Helminths (from the Greek helminthos, meaning worm) refers to all parasitic worms of humans. They are complex, multicellular organisms, ranging in size from the microscopic filarial parasites to the giant tapeworms, several metres in length. Sexual reproduction occurs in all cases, usually by mating between male and female larvae. However, some helminths are hermaphroditic, possessing both male and female reproductive organs, and can reproduce by self-fertilisation, termed parthenogenesis.
Human helminth diseases occur world-widebut are most prevalent in countries with poor socio-economic development. They seldom cause acute disease but produce chronic infections that can have a severely debilitating effect on the host
Parasitic helminths comprise the nematodes (roundworms, filaria), cestodes (tapeworms) and trematodes (flukes)
Nematodes: These are typically worm-like in
appearance and have the following characteristics:
1.They possess a mouth, digestive tract, anus and sexual organs.
2.They occur as male or female forms.
3.They reproduce by mating or through parthenogenesis of hermaphroditic forms.
The intestinal pathogenic nematodes may be divided into those that develop in the soil (larvae being the infectious stage) and those that do not (eggs are the infectious stage):
Development in soil – larvae being infective. The larvae are shed in the faeces and mature in the soil. They infect humans by burrowing into the skin (usually through the soles of the feet) and enter the blood stream to be carried to the heart and lungs. They then force their way into the alveolus and trachea, and, on reaching the epiglottis, are swallowed. The life cycle then continues in the small intestine.
Survival in soil – eggs being infective. The eggs are the infectious form in which the larvae develop. When ingested the larvae hatch in the small intestine, penetrate the mucosa and are carried through the blood stream to the heart and lungs. The rest of the life cycle is as described above.
In other nematodes the eggs hatch in the intestine where the worms develop and produce eggs that are shed in the faeces. The larvae can sometimes migrate through the body to infect other organs.
Filaria: The filaria are microscopic nematodes, transmitted by biting insect vectors in which part of the organism’s life cycle is completed. On infecting humans the larvae mate and the females produce microfilariae which develop in the blood, lymphatic system, skin and eye. This can result in gross swelling of infected tissues, most notably in the groin and legs.
Cestodes (tapeworms): The tapeworms are flat, ribbon-like worms that can grow up to 10 metres in length. They produce eggs which are excreted into the environment and can infect a variety of hosts in which the life cycle continues. Humans become infected from consuming contaminated meat. They are characterised by:
1.absent mouth, digestive tract and vascular system
2.a scolex (head) that attaches to the intestinal wall by suckers
3.a tegument (body) of the scolex through which nutrients are absorbed
4.proglottids (segments) forming the tegument, each containing male and female reproductive organs producing infective eggs
5.eggs that hatch in the gut, releasing motile larvae that migrate through the gut wall and blood vessels to encyst in muscle forming cysticerci (fluid-filled cysts each containing
Trematodes (flukes): The trematodes are flat, leaf-like organisms. They have complicated life cycles, alternating between a sexual reproductive cycle in the final host (man) and an asexual multiplicative cycle in a snail host. They cause infection of the liver, bladder and rectum. Their major features are:
1.They possess a mouth and digestive tract but no anus.
2.They are hermaphroditic, except for the schistosomes which have a boat-shaped male
and a cylindrical female form.
3.Part of their life cycle is completed in an aquatic snail host.
Diagnosis and treatment: Intestinal helminths are identified by microscopic examination of faeces. Filaria are detected in blood and tissue samples after staining. As with the protozoa, helminth infections are difficult to treat, because of the lack of effective agents. Those that are available are toxic and unable to destroy all the biological forms.