Onchocerca volvulusRoss Boreen and Ellyn Krieg

TaxonomyOnchocerca volvulus is a filarial

wormThe diseases it causes is

onchocerciasisDepending on where it infects the

host it can be further classified as river blindness or filariasis

It is one of the three nematode worms that causes subcutaneous filariasisLoa loaMansonella streptocercaOnchocerca volvulus

Geographical Range and Hosts

Found in 36 countries endemically with 30 of them being in sub-Saharan Africa commonly found in Central America as well

Roughly 80 million people are infected

• In hyperendemic areas more than 90% of people can have the microfilariae

Hosts

Definitive Host: Humans

Intermediate Host: Female Simulium flies (black flies)

Morphology Adults:

Found in pairs or groups Slender and blunt at both ends No lips or buccal capsule Two circles of four papillae which surround the mouth Males are 19 cm to 42 cm long Females are 33.5 cm to 50 cm long Male posterior end is curled ventrally and has four pairs of

adanal and 6-8 pairs of postanal papillae Microfilariae:

Unsheathed Sharply pointed and curved tails

Life Cycle

Life Cycle Continued

Simulium fly introduces L3 larvae into the skin of a human as it takes a blood meal

In subcutaneous tissues the larvae develop into adults both male and female commonly forming nodules called onchocercomas

The adults can live in these nodules for 14-16 years. While here the female will produce up to 1000 microfilariae/day for up to 9 years.

Microfilariae travel throughout the skin and lymphatics of connective tissue, but can also be found in the peripheral blood in heavy infections.

Black fly ingests the microfilariae during a blood meal

Life Cycle Continued

Microfilariae migrate from the midgut through the hemocoel to the thoracic muscles where they develop into L1 larvae. They then molt twice into L3 larvae

The L3 larvae migrates to the proboscis of the fly.The L3 is transmitted to another human when the

fly takes another blood meal. The time the microfilariae takes to develop into

an L3 larvae in the fly is about 10 days.

Pathogenesis of Adults Not very pathogenic, often no symptoms Can cause subcutaneous nodules called

onchocercomas over bony prominences In African strains tend to be on lower body such as

pelvic area, and some along the spine, chest, and knees

In Central America they are on the upper body with most being on the neck and head

Nodules are pretty benign just cause disfigurement with no pain

They are composed of collagen fibers surrounding the adult worms

Occasionally the nodule can degenerate to form an abscess or the worms can become calcified

Pathogenesis of Microfilariae

Live ones cause little inflammation Can cause a dermatitis or eye complications Dermititis (Filariasis)

When they die they start to degenerate which causes severe dermatitis

Dermatitis thought to be caused by release of a type of bacteria called Wolbachia, which can be treated with doxycycline to help reduce inflammation

First sign is intense itching, which can lead to secondary bacterial infections and death of patches of skin

After the itching the, skin thickens, becomes discolored, and cracks, a process known as lichenification Characterized by loss of elasticity making the patient look

like they are aging prematurely Lymph glands that serve the area of infected skin can

become inflamed as well

Riverblindness Eye lesions take many years to develop so most often

not found in anyone under 40 in Africa however in Central America can be found in younger adults

Microfilariae can invade many parts of the eye but do not cause many problems until they die

Once dead a similar inflammation reaction to the skin reaction occurs causing lesions

Most common cause of blindness is sclerosing keratitis (a type of inflammation of the cornea that leads to hardening of the cornea)

Inflammation in the eye is the result of de-granulating eosinophils disrupting the arrangement of the cornea

Also causing inflammation is activation of toll-like receptor 4 by Wolbachia antigens which produces many proinflammatory cytokines

Diagnosis

Most common method is a skin snip Small piece of skin is pulled up

and cut off with razor or scissors Placed in saline on a slide and

examined for emerging microfilariae

Nodules can be aspirated but only adults are found this way

DEC (diethylcarbamazine) as a confirmation of diagnosis

Treatment Removal of nodules can help

with lowering rate of eye damage and rate of infection

Ivermectin now replacing DEC and suramin due to its low rate of serious side effects A microfilaricide- a single

dose eliminates the microfilariae however it does not kill the adults but stops the female from releasing microfilariae for a year

Repeated dosing of Ivermectin can slowly kill the female

PreventionVector control

DDT Avoid fast flowing rivers since vector breeding

ground Larvicide in fast flowing rivers

Combine with drug campaigns Onchocerciasis Control Programme from ended in

2002 but prevented 125,000-200,000 people from going blind and protected 30 million people from ocular and skin lesions

Review

What are the hosts for Onchocerca volvulus?What are the two types of disease that the

microfilariae can cause?Where is Onchocerca volvulus found?What is the most common way to diagnose

it?What is the name of the drug patch that can

be used to confirm infection with Onchocerca volvulus?

References

http://www.dpd.cdc.gov/dpdx/html/frames/a-f/filariasis/body_Filariasis_o_volvulus.htm#Life%20Cycle

http://www.science.smith.edu/departments/Biology/SWILLIAM/fgn/pnb/oncvol.html

http://www.who.int/apoc/onchocerciasis/lifecycle/en/index.html

http://plpnemweb.ucdavis.edu/nemaplex/taxadata/Ovolvulus.HTM

Foundations of Parasitology Eight Edition by Roberts and Janovy