Haemophilus influenzae is a bacteria that commonly infects the respiratory tract. Dr. Kenneth Todar explained that the name comes from the influenza epidemic of 1890, when the bacteria was first isolated. Originally H. influenzae was thought to be the cause of influenza, but it is now recognized as an important secondary infection. The non-encapsulated form is present in approximately 75 percent of healthy adults and children, generally in the nose and throat.
The encapsulated form designated Haemophilus influenzae type b, or Hib, is more likely to cause active disease. The Centers for Disease Control and Prevention (CDC) reports that Hib can affect many organ systems, causing pneumonia, meningitis, ear infections, and other less common infections. The Hib vaccine has been in routine use since 1990, but unvaccinated children remain at risk for Hib disease. The CDC recommends Hib vaccination for all children under age five, and for adults at risk because of certain conditions including sickle cell disease, HIV/AIDS, removal of the spleen, bone marrow transplant, and cancer chemotherapy.
Haemophilus influenzae, Streptococcus pneumoniae, and Staphylococcus aureus can all cause secondary bacterial pneumonia in patients infected with influenza virus. Dr. Lian Ni Lee and colleagues at the Torrey Pines Institute for Molecular Studies in San Diego, California, and the University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, studied these infections in a mouse model. As background information, they reported that secondary bacterial pneumonia appeared to be the cause of death for the majority of the approximately 50 million people who were killed in the 1918 influenza pandemic. H. influenzae was isolated from the autopsied lungs of many young adults who died in this pandemic.
Lee and colleagues infected mice with a relatively mild strain of influenza virus, and then with Hib. They found a 100 percent death rate when the interval between infections was three to four days. Neither infection killed any of the mice when the time between infections was at least 10 days.