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Some of the nematode infections we'll cover are ascariasis, trichuriasis, hookworm, and toxocariasis. The first three are often referred to as the soil-transmitted helminths and together are perhaps the biggest contributor to overall morbidity in the developing world. Approximately 3 billion people are chronically infected with these worms across the globe. The last infection is a relatively common helminth infection in the developed world in poor populations in rural and inner-city communities, and may be a contributor to childhood asthma in the United States in socioeconomically disadvantaged populations.
Some of the cestode infections we'll cover are taeniasis, caused by both the pork and beef tapeworms, and diphyllobothriasis, caused by the fish tapeworm.
We'll cover two major trematode infections, schistosomiasis, and the infections caused by the incredibly varied liver flukes, which are comprised of several distinct genera and species of trematode all with highly varied disease ecology and landscape epidemiology.
The landscapes of infection that these helminths delineate are wildly varied. In part, this is because the modes of transmission are quite different across the helminths. For example, those that can be transmitted by the fecal-oral route may be highly influenced by the level of sanitation and water infrastructure in certain geographies. While, on the other hand, those helminths that are transmitted by a mosquito vector may be strongly influenced by fluctuations in climate and the specific ecology of the vector and not at all influenced by municipal sanitation.
Another critical aspect that defines the microgeographies of helminth infection is the fact that these parasites require multiple hosts, both definitive and intermediate, to complete their life cycles. As such, these organisms exist at the interface between the ecologies of different animals, including humans, which can make blocking infection extraordinarily difficult.
This week at Infection Landscapes I am beginning a new series on infections with helminths, which are parasitic worms. There are a staggering number of helminths that infect humans, which employ even more varied, and often mind boggling, strategies to complete their life cycles. Some helminths are transmitted fecal-orally, some are ingested in the muscle tissue of various animals, some are vector-borne, and some are transmitted by directly penetrating the surface of the skin. Almost all of these helminths require additional animal hosts to complete their life cycle. These organisms present some of the most fascinating disease ecologies and diverse landscape epidemiology in the world of infectious diseases. Unfortunately, many of these infections exact a high cost on the populations most affected. These make up the majority of the neglected tropical diseases, and together they comprise the largest overall disease burden to human beings on the planet.
Helminths are eukaryotic multicellular endoparasites, so they must live within their hosts.
There are two very broad over-arching categories of helminths: the roundworms and the flatworms. The roundworms are known collectively as nematodes, and the flatworms are divided into the cestodes (tapeworms) and trematodes (flukes).
Nematodes vary greatly in size from microscopic micro-filariae to the massive Ascaris worms, but they are all cylindrical in shape and thus their designation as “round”. They have an alimentary tract, a nervous system, and they develop a cuticle as their outer covering.
Ascaris lumbricoides
Some of the nematode infections we'll cover are ascariasis, trichuriasis, hookworm, and toxocariasis. The first three are often referred to as the soil-transmitted helminths and together are perhaps the biggest contributor to overall morbidity in the developing world. Approximately 3 billion people are chronically infected with these worms across the globe. The last infection is a relatively common helminth infection in the developed world in poor populations in rural and inner-city communities, and may be a contributor to childhood asthma in the United States in socioeconomically disadvantaged populations.
Cestodes also vary in size but they are all planar in shape, thus their inclusion in the flatworms, and they are readily distinguished by their segmented bodies. These helminths have no digestive tract, but they do have a nervous system, and they develop a tegument as their outer covering
Taenia solium (pork tapeworm)
Some of the cestode infections we'll cover are taeniasis, caused by both the pork and beef tapeworms, and diphyllobothriasis, caused by the fish tapeworm.
Trematodes are, again, of variable size, but most trematode forms tend to be very small. Trematodes are also planar in shape, which is why they too are considered flatworms, however they are not segmented. These helminths do have an alimentary tract but they lack an anus. Trematodes also possess a nervous system and develop a tegument as their outer covering.
Schistosoma mansoni (blood fluke)
We'll cover two major trematode infections, schistosomiasis, and the infections caused by the incredibly varied liver flukes, which are comprised of several distinct genera and species of trematode all with highly varied disease ecology and landscape epidemiology.
The landscapes of infection that these helminths delineate are wildly varied. In part, this is because the modes of transmission are quite different across the helminths. For example, those that can be transmitted by the fecal-oral route may be highly influenced by the level of sanitation and water infrastructure in certain geographies. While, on the other hand, those helminths that are transmitted by a mosquito vector may be strongly influenced by fluctuations in climate and the specific ecology of the vector and not at all influenced by municipal sanitation.
Another critical aspect that defines the microgeographies of helminth infection is the fact that these parasites require multiple hosts, both definitive and intermediate, to complete their life cycles. As such, these organisms exist at the interface between the ecologies of different animals, including humans, which can make blocking infection extraordinarily difficult.