The Zika virus is named for the Ugandan forest where it was first identified, in 1947, by Rockefeller Foundation-funded scientists studying Yellow Fever Virus. Zika isn’t really a problem in Uganda, perhaps because the mosquitoes that carry it there do not bite humans, or perhaps because the existing tests have failed to distinguish between Zika and other viruses. It is this type of uncertainty, coupled with the evidence that Zika infection is associated with microcephaly in foetuses and newborns and neurological symptoms including Guillain-Barre Syndrome in adults, which is driving the development of new detection assays – for Zika, but also for other, related viruses.
When infected with the Zika virus, 80% of patients show no symptoms, while others present with a rash, a high temperature, and sometimes joint pain- symptoms that are very common, and can be caused by many other viruses, including some that are closely related to Zika (such as Dengue or Yellow Fever virus) and some that belong to different families altogether but that are also endemic in the same areas (such as Chikungunya virus). This immediately gets us to the heart of the problem: infection can lead to more severe, even life-threatening symptoms which differ between viruses, such as haemorrhagic fever in Dengue, or neurological disorders such as Guillain-Barre Syndrome with Zika. However, currently there is no way to know which virus is responsible for a rash or a fever, or which clinical signs should be monitored in each patient to provide early alert of severe complications such as haemorrhage or, in a pregnant woman, foetal microcephaly. Even worse, the standard treatment for fever (aspirin and similar non-steroidal anti-inflammatory drugs) can affect platelet function and so worsen the outcome for patients with haemorrhagic fever- or even simply trigger a haemorrhage in patients infected with Dengue virus. Early and accurate diagnosis is therefore essential.
Current tests are either based on RT-PCR, on ‘sero-conversion’, or on plaque neutralisation tests. Reverse-transcription-PCR is necessary because Zika, like the other Flaviviruses, is an RNA virus; hand in hand with this is the problem that RNA viruses are generally more highly mutagenic than DNA viruses, meaning that RT-PCR assays, which already rely on subtle differences between related strains, are at the mercy of the stability of the regions of the genome they target. Sero-conversion measures the presence in a patient’s blood of antibodies against an infectious agent; these can be IgM antibodies (present early in infection) or IgG antibodies, which persist for months after the infection. The problem here is that the highly related nature of the Flaviviruses means that antibodies to one virus often cross-react with another, and it can be difficult to determine whether a positive result in a Zika test is actually due to a previous infection with Dengue or one of the other Flaviviruses. Finally, plaque neutralisation assays ask whether antibodies present in the patient’s blood can interfere with viral infection of cultured cells. Although considered to be the gold standard assay in terms of sensitivity and perhaps specificity, these tests are constrained by the need to use the correct cells for each type of virus, meaning that the correlation between effective antibodies and protection from cellular infection is not watertight.
Following its declaration that the Zika virus outbreak constitutes a Public Health Emergency of International Concern, the WHO has published a review of the state of the art for Zika virus detection (Charrel et al, 2016), which identified multiple gaps in the development and validation of diagnostic tests and more recently (16 June 2016) a Strategic Response Plan, WHO/ZIKV/SRF/16.3. One of these is of key relevance to Avacta Life Sciences and our customers: “enhancing surveillance globally, not just in the Americas, since it is uncertain to what extent and where Zika virus will spread, including making point- of-care tests available to frontline providers for accurate diagnoses”. The very high specificity of Affimer binders, together with the speed with which we can identify binders (9 weeks) and build new assays for their validation (a further 8 weeks to build two assays in the case of our internal Zika project)
should allow the use of Affimers to develop assays within a handful of months after the detection of a new outbreak of Zika, or any other emerging infectious organism.
Avacta recently announced (link is external) it has identified three Affimer proteins capable of binding to a recombinant form of a secreted Zika virus NS1 protein, which is diagnostic of Zika virus infection at the early, acute stage. These Affimer binders were identified and characterised within just thirteen weeks of receiving the virus target and have the potential to be developed into new rapid point-of-care diagnostic tests for Zika infection.
Sources:
- WHO: http://apps.who.int/iris/bitstream/10665/246091/1/WHO-ZIKV-SRF-16.3-eng.pdf?ua=1&ua=1
- CDC: https://www.cdc.gov/zika/symptoms/
- BBC: http://www.bbc.co.uk/news/world-africa-35431181
- WHO: Charrel RN, Leparc-Goffart I, Pas S, de Lamballerie X, Koopmans M & Reusken C. State of knowledge on Zika virus for an adequate laboratory response Bulletin of the World Health Organisation E-pub 10th Feb 2016, doi http://dx.doi.org/10.2471/BLT.16.171207
- Plaque reduction neutralization antibody test does not accurately predict protection against dengue infection in Ratchaburi cohort, Thailand. Sirivichayakul, C, Sabchareon, A, Limkittikul, K, and Yoksan, S Virol J. 2014; 11: 48. doi: 10.1186/1743-422X-11-48.
- Zika virus outbreak on Yap Island, Federated States of Micronesia. Duffy MR, Chen TH, Hancock WT, Powers AM, Kool JL, Lanciotti RS, Pretrick M, Marfel M, Holzbauer S, Dubray C, Guillaumot L, Griggs A, Bel M, Lambert AJ, Laven J, Kosoy O, Panella A, Biggerstaff BJ, Fischer M, Hayes EB. N Engl J Med. 2009 Jun 11;360(24):2536-43. doi: 10.1056/NEJMoa0805715.