Influenza Virus and Testing
There are three types of influenza viruses: A, B, and C. All are found in humans and other animals. This is why we can get infected from other animals, most commonly from birds and pigs. Type C is usually found in young children, and those who have had Influenza C will develop a lifelong immunity towards it. Types A and B are much more common; and it is Type A that caused the major pandemics of influenza in history.
The different types are also subclassified by different properties called antigens, found on the viral surface. It is these different properties that can change and give the virus resistance to well-established vaccines and standard therapies. The changes to these antigens, or reassortment, can cause a phenomenon called shifting. Each year, scientists aim to predict which of these reassorted strains will cause the next epidemic. Also, they monitor the viral activity to try and predict when the next pandemic will occur. These pandemics usually occur several decades after there is a major shift in antigen. The last major shift occurred in the 1970s, so scientists are still waiting for and expecting a major outbreak.
Type A is normally found in aquatic birds and some of its variants are able to circulate among the human population. This is the cause of the Avian Flu, which may spread to humans after undergoing changes in its antigen. So far, widespread circulation among humans has not occurred in recent years.
Many factors affect whether or not doctors will order tests for influenza. Normally, no blood tests are necessary if suspicion for influenza is high. However, doctors may choose to check blood tests if they suspect more serious infections or they are concerned about some other condition. Chest x-rays are not ordered unless an underlying pneumonia is suspected.
A definitive test for influenza is available via nasal swab for viral culture and rapid testing. Several rapid tests are available but only two can be performed in the office. The sensitivity of the rapid tests is quite high, but the specificity is variable, which makes it difficult to determine if a positive test really indicates influenza or another viral illness. However, taken in context with other clinical findings, it can be very useful in determining the need for antiviral therapy. Unfortunately, rapid tests are not widely available. Therefore, doctors often choose to treat based on symptoms, examination, and current epidemiologic evidence of influenza.