Preventing Infectious Diseases

PREVENTING INFECTIOUS DISEASES

Infections are the commonest cause of human morbidity and mortality. In developing countries at the beginning of the last century diseases, such as TB, pneumonia and bacterial infections secondary to influenza, were major causes of death. However, in the developed world their impact has been lessened by public health measures such as improved housing, better sanitation and advanced social and economic conditions. Unfortunately, in developing countries infectious diseases, such as malaria, TB, respiratory and GIT infections, are still major causes of death, particularly in children.

The prevention of infectious disease is achieved by the use of vaccines . Vaccination works by stimulating the immune system to produce antibodies against the pathogenic organism by introducing bacteria or viruses that have been rendered ‘harmless’ in some way. The simplest, though not necessarily the most effective, method of preparing vaccines is by killing whole microorganisms. These are then injected into the host to induce an immune response. An example of a ‘killed’ vaccine is that used to protect against whooping cough caused by Bordetella pertussis. In some instances the immune response produced against dead organisms is insufficient to induce good immunity. This is usually because killing the microorganism often involves denaturing their proteins so while the immune response recognizes the denatured proteins it does not react to the native proteins on the pathogen. To overcome this problem, live but attenuated (weakened) microorganisms may be used. These microorganisms are less virulent and, in most cases, stimulate an effective immune response in the host. This type of approach is used for the combined vaccine against measles, mumps and rubella (MMR) and for the oral vaccine against poliomyelitis (Margin Note 3.4).

Some patients do not develop an effective immune response to these weakened microorganisms and, unfortunately, the weakened microorganisms can become virulent again, a phenomenon known as reversion. To overcome this problem, selected proteins, from, for example, bacterial capsules or viral envelopes, are extracted from the microorganism and used as vaccines. These are known as subunit vaccines. Unfortunately, bacterial capsular polysaccharide is often poor at stimulating immunity and a recent development is to render the vaccine more immunogenic by attaching the polysaccharide to an immunogenic protein. Such vaccines are known as conjugate vaccines: examples include the most recent vaccines against Neisseria meningitidis serogroup C, and Haemophilus influenzae.

Subunit vaccines against viral proteins can now be produced more cheaply by employing recombinant DNA techniques. In such cases, nucleic acid coding for the protein in question is isolated and cloned. This DNA is then transfected into a suitable microorganism which can be cultured and induced to synthesize the protein in vitro. An example is the recombinant vaccine against Hepatitis B produced in the yeast Saccharomyces cerevisiae that have been transfected with a gene encoding the S (surface) protein of the virus. More recently, DNA vaccines, which consist of viral genes transfected into bacterial plasmids and injected directly into muscle, have been undergoing clinical trials, although, as yet, none of these is routinely available.

For those infectious diseases where toxins , rather than the microorganism, are responsible for the disease, vaccines may be prepared against chemically modified or heat inactivated toxins. These inactive toxins, known as toxoids, are then used for vaccination purposes and, indeed, this is the approach used for diphtheria and tetanus vaccines.