Onchocerciasis

Question 1

What is the pathogen that causes onchocerciasis

Answer 1

Onchocerca volvulus

Question 2

What is the global distribution of onchocerciasis

Answer 2

• Tropical Africa (blindness)
• Africa (skin disease)
• Yemen (skin disease)
• Central and South America (skin disease)

Question 3

What is the vector for onchocerciasis

Answer 3

Black Fly
Genus: Simulium

Question 4

What is the animal reservoir for Onchocerca volvulus

Answer 4

There is no animal reservoir

Question 5

What are the patterns of disease and locations of onchocerciasis in Africa

Answer 5

• Blinding - savannah areas
• Skin-dominant/non-blinding - rainforested areas

Question 6

How long to adult O. volvulus worms live for on average? The microfilaria?

Answer 6

• Adult = 12 years
• Microfilaria = 1-2 years

Question 7

Describe the lifecycle of Onchocerca volvulus

Answer 7

1. During a blood meal, an infected blackfly (genus Simulium) introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound
2. In subcutaneous tissues the larvae develop into adult filariae, which commonly reside in nodules in subcutaneous connective tissues
3. Adults can live in the nodules for approximately 15 years. Some nodules may contain numerous male and female worms. Females measure 33 to 50 cm in length and 270 to 400 µm in diameter, while males measure 19 to 42 mm by 130 to 210 µm. In the subcutaneous nodules, the female worms are capable of producing microfilariae for approximately 9 years.
4. The microfilariae, measuring 220 to 360 µm by 5 to 9 µm and unsheathed, have a life span that may reach 2 years. They are occasionally found in peripheral blood, urine, and sputum but are typically found in the skin and in the lymphatics of connective tissues
5. A blackfly ingests the microfilariae during a blood meal
6. After ingestion, the microfilariae migrate from the blackfly’s midgut through the hemocoel to the thoracic muscles
7. There the microfilariae develop into first-stage larvae
8. And subsequently into third-stage infective larvae
9. The third-stage infective larvae migrate to the head and blackfly’s proboscis and can infect another human when the fly takes a blood meal

Question 8

What is the typical incubation period for onchocerciasis?

Answer 8

15-18 months

Question 9

What are the common clinical presentations of onchocerciasis?

Answer 9

1. Skin disease
   * Acute papular onchodermatitis
   * Chronic papular onchodermatitis
   * Lichenified onchodermatitis
   * Skin atrophy and depigmentation
2. Subcutaneous nodules
   * Onchocera nodule
3. Anterior eye disease
   * Punctate keratitis
   * Sclerosing keratitis
   * Iritis
4. Posterior eye disease
   * Chorioretinitis
   * Optic atrophy

Question 10

What are the main pathological mechanisms for disease in onchocerciasis?

Answer 10

1. Almost entirely caused by immunological reactions
   to dying and dead microfilariae
2. And their endosymbiotic Wolbachia which release bacterial mediators that trigger the innate immune system resulting in clinical pathology.
3. Activated eosinophils release cellular proteins that cause connective tissue damage.

Question 11

What are the typical sizes of adult O. volvulus worms (male/female) and microfilaria?

Answer 11

• Adult female = 40cm
• Adult male = 25 mm
• Microfilaria = 350 um

Question 12

What are the typical identifying features of O. volvulus micofilariae?

Answer 12

• Unsheathed
• Tail ends in a point
• Tail does not contain nuclei

Question 13

How is onchocerciasis diagnosed?

Answer 13

1. Skin snips
   * Place in saline and look for microfilaria
   * Snip should not contain blood
   * Does not depend on time of day
2. Nodulectomy
   * Not recommended or done routinely
3. Slit lamp eye exam
   * Patient lies face down for 2-5 minutes and then microfilaria may be visualised on slit lamp
4. Serological tests
   * Cannot distinguish between previous or active infection

Question 14

What is the preferred treatment and dose for onchocerciasis? What precautions are necessary? What is another treatment available?

Answer 14

• Ivermectin single dose (150-200 µg/kg) every 6-12 months for up to 10-15 years (lifespan of adult worms)
• Risk of encephalopathy in patients with high L. loa microfilaremia
• Doxycycline 4-6 weeks

Question 15

In what countries are onchocerciasis and loa loa co-endemic?

Answer 15

1. Cameroon
2. Central African Republic
3. Congo
4. Democratic Republic of the Congo
5. Nigeria
6. South Sudan

Question 16

What is the effect of ivermectin on O. volvulus adult worms? On microfilaria?

Answer 16

• Sterilizes adult female worms for 3-6 months
• Kills microfilariae

Question 17

What is the effect of doxycycline on O. volvulus adult worms? On microfilaria?

Answer 17

• Kills adult worms by killing Wolbachia endosymbionts
• > 60% adult worms killed and 80-90% of female worms sterilized 20 months after treatment
• No effect on microfilaria
• May be used in combination with ivermectin
• Can’t be mass administered due to length of course
• No immediate symptom improvement since microfilariae cause symptoms