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April 2008: Infestations

By Dr Nigel Stollery

By Dr Nigel Stollery

1 Threadworms

Enterobius vermicularis or threadworm is the most common human intestinal worm in the UK. It is more common in children and is associated with poverty and overcrowding.

Infestation occurs when the eggs are swallowed following contact with an infected individual. The worms take 2-8 weeks to mature in the caecum and grow to a length of 1cm. They then migrate to the anal margin, where the female deposits eggs. This causes the characteristic pruritis ani, which tends to be worse at night. Scratching transfers the eggs to the fingers, from where they are reintroduced to the body or passed on to another individual. Adult worms have a lifespan of 6-12 weeks.

The treatment of choice is oral mebendazole, taken as a single dose followed by a second dose 14 days later if the infestation persists. Patients should be advised that hand washing is important, especially before eating, and that recurrence is common.

2 Roundworm

Ascaris lumbricoides or roundworm is the most common intestinal nematode in the world. The vast majority of patients are asymptomatic and many are unaware of its presence.

Roundworms are much larger than threadworms and can measure 15-35cm in length. They enter the body after contact with soil contaminated by faeces. Once ingested, the eggs hatch and the larvae penetrate the gut wall, migrating to the pulmonary bed via the portal veins, causing coughing or wheezing. The roundworms then travel along the respiratory tract, re-entering the gut.

Roundworms can live in the gut for 6-24 months, during which time they lay eggs. Rarely, their presence can lead to acute intestinal obstruction.1

Treatment is with an oral antihelminth such as mebendazole.

3 Larva migrans

Various types of hookworm cause larva migrans when they enter the skin, resulting in a characteristic rash. In the UK, the condition tends to be seen in patients who have recently returned from holiday destinations such as the Caribbean.

Hookworms live in the gut of animals such as dogs and cats, and their eggs are deposited in faeces. In warm and humid conditions, the eggs hatch. The hookworms burrow into human skin, usually when people are sitting or lying on a sandy beach.

Once in the skin, the larvae burrow, leaving a characteristic tortuous erythematous track and causing a nonspecific dermatitis. The larvae move at a rate of 1-2mm per day, which results in intense pruritis in the affected area.

Larva migrans is self-limiting and treatment is not always necessary. However, ivermectin or albendazole can be used if treatment is required.

Another option is cryotherapy. This needs to be applied 2-3 mm ahead of the visible track, which is usually where the worm will be.

4 Head lice

Lice are ectoparasites that feed on human blood after piercing the skin and injecting their saliva. There are three types: pediculosis capitis (head lice), pediculosis corporis (body lice) and pediculosis pubis (crab lice).

Head lice are the most common cause of lice infestation and are morphologically similar to body lice. They are spread by close physical contact and by sharing combs, brushes and bedding. A survey of 14 English schools found that 4-22% of pupils were infested.2

Adult lice live for 5-6 weeks on the scalp, but only 1-2 days away from a host. The females lay eggs (nits) on the hair shafts. These usually take about eight days to hatch, but may take longer in cooler temperatures.

Once infected, the patient's scalp becomes intensely itchy and excoriations may be seen, commonly in the postauricular and occipital areas.

Many public health consultants now advocate combing with a fine-toothed nit comb after the application of conditioner to the hair,3 as there are concerns that drug treatments may be absorbed systemically. Combing requires commitment but is usually effective.

Recently, a new treatment dimeticone has been developed. This coats the lice and eggs in a silicon compound, effectively suffocating the lice, although it has little effect on eggs and should be reapplied seven days later. It is available over the counter and can be used in patients aged six months and older.

5 Scabies

Scabies is caused by the arthropod Sarcoptes scabiei var. hominis and affects up to 300 million people worldwide annually. It is an obligate parasite that completes its whole lifecycle in human skin.

Female mites are 0.3mm in length and can just about be seen with the naked eye, whereas the males are half this size and tend not to be visible.

The male dies after fertilising the female. The latter burrows into the skin using proteolytic enzymes, which dissolve the stratum corneum. Eggs are deposited in the burrows and hatch 3-5 days later. About 90% of these mites die, and the survivors mature over two weeks. Once in their burrow, female mites never leave and die there after 1-2 months.

Normal infestations consist of 5-15 mites and the treatment of choice is topical 5% permethrin cream, which is washed off after 8-12 hours.

However, in the elderly or immunocompromised this may increase to many millions and is called Norwegian scabies. This condition results in a thick, crusted eruption over the skin.

Patients with Norwegian scabies need to be isolated. Gowns and gloves should be worn and all medical staff and carers should be treated as well as the patient. Treatment needs to include the whole body, including the head and neck.

Oral ivermectin 200mg/kg is an effective treatment. Although it is unlicensed for this indication it can be used on a named patient basis.

6 Ticks

Ixodid or hard ticks are very common, especially in wooded and rural areas where there are deer. After attaching to the skin with barbed mouthparts, they feed on the host's blood and remain attached by secreting a cement-like compound.

Ticks are easily visible with the naked eye and should be removed carefully. Simply pulling them off will leave the mouthparts attached, which can cause a foreign body reaction.

Various techniques for removing ticks are recommended, including applying heat to the back of the tick. Probably the best method is to grasp the tick with a pair of forceps and apply gentle traction, after which the tick will eventually let go of the skin.

7 Lyme disease

Lyme disease is a tick-borne illness caused by the spirochaete Borrelia burgdorferi. The common deer tick Ixodes scapularis is the usual vector, carrying the spirochaete from mammals such as mice to humans.

Following a tick bite, a characteristic erythema migrans rash is seen in around two-thirds of cases. Over a period, which ranges from days to months, the spirochaete may invade the bloodstream and spread to other areas of the body, such as the eyes, muscles, heart or the central nervous system. Synovial penetration is also common, leading to arthritis in affected joints.

Lyme disease should be treated with a course of oral antibiotics, such as amoxicillin or doxycycline.4 After treatment the prognosis is usually excellent.

8 Maggots

Maggots are the larvae of flies. Unfortunately, they can be attracted to living human tissue, as shown in the picture. In this case eggs were laid between an elderly patient's toes. Eggs can also be deposited through bandages into leg ulcers, especially during the summer.

Maggots can be used therapeutically to debride and deslough chronic leg ulcers. In these cases, sterile juvenile maggots are introduced to an ulcer and removed a few days later.5,6 Maggot therapy is not ideal for every patient and requires counselling and detailed preparation. However, it does seem to be effective.7

Author

Dr Nigel Stollery
MB BS
GP, Kibworth, Leicestershire and clinical assistant in dermatology, Leicester Royal Infirmary

Roundworm Head lice Tick Threadworm Lava Migrans Scabies Lyme disease Maggots

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