Population biology of hookworms in children in rural West Bengal. I. General parasitological observations.

Hookworm infection was studied over a 22-month period in 31-49% of a population of 1,803 1- to 10-year-old children in a rural area near Calcutta where both Necator americanus and Ancylostoma duodenale were prevalent. Half of the children were infected before age 5 and 90% were infected by age 9, when the mean egg count reached 2,000 eggs/g of feces. Infection was significantly heavier and more prevalent among males than among females, and greater among Muslims than among Hindus. These differences were apparent in children less than 2 years of age. Numerical factors were devised to adjust fecal egg counts for both the smaller fecal output of children and the increased dilution of eggs in watery feces as compared to formed feces. It is proposed that egg counts from 1- to 3-year-olds be multiplied by 0.3, those from 4- to 6-year-olds by 0.5, those from 7- to 9-year-olds by 0.6, and those from 10 to 12-year-olds by 0.7; differences in mean egg density among various fecal consistencies produced factors of 1, 1.5, 2, 3, and 3.5 by which the egg counts in formed, mushy-formed, mushy, mushy-diarrheic, and diarrheic feces should be increased.

Population biology of hookworms in children in rural West Bengal. II. Acquisition and loss of hookworms.

From changes in hookworm egg counts in feces of children in Gangetic West Bengal we estimate that, on the average, each child lost about 11.1 female hookworms during the 1969 post-monsoon dry period and gained about 10.3 females during the subsequent premonsoon and monsoon seasons, resulting in a net loss of worms over the 22-month study period. However, the mean fecal egg count for each 1-year age group of children from 3-11 years suggests an average net gain of 2.7 female worms/year. From seasonal changes in the relative abundance of larvae recovered from fecal cultures, it is estimated that at least 82% of the yearly gain in Ancylostoma duodenale females is a result of the maturation of hypobiotic (latent) larvae acquired during the previous wet season. A duodenale was present about equally with Necator americanus but it was significantly more common among males than among females, and more common among Muslims than among Hindus. Using damp gauze pads, we sampled the soil surface around freshly-passed feces for infective hookworm larvae and estimated that 252 larvae were contacted by each child durint the transmission season. Efficiency of penetration and maturation of each hookworm species was calculated from estimated larval densities on the soil surface, from seasonal changes in fecal egg counts, and from variations in the number of larvae recovered from fecal cultures by season and age of the host; it is estimated that N. americanus is 17.5% efficient and A. duodenale 5.3%. Children negative for hookworm became positive at all times of the year and at a significantly higher rate in 1970 than in 1969; incidence averaged 4.2% per 2-month period and mean initial fecal egg count suggested infection with about 2.0 female worms. The average life span for N. americanus is calculated to be 3-4 years and that of A. duodenale about 1 year under the conditions of reinfection found in the study area.

Toxicologic and teratologic studies of oxibendazole in ruminants and laboratory animals.

Acute toxicity of oxibendazole was assessed with single oral doses given to mice (4 to 32 g/kg of body weight), sheep (230 to 600 mg/kg), and cattle (600 mg/kg); there were no ill effects. Subacute toxicity did not occur with multiple doses given 5 days to cattle (30 to 75 mg/kg/day) and to sheep (10 to 50 mg/kg/day). Chronic effects did not occur with daily doses of 3 to 30 mg/kg given 98 days to rats and dogs. Teratogenicity of the compound was studied in mice, rats, and sheep medicated at a dose level of 30 mg of oxibendazole/kg and in cattle given 75 mg/kg on selected dates during pregnancy. Microscopically, rodent fetuses seemed normal, and on gross physical examination, lambs and calves were free of malformations and ossification variations.

Efficacy of albendazole against gastrointestinal nematodes of cattle.

Forty-five calves with artificial and pasture-acquired nematode infections were medicated with albendazole at dose levels of 0, 2.5, 5.0, or 10 mg/kg of body weight. A dose level of 2.5 mg/kg removed at least 99% of adult Trichostrongylus axei, Trichostrongylus colubriformis, Cooperia oncophora, and Bunostomum phlebotomum. Burdens of Haemonchus contortus, Strongyloides papillosus, and Ostertagia ostertagi were reduced 79, 88, and 97%, respectively. At a dose level of 5.0 mg/kg, at least 95% of all adult nematodes were removed; at 10 mg/kg, at least 97% were removed. At least 99% of 4th-stage larvae of O ostertagi, T axei, C oncophora and T colubriformis and 96% of H contortus were expelled at a dose level of 2.5 mg/kg. At 5.0 and 10 mg/kg, 99 to 100% of all species of larvae were removed. Trichuris spp adults were slightly susceptible at all dose levels; larvae were susceptible (83%) only at 10 mg/kg.

Efficacy of albendazole against Haemonchus, Nematodirus, Dictyocaulus, and Monieza of sheep.

Albendazole was highly efficacious in the removal of monospecific and mixed infection of Haemonchus contortus, Nematodirus spathiger, and Dictyocaulus filaria from sheep. A dose level of 5 mg/kg removed nearly all gastrointestinal nematodes, and 10 mg/kg removed all lungworms. Tapeworms of the genus Moniezia were completely removed by a dose level of 10 hg/kg.

Efficacy of oxibendazole against gastrointestinal nematodes of cattle.

In 3 separate studies, oxibendazole in drench and premix formulation was shown efficacious against larvae and adults of the genera Haemonchus, Ostertagia, Trichostrongylus, Strongyloides, Nematodirus, Cooperia, Bunostomum, Capillaria, Oesophagostomum, and Trichuris. In the first study, artificially infected calves were cleared of virtually all histotrophic larvae and adult parasites after medication with oxibendazole drench at a dose level of 10 mg/kg of body weight. Smaller doses (7.5 and 5.0 mg/kg) expelled 84 to 100% of the parasites. Oxibendazole at a dose level of 15 mg/kg in premix form was given to artificially infected calves 3, 7, or 42 days after infection in a 2nd study. Third stage and 4th stage Cooperia oncophora larvae were, respectively, 92 and 98% susceptible to the drug at day 3 and day 7 after infection; neither larval stages of Ostertagia ostertagi and of Oesophagostomum radiatum were susceptible. Premix given on day 42 after infection removed 83 to 100% of adult O ostertagi, Nematodirus spp, C oncophora, O radiatum, and Trichuris spp. In a 3rd study, calves harboring a mixture of parasitic stages from artificial and pasture-acquired infections were medicated with oxibendazole at a dose of 15 mg/kg in premix form. The burdens of larvae from the abomasum and small intestines were reduced 93 to 95%, respectively; the burdens of adults of 11 species of worms were reduced 87 to 100%.

Critical tests with oxibendazole against gastrointestinal parasites of ponies.

Twenty ponies were allotted to 4 groups of 5 ponies each, and oxibendazole was orally administered at dose levels of 5, 10, 15, and 20 mg/kg. In the 3 groups of ponies given the largest doses, efficacy against 3 species of Strongylus was between 92 and 100% and that against small strongylids of the genera Cyathostomum, Cylicocyclus, Cylicodontophorus, and Cylicostephanus was more than 99%. All adults and 95 to 100% of larvae of the pinworm Oxyuris equi were removed. In the group of ponies given 5 mg/kg, 86 to 100% of the large strongylids and 84 to 100% of the small strongylids were removed, as were all larval and adult pinworms. Trichostrongylus axei was found only in 4 of the 5 ponies given 5 mg/kg; results were encouraging, but not consistent. Almost all of the Parascaris equorum were found in this group of ponies; no anthelmintic activity was detected at this dose level. Oxibendazole removed approximately 99% of small strongylid 4th-stage larvae. No efficacy against the larval stages of Gasterophilus intestinalis and against Habronema spp and Setaria equina was observed.

Arrested development in Ancylostoma duodenale: course of a self-induced infection in man.

The course of self-induced infection with about 100 Ancylostoma duodenale larvae was followed by twice-weekly blood and stool examinations. After a small initial increase, no further change in eosinophil counts occurred until the 33rd week of infection when they began to increase sharply. Seven weeks later, eggs were first observed in stools. Thus, in this infection, the prepatent period was five times the expected. It is suggested that a strain of A.duodenale exists which either 1) has an inherent, abnormally long prepatent period or 2) has a prepatent period which is susceptible to change depending upon the environmental conditions experienced by the free-living larvae. In either case, the third stage larvae probably enter a hypobiotic state within the host which lasts for about 8 months.

Arrested development in human hookworm infections: an adaptation to a seasonally unfavorable external environment.

Contrary to general belief, larvae of Ancylostoma duodenale do not always develop directly to adulthood upon invasion of man. In West Bengal, India, arrested development appears to be a seasonal phenomenon which results in (i) reduction of egg output wasted in seeding an inhospitable environment and (ii) a marked increase in eggs entering the environment just before the monsoon begins.