Blood and Tissue Nematodes of Human Beings

FILARIAL PARASITES OF HUMAN BEINGS

Filariae The slender filarial worms of the family FILARIIDAE are arthropod-transmitted parasites of the circulatory and lymphatic systems, muscles, connective tissues, or serous cavities of vertebrates.

Principal species in humans are: Wuchereria bancrofti, Brugia malayi, Onchocerca volvulus, Loa loa, Dipetalonema perstans and Mansonella ozzardi. A distinctive feature filarial worms is that the viviparous female gives birth to prelarval microfilariae.

Wuchereria bancrofti and Brugia malayi CLASS: SECERNENTEA SUBCLASS: SPIRURIA ORDER: SPIRURIDA SUPERFAMILY: FILARIOIDEA FAMILY: FILARIIDAE Scientific name - Wuchereria bancrofti Disease - filariasis, elephantiasis Scientific name - Brugia malayi Disease - filariasis, elephantiasis

Life Cycle of Wuchereria bancrofti:

A microfilaria of Wuchereria bancrofti as it would appear in a blood smear; approximate size = 200 µm in length.

Elephantiasis of Legs and Scrotum

Elephantiasis of the legs

Historical Elephantiasis has been written about since the time of the early Greeks and Romans. Hosts humans. Distribution Tropical or subtropical (41N. latitude to 31S. latitude). South America, Cuba, Puerto Rico, West Indies, Africa, Spain, Turkey, Asia, Australia, many South Pacific Islands.

Characteristics Adult worms are long and slender with a smooth cuticle and bluntly rounded ends. The head is slightly swollen and bears two circles of well-defined papillae. The mouth is small; a buccal cavity is lacking. The male is about 40 mm long and 100 um wide. Its tail is fingerlike. The female is 6 to 10 cm long and 300 um wide. The vulva is near the level of the middle of the esophagus.

Symptoms-Pathogenicity Four stages of the disease are recognized. The incubation period of 3 to 12 months in which there are no symptoms. The acute symptomatic stage in which some swelling of the extremities may occur and this may be accompanied by pain, weakness of arms and legs, headache, insomnia. Fever is usually not present.

There is a period of recovery which is permanent if reinfection does not occur. If there is continued reinfection the cycle repeats and elephantiasis may result. The worms in the lymphatic system cause tissue changes which restrict normal flow of lymph and result in swelling, fibrosis and eventually secondary infections in the affected tissues. The lower extremities and groin are the parts most likely to be affected. The adult worms live for several years.

Onchocerca volvulus

Human onchocerciasis is caused by the filarial parasite Onchocerca volvulus whose life cycle occurs in two different hosts: black flies, and human. The infective larvae (a: stage L3) are normally transmitted by the bite of Simulium flies. Once in the human body, the larvae undergo molting to stage L4 (e), to then reach the adult stage in about one year (f). Adult female are able to produce millions of microfilariae (h) that they shed in the blood of their human host. When female blackflies take a bloodmeal they ingest those microfilariae that are going to undergo in the fly host a first transition to L2 life stage (j). L2 larvae then molt to L3 (a), the infective stage for human.

Simulium flies breed in fast flowing rivers i Simulium flies breed in fast flowing rivers i.e well oxygenated water, because their larvae have an obligatory aquatic stage during which they require high oxygen tension (see picture below). Hence, Onchocerciasis is associated with fast flowing rivers including rapids. That's why the blindness Onchocerciasis can lead to is often referred to as 'river blindness'.

The infective larvae of Onchocerca (stage L3) enter the body through the wound made by the bite of its host fly. The larvae then move to the subcutaneous tissues where they become encapsulated within nodules and mature into adults in approximately one year

After mating the female sheds microfilariae 300 mm in length and 0 After mating the female sheds microfilariae 300 mm in length and 0.8 mm in diameter. The microfilariae are sheathless with sharply pointed, curved tails.

The microfilariae can be found free in the fluid within the nodules and in the dermal layers of the skin spreading away from the nodules containing the adults. Microfilariae also can be found in the blood and eye during heavy infections. They infect their fly vectors while the flies are feeding on the human host and mature into third stage infective larvae in the flies' flight muscles (about 10 days total).

Loa loa CLASS: SECERNENTEA SUBCLASS: SPIRURIA ORDER: SPIRURIDA SUPERFAMILY: FILARIOIDEA FAMILY: ONCHOCERCIDAE Scientific name - Loa loa Common name - eye worm Disease - loaisis, fugitive or Calabar swellings

Hosts: Humans Distribution: Rain forest areas of west Africa and equatorial Sudan Life Cycle: Adults live in subcutaneous tissues. The microfilariae are periodic, appearing in the peripheral blood in maximal numbers during daylight hours and concentrating in the lungs at night. The intermediate host is the deer fly Chrysops. The nematodes develop to the third-stage, filariform juveniles in the fat body of the fly, after which they migrate to the mouthparts. The prepatent period in humans is about a year, and adult worms may live at least 15 years.

Characteristics Simple head with no lips and eight cephalic papillae; a long, slender body; and a blunt tail. The cuticle is covered with irregular, small bosses, except at the head and tail. Males are 20 to 34 mm long by 350 to 430 um wide. The three pairs of preanal and five pairs of postanal papillae are often asymmetrical. The spicules are uneven and dissimilar, 123 and 88 um long. Females are 20 to 70 mm long and about 425 um wide. The vulva is about 2.5 mm from the anterior end, and the tail is about 265 to 300 um long.

Symptoms-Pathogenicity Adults have a tendency to wander through the subcutaneous connective tissues. When they are still, the host reaction results in localized "Calabar swellings", which disappear when the worm moves on. Adults also migrate through the conjuctiva and cornea with swelling of the orbit and psychosomatic results to the host. Management Diagnosis is by demonstration of microfilariae in the blood. Surgical removal of swellings. Chemotherapy as in bancroftian filariasis. Control of deer flies is difficult.