Influenza C: Description, Causes and Risk Factors:
Influenza caused by strains of type C influenza virus; the disease is milder than that caused by types A and B and has become uncommon in recent years.
Influenza C virus was first isolated in the forties, with sporadic cases reported. In a seroepidemiological study carried out in France in 1992, 61 to 70% of the population was found to have been previously exposed to the virus, the highest rates for positive samples being found in the 16- to 30-year-old group. The results indicated intense circulation of influenza C virus in the population.
Influenza C viruses are somewhat different. The enveloped virions have hexagonal structures on the surface and form long (500 microns) cordlike structures as they bud from the cell. Like the influenza A and B viruses, the core of influenza C viruses consists of a ribonucleoprotein made up of viral RNA and four proteins. The M1 protein lies just below the membrane, as in influenza A and B virions. A minor viral envelope protein is CM2, which functions as an ion channel. The major influenza C virus envelope glycoprotein is called HEF (hemagglutinin-esterase-fusion) because it has the functions of both the HA and the NA. Therefore the influenza virion contains 7 RNA segments, not 8 RNAs like influenza A and B viruses.
During the winter, infection with influenza C virus coincides with influenza A (H3N2 and H1N1) and B virus activity. The influenza C cases may exhibit symptoms with a severity similar to that caused by influenza A or B virus, and it is therefore important to better understand the epidemiology of this virus and its exact role in acute viral respiratory infections.
Influenza C viruses occur primarily in a pattern of sporadic cases or in limited outbreaks of mild illness involving children or young adults.
Influenza C virus was thought to infect humans only but was isolated from swine. The close relationship of strains from humans to isolates from pigs that was found and the possibility of interspecies transmission of the influenza C virus between humans and pigs further indicate the potential involvement of the virus in respiratory infections. The finding of persistent infection in chicken lungs in vitro as well as the fact that frequent genetic re-assortment between influenza C virus strains occurs in nature calls for broadening this study.
During the winter of 1996 to 1997 two cases of influenza C were confirmed, one by isolation and the second by serology. The cases of influenza C occurred during an outbreak of influenza A (H3N2) and B viruses. The positive isolation was from one of three throat washings sent to the laboratory, and the other case was from a group of 51 students participating in a study of influenza virus vaccination. It seems, therefore, that influenza C virus should also be considered when examining patients with respiratory infections during the influenza season.
It is possible that because of more efficient control of influenza A and B viruses by vaccination and/or antiviral drugs, influenza C virus could have a better chance of persisting, leading to more cases being identified.
Laboratory identification of human influenza virus infections is commonly performed using direct antigen detection, virus isolation in cell culture, or detection of influenza-specific RNA by reverse transcriptase-polymerase chain reaction (RT-PCR). In recent years Commercial influenza rapid diagnostic tests have become available. These are mostly antigen detection tests, which can produce results within 30 minutes. They can provide results in a clinically relevant time frame to complement the use of antiviral medications for treatment and chemoprophylaxis of influenza. Their wide availability has resulted in their increasing application to clinical situations, which may be inappropriate or where scientific data are lacking.
Acceptable respiratory specimens: Most tests can be used on a variety of respiratory specimen types, however not all specimen types yield equivalent results, and other factors can influence specimen quality. Nasal aspirates, nasal washes, sputa and nasopharyngeal swabs, especially those specimens containing cellular material, are preferable to nasal swabs and throat swabs. They should be collected as close to the onset of symptoms as possible and not after 4-5 days in adults as virus shedding typically diminishes. In young children, viral shedding may occur for longer periods, and the collection of specimens for testing after 5 days of illness may still be useful.
Rapid tests vary in complexity with the number of steps required to perform each test ranging from 2-8. The United States is currently the only country to categorize their complexity. The rapid diagnostic tests, which are easy to use and interpret, are waived from approval by the FDA, for use in a clinical/office setting, while others are classified as moderately complex and must be used in a diagnostic laboratory setting. Other countries may require specific agency approval for rapid test use. Training in the use of rapid diagnostic tests is highly recommended because of their varying complexity and the importance of specimen type and quality. The accuracy of an influenza diagnostic test is determined by the sensitivity, specificity, positive and negative predicted value of the test to detect an influenza virus infection compared with a "gold" standard (usually culture).
Immunofluorescent antibody staining: The sensitivity of influenza antigen detection in respiratory specimens by immunofluorescent staining in comparison to cell culture ranges between 70-100%; specificity, 80-100%; PPVpositive predictive value, 85-94%; NPV (negative predictive value, 96-100%.
- Diffuse lower abdominal pain.
- Nausea and vomiting.
- Runny nose.
- Sore throat.
Antiviral drugs for influenza are available in some countries and effectively prevent and treat the illness. There are two classes of such medicines, 1) adamantanes 2) neuraminidase inhibitors. Some influenza viruses develop resistance to the antiviral medicines, limiting the effectiveness of treatment.
Nearly all adults have been infected with influenza C virus, which causes mild upper respiratory tract illness. Lower respiratory tract complications are rare. There is no vaccine against influenza C virus.
NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.
DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.