Periodic reproduction of onchocerca volvulus

Schulz‐Key, H.; Karam, M

doi:10.1016/0169-4758(86)90138-9

Cited by 77 publications

(59 citation statements)

References 7 publications

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“…There is, however, some positive correlation between the number of worms harboured and the resulting concentration of microfilariae in the blood, with other research also involving experimental infection of primates demonstrating that, whilst monkeys inoculated with 200 L3 larvae had, on average, higher concentrations of microfilariae than those inoculated with 75 L3 larvae, there was significant overlap between infection intensities observed across the two groups [25]. The results are, therefore, suggestive of some non[ 2 9 5 _ T D $ D I F F ] -linear (and possibly density-dependent) relationship between worm burden and microfilaraemia, a phenomenon reminiscent of that directly observed for Ascaris lumbricoides [31] and suggested for O. volvulus [32] (although, in the latter case, other studies have failed to substantiate these findings [33]). Although it is apparent that worm burden and microfilarial intensity are not linearly related, it remains to be established whether this is a product of density-dependent processes acting on female worm fecundity, or on other aspects of parasite or host biology.…”

Section: Extrinsic Incubation Period (Eip)supporting

confidence: 49%

The Population Biology and Transmission Dynamics of Loa loa

Whittaker

Walker

Pion

et al. 2018

Trends in Parasitology

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African eye worm (caused by the filarial nematode Loa loa) -affects more than 10 million people. Despite causing ocular and systemic symptoms, it has typically been considered a benign condition, only of public health relevance because it impedes mass drug administration-based interventions against onchocerciasis and lymphatic filariasis in co-endemic areas. Recent research has challenged this conception, demonstrating excess mortality associated with high levels of infection, implying that loiasis warrants attention as an intrinsic public health problem. This review summarises available information on the key parasitological, entomological, and epidemiological characteristics of the infection and argues for the mobilisation of resources to control the disease, and the development of a mathematical transmission model to guide deployment of interventions.

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Section: Extrinsic Incubation Period (Eip)supporting

confidence: 49%

The Population Biology and Transmission Dynamics of Loa loa

Whittaker

Walker

Pion

et al. 2018

Trends in Parasitology

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“…Although densitydependent regulation of mf production has been suggested, 15 this phenomenon has not been substantiated by other researchers in either O. volvulus 16 or O. ochengi. 17 In other filarial species such as Wuchereria bancrofti in humans 18 and Brugia pahangi in the cat model, 19,20 there is no evidence of saturation of mf output at high parasite densities (a pattern that would indicate the operation of density-dependent checks on per capita fecundity).…”

Section: Introductionmentioning

confidence: 82%

Transmission intensity and the patterns of Onchocerca volvulus infection in human communities.

Basáñez¹,

Collins²,

Porter³

et al. 2002

The American Journal of Tropical Medicine and Hygiene

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Abstract.We focus on possible constraints upon Onchocerca volvulus establishment in humans in relation to exposure rates to infective larvae (L3) as measured by the annual transmission potential (ATP). We use mathematical and statistical modeling of pre-control west African (savanna), Mexican, and Guatemalan data to explore two hypotheses relating human infection to transmission intensity: microfilarial (mf) loads either saturate with increasing ATP or become (asymptotically) proportional to the ATP. The estimated proportion of L3 developing into adult worms ranged from 7% to 0.3% (low and high intensity areas, respectively). Relationships between mf prevalence and both mf and transmission intensity were nonlinear and statistically similar between west Africa (Simulium damnosum s.l.) and Meso America (S. ochraceum s.l.). This similarity extended to the relationship between mf intensity and ATP. The critical biting rates for onchocerciasis introduction and persistence (which depended on vector competence and host preference), were approximately 10-fold higher in settings where onchocerciasis is transmitted by S. ochraceum than in those where the vector is S. damnosum. A role for focal vector control in Mexico and Guatemala, in addition to nodulectomy and ivermectin, is suggested. INTRODUCTIONAt the start of the 21st century onchocerciasis or "river blindness" still poses a threat to public health in many tropical regions of Africa and more focally in Latin America despite a concerted research effort and high expenditure on control programs. Although much research has been conducted on the biology and epidemiology of the host-parasite interaction, relatively little is known about the population dynamics of the parasite in terms of the key regulatory constraints on population growth. In an attempt to further understanding of such processes, a number of researchers have explored the properties of simple and complex mathematical models of the transmission dynamics of human onchocerciasis.1 Considerable progress has been made concerning the regulatory constraints operating within the vector, 2-6 with some progress being recently made in investigating those acting within the human host.

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“…For example, some studies demonstrate a significant reduction of the number of male worms [7,9]. Although these lower counts may be the result of the ability of male worms to leave nodules [31], it may at least temporarily lead to reduced mating chances and a lower Mf output of female worms. Hence, we cannot exclude that part of the effect estimated for female worms should be attributed to males.…”

Section: Resultsmentioning

confidence: 99%

Irreversible Effects of Ivermectin on Adult Parasites in Onchocerciasis Patients in the Onchocerciasis Control Programme in West Africa

Plaisier

Alley²,

Boatin³

et al. 1995

Journal of Infectious Diseases

101

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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-

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Periodic reproduction of onchocerca volvulus

Cited by 77 publications

References 7 publications

The Population Biology and Transmission Dynamics of Loa loa

The Population Biology and Transmission Dynamics of Loa loa

Transmission intensity and the patterns of Onchocerca volvulus infection in human communities.

Irreversible Effects of Ivermectin on Adult Parasites in Onchocerciasis Patients in the Onchocerciasis Control Programme in West Africa

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