fue determinar la prevalencia de Haemonchus sp. y la estimación de los valores de hematocrito (Hto) y hemoglobina (Hb) en ovinos pertenecientes al municipio de Oicatá, Colombia. Se realizó un muestreo aleatorio de 201 animales criollos que pertenecían a explotaciones extensivas. Se tomaron muestras de materia fecal que fueron analizadas por medio del método de concentración formol-éter y posteriormente se realizó la identificación de Haemonchus sp. En los mismos individuos se obtuvo una muestra de sangre de la vena cefálica, se estimaron y se realizaron hemogramas completos. Se escogieron cuatro parámetros hematológicos: hemoglobina (Hb), hematocrito (Hto), recuento de glóbulos rojos (RBC) y volumen corpuscular medio (VCM). Se observó una prevalencia de Haemonchus sp. del 31,3%. Solo el 6% de los animales mostró Hto por debajo de los valores normales, mientras que el 6,5% tenía valores de Hb anormal en presencia del helminto. Los valores de RBC y VCM no demostraron alteraciones en presencia de la infección. No se observó ninguna relación entre los parámetros hemáticos y la presencia de huevos de Haemonchus sp. en las heces ovinas.

Haemonchosis caused by the abomasal nematode of ruminants, Haemonchus contortus, represents one of the biggest problems in sheep farming by its impact on production losses and cost of anthelmintic treatments. The aim of the present study was to determine the prevalence of Haemonchus sp. and the estimated values of the hematocrit (Hto) and hemoglobin (Hb) of sheep from the municipality of Oicatá, Colombia. A random sampling of 201 creole animals belonging to extensive farms was carried out. Individual fecal samples were analyzed by the method of concentration formol-ether and the Haemonchus sp. infection determined. Blood samples from the same animals were obtained from the cephalic vein in EDTA containing tubes. Four hematological parameters were estimated: haemoglobin (Hb), haematocrit (Hto), red blood cells count (RBC) and mean corpuscular volume (MCV). Results were analyzed with the statistical package SPSS18 with a level of statistical significance of p<0.05. A prevalence of 31.3% was found for Haemonchus sp. Only 6% of the animals showed Hto below normal values, while 6.5% had abnormal Hb values in the presence of the helminth. RBC presented normal values in the presence of the parasite. In this study no correlation was found between the presence of the parasite and the haematological parameters determined.

Clinical Characteristics of Gastroallergic Anisakiasis and Diagnostic Implications of Immunologic Tests

PURPOSE: Recent studies have used the term "gastroallergic anisakiasis" to describe incidental gastrointestinal infection with Anisakis spp. larvae, proposed as a causative agent of food hypersensitivity. However, it is unknown whether this condition represents an independent disease entity distinguishable from acute gastric anisakiasis. To better understand the role of the allergic response in Anisakis infections we examined the clinical and immunological implications of Anisakis-specific IgE. METHODS: A prospective study was performed in a geographic region where the consumption of raw seafood is common. Case subjects who had been clinically diagnosed with gastroallergic anisakiasis were selected, along with controls who frequently ate raw seafood but had never experienced gastroallergic anisakiasis-like symptoms. Clinical and immunological features were compared based on atopic status, sensitization rates to Anisakis, and serum titer of Anisakis-specific IgE. RESULTS: Seventeen case subjects and 135 controls were included in this study. The case subjects had experienced gastrointestinal symptoms after raw seafood ingestion, along with additional mucocutaneous, respiratory, or multisystemic symptoms. Case subjects were significantly sensitized to Anisakis excretory-secretory product and crude extract compared with controls (76.5% vs. 19.3%, P17.5 kU/L) were found to be reliable indicators for the diagnosis of gastroallergic anisakiasis. CONCLUSIONS: Among patients presenting acute gastric anisakiasis-like symptoms, a diagnosis of gastroallergic anisakiasis may be strongly supported by a high Anisakis-specific IgE titer.

Prevalencia y Abundancia de Huevos de Estróngilos Digestivos y Ooquistes de Eimeria spp. en Búfalos de Agua Infectados Naturalmente / Prevalence and Abundance of Digestive Strongyles Eggs and Eimeria spp. Oocysts in Water Buffaloes Naturally Infected

La cría de búfalos de agua (Bubalus bubalis) se ha convertido en una ganadería cada vez más importante y común en nuestro país; sin embargo, afecciones como las gastroenteritis parasitarias representan una de las limitantes en el desarrollo de esta especie, provocando un impacto negativo sobre la producción cárnica y lechera. Los principales agentes etiológicos responsables de esta afección son los estróngilos digestivos (Nematoda: Strongylida) y coccidias del género Eimeria (Sporozoa: Eucoccidida). Con el objetivo de determinar el comportamiento de estas infecciones, se evaluaron mediante exámenes coprológicos mensuales a través de la técnica cuantitativa de McMaster, búfalos de cuatro grupos etarios (bumautes entre 8 y 18 meses de edad, bumautes mayores de 18 meses, sementales y búfalas), de ambos sexos, pertenecientes a un rebaño comercial del estado Falcón. Los estróngilos digestivos y las coccidias fueron los más prevalentes, con 25,2 y 46,2%, respectivamente. La abundancia del rebaño fue de 38±41,1 huevos por gramo (HPG) de heces para estróngilos y de 142,4±117,2 ooquistes por gramo (OPG) de heces para coccidias. Hubo diferencias estadísticas (p<0,05) al comparar los valores de prevalencia y abundancia de las infecciones por estróngilos digestivos y coccidias entre los grupos etarios y el sexo. Los valores de prevalencia y abundancia disminuyeron con la edad de los animales evaluados (estróngilos: prevalencias de 39,5; 34,0; 42,3; 15,8% y abundancias de 79,8; 47,2; 30,8; 12,2 HPG; coccidias: prevalencias de 71,3; 54,8; 38,5; 34,0% y abundancias de 379,1; 121,0; 44,2; 47,6 OPG), siendo superiores en machos que en hembras (estróngilos: prevalencias de 36,2 y 22,4%, y abundancias de 70,5 y 30,9 HPG; coccidias: prevalencias de 57,3 y 43,6% y abundancias de 166,3 y 131,5 OPG, respectivamente). Las mayores cargas parasitarias y las infecciones de importancia clínica se observaron en los bumautes entre 8 y 18 meses de edad (contajes mayores a 500 HPG y 5050 OPG). Las condiciones climáticas se encontraron en rangos favorables (25,6-29,3ºC, 77,0-89,0% de humedad relativa y 1,0-151,3 mm de precipitación) para el desarrollo de las formas preparasíticas de estróngilos digestivos y coccidias.

The breeding of water buffaloes (Bubalus bubalis) has become an increasingly important and common livestock in Venezuela; however, conditions such as gastrointestinal parasitism are one of the major constraints in the development of this species, with a negative impact on meat and milk production. The main etiologic agents responsible for this condition are digestive strongyles (Nematoda: Strongylida) and coccidia of the genus Eimeria (Sporozoa: Eucoccidida). This investigation evaluated the behavior of these infections in buffaloes of different age and sex. Samples were collected and examined monthly by the quantitative McMaster technique. The buffaloes were subdivided into four age groups (animals between 8 and 18 months of age, over 18 months, males and female buffaloes), the animals came from a commercial flock of the State of Falcon, Venezuela. The results show that the most prevalent parasites were strongyles and coccidian, 25.2% and 46.2%, respectively. The flock abundance was 38±41.1 eggs per gram (EPG) for strongyles and 142.4±117.2 oocysts per gram (OPG) forcoccidia. There were statistical differences (p <0.05) between age groups and sex of the buffalo, when comparing the values of prevalence and abundance of infections for gastrointestinal strongyles and coccidia. The prevalence and abundance values decreased, with increasing age of the animals tested (strongyles: prevalence of 39.5; 34.0; 42.3; 15.8% and abundance of 79.8; 47.2; 30.8; 12.2 EPG; coccidian: prevalence of 71.3; 54.8; 38.5; 34.0% and abundance of 379.1; 121.0; 44.2; 47.6 OPG), being higher in males than in females (strongyles: prevalence of 36.2 and 22.4%, and abundance of 70.5 and 30.9 EPG; coccidian: prevalence of 57.3 and 43.6% and abundance of 166.3 and 131.5 OPG). The highest parasitic loads and the clinically important infections were seen in 8 to 18 months old buffaloes (counts higher than 500 EPG and 5,050 OPG). Environmental conditions fell within favorable ranges (25.6-29.3ºC of temperature, 77.0-89.0% of relative humidity and 1.0-151.3 mm of precipitation) for the development of infective parasite forms of both digestive strongyles and coccidian.

Larva migrans cutánea en un adolescente / Cutaneous larva migrans in a teenager

Introducción: larva migrans cutánea, es una zoonosis parasitaria causada por la penetración y migración a través de la piel de larvas de nematodos, adquiridos por contacto corporal con tierra, arena y materiales de construcción contaminados con heces de perros y gatos infestados. Objetivo: identificar correctamente el diagnóstico clínico de larva migrans cutánea en pacientes con sospecha de la enfermedad y contribuir a su mejor manejo. Caso clínico: paciente de 14 años de edad con lesiones serpiginosas en ambos miembros inferiores, y región glútea, acompañadas de dolor local intenso, prurito e irritabilidad. El diagnóstico se realizó mediante la clínica y los antecedentes epidemiológicos, se le aplicó como tratamiento antiparasitario el tiabendazol con una evolución satisfactoria del paciente.

Introduction: cutaneous larva migrans is a parasitic zoonosis caused by the penetration and migration through the skin of nematodes larvae, acquired by corporal contact with earth, sand and polluted construction materials infested with excrements of dogs and cats. Objective: to identify correctly the clinical diagnosis of cutaneous larva migrans in patients with suspicion of the disease and to contribute to their best handling. Clinical Case: a 14 year-old patient with serpiginous lesions in both inferior limbs and in the gluteal region, accompanied by local intense pain, pruritus and irritability. The diagnosis was carried out by means of the clinic and the epidemical antecedents. The patient was administered the thiabendazole as part of the antiparasitic treatment with satisfactory outcomes.

Analysis on the infection status of soil-transmitted nematodes in Shanxi Province in 2006 / 中华疾病控制杂志

Objective To analyze the distribution status of soil-transmitted nematodes infection for prevention and treatment.Methods The inquiry field test and grade random sampling were emplyed on a group of people naturally selected in Shanxi Province.The people were examined through pathogen test and statistical diagnosis.Results A total of 7 726 people were investigated.Soil-transmitted nematodes infection rate was 18.86%.Ascaris,enterobius,trichuris,and hookworm was 15.87%,11.46%,0.82%,and 0.04% respectively.The intensity of ascaris infection was 97.74%,1.60%,and 0.66% respectively in light,moderate,and serious conditions,and that of trichuris infection was 88.89%,7.41%,and 3.70% respectively in light,moderate,and serious conditions.Conclusions Soil-transmitted nematodes infection rate is lower than that in 1992.Its cases are mainly distributed in rural areas and schools,closely related to education level.Improving water-drinking conditions and personal hygiene is still key to the prevention and control of the infections.

Acondicionamento e coleta de resíduos sólidos domiciliares e impactos na saúde de crianças residentes em assentamentos periurbanos de Salvador, Bahia, Brasil / Household solid waste bagging and collection and their health implications for children living in outlying urban settlements in Salvador, Bahia State, Brazil

O artigo apresenta estudo do acondicionamento e coleta de resíduos sólidos domiciliares e o impacto na saúde de crianças, por meio de pesquisa realizada em nove assentamentos humanos localizados em área periurbana da cidade de Salvador, Bahia, Brasil. Como indicadores epidemiológicos foram utilizados, em 1.893 crianças entre 5 e 14 anos, a infecção por nematóides intestinais, expressa pela prevalência de Ascaris lumbricoides, Trichuris trichiura e ancilostomídeos, e em 1.204 crianças menores de cinco anos, a incidência de diarréia e o estado nutricional, este expresso por indicadores antropométricos. Os resultados apresentam a maior prevalência dos três nematóides nas crianças dos domicílios que não dispõem de acondicionamento adequado e de coleta de resíduos sólidos que naquelas de domicílios com acondicionamento adequado e coleta regular, sendo a diferença encontrada estatisticamente significante, mesmo quando outros fatores de risco sócio-econômicos, culturais, demográficos e ambientais são considerados. Resultado semelhante é também observado com relação aos indicadores epidemiológicos, incidência de diarréia e estado nutricional.

This paper presents a study on the bagging and collection of household solid waste and the health implications for children. The research was conducted in nine human settlements on the outskirts of Salvador, Bahia State, Brazil. Intestinal nematode infection, predominantly involving Ascaris lumbricoides, Trichuris trichiura, and hookworms, was used as an epidemiological indicator in 1,893 children from 5 to 14 years of age. The study also included diarrhea incidence and nutritional status as shown by anthropometric indicators in 1,204 children less than 5 years of age. There was a higher prevalence of the three nematodes in children living in households without proper bagging/isolation and collection of household solid waste as compared to those in areas with regular garbage collection and adequate isolation of solid waste. The differences were statistically significant when other socioeconomic, cultural, demographic, and environmental risks factors were considered in the analysis. Similar results were also observed for epidemiological indicators, diarrhea incidence, and nutritional status.

El grado de agregación de Ascaris lumbricoides según grupos de edad, después de una intervención antihelmíntica de masas / The degree of age-aggregation of Ascaris lumbricoides in a community after a mass antihelminthic intervention

Objetivo. Descubrir la heterogeneidad de la reinfección por Ascaris lumbricoides según grupos de edad, en distintos intervalos, después de un primer tratamiento antihelmíntico masivo. Material y métodos. A partir de datos de la prevalencia e intensidad de la infección por A. lumbricoides obtenidos en un estudio epidemiológico de ascariasis, realizado en Coatzacoalcos, Veracruz, México, se deriva el grado de agregación por medio del método de máxima verosimilitud, asumiendo una distribución binomial negativa del parásito en el huésped. Resultados. Se muestra que la distribución del parásito en el huésped es distinta en el ámbito poblacional que en el de grupos de edad, que la recuperación de la infección no es uniforme; y que la dinámica de asociación huésped-parásito exhibe una marcada desestabilización de su equilibrio endémico después de una primera intervención de masas. Conclusiones. Lo anterior sugiere que existe una alta variabilidad en la agregación entre los distintos grupos de edad y, por tanto, la infección y la reinfección son heterogéneas respecto a la edad. Después de la desestabilización del equilibrio endémico, los estados que alcanza el sistema huésped-parásito son transitorios. Dada la heterogeneidad de la infección, los tratamientos selectivos parecen ser la estrategia más adecuada para disminuirla

Objective. The degree of aggregation is determined for the whole population and for different age-specific strata and is used for assessing the reinfection rate after a mass antihelminthic chemotherapeutic intervention. Material and methods. The degree of aggregation of Ascaris lumbricoides is derived from prevalence and intensity of infection data obtained from an epidemiological study of ascariasis carried out in Coatzacoalcos, Veracruz, México by means of maximum likelihood estimation of the negative binomial distribution. Results. The results show that the distribution of the parasite for the whole population differs from the distribution of the parasite within each age-specific stratum; the reinfection rate is not uniform; the dynamics of the hostparasite association exhibits a clear destabilization of its endemic equilibrium after a first round of mass treatment.Conclusions. It is suggested that the aggregation among the different age-groups is highly variable and hence the infection and reinfection rates are heterogeneous with regard to age; these states of the host-parasite system are transient. Given the heterogeneity of the infection, the selective treatment seems to be the most adequate strategy to diminish helminth infection rates.