2.5 Cellular pathology of influenza infection
Flu viruses can infect a number of different cell types from different species. This phenomenon is partly because the cellular glycoproteins which are recognised by viral haemagglutinin are widely distributed in the infectious agent.
What is the term for the property of viruses that allows them to only replicate in particular cell types?
This property is viral tropism. Hence we can say that flu viruses have a broad tropism.
A second reason why the virus can infect a variety of cell types is that the replication strategy of flu is relatively simple: ‘infect the cell, replicate as quickly as possible and then get out again’. This is the cytopathic effect of the virus. Cell death caused directly by the virus can be distinguished from cell death caused by the actions of the immune system as it eliminates infected cells.
The effects of cell death
Cell death impairs the function of an infected organ and often induces inflammation, a process that brings white cells (leukocytes) and molecules of the immune system to the site of infection. In the first instance, the leukocytes are involved in limiting the spread of infection; later they become involved in combating the infection, and in the final phase they clear cellular debris so that the tissue can repair or regenerate.
The symptoms of flu experienced by an infected person are partly due to the cytopathic effect of the virus, partly due to inflammation and partly a result of the innate immune response against the virus. The severity of the disease largely depends on the rate at which these processes occur.
- In most instances, the immune response develops sufficiently quickly to control the infection and patients recover.
- If viral replication and damage outstrip the development of the immune response then a fatal infection can occur.
In severe flu infections, the lungs may fill with fluid as the epithelium lining the alveoli (air sacs) is damaged by the virus. The fluid is ideal for the growth of bacteria, and this can lead to a bacterial pneumonia, in which the lungs become infected with one or more types of bacteria such as Haemophilus influenzae. Damage to cells lining blood vessels can cause local bleeding into the tissues, and this form of ‘fulminating disease’ was regularly seen in post-mortem lung tissues of people who died in the 1918 pandemic.