Ascaris The Neglected Parasite

This book tackles a number of different perspectives concerning the parasitic helminth Ascaris, both in animals and in humans and the disease known as ascariasis. It seeks to identify interesting, exciting and novel aspects, which will interest readers from a broad range of disciplines. Over a quarter of the world's population are infected with the human roundworm, and the equivalent in pigs is equally ubiquitous. Both contribute to insidious and chronic nutritional morbidity, and this has been quantified, in humans, as disability adjusted life years approximating 10.5 million. Ascaris larvae develop in host parenteral tissues, and the resultant pathology has been condemnation. Ascariasis, despite its staggering global prevalence and the sheer numbers of people it infects, remains a classic neglected disease. However, renewed interest in the consequences of early infection with worms from the perspective of immune modulation, co-infections and the development of allergy further enhances the relevance of these parasites. Brings together a wide range of topics and approaches and recent, comprehensive and progressive research concerning the neglected parasite Ascaris Provides a blueprint of how a single parasite entity can stimulate interest in basic biology, clinical science, veterinary science, public health and epidemiology Presents a wealth of new insights given that a book on this parasite has not been published for over 20 years 16 chapters from a range of top authors from around the world

The fields of conservation genetics and genetic epidemiology share many parallel questions. Hence, methodologies can be adapted from the former to the latter. Three applications of population genetics are presented to further our understanding of Ascaris epidemiology. (1) I address if Ascaris infections in humans and pigs constitute a single population or subdivided host-affiliated populations with potential for cross-transmission and hybridization. (2) I discuss the use of landscape genetics as a means to identify transmission foci and epidemiologically relevant variables correlated to parasite population substructure. (3) I propose the novel integration of the effective population size (Ne) parameter into population monitoring and epidemiological studies of parasites in general. Using microsatellite data from a metapopulation of A. lumbricoides in Nepal, I demonstrate the utility of estimating Ne with single-sample contemporary estimators, offer epidemiologically related questions that can be addressed with Ne, and highlight assumptions of Ne estimation related to Ascaris biology.

Mathematical models are playing an increasing role in the design and optimization of control programs for many infectious diseases. In the case of Ascaris lumbricoides, there is a long history of models to understand the biology and epidemiology and to design control programs. We outline the key insights which models have given on the dynamics of Ascaris transmission, describe the model structures and discuss how current and future models are informing the design of control programs in the era of both increased access to donate drugs and new statistical techniques to validate these models against reinfection data. We show how the “bounce-back” time following a treatment round, when infection levels monotonically increase to baseline levels, are driven by the life expectancy of the parasite and the basic reproductive number. We also show that the effect of treatment in subsets of the population, for example school children, depends on the extent to which different subpopulations interact with the egg output of the treated group. We discuss the outstanding policy questions which models can be used to answer and the data and model development required to address them.

Roundworms are the most prevalent parasite of swine in the world today. Their presence on a pig farm can have detrimental effects on farm profitability due to their impact on growth rate, feed conversion, and general health of the produced pigs. For farmers, it is therefore essential to be aware of the “worm status” of their herd in order to make informed decisions on how to control this disease on their farm. This chapter concentrates on the diverse aspects involved in the diagnosis and control of Ascaris suum infections on pig farms. It focuses on how the results obtained from different detection methods should be interpreted and what efforts could be made to control parasite transmission and consequentially reduce the prevalence and economic impact of this parasite on a pig farm.

Experiences from Japan and Korea demonstrated that when a country is developing economically, the periodic administration of anthelmintic (preventive chemotherapy) drastically reduces soil-transmitted helminthiasis (STH) and these advances are then maintained by increasing sanitation standards; both countries eliminated the morbidity due to STH in less than a decade. The situation is more complex in countries with slow economic development in which preventive chemotherapy should be maintained over a longer period of time in order to reduce morbidity due to STH. Preventive chemotherapy is characterized by population-based diagnosis, population-based treatment, and implementation at regular intervals. WHO recommends regular monitoring of the implementation of this strategy in order to obtain maximal benefit for the target populations. In 2010, preventive chemotherapy for the control of STH reached over 33% of the population in need of treatment and rapid expansion of the coverage is expected in 2012–2015 as a consequence of the drug donation provided by the original producers. WHO is monitoring this trend and facilitating the process providing direct support to member states, facilitating the drug donation and developing guiding material and strategic plans.

The roundworm Ascaris suum is a highly prevalent intestinal parasite of pigs worldwide, and the infection may have low to moderate pathogenic effects on health and productivity. Besides these direct effects, there may be indirect effects on enhanced susceptibility or pathogenicity to bacterial or viral infections, related to the migratory and immune-modulatory capacity of A. suum. Losses due to ascariasis in pigs can thus be summarized as (1) farm economic losses due to clinical effects (although limited), reduced growth and feed conversion efficiency and costs of control (e.g. use of anthelmintics), (2) abattoir operator losses due to condemnation or downgrading of livers and lower product quality, and (3) potential interference with vaccinations and higher risk of co-infections. Apart from the cost of liver condemnations that can be extrapolated from available data, it is difficult to quantify losses.

To better understand the epidemiology of ascariasis, molecular genetic methods have been applied to differentiate between worms found infecting people and pigs, and other occasional hosts. Owing to difficulties in applying species concepts, the longstanding debate of the distinction(s) between Ascaris lumbricoides and Ascaris suum continues. From an applied perspective, however, molecular markers help to “tag” and “track” worms during their transmission cycle(s), providing new insights into host range, as well as methods for assessing parasite population dynamics through time and contingent upon disease control. While useful in determining dynamics at the tips of the evolutionary tree, these molecular tools also provide insights into deeper evolutionary branches. Although Ascaris is found throughout the globe, molecular analysis of worms retrieved from sub-Saharan Africa point towards a significant center of genetic diversity, possibly denoting a likely center of evolutionary origin with subsequent parasite diaspora. Resolving these issues precisely, however, requires greater scrutiny of genetic variation within Parascaris and Baylisascaris.

Natural infections with Ascaris lumbricoides and A. suum of pigs and humans, respectively, have a worldwide distribution. Ascaris infections induce strong Th2 responses in the host that appear to be central to the development of protective immunity. Th2 responses may be modulated actively through a complex host–parasite interaction that allows the parasite to survive but also limits severe Th2-mediated pathology in the host. The increased production of IL-10 by regulatory cell populations may have a key role in the modulation of the immune response during chronic ascariasis although most evidence in support is derived from experimental infections with A. suum and A. suum antigen extracts. Antigen fractions of A. suum have potent anti-inflammatory effects in experimental models of arthritis and asthma, but other antigen fractions are strong inducers of inflammation. The clinical effects of such immune modulation in humans is uncertain although there is some evidence that chronic infections may be associated with impaired immune responses to vaccines indicating potentially important effects on both mucosal and systemic immunity. Human ascariasis has been associated with an increased risk of asthma and other allergic reactions, observations that preclude the use of experimental infections for the treatment of inflammation in favor of the study of the anti-inflammatory properties of specific molecules derived from Ascaris.