Onset is sudden with fever, rhinorrhea, sore throat and headache. Severe myalgias coupled with respiratory tract symptoms are typical of influenza [7]. Patients may also have red, watery eyes and cough. There can also be tachycardia and dyspnea but with hypotension. Vomiting and diarrhea may also occur, but are rare findings associated with the disorder.

Laboratory tests

• Complete blood count
• Nasal or throat wash
• Nasal or throat swab
• PCR
• Cell-culture based tests
• FLU OIA test and QUICKVUE Influenza test based on detection of viral antigens by using monoclonal antibodies [7].
• ZSTATFLU test based on detection of viral neuraminidase using a substrate of colour-changing enzyme.
• Alere I Influenza A & B test a new point-of-care influenza test that delivers highly accurate molecular results in less than 15 minutes, approved by FDA in June 2014.

Imaging 

Imaging studies are generally not required. In case of complications such as bacterial pneumonia, chest radiography may be advised.

Test results

A clinical examination and history is sufficient to form a diagnosis. Laboratory tests can help confirm the diagnosis.

Pharmacological treatment

Lam et al suggested that cases of suspected severe influenza infection should be treated early and aggressively, even before diagnostic tests can be confirmed [8]. Commonly used antivirals include neuraminidase inhibitors like oseltamivir and zanamivir. M2 protein ion channel inhibitors, which are adamantine derivatives, like amantidine and rimantidine can also be used. Due to a marked increase in resistant isolates, the Advisory Committee on Immunization Practices (ACIP) recommends that adamantanes not be used in the United States for the treatment of influenza, except in selected circumstances [9].

Non-pharmacological treatment

It includes bed rest, the increased intake of fluids and a light but healthy diet during the flu. Typically, flu will resolve within a few days without any treatment.

Influenza has an excellent prognosis. Like all infectious diseases, it has a certain duration of action after which the symptoms begin to slowly subside. When there are no complications, acute symptoms subside within 3-7 days. Post-influenza asthenia may last for a few weeks.

The mortality associated with influenza is due to the complications associated with the disease. As a result of these complications, influenza can be deadly, especially for the weak, young and old, or chronically ill [6]. They include bacterial pneumonia, bronchitis, myositis, Reye syndrome in children, myocarditis and in some rare cases, multiorgan failure.

Causative factors

There are 3 basic causative factors of this disease: Influenza A, B and C virus. They belong to the Orthomyxovirus family. Of the three, Influenza A virus is considered to be the most pathogenic and has at least 4 different recognized strains.

Transmission

Although the respiratory tract is presumed to be the primary site for the acquisition of infection, one study has suggested that transocular entry of influenza virus can occur [2]. Infection can be transmitted through sneezing and coughing and is thought to be transmitted primarily via large particle droplets (>5 microns) [3]. It is an airborne infection and close contact with a person with an active infection can transmit the disease.

Incidence

According to a recent estimate by World Health Organization, 3 to 5 million cases of severe cases occur worldwide annually. 250,000 to 500,000 deaths have also been reported [4].

Age

Influenza affects young and old alike. Elderly patients with low immunity and immunocompromised individuals are particularly susceptible.

Sex

Influenza occurs equally in both sexes.

Influenza A is a genetically labile virus with mutation rates as high as 300 times that of other microbes [5]. It is known to cause pandemics worldwide. Influenza B is usually associated with smaller outbreaks. Influenza C has been reported to cause epidemics in children but mainly causes only mild respiratory infections.

After the virus has been inhaled, an incubation period of 12-24 hours ensues. A special substance present in the form of spikes on the virus is hemagglutinin. It binds to cell surface receptors to initiate infection. Another substance, neuroaminidase, present in the virus is an enzyme that degrades the protective layer of mucus in the respiratory tract. It also cleaves neuraminic acid to release progeny virus from the infected cells. It is the action of these two moieties that result in the symptoms of influenza as well as necrosis of the superficial layers of the respiratory epithelium.

The main mode of prevention is the influenza vaccine that typically contains two strains of Influenza A virus and one strain of Influenza B virus. Annual influenza vaccination is an important public health measure for preventing influenza infection [10]. Currently, there are two types of vaccines available in the US. First is a killed vaccine containing purified protein subunits of the virus (hemagglutinin and neuraminidase). It has two versions, namely ‘split virus’ vaccine and ‘purified subunit’ vaccine. Both are administered intramuscularly. The second type is a live vaccine which contains temperature sensitive mutants of Influenza A and B virus. 

Some antiviral drugs may also help in preventing the infection, particularly in elderly, non-vaccinated individuals.

Influenza is a common household occurrence. Commonly known as the ‘flu’, influenza is an infectious disease caused by a virus. Although it is often confused with other influenza-like illnesses, especially the common cold, influenza is a more severe disease [1]. It has been known to cause epidemics and pandemics worldwide. Influenza has a seasonal variation, being much more common in winter and in the rainy season. It has variations which pose to be a much bigger threat like the avian flu and the swine flu, however, our section deals with the general concept of this disease.

Definition

Influenza, simply called the flu, is a very common infectious disease of viral origin.

Cause

It is caused by Influenza virus A, B or C. Transmission is through airborne aerosol droplets that contain the infectious particles. It can be spread by close contact with a person having an active infection. Simple acts such as coughing, sneezing and even laughing can transmit the infection.

Signs and symptoms

Signs and symptoms include sudden high grade fever associated with chills, sore throat, runny nose and cough. Headache and body aches are commonly present. There may also be difficulty in breathing, increased heart rate, vomiting and diarrhea. Symptoms may go away on their own eventually within 3-7 days.

Diagnosis

Diagnosis is made based on clinical findings. Some laboratory tests may be performed to confirm the diagnosis. In case of complications such as pneumonia, a more through series of tests will be required.

Treatment

Treatment is not necessary as symptoms often resolve by themselves. Bed rest and fluids help speed up recovery. Some antiviral drugs may be used if needed along with painkillers to relieve headaches and muscle pain.

1. Eccles R. Understanding the symptoms of the common cold and influenza. Lancet Infect Dis 5 (11): 718–25, 2005. 
2. Bischoff WE, Reid T, Russell GB, Peters TR. Transocular entry of seasonal influenza-attenuated virus aerosols and the efficacy of n95 respirators, surgical masks, and eye protection in humans. J Infect Dis 2011; 204:193.
3. Mubareka S, Lowen AC, Steel J, et al. Transmission of influenza virus via aerosols and fomites in the guinea pig model. J Infect Dis 2009; 199:858. 
4. Lozano R. Global and regional mortality from 235 causes of death for 20 age groups in 1990 and 2010: a systematic analysis for the Global Burden of Disease Study 2010. Lancet 380 (9859): 2095–128, 2012
5. Drake JW. Rates of spontaneous mutation among RNA viruses. Proc Natl Acad Sci U S A. May 1 1993;90(9):4171-5.
6. Hilleman M. Realities and enigmas of human viral influenza: pathogenesis, epidemiology and control. Vaccine. Aug 2002. 20 (25–26): 3068–87. 
7. Warren L. Review of Medical Microbiology and Immunology. 11th ed. Pa: McGraw Hill, 2010. ISBN 978-0-07-170028-3. 
8. Lam J, Nikhanj J, Ngab T, et al. Severe Cases of Pandemic H1N1 Pneumonia and Respiratory Failure Requiring Intensive Care. J Intensive Care Med. 5;26:318-25.
9. Fiore AE, Fry A, Shay D, et al. Antiviral agents for the treatment and chemoprophylaxis of influenza --- recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Recomm Rep 2011; 60:1. 
10. Grohskopf LA, Olsen SJ, Sokolow LZ, et al. Prevention and control of seasonal influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP) -- United States, 2014-15 influenza season. MMWR Morb Mortal Wkly Rep 2014; 63:691.