Arthropods

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Numerous species of arthropods play a role in human disease. Most of these are as vectors of different pathogens and, in other sections of this text, we have deal with such disease vectors. There are also a number of arthropods that cause harm due their venom but these are not parasites. Here, we shall deal with arthropods that are parasitic to and cause disease in man. For example, myiasis (burial of larvae in tissue) is an obligatory step in the life cycle of some flies and incidental for others. Species that cause myiasis in the Americas are Cochliomyia (Screw worm fly), Calliphora, Oestrus, Sarcophaga, Gastrophilus, etc. Myiasis may be cutaneous, arterial, intestinal or urinary in normal tissue or in pre-existing wounds, some of which may result from other infections. Larvae can burrow through necrotic or healthy tissue using their mandibular hooks aided by proteolytic enzymes. They can cause mechanical damage and the affected area may be the site of a secondary infection. Cutaneous myiasis may require surgical removal of burrowed larvae. Eggs and maggots may be washed from hair, skin and wounds with soap and water. Urinary myiasis usually clears itself. Purgation with anti-helminths may be necessary for gastrointestinal myiasis.

Lice

Three types of sucking lice are important for human health: Pediculus humanus capitis (head louse), P. humanus humanus (body louse) and Pthirus pubis (crab louse). Lice spend all of their life on one very specific host and both male and female feed on blood and leave one host only to transfer to another.

Head Lice

In the developed world, 2 to 10% of children are infested with head lice. Light infestations cause moderate itching of the scalp due to sensitization to louse saliva. Heavy infestations may result in fever, aches and secondary infections. Diagnosis is based on finding live lice or empty egg shells (nits measuring 0.8 x 0.3 mm) attached to hair, often behind the ears. Topical application of soothing lotions relieves irritation. Hair should be washed with shampoo containing 1% benzene hexachloride (Kwell). Application of a mixture of pyrentins (0.2%) and pipronyl butoxide (2%) or copper oleate may be as effective and less toxic than benzene hexachloride.

Body Lice

The body louse is similar to the head louse except that it is found on the body and clothes. Diagnosis is based on finding eggs or nits in seams of clothing. Symptoms and treatment are the same as those for head lice.

Pubic Lice

Pubic lice also known as crab lice infest widely-spaced coarse hair in the pubic area in adults or eye lashes in children. Transmission in adults is usually by sexual contact. Diagnosis is based on finding lice or nits in the infested area; crab lice may be difficult to see at the base of the hair. The pubic area is treated in the same manner as the infested head. Nits and lice may be removed from eye lashes with forceps. Ointments with physostigmine (0.25%) or yellow mercury oxide are effective.

Fleas

Most fleas are of clinical significance to man because they are vectors for other parasites. However, the jigger flea or chigoe (Tunga penetrans) is a serious pest in the tropical and subtropical regions of the Americas and Africa. Both sexes feed on blood. The female flea, after insemination, burrows itself in the skin of the toes and the sole of the foot. The female swells to the size of a pea, produces eggs and dies in the tissue. There is local reaction to the bite and the eggs and dead flea produce reaction. The infested tissue can get infected and gangrenous; auto-amputation is not uncommon. Treatments are symptomatic: infestation may be physically removed; secondary infections are treated appropriately. Shoes should be worn in infested areas.

Mites

Scabies mite (Sarcoptes scabei) is the cause of scabies and is distributed worldwide. Epidemics of the disease may occur for long periods but mites may be common at all times in very poor communities with inadequate washing facilities. The mite transmitted by contact burrows into the skin on the webbing side of fingers, later spreading to the wrists, elbows and the rest of the body. The buttocks, women's breasts and external genitalia may be involved. The mite tunnels itself through the upper layer of the skin depositing eggs. Larvae escape the tunnel and wander on the skin and start new burrows and mature there to continue the cycle. Scabies itch is due to the sensitization of the patient to the mite and eggs and is characteristically nocturnal. Septic pustules may develop after scratching, if the hygiene is poor. Diagnosis is made by the characteristic rash and by smearing black ink on the skin and observing burrows when the ink is wiped away. Microscopic examination of a skin scraping show mites. Treatment involves swabbing of the whole body from the neck down with 1% malathion or benzene hexachloride (crotamiton for infants). Topical steroids must not be used. If possible, the whole family should be treated. Contact with an infested person should be avoided. Clothes should be washed in hot water.

Chiggers

The chiggers or red-bugs (Trombiculidae) are an important group of ectoparasites affecting humans. They attach to the skin in the ankles, waistline, armpits and perianal area after walking through a grassy environment. These mites do not feed on blood but on partially digested skin cells using enzmes in the chigger's saliva. The host reacts to the mouth parts and saliva of the mite. The bite causes severe irritation and sometimes fever. Treatment with a local anesthetic is useful. Insect repellents may be effective in avoiding chigger bites.