Influenza: Clinical sciences

Influenza is a highly transmissible viral infection that can primarily affect the upper and lower respiratory tract. It spreads via aerosols and directly infects the respiratory epithelium, causing inflammation. If you suspect influenza, you should determine whether your patient has a high risk of complications, and perform viral testing to confirm the diagnosis and guide treatment decisions.

Now, if a patient presents with signs and symptoms of influenza, you should first perform an ABCDE assessment to determine if they are unstable or stable. If they’re unstable, begin acute management. First stabilize their airway, breathing, and circulation. Next, obtain IV access and put your patient on continuous vital sign monitoring including heart rate, blood pressure, and pulse oximetry. Provide supplemental oxygen if they’re hypoxic, to maintain oxygen saturation above 90%.

Now let's go back to the ABCDE assessment and discuss the approach to stable patients. First, perform a focused history and physical examination. Your patient may report a sudden onset of fever, chills, myalgia, malaise, and headache. Additionally, local inflammation can result in dry cough, sore throat, and rhinorrhea. Your patient might also report exposure to a possible influenza contact within the previous 1 to 4 days, especially if there’s high transmission of influenza in the community.

Physical exam may reveal nonpurulent conjunctivitis, diaphoresis, and pharyngeal erythema, as well as abnormal lung sounds, such as crackles, rhonchi or wheezing.

At this point you can suspect influenza, so your next step is to assess whether your patient presents with high-risk features. Individuals are considered to be at high risk if they have chronic medical conditions, like diabetes, obesity, or heart disease, or if they are immunocompromised, like those with HIV or an organ transplant. In addition, certain populations are at higher risk, such as patients who are pregnant or less than 2 weeks postpartum, and those over the age of 65. Finally, individuals with severe or progressive symptoms, as well as patients who are currently hospitalized or living in a chronic care facility are also at high risk for influenza complications.

Ok, now let’s first consider patients who have no high-risk features. Your next step is to start supportive care, including the use of antipyretics as needed, promoting rest, and adequate hydration. Then, determine if testing for influenza will change your clinical management. Viral testing isn’t necessary in all situations; in fact, factors that indicate there’s no need for testing include patients without high-risk features who have symptoms that are highly suggestive of influenza, as well as if community transmission is known to be high like during the “flu season”, or if they don’t have any high-risk household contacts.

For most patients, all factors are present and testing is not indicated, so you can make a clinical diagnosis of influenza and continue supportive care. In addition, if their symptom onset was within 48 hours, you can consider offering them a neuraminidase inhibitor, or NAI, like oral oseltamivir. Lastly, encourage precautions for infection control until they have been fever-free for at least 24 hours.

However, if any of these factors is not present, testing for influenza will indeed change your clinical management, so you should order a nucleic acid amplification test, or NAAT. If the NAAT result is negative for influenza, you should consider alternative diagnoses, such as respiratory syncytial virus or RSV, parainfluenza virus, and SARS-CoV-2 infection. On the other hand, if the NAAT is positive, you can confirm the diagnosis of influenza. In this case, in addition to continuing supportive care, if your patient’s symptom onset was within 48 hours, you can prescribe a neuraminidase inhibitor, or NAI, like oral oseltamivir. In addition, patients with confirmed influenza should implement precautions for infection control until they have been fever-free for at least 24 hours. This includes standard precautions like handwashing, as well as droplet precautions like isolation or wearing a face mask

Alright, so now that we’ve reviewed what to do with a patient with no high risk features, let’s go back and talk about those who have 1 or more high risk features. The first thing you should do is obtain a nasopharyngeal specimen and send it for NAAT or, if your patient is hospitalized, you might order a polymerase chain reaction test, or PCR test for short. Start your patient on supportive care and an empiric NAI immediately, so don’t wait for the lab results. You can give oral oseltamivir, inhaled zanamivir, or a single dose of IV peramivir and consider hospitalization for patients with severe signs and symptoms.