Infecting and affecting approximately two billion people worldwide per year, helminths are important pathogens on the global scale. Commonly known as parasitic worms, the term “helminth” covers all worms — both parasitic and free-living organisms that often use living hosts as sources for nourishment and protection.
Biologically, helminths are both eukaryotic and multicellular organisms, which are usually large enough by their adult stage to be seen by the human eye. All helminths are symmetrical along a line running from head to tail, and either have tube-like or flattened body structures. While helminth is a group covering many different species, the classification of these worms can be broken down further into three main phyla:
Most commonly known as roundworms, the phylum Nematoda contain the tube-like helminths, clearly distinguishable from the other phyla on account of their cylindrical bodies. The main feature interrupting a nematode’s outer skin is its anterior mouth, its main source of nutrient intake. The mouth feeds into a fluid-filled cavity which then acts as a skeleton, providing rigidity for the organism when needed. On a more specific level, the orders of nematodes that infect humans and domestic animals include Trichocephalida, Oxyurida, Ascaridida, Strongylida, Rhabditida, Camallanida, and Spirurida. Nematodes move by using longitudinal muscles to thrash sideways. Although nematode eggs do not have a defined gender, adult organisms form separate sexes with well-developed reproductive systems.
Also referred to as tapeworms, the phylum Cestoda contain some of the flatter helminths, and are generally longer and ribbon-shaped. Cestodes are broken into numerous segments, and have a scolex (an organ on the head used to attach to the host). Cestodes have evolved to become flat enough to perfuse to internal tissues — such as the lining of a digestive tract — and as a result do not have a stomach or any internal cavities. Because cestodes will attach themselves to the lining of a tissue, they do not have any demand for complex muscle structures and absorb nutrients in the most efficient manner: through their skin. All cestodes are hermaphroditic, with each segment housing both male and female reproductive organs.
Commonly called flukes, the phylum Trematoda contain the smallest of the helminths: these helminths are both flat and leaf-like. Trematodes closely resemble cestodes, and also lack any internal cavities, as well as (most) Trematodes being hermaphroditic, with only a few exceptions. Trematode motion is the result of a complex muscle systems, which help the worms glide over objects easily.
Although helminths can come in different structures and species, they go about infecting humans and animals in fairly similar ways. Helminths make their way into the human body through one of four main ways: fecal-oral transmission (eggs or larvae passed in the faeces of one host and ingested with food/water by another); transdermal transmission (larvae in the soil actively penetrate the skin through contact and migrate through the tissues to the gut where adults develop and produce eggs); vector-borne transmission (larvae are taken up by another animal, such as a fly, and injected into new human hosts); and predator-prey transmission (larvae within animals are then eaten by predators, within which adult worms can develop and produce eggs).
Once helminths have successfully entered the body, they can impair the nutritional status of the host in a variety of ways, namely:
- A loss of iron and protein caused by the worms feeding on host tissues and blood.
- Malabsorption of nutrients.
- Vitamin A deficiency, caused by nematodes (roundworms) competing for vitamin A in the intestine.
- Loss of appetite and, therefore, a reduction of nutritional intake and physical fitness.
However, these symptoms will mainly occur in humans carrying a large quantity of helminth larvae, and most people infected with smaller quantities will never experience any of these effects.
The main effects of any helminth infection will only manifest as the worm makes its way out of the host in order to continue living and breeding. The most visually jarring of these exits is made by the Guinea Worm (Dracunculus medinensis), which burrows out of the skin in a painful blister, before releasing its eggs in the body of water the host will inevitably put the blister in due to the burning sensation. The more common method of exiting the body is through feces or urine, as the Trematode Schistosoma mansoni does (see this figure).
As parasitic organisms, helminths are often not given the attention they deserve. Unlike many other infectious organisms, there are actually many ways to prevent and treat helminths. The first step to prevent any helminth-induced infection is to follow the directions hidden in the acronym “W.A.S.H.” This stands for water supply, sanitation and hygiene. By defecating and handling waste far from the water supplies, people can better their chance that water supplies stay free of helminth contamination from humans. Next, people can wear shoes and garments in areas meant for waste to avoid infection by burrowing through skin, as helminthes frequently do. By preparing food in clean and sanitized areas, there is a much smaller chance that helminth eggs are ingested.
In terms of treatment options, there seem to be various, but similar options. Antiparasitic drugs are regarded as the best and most effective option of treatment. Too frequently do people try and let the parasite make its way out of the body without physically treating themselves. Antiparasitic drugs have different ways of killing the Helminths and most drugs send the invaders into paralysis. The way that drugs kill the helminths is usually by affecting the nervous system of the worms, and altering the ion-channels on, and inside the parasites membrane. Drugs like Ivermectin work by binding to glutamate-gated chloride ion channels in the helminths nerve and muscle cells causing paralysis of peripheral motor function and death of the worm. It seems that most antiparasitic drugs work this way. Whereas Praziquantel is an antiparasitic which affects the helminths ability to maintain sufficient calcium levels ultimately killing the helminth. An outlier in the medicines is Albendazole, which causes degeneration of cytoplasmic microtubules in intestinal helminths. Most of the drugs work by limiting the helminths ability to live sufficiently, in order to keep the drugs safe enough for human use, while effectively killing off parasites like helminths.
Diseases caused by helminth species include:
- Hookworm disease: Caused by the nematodes Ancylostoma duodenale or Necator americanus. When infected, hookworm disease can result in anemia and malnutrition.
- Dracunculiasis or Guinea Worm disease: Caused by the nematode Dracunculus medinensis, this disease is transmitted through contaminated water. The worm burrows out from the skin causing severe inflammatory reactions and discomfort.
- Loiasis: Caused by the nematode Loa loa, Loiasis is transmitted through Deer fly or Mango fly bites (where the flies contain Loa loa larvae). Once inside of a human host, adult worms move through tissue towards the inner eye. Loiasis causes red, itchy swellings in the skin referred to as Calabar swellings.
- Cysticercosis:Caused by the cestode Taenia solium. Symptoms only become evident when enough worms have taken refuge in their host — a process which often takes years — eventually resulting in painless bumps on the skin and muscles, or neurological problems.
- Echinococcosis: Caused by the cestode genus Echinococcus, Echinococcosis usually targets the liver first, before moving to the lungs and brain. Early stages can be felt by abdominal pain and jaundice, while the disease’s spread to the lungs is accompanied by breathlessness and coughing.