Parasitic infections

34. Parasitic infections




Questions


image Which adult nematodes live in the small bowels of humans?

image Which parasites cause liver pathology?

image By which insect is onchocerciasis transmitted?

The helminths are subdivided into nematodes, cestodes and trematodes.



Soil-transmitted nematodes (roundworms)


The roundworms, apart from Enterobius vermicularis, can survive outside of the host and are transmitted by faecally contaminated soil. Their larvae require migration through lungs and tissues before maturing to an adult worm. This migration induces an eosinophilia. Diagnosis is usually made by stool concentration for ova and parasites. For treatment, many options are available (Ch. 44).


Enterobius vermicularis (threadworm, pinworm, oxyuris) lives in the caecum and passes to the anus to lay eggs, which causes anal pruritus. Eggs under fingernails lead to faecal–oral transmission in households. Diagnosis is made by microscopy of tape preparations from the anus.


Ascaris lumbricoides is common in children living in poor hygienic conditions. Ascaris eggs contaminating food are swallowed and liberate larvae in the small bowels, which migrate through the lung to get back through the oesophagus to the bowels, where they remain as adult worms. Heavy infections can cause intestinal obstructions and wandering worms can block the bile duct.


Hookworm (Ancylostoma duodenale, Necator americanus) eggs hatch in the soil and the larvae penetrate the skin of the host. After migration through the lungs and oesophagus, the adult worm finds its final destination in the small bowels, where it sucks blood through the mucosa causing anaemia.


Whipworm (Trichuris trichiuria) eggs are swallowed with contaminated food and liberate larvae in the caecum, from where they colonize the large bowels to feed on tissue juices. Heavy infestation can lead to dysentery and rectal prolapse.


Toxocara canis and T. cati are the roundworms of dogs and cats. Children get infected in fouled sandpits in public parks. After swallowing the eggs, the larvae are liberated in the intestines, from where they migrate for 1 or 2 years through organs without final destination in humans. Sometimes larval migration becomes symptomatic as hepatosplenomegaly (visceral larva migrans) or as chorioretinitis or strabism (ocular larva migrans). Visceral larva migrans can be treated with anthelminthic drugs, whereas ocular larva migrans requires laser photocoagulation.


Strongyloides larva are excreted by faeces and develop adult worms in soil. Their larvae penetrate the skin of the human host in order to migrate via the lung to the bowels. These infective larvae can penetrate the bowel or anal skin and autoinfect the host, causing the characteristic larva currens (Fig. 3.34.1). Immunosuppression can induce a hyperinfection syndrome, with peritonitis and Gram-negative sepsis.


image

Fig. 3.34.1 Larva currens. Strongyloides stercoralis larva in skin; these are visible for minutes.

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Epidemiology of infectious diseases Infections of the immunocompromised host Pathogenesis of infectious disease Infections of bone, joints and muscle Tuberculosis and other mycobacterial diseases Innate host defences to infectious disease

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Jul 3, 2016 | Posted by in MUSCULOSKELETAL MEDICINE | Comments Off on Parasitic infections

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