However, until now no correlation between this fact and the occurrence of other diseases had been investigated.
Dengue and zika are both transmitted by the same mosquito (Aedes aegypti), and have similar symptoms, often making diagnosis difficult.
Dengue is more serious because in addition to fever, headache, muscle and joint pain, rash and nausea, it can cause bleeding and even death, the researchers said.
The symptoms of zika are milder, but the virus can cause severe problems in pregnant women and in babies, such as microcephaly and possibly Guillain-Barre syndrome, a neurological disorder that leads to paralysis, they said.
The latest study, published in the journal PLOS Neglected Tropical Diseases, found the mechanism that exacerbates dengue infection following a case of zika differs from that of two consecutive infections by the dengue virus.
The viral load is higher in the second dengue episode, with high levels of inflammatory cytokines not seen in zika, the researchers at Sao Jose do Rio Preto Medical School (FAMERP) in Brazil said.
Cytokines are small proteins that are crucial in controlling the growth and activity of other immune system cells and blood cells.
Detection of other markers suggested that the increase in severity may be due to activation of T cells – key parts of the immune system that help produce antibodies – in an immune response that has been termed the “original antigenic sin”.
The process involves so-called T-cell memory, a response in which T cells produced during a previous infection stimulate the production of more T cells to combat a new infection, the researchers said.
Because these new cells are not specific to the virus, they trigger an excessive release of inflammatory cytokines, which attack the organism’s proteins and tissues, potentially leading to hemorrhage, they said.
The researchers analysed samples from 1,043 laboratory-confirmed dengue patients, identifying those with prior zika and dengue infections.
The cases occurred in 2019 in Sao Jose do Rio Preto, a large city in Sao Paulo, Brazil, considered hyperendemic for dengue since more than 70 per cent of the population has had the disease.
Its climate and geography favour the circulation of arboviruses throughout the year. Dengue epidemics occurred there in 2010, 2013, 2015, 2016 and 2019, with a record number of cases involving serotype 2, the researchers said.
The team concluded that a prior dengue infection was not a risk factor for severity, probably because the patients were already into their third or fourth infection.
Prior zika infection, however, was important and an aggravating factor in a second dengue episode, they said.
The study also showed that antibody-dependent enhancement (ADE), in which, instead of providing protection, antibodies enhance viral entry into host cells and can exacerbate the disease, is not applicable in this case.
This raises questions about the type of zika vaccine that should be used and the optimal timing: should it be administered with a dengue vaccine in order to avoid this problem of one following the other, they added.