The real burden behind Zika infection

Zika: a clear and present threat

Believe it or not, Zika virus (ZIKV) was once considered rather harmless. This is because most Zika-infected individuals are asymptomatic. But things can change rapidly in the world of viruses. ZIKV is now considered a much more frightening prospect. Worse, it is now spreading rapidly.

The subtle consequences of infection

The mosquito-borne ZIKV is not only blamed for deadly congenital disabilities (microcephaly and general fetal growth restriction) but also seems to have consequences that go beyond the immediately evident physical malformations.

Using immunocompetent ZIKV-infected mice, Nem de Oliveira Souza et al. show how congenital ZIKV exposure can cause a spectrum of long-term severe neurological abnormalities and dysfunctions.

ZIKV is highly neurotropic: neural stem cells are particularly vulnerable to the virus, making ZIKV especially dangerous during pregnancy, when the baby’s brain is developing. However, a number of babies exposed to ZIKV during gestation and born with a normal head size have been reported to have motor impairment, cortical dysplasia, and frequent seizures. Also, normocephalic babies congenitally exposed to ZIKV show decreased head growth after birth, which results in postnatal onset microcephaly.

A mouse model of ZIKV infection in newborns

In their effort to better understand the behavioral, neuropathological, and molecular consequences of Zika infection in a model that better represents the real situation, Souza et al. systematically infected newborn wild-type mice with ZIKV. The virus was subcutaneously inoculated at postnatal day 3, a period of rodent brain and immune system development that resembles the second and third trimester of gestation in humans. Then, the weight, lifespan, brain size, and viral load of these mice were assessed during short and long-term follow-up.

ZIKV-infected mice showed reduced weight gain (that was not recovered during adulthood) and marked macroscopic brain atrophy, with reduced dorsal brain area compared to mock-infected mice. Both positive and negative RNA strands of ZIKV were still detectable in adult mice (100 days after infection), indicating persistent replication of the virus long after the acute phase of the disease. Also, the 40% mortality rate in the ZIKV infected mice suggests that peripherally administered ZIKV can reach and replicate in the developing mouse brain.  Spontaneous seizures and several persistent motor dysfunctions have been reported in babies congenitally exposed to ZIKV. In line with this, ZIKV-infected mice showed behavioral seizures within just 9–12 days post-infection, coinciding with the peak of viral replication in the brain. These seizures are likely to be dependent on active viral replication in the brain because they were reduced in mice inoculated with an inactivated form of ZIKV. ZIKV has also been shown to infect peripheral neurons and spinal cords, causing cell death. In line with this, ZIKV-infected mice showed motor impairments, decreased muscle strength, impaired locomotor behavior, and cognitive deficits.

These effects are specific to ZIKV

Dengue-infected mice were used as a control to assess whether these effects were zika-specific (and just not a more general consequence of infection by any flavivirus). However, seizures and the late behavioral effects were not seen in dengue-infected mice, suggesting that these effects are specific to infection with ZIKV (a neurotrophic flavivirus). When the researchers performed a closer examination of the brains of the infected mice, they found a spectrum of neuropathological changes, including severe brain atrophy, multiple necrotic areas (in hippocampus, thalamus, striatum and cortex), nerve cell degeneration, microgliosis, infiltration of inflammatory cells, and a general disruption of the normal cytoarchitecture of many of the brain regions. These neuropathological findings are analogous to those seen in fetuses and new-borns with ZIKV congenital syndrome. To assess whether these changes were caused by cell death or impaired proliferation of neural precursor, the researchers looked at the expression of Ki67 – a marker of neuronal proliferation – in infected and control brains (focusing on the subgranular zone of the hippocampal dentate gyrus and the subventricular zone, where neurogenesis occurs). Comparing infected and control mice, the authors detected no difference in the proportion of Ki-67 positive cells, suggesting that the neuropathological alterations found in mice caused by cell death rather than a defect in progenitor proliferation.

Also, compared to mock infected-mice, the authors detected increased expression of pro-inflammatory mediators and an excessive generation of reactive oxygen species in the ZIKV-infected mice. Because UV-inactivated virus particles were unable to induce these same damages, the authors were able to conclude that the ZIKV-induced damage seen in infected mice brains are a direct result of active viral replication, and not simply due to the presence of the viral particle.

Armed with these data, could novel pharmacological approaches help?

Next, the authors tested whether they could improve the long-term behavioral impairment induced by ZIKV using pharmacological approaches (targeting oxidative stress and inflammation using the antioxidant drug N-acetylcysteine and TNF-alpha neutralizing antibody infliximab, respectively). Treatments were started on the day of infection (P3) and continued until 12 days after infection when the mice were tested for seizure occurrence. Although the monoclonal treatment was effective at preventing the development of seizures, no effect on the performance of ZIKV-infected mice could be detected. TNF-alpha is a key signaling molecule underling Zika-induced seizures and patients exposed to the virus during the perinatal period might benefit from the use of TNF-alpha blocking strategies. However, the ineffectiveness of drugs at preventing the viral-induced damage further demonstrates the importance of identifying antiviral agents against ZIKV. In case of infection, such antivirals could be used to contain the virus and limit its ability to damage the host.

A paradigm shift in the consequences of viral infection

The work of Souza et al. clearly shows how a wide range of neurological and neuropsychiatric manifestations might be expected following perinatal ZIKV exposure. These findings are in line with other data linking the early life exposure to various viruses and the development of cognitive impairment and neuropsychiatric disorders later in life. This, together with the recent findings addressing herpes virus in a large cohort of Alzheimer’s disease patients, seems to indicate a stronger than previously thought connection between viruses and neuronal diseases.

This certainly seems a topic to follow!