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over plants grown without competition (P < .05). Plants suffering from both root and shoot competition were not significantly smaller than those suffering from root competition only, but they showed the greatest size variability. In plants competing both above and below ground, root competition accounted for the reduction in mean plant size, but shoot competition accounted for the increase in size inequality. The results support the hypothesis that competition for light is "asymmetric" and that the observed increases in plant size variability with increasing density may have been primarily due to competition for light.There may be situations in which competition for nutrients is also asymmetric, i.e., situations in which nutrients can be preempted by individuals with larger roots. Because a leaf is shaded only by leaves above it, competition for light appears to be inherently asymmetric.Acknowledgments:

The outcome of Helicobacter pylori infection has been associated with specific virulence-associated bacterial genotypes. The present study aimed to investigate the gastric histopathology in Portuguese and Colombian patients infected with H. pylori and to assess its relationship with bacterial virulence-associated vacA, cagA, and iceA genotypes. A total of 370 patients from Portugal (n = 192) and Colombia (n = 178) were studied. Corpus and antrum biopsy specimens were collected from each individual. Histopathological features were recorded and graded according to the updated Sydney system. H. pylori vacA, cagA, and iceA genes were directly genotyped in the gastric biopsy specimens by polymerase chain reaction and reverse hybridization. Despite the significant differences between the Portuguese and Colombian patient groups, highly similar results were observed with respect to the relation between H. pylori genotypes and histopathology. H. pylori vacA s1, vacA m1, cagA+ genotypes were significantly associated with a higher H. pylori density, higher degrees of lymphocytic and neutrophilic infiltrates, atrophy, the type of intestinal metaplasia, and presence of epithelial damage. The iceA1 genotype was only associated with epithelial damage in Portuguese patients. These findings show that distinct H. pylori genotypes are strongly associated with histopathological findings in the stomach, confirming their relevance for the development of H. pylori-associated gastric pathology.

Background-Virulence factors of Helicobacter pylori are associated with peptic ulcer disease and may be also associated with the eYcacy of treatment. Aims-To determine the relation between the vacA and the cagA status of H pylori, clinical disease, and treatment outcome. Patients-121 patients with H pylori infection and peptic ulcer disease or functional dyspepsia were treated by quadruple antibiotic therapy in two groups for one and two days, respectively. Methods-DNA was isolated from gastric antral biopsy specimens, taken before and after treatment, and the vacA and cagA status was determined by polymerase chain reaction and reverse hybridisation. Results-Peptic ulcer disease was significantly associated with the vacA s1 type, and cagA positivity, but not with the vacA m type. Treatment eYcacy was significantly higher in patients with peptic ulcer disease, or infected with cagA+/vacA s1 strains. Conclusions-The strong association between the cagA and vacA status and peptic ulcer disease was confirmed. Cure rates seem to be higher for patients with cagA+/ vacA s1 H pylori strains, which is consistent with the higher cure rate observed among ulcer patients compared with functional dyspepsia patients. Therefore, treatment studies may require stratification for presence of ulcers as well as H pylori genotypes. (Gut 2000;46:321-326)

ONCHOSIM is a computer program for modelling the transmission and control of the tropical parasitic disease onchocerciasis, or river blindness. It is developed in collaboration with the Onchocerciasis Control Programme in West Africa (OCP), and is used as a tool in the evaluation and planning of control operations. The model comprises a detailed description of the life history of the parasite Onchocerca volvulus and of its transmission from person to person by Simulium flies. The effects of different control strategies, based on larvicide application and chemotherapy (ivermectin), on the transmission and on the disease symptoms can be evaluated and predicted. In the program two simulation techniques are mixed. Stochastic microsimulation is used to calculate the life events of individual persons and inhabitant parasites, while the dynamics of the Simulium population and the development of the parasite in the flies are simulated deterministically. Output of ONCHOSIM conforms to the format in which data collected by the OCP are reported. This enables detailed checking of model specifications against empirical data. Output can also consist of summarizing key indices for the intensity of onchocerciasis infection, which is especially useful for comparing the effectivity of control strategies.

The epidemiological model ONCHOSIM--a model and computer simulation program for the transmission and control of onchocerciasis--has been used to determine the range of plausible values for the reproductive lifespan of Onchocerca volvulus. Model predictions based on different lifespan quantifications were compared with the results of longitudinal skin-snip surveys undertaken in 4 reference villages during 13 to 14 years of successful vector control in the Onchocerciasis Control Programme in West Africa. Good fits between predicted and observed trends in skin microfilarial loads could be obtained for all villages. It is concluded that the reproductive lifespan of the savanna strain of O. volvulus lies between 9 and 11 years, and that 95% of the parasites reach the end of reproduction before the age of 13 to 14 years.

Ivermectin is an effective drug for the treatment of human onchocerciasis, a disease caused by the parasitic filarial nematode Onchocerca volvulus. When humans are treated, the microfilariae normally found in the skin are rapidly and very nearly completely eliminated. Nonetheless, after a delay, microfilariae gradually reappear in the skin. This study is concerned with the causes of this delay. Hypotheses are tested by comparing the results of model calculations with skin microfilaria counts collected from 114 patients during a trial of five annual treatments in the focus area of Asubende, Ghana. The results obtained strongly suggest that annual treatment with ivermectin causes an irreversible decline in microfilariae production of~30%/treatment. This result has important implications for public health strategies designed to eliminate onchocerciasis as a significant health hazard.The registration ofthe anthelminthic drug ivermectin (Mectizan; Merck, Rahway, NJ) in 1987 was a landmark in the control of human onchocerciasis or river blindness, a parasitic disease caused by the filarial nematode Onchocerca volvulus. Oral administration in a standard dose of 150-200 j.Lg/kg of body weight is followed by rapid elimination of microfilariae (Mf) from the skin and gradual reduction of ocular Mf levels [1]. Side effects are generally mild. This makes ivermectin a better therapeutic option than diethylcarbamazine, which is often accompanied by severe Mazotti reactions and ocular damage. Ivermectin also produces a longer suppression ofMfrepopulation of the skin [2,3]. To explain this difference, which was obvious in all studies done so far, the effects of ivermectin on adult parasites were studied.Adult female parasites in treated persons show an interruption of the normal embryogenesis, but after a single treatment, this appears to be reversible for most of the worms [4,5]. Excess worm mortality was not observed after a single treatment [1, 2, 6], although significant numbers of dead and mori-

This paper presents a model-based analysis of longitudinal data describing the impact of integrated vector management on the intensity of Wuchereria bancrofti infection in Pondicherry, India. The aims of this analysis were (1) to gain insight into the dynamics of infection, with emphasis on the possible role of immunity, and (2) to develop a model that can be used to predict the effects of control. Using the LYMFASIM computer simulation program, two models with different types of immunity (anti-L3 larvae or anti-adult worm fecundity) were compared with a model without immunity. Parameters were estimated by fitting the models to data from 5071 individuals with microfilaria-density measurement before and after cessation of a 5-year vector management programme. A good fit, in particular of the convex shape of the age-prevalence curve, required inclusion of anti-L3 or anti-fecundity immunity in the model. An individual's immune-responsiveness was found to halve in approximately 10 years after cessation of boosting. Explanation of the large variation in Mf-density required considerable variation between individuals in exposure and immune responsiveness. The mean life-span of the parasite was estimated at about 10 years. For the post-control period, the models predict a further decline in Mf prevalence, which agrees well with observations made 3 and 6 years after cessation of the integrated vector management programme.