Haemophilus influenzae is a bacterium that can cause upper respiratory disease mainly in young children. H. influenzae type b, or Hib for short, is a particular cause for meningitis.
H. influenzae are Gram negative—this means that their cell wall consists of two membranes that are on either side of a thin, but strong layer called the peptidoglycan. The bacteria can be shaped like ovals or can adopt different shapes, and so are described as being pleomorphic. When grown on a solid nutrient, clumps of bacteria tend to form in the vicinity of another bacterium called Staphylococcus when the latter are present. This behavior can be important in identifying H. influenzae.
While vaccination against Hib has reduced the occurrence of infections in developed countries, H. influenzae remains responsible for many lower respiratory infections in children in other regions of the world.
H. influenzae was first described Richard Johannes Pfeiffer (1858–1945) in 1892 during an influenza epidemic. The name for the bacterium reflects an early misunderstanding that it was the cause of the flu. In 1933, scientists demonstrated that influenza was instead caused by a virus. H. influenzae, however, was subsequently shown to be the cause of several other diseases.
Some types (strains) of H. influenzae are surrounded by a sugary coat called a capsule, while other strains do not have a capsule. The strains with a capsule tend to be more of a health concern, since the capsule helps protect a bacterium from attack by a host's immune system. Nonetheless, some strains without a capsule are also pathogenic (disease-causing) and can cause bronchitis, ear infections, and epiglottitis, an inflammation in the esophagus. Complications of epiglottis can produce a blockage of the airway that can be fatal in children under the age of five.
H. influenzae is normally found in the throat and nose of many people. The bacteria residing there are usually harmless. But, if the bacteria spreads to other areas of the body or if a person's immune system is compromised, the bacteria can cause infection. Thus, H. influenzae represents an opportunistic pathogen.
Only humans are known to be susceptible to H. influenzae infections. This has complicated research on the mechanisms of infection and vaccine development, since animal models of the disease cannot be established.
H. influenzae can be spread from person to person in the droplets that are expelled when someone coughs or sneezes. The bacterium most often affects children, where strains that possess a capsule can cause a lung infection (pneumonia). More seriously, the bacteria can infect the blood and spread. Joints can be affected, producing arthritis. A heart infection called pericarditis can occur. H. influenzae infections may also attack the lining of nerves such as those in the brain. The resulting inflammation is a form of bacterial meningitis, a potentially life-threatening complication. Stiffness in the neck accompanied by flulike symptoms can be an early indication of bacterial meningitis.
Hib infections were previously a much more common and dangerous threat. The availability of infant-and childhood-based vaccines against Hib, and their widespread use beginning in the late 1990s, included in the series of vaccinations that many children receive during their first decade of life, has reduced the prevalence of Hib meningitis dramatically. In the United States, the incidence of Hib infections in children less than five years old has dropped from 40–100 children per 100,000 in the 1990s, to less than two of every 100,000 children in 2006.
WORDS TO KNOW
DROPLETS: A drop of water or other fluid that is less than 5 microns (a millionth of a meter) in diameter.
GRAM NEGATIVE BACTERIA: Gram-negative bacteria are bacteria whose cell wall is comprised of an inner and outer membrane that are separated from one another by a region called the periplasm. The periplasm also contains a thin but rigid layer called the peptidoglycan.
STRAIN: A subclass or a specific genetic variation of an organism.
HATTIE ELIZABETH ALEXANDER (1901–1968)
Hattie Elizabeth Alexander was a pediatrician and microbiologist who made fundamental contributions in the early studies of the genetic basis of bacterial antibiotic resistance, specifically the resistance displayed by Hemophilus influenzae, the cause of influenzal meningitis (swelling of the nerves in the spinal cord and brain). Her pioneering studies paved the way for advances in treatment saved countless lives.
Alexander pioneered studies of the antibiotic resistance and susceptibility of Hemophilus influenzae. In 1939, she successfully utilized an anti-pneumonia serum that had been developed at Rockefeller University to cure infants of influenzal meningitis. Until then, infection with Hemophilus influenzae type b almost always resulted in death. Her antiserum reduced the death rate by almost 80%. Further research led to the use of sulfa drugs and other antibiotics in the treatment of the meningitis.
In addition to her research, Alexander devoted much time to teaching and clinical duties. She was elected the first woman president of the American Pediatric Society in 1965.
H. influenzae is worldwide in distribution. Most affected are children who are in close contact with one another. There is no indication that girls, boys, or members any particular race group are more susceptible to infection.
H. influenzae pneumonia and meningitis are greater problems in developing countries. Agencies including the United States Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) assist countries in determining the prevalence and geographical distribution of infections, which helps in infection control programs.
H. influenzae can be treated by a number of different antibiotics, although there have been reports of antibiotic resistance.
Hib pneumonia and meningitis are preventable. Vaccination in a series of inoculations, which can begin as early as six months of age, protects children. The discovery and widespread use of the three Hib vaccines have been invaluable in reducing the cases of childhood pneumonia and meningitis. Prior to the use of the vaccine, Hib was the most common cause of bacterial meningitis in countries such as Canada, where it caused more cases than all other forms of bacterial meningitis combined. The illness ravaged the young; almost 70% of cases involved children less than 18 months of age. Up to 30% of those affected had permanent brain damage.
IN CONTEXT: DISEASE IN DEVELOPING NATIONS
The World Health Organization (WHO) estimates that the “ Haemophilus influenzae type b, or Hib, is a bacterium estimated to be responsible for some three million serious illnesses and an estimated 386,000 deaths per year, chiefly through meningitis and pneumonia. Almost all victims are children under the age of five, with those between four and 18 months of age especially vulnerable.”
“In developing countries, where the vast majority of Hib deaths occur, pneumonia accounts for a larger number of deaths than meningitis. However, Hib meningitis is also a serious problem in such countries with mortality rates several times higher than seen in developed countries; it leaves 15 to 35% of survivors with permanent disabilities such as mental retardation or deafness.”
SOURCE: World Health Organization (WHO)
Following the introduction of vaccine formulations for children (1987) and infants (1990), the number of cases of Hib meningitis decreased by over 80% in Canada and the United States within five years.
Despite the overwhelming success of Hib vaccines in combating meningitis, H. influenzae continues to be a problem in developing countries. The World Health Organization (WHO) estimates that Hib causes three million serious illnesses and almost 400,000 deaths each year, mainly due to pneumonia. Meninigitis also is a health and economic threat is these countries; 15–35% of survivors are left with brain damage and hearing loss, which impairs their ability to function in family care and to work.
The vaccination rate for the approximately 90 countries who have Hib vaccination programs is over 90%. However, in developing countries, only about 42% of people are vaccinated, and in under-developed regions such as Sub-Saharan Africa the vaccination rate is less than 10%.
The major reason for the gulf between the developed and developing world in the prevention of H. influenzae disease is the cost of the vaccines. International assistance through agencies that includes CDC and WHO are aimed at increasing the availability and use of these vaccines in less-developed regions. For example, WHO is actively involved in implementing the GAVI Hib Intitiative, which will help countries most effectively vaccinate children. Agencies including WHO and UNICEF have developed the Global Immunization Vision and Strategy (GIVS), which seeks make vaccination against diseases including Hib more efficient.
Bloom, Barry R., and Paul-Henri Lambert. The Vaccine Book. Oxford: Academic Press, 2002.
Ferreiros, C. Emerging Strategies in the Fight against Meningitis. Oxford: Garland Science, 2002.
Centers for Disease Control and Prevention. “Haemophilus influenzae/e Serotype b (Hib) Disease.” <http://www.cdc.gov/ncidod/dbmd/diseaseinfo/haeminfluserob_t.htm> (accessed April 10, 2007).