Zika wasn't even on Dr. Sankar Swaminathan's mind when he first examined a severely ill 73-year-old man in a Salt Lake City hospital in June. The patient had just returned from a visit to Mexico when he suddenly fell violently ill.
"We were not thinking about Zika at all because Zika usually does not cause severe illness, in fact it almost never does," says Swaminathan, chief of the division of infectious diseases at the University of Utah.
"His symptoms were really what is often the end point of many, many severe infections."
He had a high fever and low blood pressure. His kidneys and lungs were failing. "This happens with many bacterial infections," says Swaminathan, "and it's often referred to as sepsis."
In addition to the patient's bacterial infection, Swaminathan thought the man might have picked up a powerful case of malaria or dengue. But tests for those diseases came back negative. Actually every test they tried came back negative.
Then they finally tested for Zika. The result came back positive — and the level of virus in his blood was astronomical.
"In this particular case the level of virus was probably a hundred thousand times what you or I might have in our blood if we were having acute Zika infection," Swaminathan says. "We estimated 200 million copies [of virus] in a milliliter of blood. That level is really quite high. We see that in some other viral illnesses but not many. For instances in most viral illnesses a million would be considered very, very high."
Swaminathan writes about this highly unusual Zika case in the New England Journal of Medicine.
Four days after the patient was admitted to the hospital, he died. Five days later his 38-year-old son, who had helped care for him in the ward, also came down with Zika. The son hadn't recently traveled to areas with Zika, and mosquitoes that spread Zika aren't present in Utah so that ruled out transmission by mosquito.
According to the NEJM article, the son had helped a nurse to reposition his father in his bed and wiped his father's eyes. This raised the question: Could Zika also spread through sweat or tears?
"I should stress this was an extremely rare event superimposed on a rare event," Swaminathan says of the younger man's Zika case. "We've never seen transmission outside of mosquitoes or sexual transmission. This is one of the areas where research needs to be done on whether people who are severely ill pose an increased risk of transmission."
So it's still unclear how the son was infected.
The son had what Swaminathan calls "a very normal" case of Zika. He wasn't extremely sick and he quickly recovered.
The bigger question is what caused his dad to get such a severe form of the disease.
"There are hundreds of thousands of people who've been infected in this outbreak but only about a dozen people have actually died from it," says Swaminathan. The dozen deaths he mentions don't include people who got hit with a post-infection syndrome called Guillain-Barre in the wake of a Zika virus infection, which can lead to death.
The 73-year-old was originally from Mexico and immigrated to the U.S. in 2003. Swaminathan says testing at the Salt Lake hospital showed that the man had had dengue "sometime in the distant past."
Prior exposure to one strain of dengue can make people more likely to get severely ill if they're hit with another strain of dengue. Swaminathan says exposure to dengue may also make people more susceptible to a severe case of Zika. "There's laboratory evidence that if you take blood from people who've had dengue before and you add it to a culture system with Zika, you increase the infection by Zika virus," he says.
Even looking at those studies, he says that the case of this 73-year-old man was highly unusual and raises more questions about Zika than it answers.
Indeed, ever since Zika took off in the Americas last year, it has surprised researchers, first with the devastating effects it can have on fetuses, then by turning out to also be a sexually transmitted disease — and now appearing to find potentially even other ways to spread.