Why does the coronavirus seem to cause so many different symptoms?

WATCH: Shortness of breath, a fever and a dry cough are the most common symptoms of the new coronavirus but medical experts are discovering patients are experiencing additional symptoms.

The major symptoms of COVID-19, according to public health authorities, are a cough, fever and difficulty breathing.

But other symptoms have since joined the list.

Reports of COVID-19 patients who have lost their sense of taste and smell, who are showing signs of unusual blood clots, lesions on their feet that are sometimes called “COVID toes,” headaches, nausea and rare inflammatory diseases in pediatric patients have all hit the news over the last few weeks. It seems like the list of possible symptoms of COVID-19 just keeps getting longer.

So how does a single virus cause so many symptoms?

One possible explanation: it doesn’t.

Dr. Gerald Evans, chair of the division of infectious diseases at Queen’s University, suggests that many of the symptoms currently being ascribed to COVID-19 are already present at similar levels in the general population, or are the effects of any severe infection — not just infections from the novel coronavirus.

For example, he said, conjunctivitis is present in about 0.8 per cent of COVID-19 cases according to one study. “So the interesting thing is, if you look at what is the incidence of conjunctivitis in the general population, it’s 0.8 percent. So I think it encapsulates the problem that you have.”

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Other symptoms, like nausea or headaches, are already experienced by millions around the world for all kinds of reasons, he said, and it can be hard to ascribe those specific things to coronavirus alone. Even conjunctivitis is associated with many viral infections, including measles.

However, as doctors ask their patients questions to try to figure out what’s going on, many common or general symptoms might mistakenly be associated with the disease.

Another possible explanation, according to Jason Kindrachuk, who holds the Canada Research Chair in Emerging Viruses at the University of Manitoba, is that because 3 million people have now been diagnosed with COVID-19, we’re noticing more cases of very rare symptoms — simply because the numbers are so high.

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He saw a similar effect when studying Ebola, he said. Early on, when there were relatively few cases, doctors thought they had a good idea of what the disease looked like.

But in the 2014 outbreak in West Africa, so many people were infected that doctors got a much better idea of all the possible effects of Ebola.

“2014 hits and we have tens of thousands of cases. And now you start to get a little bit more of an indication of the nuances of what the disease actually looks like,” he said.

“It doesn’t maybe fit the blueprint that we had, because now we’re actually seeing this across a much broader number of patients with very variable stages of disease. You’re not seeing just the people that are ultra-sick, you’re seeing people that maybe are more mildly sick.”

Similarly, with all these millions of COVID-19 cases, he doesn’t think we’re seeing new disorders cropping up, but instead the relatively rare effects that only occur in a handful of people.

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Why a virus might cause so many different effects is the “million dollar question,” Kindrachuk said.

Primarily, the coronavirus is thought to target the lungs, he said. “But what we start to see with some of these different viruses is that in more severe cases of disease, these viruses actually get into many different organs.”

When that happens, a patient’s immune system can go into “overdrive” to try to fight off the infection — something he says can lead to a lot of the more severe complications associated with COVID-19, like kidney failure.

“What we think is happening is that the virus gets in. It basically causes the immune system to overreact or start to basically misfire or dysfunction. And what will happen is that then the disease starts getting driven by basically this over-activation of the immune system.”

The immune system response can damage organ tissues too, which can lead to serious consequences, he said. For this reason, future therapies for COVID-19 may also need to address the immune system.

According to Evans, it’s important to distinguish the direct symptoms of COVID-19 from complications, like kidney failure. Organ failure can happen when a patient is sick with many different kinds of severe infections though, not just the coronavirus, he said.

“There are a lot of things that happen to your body when you’re that sick. And one of them is certain organs which are very susceptible to problems of blood flow, like the kidneys, your brain. Things are you begin to develop what are really complications of COVID-19.”

“The problem is that all of us in medicine know that’s not an uncommon thing to happen with very sick people in the ICU.”

Even blood clots, which have been reported with COVID-19, can happen in other very sick patients, he said, though other doctors have told Global News they are happening at higher rates in patients suffering from COVID-19.

These complications should be managed the same way, whether or not the patient has COVID-19, Evans said.

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So where does this leave a diagnosing physician? Evans believes the list of symptoms is too long, and as long as there remains a limited supply of test kits, doctors should ask more questions to try to determine whether or not someone likely has COVID-19 before getting them tested.

For Kindrachuk, the biggest issue isn’t the long list of possible symptoms, but that patients can have very mild, or no symptoms at all. This makes them very difficult to separate from the rest of the population, or even to treat them.

But more information is a good thing, he thinks, and our knowledge of the virus keeps growing.

“People are getting more in tune of what to look for when people are reporting symptoms. All these things are going to continue to evolve over the days and weeks and months as we move ahead.”

© 2020 Global News, a division of Corus Entertainment Inc.

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