A stochastic model for the aggregation of Onchocerca volvulus in nodules

Designing antifilarial drug trials using clinical trial simulators

Lymphatic filariasis and onchocerciasis are neglected tropical diseases (NTDs) targeted for elimination by mass (antifilarial) drug administration. These drugs are predominantly active against the microfilarial progeny of adult worms. New drugs or combinations are needed to improve patient therapy and to enhance the effectiveness of interventions in persistent hotspots of transmission. Several therapies and regimens are currently in (pre-)clinical testing. Clinical trial simulators (CTSs) project patient outcomes to inform the design of clinical trials but have not been widely applied to NTDs, where their resource-saving payoffs could be highly beneficial. We demonstrate the utility of CTSs using our individual-based onchocerciasis transmission model (EPIONCHO-IBM) that projects trial outcomes of a hypothetical macrofilaricidal drug. We identify key design decisions that influence the power of clinical trials, including participant eligibility criteria and post-treatment follow-up times for measuring infection indicators. We discuss how CTSs help to inform target product profiles.

Drugs for filariases are under development and clinical trial simulators could help to inform the design of clinical trials. Here, Walker et al. use an individual-based onchocerciasis transmission model to project trial outcomes of a hypothetical macrofilaricidal drug, resolving key design choices.

Soil-Transmitted Helminths: Mathematical Models of Transmission, the Impact of Mass Drug Administration and Transmission Elimination Criteria

Infections caused by soil-transmitted helminthias (STHs) affect over a billion people worldwide, causing anaemia and having a large social and economic impact through poor educational outcomes. They are identified in the World Health Organization (WHO) 2020 goals for neglected tropical diseases as a target for renewed effort to ameliorate their global public health burden through mass drug administration (MDA) and water and hygiene improvement. In this chapter, we review the underlying biology and epidemiology of the three causative intestinal nematode species that are mostly considered under the STH umbrella term. We review efforts to model the transmission cycle of these helminths in populations and the effects of preventative chemotherapy on their control and elimination. Recent modelling shows that the different epidemiological characteristics of the parasitic nematode species that make up the STH group can lead to quite distinct responses to any given form of MDA. When connected with models of treatment cost-effectiveness, these models are potentially a powerful tool for informing public policy. A number of shortcomings are identified; lack of critical types of data and poor understanding of diagnostic sensitivities hamper efforts to test and hence improve models.

Modelling Neglected Tropical Diseases diagnostics: the sensitivity of skin snips for Onchocerca volvulus in near elimination and surveillance settings

BACKGROUND

The African Programme for Onchocerciasis Control has proposed provisional thresholds for the prevalence of microfilariae in humans and of L3 larvae in blackflies, below which mass drug administration (MDA) with ivermectin can be stopped and surveillance started. Skin snips are currently the gold standard test for detecting patent Onchocerca volvulus infection, and the World Health Organization recommends their use to monitor progress of treatment programmes (but not to verify elimination). However, if they are used (in transition and in parallel to Ov-16 serology), sampling protocols should be designed to demonstrate that programmatic goals have been reached. The sensitivity of skin snips is key to the design of such protocols.

METHODS

We develop a mathematical model for the number of microfilariae in a skin snip and parameterise it using data from Guatemala, Venezuela, Ghana and Cameroon collected before the start of ivermectin treatment programmes. We use the model to estimate sensitivity as a function of time since last treatment, number of snips taken, microfilarial aggregation and female worm fertility after exposure to 10 annual rounds of ivermectin treatment.

RESULTS

The sensitivity of the skin snip method increases with time after treatment, with most of the increase occurring between 0 and 5 years. One year after the last treatment, the sensitivity of two skin snips taken from an individual infected with a single fertile female worm is 31 % if there is no permanent effect of multiple ivermectin treatments on fertility; 18 % if there is a 7 % reduction per treatment, and 0.6 % if there is a 35 % reduction. At 5 years, the corresponding sensitivities are 76 %, 62 % and 4.7 %. The sensitivity improves significantly if 4 skin snips are taken: in the absence of a permanent effect of ivermectin, the sensitivity of 4 skin snips is 53 % 1 year and 94 % 5 years after the last treatment.

CONCLUSIONS

Our model supports the timelines proposed by APOC for post-MDA follow-up and surveillance surveys every 3-5 years. Two skin snips from the iliac region have reasonable sensitivity to detect residual infection, but the sensitivity can be significantly improved by taking 4 snips. The costs and benefits of using four versus two snips should be evaluated.

Dynamics of Onchocerca volvulus microfilarial densities after ivermectin treatment in an ivermectin-naïve and a multiply treated population from Cameroon

Background/Objective

Ivermectin has been the keystone of onchocerciasis control for the last 25 years. Sub-optimal responses to the drug have been reported in Ghanaian communities under long-term treatment. We assessed, in two Cameroonian foci, whether the microfilaricidal and/or embryostatic effects of ivermectin on Onchocerca volvulus have been altered after several years of drug pressure.

Methods

We compared the dynamics of O. volvulus skin microfilarial densities after ivermectin treatment in two cohorts with contrasting exposure to this drug: one received repeated treatment for 13 years whereas the other had no history of large-scale treatments (referred to as controls). Microfilarial densities were assessed 15, 80 and 180 days after ivermectin in 122 multiply treated and 127 ivermectin-naïve individuals. Comparisons were adjusted for individual factors related to microfilarial density: age and number of nodules.

Findings

Two weeks post ivermectin, microfilarial density dropped equally (98% reduction) in the ivermectin-naïve and multiply treated groups. Between 15 and 180 days post ivermectin, the proportion of individuals with skin microfilariae doubled (from 30.8% to 67.8%) in controls and quadrupled (from 19.8% to 76.9%) in multiply treated individuals but the mean densities remained low in both sites. In fact, between 15 and 80 days, the repopulation rate was significantly higher in the multiply treated individuals than in the controls but no such difference was demonstrated when extending the follow-up to 180 days. The repopulation rate by microfilariae was associated with host factors: negatively with age and positively with the number of nodules.

Conclusion

These observations may indicate that the worms from the multi-treated area recover mf productivity earlier but would be less productive than the worms from the ivermectin-naïve area between 80 and 180 days after ivermectin. Moreover, they do not support the operation of a strong cumulative effect of repeated treatments on the fecundity of female worms as previously described.

Millions of Africans and thousands of Latin Americans are infected with Onchocerca volvulus, the filarial worm responsible for onchocerciasis. Since the mid-1990s, control programs rely on annual or six-monthly community treatments with the only safe drug available, ivermectin. If sustained for another 10–15 years, this strategy could lead to elimination of onchocerciasis. Unfortunately, there have been reports of low response to the drug in Ghanaian communities under long-term treatment. Here, we compared the response of O. volvulus to ivermectin between a Cameroonian population repeatedly treated and an ivermectin-naïve population. Skin parasite density was assessed before and 15, 80 and 180 days after treatment. Parasite density dropped equally in the two groups (∼98% reduction) by 15 days, in accordance with the expected effect of ivermectin at this time point. In the multi-treated subjects, the repopulation rate of the skin by microfilariae was higher than in the controls from 15 to 80 days after treatment but the microfilarial levels reached similar levels six months after treatment in the two groups. Thirteen years of large-scale treatments may have selected worms less sensitive to the drug. In addition, those treatments had little if any cumulative effect on skin parasite repopulation after an additional treatment.

Individual host factors associated with Onchocerca volvulus microfilarial densities 15, 80 and 180 days after a first dose of ivermectin

Reduction in Onchocerca volvulus skin microfilarial densities after treatment with ivermectin shows wide between-host variation. Data from two separate studies conducted in Cameroon on onchocerciasis patients treated for the first time with ivermectin were analyzed to identify host factors associated with microfilarial density at different time-points after treatment. In one site (Nkam valley), the dataset included 103 adult males for whom age, number of palpable onchocercal nodules and microfilarial densities on D0 (pre-treatment), D15, D80 and D180 were available. In the other site (Vina valley), analyses were conducted on 965 individuals of both sexes aged 5 years and over; in this dataset, available information included age, gender, exact dose of ivermectin received, onchocerciasis endemicity level in the village of residence and microfilarial densities on D0 and D180. Negative binomial regression models of microfilarial density at the different intervals post-treatment were fitted, using maximum likelihood, with the available independent variables. Gender and age were found to be associated with microfilarial density on D180. The initial microfilarial density influenced post-treatment densities at all the time-points. All other things being equal, microfilarial densities on D180 were higher in individuals harbouring a higher number of nodules or living in communities with high endemicity levels. This study demonstrates that O. volvulus microfilarial density measured after a first treatment with ivermectin, and thus probably the rate of skin repopulation by microfilariae (mf) varies according to several host factors. Should such factors also influence ivermectin efficacy after repeated treatment, then they should be taken into account to determine whether sub-optimal responses to treatment reported from various areas in Africa are actually due to parasite-related factors, particularly to the emergence of resistant populations.

Epidemiology and control of onchocerciasis: the threshold biting rate of savannah onchocerciasis in Africa

Control of onchocerciasis currently focuses on community-directed treatment with the microfilaricide ivermectin which effectively kills Onchocerca volvulus microfilariae in the human host. The feasibility of elimination by this control strategy has recently been reported for some foci in Africa which has rekindled discussions on evaluating the threshold conditions of elimination of onchocerciasis. We developed a stochastic model based on a master equation which predicts, based on data from West and Central Africa, that elimination of savannah onchocerciasis can be expected around a threshold biting rate of 730 bites per person per year, ranging region-specifically roughly from 230 to 2300 bites per person and year. The threshold values give rise to optimism that elimination of onchocerciasis is feasible, but the associated measures of parasite prevalence and density suggest that onchocerciasis can remain endemic at very low infection intensities. Endemicity at a low level is a risk factor for elimination strategies, and we point to the necessity of investigating these issues on the basis of breakpoints which refer to threshold conditions based on parasite prevalence and density.

Rates of microfilarial production by Onchocerca volvulus are not cumulatively reduced by multiple ivermectin treatments

Regular distribution of ivermectin reduces onchocerciasis transmission and morbidity by killing, within humans, the microfilarial stage of the parasite (microfilaricidal effect). In addition, ivermectin exerts a so-called embryostatic effect by which microfilarial production by the adult female worm becomes suppressed during a number of weeks after treatment. To assess the overall effect of ivermectin on onchocerciasis transmission and evaluate the likelihood of local elimination of the infection it is important to estimate the magnitude of the anti-fertility effect over the course of a treatment programme. Estimates of the effect of repeated drug treatments on the production of microfilariae by adult Onchocerca volvulus were obtained by developing a model that was fitted to data collected from three hyperendemic communities in Guatemala, where eligible residents received ivermectin twice per year for two and a half years. The data consist of microfilarial load measurements in the skin, collected just before each six-monthly treatment during the programme. The model that is developed describes the dynamics of an individual host's expected microfilarial load over the 30-month study period. We adopt a Bayesian approach and use Markov chain Monte Carlo (McMC) techniques to fit the model to the data. Combining estimates from the three villages, average microfilarial production in the first six months post-treatment was reduced by ~64% of its pre-treatment level, regardless of values chosen for the pre-ivermectin fertility rate within plausible ranges. Increased adult worm death rate after treatment (to mimic removal of macrofilariae via nodulectomy during the programme) resulted in a smaller estimated magnitude of the embryostatic effect (rate of microfilarial production was reduced by ~58% of pre-ivermectin value). After subsequent treatments, the rate of microfilarial production appeared to be similarly decreased. The data and analyses therefore do not support the hypothesis of a cumulative effect of multiple ivermectin treatments on microfilarial production by female worms.

Diagnostic value of nodule palpation in onchocerciasis

Nodule palpation is the major diagnostic tool for determining the prevalence of infection in areas of the African Programme for Onchocerciasis Control (APOC) and is recommended for identifying communities at risk and selecting them for mass drug administration. The diagnostic value of palpation, however, has not been quantified in terms of sensitivity and predictive values. We derive these measures from the probability that a nodule is palpable, which has been estimated by stochastic simulations from an extensive pre-control database. We show that nodule palpation is only reliable in highly endemic areas and that false-positive diagnoses can lead to considerable misclassifications of regions where endemicity is actually low. Its diagnostic precision is poor because of large intra- and inter-individual variability. The findings underline the need for further development of available diagnostics that allow long-term monitoring when endemicity declines.

Density-dependent host choice by disease vectors: epidemiological implications of the ideal free distribution

The proportion of vector blood meals taken on humans (the human blood index, h) appears as a squared term in classical expressions of the basic reproduction ratio (R(0)) for vector-borne infections. Consequently, R(0) varies non-linearly with h. Estimates of h, however, constitute mere snapshots of a parameter that is predicted, from evolutionary theory, to vary with vector and host abundance. We test this prediction using a population dynamics model of river blindness assuming that, before initiation of vector control or chemotherapy, recorded measures of vector density and human infection accurately represent endemic equilibrium. We obtain values of h that satisfy the condition that the effective reproduction ratio (R(e)) must equal 1 at equilibrium. Values of h thus obtained decrease with vector density, decrease with the vector:human ratio and make R(0) respond non-linearly rather than increase linearly with vector density. We conclude that if vectors are less able to obtain human blood meals as their density increases, antivectorial measures may not lead to proportional reductions in R(0) until very low vector levels are achieved. Density dependence in the contact rate of infectious diseases transmitted by insects may be an important non-linear process with implications for their epidemiology and control.

High infection rates at low transmission potentials in West African onchocerciasis

Onchocerciasis has been successfully controlled for many years in endemic countries but more than 120 million people are still at risk. Factors which stabilise the persistence of the parasite in the population must be studied to minimise the future risk of re-infection. Among these factors, the relationship between the annual transmission potential and the parasite establishment rate is a main determinant which has to date not been quantified. Using entomological information and palpation data collected by the Onchocerciasis Control Programme in West Africa prior to the initiation of control activities, we derive annual transmission potential-dependent estimates of the parasite establishment rate from statistical analyses and computer simulations. Even at very low transmission intensities, the filarial parasite Onchocerca volvulus can efficiently establish in the human population, originating from an infection process which is strongly limited with respect to the annual transmission potential. Implementing the estimates into a simplified transmission model predicts that the critical annual biting rate, below which transmission is not possible, is much lower than previously assumed. We conclude that under the current strategy of mass distribution of microfilaricides without additional measures of vector control, the risk of re-infection is higher than previously assumed.

The relationships between the burden of adult parasites, host age and the microfilarial density in human onchocerciasis

We investigate the relationship between the microfilarial density in the skin and the burden of adult female Onchocerca volvulus by analysing pre-control nodulectomy data which allow for a direct approach, independent of exposure. The data of 169 patients in Burkina Faso and 182 patients in Liberia represent savannah and forest onchocerciasis in West Africa, respectively. Whereas in Burkina Faso, a saturating relationship between microfilarial density and worm burden suggests the operation of density-dependent processes within human hosts, the Liberian data show a linear relationship implying no density dependence. The differences may derive from differences between both parasite strains, i.e. the savannah or the forest strain of O. volvulus. Consistently for both parasite strains and independent of the worm burden, the microfilarial density increases with host age emphasising the concept of the acquisition of immunological tolerance. In male hosts in Liberia, the microfilarial density increases stronger with the worm burden than in female hosts, whereas such sex-specific differences cannot be found in Burkina Faso. In the methodological part of this investigation, we suggest the beta-distribution to be most appropriate for describing variability in microfilarial densities and we present an approach to consider the uncertainty in the adult parasite burden which cannot be determined precisely in helminth infections. Implications of density dependence are discussed with respect to immunological processes in the human host and with respect to the success of control programs. The relationships described show that regulatory processes between the parasite and the human host are multi-dimensional, operating within a high degree of biological variability.

Pathogenesis and host responses in human onchocerciasis: impact of Onchocerca filariae and Wolbachia endobacteria

Onchocerca volvulus is a tissue-invasive parasitic nematode causing skin and eye pathology in human onchocerciasis. The filariae habour abundant intracellular Wolbachia bacteria, now recognised as obligatory symbionts, and therefore emerging as a novel target for chemotherapy. Recent research demonstrates that both the filariae and endobacteria contribute to the pathogenesis of onchocerciasis, and molecules have been identified that promote inflammatory or counter-inflammatory immune mechanisms, divert the host's immune response or procure evasion of the parasite.

Density-dependent parasite establishment suggests infection-associated immunosuppression as an important mechanism for parasite density regulation in onchocerciasis

The modulation of human immune response by filarial parasites has yielded contradictory experimental findings and attracted much controversy. We address the unresolved question of acquisition, establishment and accumulation of Onchocerca volvulus by using a modelling approach that relates computer simulations to cross-sectional data concerning parasite burdens in 913 West African onchocerciasis patients. It is shown that the acquisition of O. volvulus is not constant with host age; instead, the analysis of age profiles of parasite burdens strongly indicate the operation of immunosuppressive processes within the human host, associated with the presence of adult parasites or microfilariae. It is suggested that these processes suppress immunity against incoming infective larvae (L3), which themselves act as an immune modulating component once they have successfully overcome the barrier of concomitant immunity. Suppression of parasite-specific immunity leads to parasite establishment rates which increase along with the parasite burden, but which hardly depend on hyperendemic annual transmission potentials. Children, still immunocompetent due to low parasite burdens, acquire 0.1-0.5 adult female parasites per year, whereas older people, immunosuppressed due to high burdens, acquire 2-4 adult female parasites per year. Differences in parasite establishment between the forest and the savannah strains of O. volvulus are quantified and dynamic aspects of density-dependent parasite establishment discussed.