Dermatology MCQ - Viral Infections - O’Nyong-Nyong virus

A 35-year-old aid worker returns from rural Uganda with an abrupt onset of high fever, severe polyarthralgia, and a maculopapular rash. She also reports significant lymphadenopathy, particularly in the cervical region. Laboratory studies show leukopenia. Which of the following is the most likely causative agent and its primary vector?

A) Chikungunya virus; Aedes aegypti mosquito
B) O’Nyong-Nyong virus; Anopheles mosquito
C) Dengue virus; Aedes aegypti mosquito
D) Zika virus; Aedes albopictus mosquito
E) West Nile virus; Culex mosquito

Correct Answer: B) O’Nyong-Nyong virus; Anopheles mosquito

Explanation

This presentation is classic for O’Nyong-Nyong (ONN) fever, an arboviral illness closely related to chikungunya but with distinct epidemiological features.

Key Clinical Features of O’Nyong-Nyong Fever:

• Acute Phase:

  • Abrupt onset of high fever.

  • Severe polyarthralgia: Involves multiple joints (e.g., knees, wrists, ankles), often with significant disability.

  • Maculopapular rash: Typically generalized and pruritic.

  • Lymphadenopathy: Prominent, especially in the cervical region (a key differentiating feature from chikungunya).

• Constitutional symptoms: Headache, myalgia, conjunctivitis.

• Laboratory Findings: Leukopenia is common.

Virologic and Transmission Features:

• Causative Agent: O’Nyong-Nyong virus (an alphavirus, Togaviridae family, closely related to chikungunya virus).

• Vector: Unique among human arboviruses as it is primarily transmitted by Anopheles mosquitoes (e.g., Anopheles funestus, An. gambiae), which are also malaria vectors.

• Geographic Distribution: Primarily East Africa (e.g., Uganda, Kenya, Tanzania). Major outbreaks occurred in the 1950s and 1990s.

Why Not the Other Options?

• (A) Chikungunya virus: Causes nearly identical symptoms (fever, polyarthralgia, rash) but is transmitted by Aedes mosquitoes and lacks prominent lymphadenopathy.

• (C) Dengue virus: Causes fever, rash, and arthralgia but is less associated with severe polyarthritis and lymphadenopathy. Transmitted by Aedes mosquitoes.

• (D) Zika virus: Causes mild fever, rash, and arthralgia but not severe polyarthritis or significant lymphadenopathy.

• (E) West Nile virus: Causes fever, rash, and arthralgia in only 20% of cases; more commonly presents with neuroinvasive disease. Transmitted by Culex mosquitoes.

Management:

• Supportive care: NSAIDs for pain and inflammation.

• No specific antiviral treatment or vaccine is available.

• Prevention: Mosquito avoidance (bed nets, repellents) is critical, especially given the Anopheles vector.

Prognosis:
Self-limiting; acute symptoms resolve within 1-2 weeks, but arthralgia may persist for months.

Note: O’Nyong-Nyong means "weakening of the joints" in the Acholi language. The presence of lymphadenopathy and transmission by Anopheles mosquitoes are key distinctions from chikungunya. Always consider ONN fever in patients with a chikungunya-like illness from East Africa.