A virologist at the Bernhard-Nocht Institute for Tropical Medicine in Hamburg, Germany, has suggested that Zika virus infection is the only vector-borne disease that may be transmitted by sexual intercourse.

The first suggestion that you could get Zika virus infection from both a mosquito as well as a sexual partner came in 2008, when an American scientist contracted Zika in Senegal and fell ill a few days after returning home to northern Colorado. His wife got Zika soon after, even though she had not travelled anywhere far for several months. More weight was placed on this suspicion by research published in 2015 that found the presence of the Zika virus in semen.

Researchers have commented, however, that this does not mean sexual transmission will take over from mosquitoes as the main means of the spread of the Zika virus, but it certainly would make prevention more difficult than simply wearing mosquito repellant. Whilst this additional route of transmission could contribute to the spread of the virus in the case of an epidemic, its effect is likely to be very small since, when a person is sick with Zika, having sexual intercourse is less likely to occur.

THE HISTORIC SPREAD OF ZIKA

The Aedes albopictus and the Aedes aegypti are the two types of mosquitoes known to spread the flavi-viruses of dengue, chikungunya and Zika, and are endemic in nearly all countries of the Caribbean and the Americas.

The Zika virus was first discovered in 1947 in a rhesus monkey with a fever and living in the Zika forest of Uganda. However, up to 2007 only 14 cases of Zika virus infection had occurred in humans. Then, in that year, it appeared in humans on the island of Yap in the south-western Pacific, and within a few months nearly three-quarters of the island’s 11,000 residents were infected.

At first, those sick with Zika developed fever, joint pains and eye inflammation, followed by a red bump-like rash. Some people also developed painful swellings of their hands and feet, while others had vomiting. The eyes of some people became sensitive to light. However, the symptoms usually resolved a few days later and nobody died.

In 2013 Zika appeared again, this time in Tahiti and other islands of French Polynesia. An estimated 28,000 (about 11 per cent of the population of those islands) felt sick enough with the virus to seek medical care. By 2014, the virus had shown up in other South Pacific spots, such as New Caledonia — east of Australia, and the Cook Islands. In 2015, the virus appeared for the first time in the Americas, being found in Easter Island, which is a remote island belonging to Chile in South America. When its presence was confirmed in Brazil in May 2015, it prompted the Pan American Health Organization (PAHO) to issue an epidemiological alert for the Americas. Since then, Zika has been reported in the Caribbean islands of Martinique and Puerto Rico.

WHY HUMANS NOW?

The virus was identified almost 70 years ago, and so why did it suddenly begin to affect so many humans and why has it spread so fast? Well, scientists believe that these types of viruses can evolve (change its form to adapt) very fast, and so Zika presumably evolved so that it could be transmitted by the mosquito. Once a local traveller, whether in Uganda or elsewhere in East Africa, served as a meal for a local mosquito, a new chain of transmission began.

The virus was able to spread quickly because people in the new location had no previous exposure or acquired immunity, hence an explosion-like transmission occurred. We now have many more people travelling globally than before, thereby bringing new viruses to non-immune populations.

In a research paper published in June 2015, a group of Brazilian scientists suggested that Zika may have been brought into their country during the 2014 Football World Cup tournament hosted there. The encouraging news, however, is that compared to the chikungunya virus, the symptoms caused by the Zika virus is relatively mild. Nevertheless, doctors should be on the lookout for the possibility of it morphing into the Guillain-Barré syndrome, an autoimmune disorder that can cause paralysis, severe pain, loss of bladder control, difficulty breathing, and sometimes death.

A director of the Institute for Human Infections and Immunity at the University of Texas Medical Branch has stated that there seems to be a fairly convincing link emerging between Zika and the syndrome based on observations in some of the locations where an outbreak of Zika had occurred. In those areas, there had been an increase in the incidence of Guillain-Barré immediately afterwards, sometimes up to 20-fold. Even more recently, on December 1, 2015, PAHO also warned of a possible link between Zika virus infection and an explosion of microcephaly (small head) in babies born to infected mothers in Brazil, even though final proof was lacking.

WHAT YOU SHOULD DO

Consequently, people should make every effort to eradicate mosquitoes from their living and working environment, and to avoid being bitten by mosquitoes. Emptying standing water from containers around your house that can become breeding grounds for mosquitoes, wearing mosquito repellents and long-sleeved shirts, and condom use for sexual intercourse are but some of the many efforts that individuals can undertake.

Derrick Aarons MD, PhD is a consultant bioethicist/family physician, a specialist in ethical issues in medicine, the life sciences and research, and is the ethicist at the Caribbean Public Health Agency – CARPHA. (The views expressed here are not written on behalf of CARPHA.)