Management of Filarial Worms in Adult

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Filariasis is a parasitic infection caused by thread-like nematodes (filariae) that belong to the roundworm super family filarioidea. They are found primarily in tropical countries such as sub-Saharan Africa, southern Asia, the western Pacific islands, Brazil and Guyana.

Generally, there are eight types of filarial nematodes that use humans as their host which these are divided into three main groups depending on the body area they tend to occupy, as follows:

Lymphatic filariasis: is infected by the following types of worm: Wuchereria bancrofti, Brugia malayi and Brugia timori. In chronic cases, the host may develop elephantiasis, a condition where parts of the body swell to massive proportions. Subcutaneous filariasis: it involves the subcutaneous tissue and is caused by the worms Loa loa, Onchocerca volvulus, Mansonella streptocerca, and Dracunculus medinensis (guinea worm).

Serous cavity filariasis: it occurs in the serous cavity of the abdomen and is caused by Mansonella perstans and Mansonella ozzardi.

Transmission and life cycle

The filariae worms are spread by black flies and mosquitoes. It has a complex life cycle that can be divided into five main stages:

Adult worms in the human give birth to thousands of live microfilariae which are taken up by the mosquito or fly during a blood meal.

Within the insect, the microfilariae migrate towards the thoracic muscles where they develop into first-stage and eventually third-stage larvae, which are capable of infecting humans.

These third-stage larvae migrate towards the mosquito’s head and proboscis and are transmitted to another human next time the mosquito takes a blood meal.

Within the new host, the larvae take around a year to complete two more stages of development and mature into adult worms.

Symptoms

Edema

Thickened skin and massive swelling

Itching

Urticaria

Skin rashes

Arthritis

Abdominal pain

Diagnosis

The infection is confirmed when a blood film shows the presence of microfilariae

Identification is done using Geimsa stain

Polymerase chain reaction (PCR)

Antigenic assays.

Treatment

The treatment of filariaisis consists of chemotherapy directed against the adult worms (macrofilaricidal) and against the microfilariae (microfilaricidal) combined with symptomatic treatment to relieve the damage caused by the body’s immunological reaction to dead or dying worms.

Macrofilaricidal drugs

Suramin: also known as Antrypol, has a lethal action on adult W. bancrofti and other filarial parasites as well as some effect on microfilariae. Its mode of action is unknown however its high toxicity and the availability of other filarialicides has drastically reduced its use.

Side effecst: Immediate reactions include nausea and anaphylactic shock; 24 hours later photophobia and peripheral neuritis may occur. Agranulocytosis and haemolytic are uncommon however death of adult worms can lead to swelling of the affected area and abscess formation.

Macro- and Micro-filaricidal drugs

Diethylcarbamazine: also known as Heterazan, Banocide, and Notezine this piperazine derivative is used mainly as a microfilaricide even though it does have macrofilaricidal properties. It is usually effective in treating Tropical Pulmonary Eosinophilia (TPE) and its mechanism of action is thought to involve sensitizing the microfilariae to phagocytosis.

Side effects: There are no toxic effects on uninfected individuals. However, systemic adverse reactions (fevers, malaise, headache) are seen in microfilaraemic patients in proportion to the intensity of blood parasitaemia. These can be minimized by spreading the treatment into weekly low doses. Localized adverse reactions also occur, apparently around dying adult worms.

Microfilaricidal drugs:

Ivermectin: also known as Mectizan (22,23-dihydroavermectin B1) this drug is a semisynthetic macrocyclic lactone derivative of avermectine. It is an effective microfilaricide for most filarial species and W. bancrofti seems to be especially sensitive to its effects. Its mechanism of action is thought to involve the activation of GABA (gamma-aminobutyric acid) pathways and chloride channel permeability.

Side effects: The lack of severe side effects, microfilaricidal activity, and single oral dose administration have encouraged its use. Adverse effects which include fever, myalgia, headache, sore throat, and cough are usually more prevalent in individuals with higher microfilaremic levels. Clinical use indicates that invermectin does not have macrofilarical effects upon adult worms unless used in multiple doses over a period of months.

Metrifonate: also known as trichlorphon this organophosphorus insecticide is also effective in treating nematode infections. It acts by inhibiting cholinesterases and thus paralysising the worm. It has been used clinically in the treatment of W. bancrofti infections and is an effective microfilaricide.

Side effects: Although plasma cholinesterase levels fall and remain low for several weeks after the treatment, side effects are not severe. If the dose is increased above 10mg/kg vomiting, nausea and abdominal pain are frequent.

Benzimidazole derivatives

Mebendazole stops the development of the embryos in onchocerciasis and when given in conjunction with levamisole both microfilaricidal and embryostatic effects can be achieved. Microfilarial counts in the blood fall slowly and the effects may last for up to six months.

Mebendazole:

Mebendazole is a potent antihelmintic with a wide range of activity against both nematodes and cestodes, and against both tissue stages of the parasite as well as against worms in the lumen and gut.As a drug it is poorly absorbed so high doses must be used when treating tissue infections (like filarial infections).

Side effects: they are unusual at lower doses but common when higher doses are given and fatalities have occurred when patients have been treated with very high doses.

Levamisole: it acts by interfering with the carbohydrate metabolism of the nematodes and inhibiting the production of succinate dehydrogenase, causing muscular paralysis of the worms. It also acts as a non-humoral immunostimulant in immunosupressed individuals. The mechanism of this stimulation is unknown.

Side effects: are unusual; however, when they occur they include nausea, vomiting, abdominal pain, dizziness, diarrhea, skin rashes, and transient neutropenia.

Symptomatic Treatment

Lymphatic incompetence due to filarial infection causes lymph stasis and allows opportunistic microbial infections to develop. Antiseptic hygiene minimises these complications. Surgery can remove and provide bypasses around the damaged lymphatics.

Prevention

There are currently no vaccines available to prevent filarial infections however control of mosquito vectors as well as the reduction of human reservoirs (through mass DEC treatment) has been effective reducing infection rates and seems to be a viable method of control for filariasis.

 



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