In the recent issue of the Morbidity Mortality weekly report (dated March 22, 2013), the CDC has published detailed recommendations for prevention and control of meningococcal disease. Meningococcal disease refers to diseases caused by the Gram-negative (stains pink by the Gram Stain), diplococcus (round-shaped cells usually occurring in clusters of two) bacterium known as Neisseria meningitidis (a.k.a. the meningococcus) – that is responsible for a variety of serious, often life-threatening, diseases, including bacterial meningitis (inflammation of the meninges, or covering of the brain and the central nervous system), bacteremia (more precisely, meningococcemia; meningococcal invasion of blood) often leading to septic shock, as well as a type of pneumonia.Neisseria meningitidis” width=”220″ height=”103″ />
Image Courtesy: CDC
The meningococcus of the world are divided into 12 serogroups, depending upon how specific antibodies recognize the antigenic characteristics of the polysaccharide covering (a.k.a. capsule) of the microbe. Some of the clinically important serogroups are A, B, C, W135, and Y. However, not all the meningococcal serogroups occur naturally in all parts of the world.
The report, authored by Cohn et al., summarizes the recommendations on prevention and control of meningococcal disease in the US, from CDC’s Advisory Committee on Immunization Practices (ACIP), a committee of 15 experts in medicine and public health; established under Section 222 of the Public Health Service Act (42 USC §2l7a), this committee develops national guidelines on the use of vaccines to prevent vaccine-preventable diseases in the US.
Meningococcal disease is largely amenable to vaccination. As indicated in the CDC report:
Serogroups B, C, and Y are the major causes of meningococcal disease in the United States, each accounting for approximately one third of cases. However, the proportion of cases caused by each serogroup varies by age group. Approximately 60% of disease among children aged 0 through 59 months is caused by serogroup B N. meningitidis…; Serogroups C, Y, or W, … cause 73% of all cases of meningococcal disease among persons aged ≥11 years (CDC, unpublished data, 2012).
Currently, in the US, four vaccines are licensed, protecting against four (quadrivalent or tetravalent; containing A, C, W, and Y serogroup polysaccharides) or two (bivalent; C and Y) serogroups. However, there is currently no vaccine against serogroup B in the US, because this serogroup has proven highly challenging to make a vaccine against.
Interestingly, the meningococcus doesn’t always cause disease in the human host. It is known to colonize (form a habitat around) mucosal surfaces of the nasopharynx (communicating channel, running from the soft palate to the base of brain) of healthy, asymptomatic carriers, as well as patients. Young adults and adolescents are often responsible for asymptomatic nasopharyngeal carriage of the microbe, which appears to be transmitted via droplets of respiratory tract or throat secretions from one person to another; naturally, situations which may result in spraying of spit or exchange of saliva (as may happen during living in close quarters, talking close to someone’s face, sharing food/drinks, or kissing) increase the risk of transmission.
Ordinarily, body’s defences can mount a successful antibody response to keep the microbe at bay; this is also the principle behind the efficacy of the vaccination, which seeks to stimulate the generation of serogroup-specific bactericidal (i.e. capable of killing the bacterium) antibodies. However, colonization may lead to invasive disease in case of individuals with impaired or immature immunity. As the CDC report indicates: “Incidence of meningococcal disease peaks among persons in three age groups: infants and children aged <5 years, adolescents and young adults aged 16 through 21 years, and adults aged ≥65 years (CDC, unpublished data...).” Therefore, these are the major target groups for the immunization program.
Image Courtesy: Left Panel: Department of Health and Human Services, Government of Tasmania; right panel: CDC; 4month old female with gangrene of hands and lower extremities due to meningococcemia, which caused caused arterial occlusions.
According to the ACIP recommendations, routine meningococcal vaccination is advised for:
- Adolescents aged 11-12 years, with a quadrivalent meningococcal conjugate vaccine, followed by a booster dose at age 16 years.
- Persons at increased risk for meningococcal disease, i.e.,
- People with chronic deficiencies of complement components (serum proteins which help focus certain antimicrobial immune responses).
- People living in close quarters, such as college freshman dormitories or military barracks.
- Children and adults with anatomic or functional asplenia (absent or non-functioning spleen).
- Professionals who are exposed to meningococcal isolates because of their profession (such as microbiologists, or military personnel who have to travel to or reside in areas endemic for meningococcal disease)
- People exposed to someone infected with meningococcus during an outbreak.
To be sure, meningococcal disease has definitive guidelines for chemoprophylaxis (prevention by antibiotic administration) and treatment. Certain antibiotics are highly (90%–95%) effective in reducing nasopharyngeal carriage of N. meningitidis, and may be administered to people who have been exposed to it. Similarly, in the event of a suspected outbreak, there are protocols in place for evaluation and management. However, in the interest of risk management and harm reduction, routine vaccination of the at-risk population makes a lot of sense.
Do read the CDC report in its entirety. It is freely available and highly informative.