Because many contagious people have no symptoms, using temperature checks to catch them is like trying to catch tennis balls in a soccer net: way too many can get through. On Tuesday, the head of the Transportation Security Administration told reporters, “I know in talking to our medical professionals and talking to the Centers for Disease Control … that temperature checks are not a guarantee that passengers who don’t have an elevated temperature also don’t have Covid-19.” The reverse is also true: Feverish travelers might not have Covid-19.
In this case, however, a growing body of science suggests a simple fix: make smell tests another part of routine screenings.
“My impression is that anosmia is an earlier symptom of Covid-19 relative to fever, and some infected people can have anosmia and nothing else,” said physician Andrew Badley, who heads a virus lab at the Mayo Clinic. “So it’s potentially a more sensitive screen for asymptomatic patients.”
In a recent study, Badley and colleagues found that Covid-19 patients were 27 times more likely than others to have lost their sense of smell. But they were only 2.6 times more likely to have fever or chills, suggesting that anosmia produces a clearer signal and may therefore be a better Covid-catching net than fever.
There is no definitive study on the predictive value of temperature checks for Covid-19. But there are clues from when that strategy was used during the SARS epidemic of 2003. Deployed at airports, especially in Asia, the devices fell far short of the ideal, an analysis found. Although contactless thermometers are quite accurate if used correctly, many other conditions (including medications and inflammatory disease) can cause fever. As a result, the likelihood that someone with a fever had SARS ranged from 4% to 65%, depending on the underlying prevalence of the disease.
The likelihood that someone with a normal temperature reading was SARS-free was at least 86%. That suggests SARS fever checks didn’t miss many infected people. Unlike SARS, unfortunately, Covid-19 can be contagious even before an infected person runs a fever, which makes missed cases more likely.
As experts have cast around for other screening tools, some have zeroed in on smell tests, which could be as simple asking people to identify a particular scent from a scratch-and-sniff card. Though not a universal symptom, loss of smell is one of the earliest signs of Covid-19 because of how the virus acts. Support cells in the olfactory epithelium, the tissue that lines the nasal cavities, are covered with the receptors that SARS-CoV-2 uses to enter cells. They become infected very early in the disease process, often before the body has mounted the immune response that causes fever.
“These support cells either secrete molecules that shut down the olfactory receptor neurons, or stop working and starve the neurons, or somehow fail to support the neurons,” said Danielle Reed, associate director of Monell Chemical Senses Center, a world leader in the science of taste and smell. As a result, “the [olfactory neurons] either stop working or die.”
In an analysis of 24 individual studies, with data from 8,438 test-confirmed Covid-19 patients from 13 countries, 41% reported that they had lost their sense of smell partly or completely, researchers reported in Mayo Clinic Proceedings. But in studies that used objective measurements of smell rather than simply asking patients, the incidence of anosmia was 2.3 times higher.
A Monell analysis of 47 studies finds that nearly 80% of Covid-19 patients have lost their sense of smell as determined by scratch-and sniff tests, Reed said. But only about 50% include that in self-reported symptoms. In other words, people don’t realize they have partly or even completely lost their sense of smell. That may be because they’re suffering other, more serious symptoms and so don’t notice this one, or because smell isn’t something they focus on.
Smell and taste dysfunction in patients with COVID-19
Michael S Xydakis,a Puya Dehgani-Mobaraki,c Eric H Holbrook,d Urban W Geisthoff,e Christian Bauer,e Charlotte Hautefort,f Philippe Herman,f Geoffrey T Manley,g Dina M Lyon,b and Claire Hopkinsh
The plural of an anecdote is not evidence, yet anecdotal international reports are accumulating from ear, nose, and throat (ENT) surgeons and other health-care workers on the front lines that anosmia, with or without dysgeusia, are symptoms frequently associated with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. The American Academy of Otolaryngology—Head and Neck Surgery and the British Association of Otorhinolaryngology are now recommending these symptoms be added to the list of primary screening symptoms for COVID-19.
Our understanding of an absent or diminished ability to smell or taste, resulting from a neurotropic or neurovirulent viral infection targeting the olfactory system, remains fragmentary and is largely historically informed. The clinical evaluation of the first cranial nerve (olfactory nerve or CN I) has all but dropped from history taking and physical examination; hence, it is often referred to by ENT professionals as the forgotten cranial nerve. To further complicate matters, immediate self-recognition of olfactory dysfunction is typically only present in the most severe cases, or it is only self-identified after a prolonged latency period.1, 2 A scarcity of acute-phase advanced neuroimaging studies, difficulties in obtaining histopathological tissue specimens, and an absence of viral cultures of infected olfactory neuroepithelium compound the difficulties in studying this phenomenon. Moreover, in the context of normal trans-nasal airflow of odorant molecules (ie, no oedema in the nasal vault or olfactory cleft), and in the absence of intranasal disease (eg, infectious rhinosinusitis, allergic or vasomotor rhinitis, or polyposis), until now patients with sensorineural viral anosmia have been seldom seen in general otolaryngology practice—on the order of approximately one to two new-onset patients each year. Hence, up until the coronavirus disease 2019 (COVID-19) pandemic, the low prevalence of sensorineural viral anosmia in society as a whole has made clinical research challenging.
Given the urgency and lethality of the current pandemic, knowledge obtained from front-line otolaryngologists who are currently managing and monitoring patients with COVID-19, and those with clinical experience in olfaction and rhinology, would have great value when transferred forward to deployed caregivers. Our multinational group, including one otolaryngologist currently infected with COVID-19 and experiencing anosmia and dysgeusia, suggest that physicians evaluating patients with acute-onset loss of smell or taste, particularly in the context of a patent nasal airway (ie, non-conductive loss), should have a high index of suspicion for concomitant SARS-CoV-2 infection. We have observed that traditional nasal cavity manifestations, as seen in other upper respiratory infections (eg, rhinovirus, influenza, and adenovirus), are commonly absent in patients with COVID-19. We have also observed that SARS-CoV-2 does not appear to generate clinically significant nasal congestion or rhinorrhoea—ie, a red, runny, stuffy, itchy nose. This observation suggests a neurotropic virus that is site-specific for the olfactory system. Although labelled as a respiratory virus, coronaviruses are known to be neurotropic and neuroinvasive.3, 4, 5, 6 Finally, we and others7 have observed that anosmia, with or without dysgeusia, manifests either early in the disease process or in patients with mild or no constitutional symptoms.
Nevertheless, it is still too early in our understanding of COVID-19 to definitively establish the incidence, as well as the full-spectrum clinical utility, of these symptoms.
Lancet Infect Dis. 2020 Apr 15
doi: 10.1016/S1473-3099(20)30293-0