Modelling heterogeneity and the impact of chemotherapy and vaccination against human hookworm

Vaccine value profile for Hookworm

Hookworm, a parasitic infection, retains a considerable burden of disease, affecting the most underprivileged segments of the general population in endemic countries and remains one of the leading causes of mild to severe anemia in Low and Middle Income Countries (LMICs), particularly in pregnancy and children under 5. Despite repeated large scale Preventive Chemotherapy (PC) interventions since more than 3 decades, there is broad consensus among scholars that elimination targets set in the newly launched NTD roadmap will require additional tools and interventions. Development of a vaccine could constitute a promising expansion of the existing arsenal against hookworm. Therefore, we have evaluated the biological and implementation feasibility of the vaccine development as well as the added value of such a novel tool. Based on pipeline landscaping and the current knowledge on key biological aspects of the pathogen and its interactions with the host, we found biological feasibility of development of a hookworm vaccine to be moderate. Also, our analysis on manufacturing and regulatory issues as well as potential uptake yielded moderate implementation feasibility. Modelling studies suggest a that introduction of a vaccine in parallel with ongoing integrated interventions (PC, WASH, shoe campaigns), could substantially reduce burden of disease in a cost - saving mode. Finally a set of actions are recommended that might impact positively the likelihood of timely development and introduction of a hookworm vaccine.

Zoonotic transmission of intestinal helminths in southeast Asia: Implications for control and elimination

Intestinal helminths are extremely widespread and highly prevalent infections of humans, particularly in rural and poor urban areas of low and middle-income countries. These parasites have chronic and often insidious effects on human health and child development including abdominal problems, anaemia, stunting and wasting. Certain animals play a fundamental role in the transmission of many intestinal helminths to humans. However, the contribution of zoonotic transmission to the overall burden of human intestinal helminth infection and the relative importance of different animal reservoirs remains incomplete. Moreover, control programmes and transmission models for intestinal helminths often do not consider the role of zoonotic reservoirs of infection. Such reservoirs will become increasingly important as control is scaled up and there is a move towards interruption and even elimination of parasite transmission. With a focus on southeast Asia, and the Philippines in particular, this review summarises the major zoonotic intestinal helminths, risk factors for infection and highlights knowledge gaps related to their epidemiology and transmission. Various methodologies are discussed, including parasite genomics, mathematical modelling and socio-economic analysis, that could be employed to improve understanding of intestinal helminth spread, reservoir attribution and the burden associated with infection, as well as assess effectiveness of interventions. For sustainable control and ultimately elimination of intestinal helminths, there is a need to move beyond scheduled mass deworming and to consider animal and environmental reservoirs. A One Health approach to control of intestinal helminths is proposed, integrating interventions targeting humans, animals and the environment, including improved access to water, hygiene and sanitation. This will require coordination and collaboration across different sectors to achieve best health outcomes for all.

Changes in parasite traits, rather than intensity, affect the dynamics of infection under external perturbation

Understanding the mechanisms that generate complex host-parasite interactions, and how they contribute to variation between and within hosts, is important for predicting risk of infection and transmission, and for developing more effective interventions based on parasite properties. We used the T. retortaeformis (TR)-rabbit system and developed a state-space mathematical framework to capture the variation in intensity of infection and egg shedding in hosts infected weekly, then treated with an anthelminthic and subsequently re-challenged following the same infection regime. Experimental infections indicate that parasite intensity accumulates more slowly in the post-anthelminthic phase but reaches similar maximum numbers. By contrast, parasite EPG (eggs per gram of feces) shed from rabbits in the post-treatment phase is lower and less variable through time. Inference based on EPG alone suggests a decline in parasite intensity over time. Using a state-space model and incorporating all sources of cross-sectional and longitudinal data, we show that while parasite intensity remains relatively constant in both experimental phases, shedding of eggs into the environment is increasingly limited through changes in parasite growth. We suggest that host immunity directly modulates both the accumulation and the growth of the parasite, and indirectly affects transmission by limiting parasite length and thus fecundity. This study provides a better understanding of how within-host trophic interactions influence different components of a helminth population. It also suggests that heterogeneity in parasite traits should be addressed more carefully when examining and managing helminth infections in the absence of some critical data on parasite dynamics.

The Global Economic and Health Burden of Human Hookworm Infection

BACKGROUND

Even though human hookworm infection is highly endemic in many countries throughout the world, its global economic and health impact is not well known. Without a better understanding of hookworm's economic burden worldwide, it is difficult for decision makers such as funders, policy makers, disease control officials, and intervention manufacturers to determine how much time, energy, and resources to invest in hookworm control.

METHODOLOGY/PRINCIPLE FINDINGS

We developed a computational simulation model to estimate the economic and health burden of hookworm infection in every country, WHO region, and globally, in 2016 from the societal perspective. Globally, hookworm infection resulted in a total 2,126,280 DALYs using 2004 disability weight estimates and 4,087,803 DALYs using 2010 disability weight estimates (excluding cognitive impairment outcomes). Including cognitive impairment did not significantly increase DALYs worldwide. Total productivity losses varied with the probability of anemia and calculation method used, ranging from $7.5 billion to $138.9 billion annually using gross national income per capita as a proxy for annual wages and ranging from $2.5 billion to $43.9 billion using minimum wage as a proxy for annual wages.

CONCLUSION

Even though hookworm is classified as a neglected tropical disease, its economic and health burden exceeded published estimates for a number of diseases that have received comparatively more attention than hookworm such as rotavirus. Additionally, certain large countries that are transitioning to higher income countries such as Brazil and China, still face considerable hookworm burden.

MODELING THE SPREAD OF HOOKWORM DISEASE AND ASSESSING CHEMOTHERAPY PROGRAMS: MATHEMATICAL ANALYSIS AND COMPARISON WITH SURVEILLANCE DATA

Soil-transmitted helminthes including hookworm infections result in a major disease burden worldwide. Periodic chemotherapy treatments with anthelminthic drugs remain major intervention strategy in highly endemic areas and currently there is no approved human vaccine. We developed a model to study the spread of the hookworm infection in the population level having a variety of parasite development stages. We investigated long-term effectiveness of selective and mass chemotherapy, scenarios in which the therapy becomes less effective, and is interrupted. We analyzed the mathematical model, which includes determining the basic reproductive number and steady states, defining model initial conditions, proving positiveness and boundedness of solutions, local and global stability of the disease-free and endemic steady states, and uniform persistence of the system. We fitted our model using field data from hookworm control programs in endemic areas in China and Zimbabwe. The model provided accurate predictions of disease prevalence for all considered locations for multiple years of utilizing deworming programs. We demonstrated that administration of the deworming therapy is an effective method in terms of controlling the hookworm infection. Nevertheless, the chemotherapy programs do not eradicate the disease in the villages and the infection occurrence promptly increases once the program is interrupted, thus ultimately leading to the pre-treatment levels. Our modeling results suggest that the deworming programs should be maintained at least once per year to control the infection.

A model to assess the efficacy of vaccines for control of liver fluke infection

Fasciola hepatica, common liver fluke, infects cattle and sheep causing disease and production losses costing approximately $3 billion annually. Current control relies on drugs designed to kill the parasite. However, resistance is evident worldwide and widespread in some areas. Work towards a vaccine has identified several antigens of F. hepatica that show partial efficacy in terms of reducing worm burden and egg output. A critical question is what level of efficacy is required for such a vaccine to be useful? We have created the first mathematical model to assess the effectiveness of liver fluke vaccines under simulated field conditions. The model describes development of fluke within a group of animals and includes heterogeneity in host susceptibility, seasonal exposure to metacercariae and seasonal changes in temperature affecting metacercarial survival. Our analysis suggests that the potential vaccine candidates could reduce total fluke burden and egg output by up to 43% and 99%, respectively, on average under field conditions. It also suggests that for a vaccine to be effective, it must protect at least 90% of animals for the whole season. In conclusion, novel, partial, vaccines could contribute substantially towards fasciolosis control, reducing usage of anthelmintics and thus delaying the spread of anthelmintic resistance.

Modeling the economic and epidemiologic impact of hookworm vaccine and mass drug administration (MDA) in Brazil, a high transmission setting

BACKGROUND

Although mass drug administration (MDA) has helped reduce morbidity attributed to soil-transmitted helminth infections in children, its limitations for hookworm infection have motivated the development of a human hookworm vaccine to both improve morbidity control and ultimately help block hookworm transmission leading to elimination. However, the potential economic and epidemiologic impact of a preventive vaccine has not been fully evaluated.

METHODS

We developed a dynamic compartment model coupled to a clinical and economics outcomes model representing both the human and hookworm populations in a high transmission region of Brazil. Experiments simulated different implementation scenarios of MDA and vaccination under varying circumstances.

RESULTS

Considering only intervention costs, both annual MDA and vaccination were highly cost-effective (ICERs ≤ $790/DALY averted) compared to no intervention, with vaccination resulting in lower incremental cost-effectiveness ratios (ICERs ≤ $444/DALY averted). From the societal perspective, vaccination was economically dominant (i.e., less costly and more effective) versus annual MDA in all tested scenarios, except when vaccination was less efficacious (20% efficacy, 5 year duration) and MDA coverage was 75%. Increasing the vaccine's duration of protection and efficacy, and including a booster injection in adulthood all increased the benefits of vaccination (i.e., resulted in lower hookworm prevalence, averted more disability-adjusted life years, and saved more costs). Assuming its target product profile, a pediatric hookworm vaccine drastically decreased hookworm prevalence in children to 14.6% after 20 years, compared to 57.2% with no intervention and 54.1% with MDA. The addition of a booster in adulthood further reduced the overall prevalence from 68.0% to 36.0% and nearly eliminated hookworm infection in children.

CONCLUSION

Using a human hookworm vaccine would be cost-effective and in many cases economically dominant, providing both health benefits and cost-savings. It could become a key technology in effecting control and elimination efforts for hookworm globally.

Human Onchocerciasis: Modelling the Potential Long-term Consequences of a Vaccination Programme

Background

Currently, the predominant onchocerciasis control strategy in Africa is annual mass drug administration (MDA) with ivermectin. However, there is a consensus among the global health community, supported by mathematical modelling, that onchocerciasis in Africa will not be eliminated within proposed time frameworks in all endemic foci with only annual MDA, and novel and alternative strategies are urgently needed. Furthermore, use of MDA with ivermectin is already compromised in large areas of central Africa co-endemic with Loa loa, and there are areas where suboptimal or atypical responses to ivermectin have been documented. An onchocerciasis vaccine would be highly advantageous in these areas.

Methodology/Principal Findings

We used a previously developed onchocerciasis transmission model (EPIONCHO) to investigate the impact of vaccination in areas where loiasis and onchocerciasis are co-endemic and ivermectin is contraindicated. We also explore the potential influence of a vaccination programme on infection resurgence in areas where local elimination has been successfully achieved. Based on the age range included in the Expanded Programme on Immunization (EPI), the vaccine was assumed to target 1 to 5 year olds. Our modelling results indicate that the deployment of an onchocerciasis vaccine would have a beneficial impact in onchocerciasis–loiasis co-endemic areas, markedly reducing microfilarial load in the young (under 20 yr) age groups.

Conclusions/Significance

An onchocerciasis prophylactic vaccine would reduce the onchocerciasis disease burden in populations where ivermectin cannot be administered safely. Moreover, a vaccine could substantially decrease the chance of re-emergence of Onchocerca volvulus infection in areas where it is deemed that MDA with ivermectin can be stopped. Therefore, a vaccine would protect the substantial investments made by present and past onchocerciasis control programmes, decreasing the chance of disease recrudescence and offering an important additional tool to mitigate the potentially devastating impact of emerging ivermectin resistance.

Novel and alternative strategies are required to meet the demanding control and elimination (of infection) goals for human onchocerciasis (river blindness) in Africa. Due to the overlapping distribution of onchocerciasis and loiasis (African eye worm) in forested areas of central Africa, millions of people living in such areas are not well served by current interventions because they cannot safely receive the antiparasitic drug ivermectin that is distributed en masse to treat onchocerciasis elsewhere in Africa. The Onchocerciasis Vaccine for Africa—TOVA—Initiative has been established to develop and trial an onchocerciasis vaccine. We model the potential impact of a hypothetical childhood vaccination programme rolled out in areas where co-endemicity of onchocerciasis and African eye worm makes mass distribution of ivermectin difficult and potentially unsafe for treating, controlling and eliminating river blindness. We find that, 15 years into the programme, a vaccine would substantially reduce infection levels in children and young adults, protecting them from the morbidity and mortality associated with onchocerciasis. Most benefit would be reaped from a long-lived vaccine, even if only partially protective. We also discuss how a vaccine could substantially reduce the risk of re-emergence of onchocerciasis in areas freed from infection after years of successful intervention.

Selection and quantification of infection endpoints for trials of vaccines against intestinal helminths

Vaccines against human helminths are being developed but the choice of optimal parasitological endpoints and effect measures to assess their efficacy has received little attention. Assuming negative binomial distributions for the parasite counts, we rank the statistical power of three measures of efficacy: ratio of mean parasite intensity at the end of the trial, the odds ratio of infection at the end of the trial, and the rate ratio of incidence of infection during the trial. We also use a modelling approach to estimate the likely impact of trial interventions on the force of infection, and hence statistical power. We conclude that (1) final mean parasite intensity is a suitable endpoint for later phase vaccine trials, and (2) mass effects of trial interventions are unlikely to appreciably reduce the force of infection in the community - and hence statistical power - unless there is a combination of high vaccine efficacy and a large proportion of the population enrolled.

The potential economic value of a hookworm vaccine

Hookworm infection is a significant problem worldwide. As development of hookworm vaccine proceeds, it is essential for vaccine developers and manufacturers, policy makers, and other public health officials to understand the potential costs and benefits of such a vaccine. We developed a decision analytic model to evaluate the cost-effectiveness of introducing a hookworm vaccine into two populations in Brazil: school-age children and non-pregnant women of reproductive age. Results suggest that a vaccine would provide not only cost savings, but potential health benefits to both populations. In fact, the most cost-effective intervention strategy may be to combine vaccine with current drug treatment strategies.

Effect of heterogeneous mixing and vaccination on the dynamics of anthelmintic resistance: a nested model

Anthelmintic resistance is a major threat to current measures for helminth control in humans and animals. The introduction of anthelmintic vaccines, as a complement to or replacement for drug treatments, has been advocated as a preventive measure. Here, a computer-based simulation, tracking the dynamics of hosts, parasites and parasite-genes, shows that, depending on the degree of host-population mixing, the frequency of totally recessive autosomes associated with anthelmintic resistance can follow either a fast dynamical regime with a low equilibrium point or a slow dynamical regime with a high equilibrium point. For fully dominant autosomes, only one regime is predicted. The effectiveness of anthelminthic vaccines against resistance is shown to be strongly influenced by the underlying dynamics of resistant autosomes. Vaccines targeting adult parasites, by decreasing helminth fecundity or lifespan, are predicted to be more effective than vaccines targeting parasite larvae, by decreasing host susceptibility to infection, in reducing the spread of resistance. These results may inform new strategies to prevent, monitor and control the spread of anthelmintic resistance, including the development of viable anthelmintic vaccines.

Interpreting malaria age-prevalence and incidence curves: a simulation study of the effects of different types of heterogeneity

Background

Individuals in a malaria endemic community differ from one another. Many of these differences, such as heterogeneities in transmission or treatment-seeking behaviour, affect malaria epidemiology. The different kinds of heterogeneity are likely to be correlated. Little is known about their impact on the shape of age-prevalence and incidence curves. In this study, the effects of heterogeneity in transmission, treatment-seeking and risk of co-morbidity were simulated.

Methods

Simple patterns of heterogeneity were incorporated into a comprehensive individual-based model of Plasmodium falciparum malaria epidemiology. The different types of heterogeneity were systematically simulated individually, and in independent and co-varying pairs. The effects on age-curves for parasite prevalence, uncomplicated and severe episodes, direct and indirect mortality and first-line treatments and hospital admissions were examined.

Results

Different heterogeneities affected different outcomes with large effects reserved for outcomes which are directly affected by the action of the heterogeneity rather than via feedback on acquired immunity or fever thresholds. Transmission heterogeneity affected the age-curves for all outcomes. The peak parasite prevalence was reduced and all age-incidence curves crossed those of the reference scenario with a lower incidence in younger children and higher in older age-groups. Heterogeneity in the probability of seeking treatment reduced the peak incidence of first-line treatment and hospital admissions. Heterogeneity in co-morbidity risk showed little overall effect, but high and low values cancelled out for outcomes directly affected by its action. Independently varying pairs of heterogeneities produced additive effects. More variable results were produced for co-varying heterogeneities, with striking differences compared to independent pairs for some outcomes which were affected by both heterogeneities individually.

Conclusions

Different kinds of heterogeneity both have different effects and affect different outcomes. Patterns of co-variation are also important. Alongside the absolute levels of different factors affecting age-curves, patterns of heterogeneity should be considered when parameterizing or validating models, interpreting data and inferring from one outcome to another.