Saturday, February 13, 2016

WHO Interim Case Definition For Zika Virus Disease

Zika Rash - Credit Wikipedia


The World Health Organization has released an interim case definition for identifying suspected, probable, and confirmed Zika Virus Disease infection. 

Since the majority of cases are expected to be asymptomatic or mild - and given the limited availability of laboratory testing - most infections will never be identified, much less confirmed.

We see a similar situation every year in the United States with the West Nile Virus.  Only about 20% ever develop symptoms, and the vast majority of those are mild cases – called West Nile Fever – with probably only 2%-3% of those being identified.

Serious illness - due to neuroinvasive WNV - accounts for less than 1% of all cases.  In 2015, 1,360 neuroinvasive WNV cases were reported in the United States, resulting in 119 deaths.

Which means that there were probably more than 100,000 mild West Nile Fever cases that went unreported last year. It is likely that more than a million Americans have been infected with the West Nile Virus over the past 16 years, and never even knew it. 

Although we are still learning about the Zika virus, the same pattern appears to be true in Central and South America, with only a very small percent of cases experiencing serious illness or complications.

Zika virus disease

Interim case definition
12 February 2015

These interim case definitions have been developed for the purpose of providing global standardization for classification and reporting of Zika virus cases. WHO guidance for the surveillance of Zika virus disease is currently being developed.
WHO will periodically review these interim case definitions and update them as new information becomes available.

Suspected case

A person presenting with rash and/or fever and at least one of the following signs or symptoms:
  • arthralgia; or
  • arthritis; or
  • conjunctivitis (non-purulent/hyperaemic).

Probable case

A suspected case with presence of IgM antibody against Zika virus1 and an epidemiological link2

Confirmed case

A person with laboratory confirmation of recent Zika virus infection:
  • presence of Zika virus RNA or antigen in serum or other samples (e.g. saliva, tissues, urine, whole blood); or
  • IgM antibody against Zika virus positive and PRNT90 for Zika virus with titre ≥20 and Zika virus PRNT90 titre ratio ≥ 4 compared to other flaviviruses; and exclusion of other flaviviruses


1 With no evidence of infection with other flaviviruses
2 Contact with a confirmed case, or a history of residing in or travelling to an area with local transmission of Zika virus within two weeks prior to onset of symptoms.

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