#55 Infectious diseases and Antibiotics

An infectious disease is one that can be passed between one person and another. Infectious diseases are caused by pathogens. These are usually microorganisms such as viruses, bacteria, fungi or protoctists.

A non-infectious disease cannot be passed between people and is not caused by pathogens. Examples include sickle cell anaemia and lung cancer.


Disease can be defined as a condition in which the body does not function normally, and which produces unpleasant symptoms such as pain, distress or feeling weak. The term disease is generally used for conditions that last for at least several days.

Important infectious diseases

1. Cholera

Cause

Cholera is caused by a bacterium, Vibrio cholerae.

Vibrio choleae.

Transmission

V. cholerae can enter the body in contaminated food or water. The bacteria breed in the small intestine, where they secrete a toxin that reduces the ability of the epithelium of the intestine to absorb salts and water into the blood. These are lost in the faeces, causing diarrhoea. If not treated, the loss of fluid can be fatal. Cholera is most likely to occur where people use water or food that has been in contact with untreated sewage, as the bacteria are present in the faeces of an infected person.

Prevention and control

Transmission is most likely to occur in crowded and impoverished conditions, such as refugee camps. Cholera is best controlled by treating sewage effectively, providing a clean water supply and maintaining good hygiene in food preparation. There is no fully effective vaccine for cholera.

2. Malaria 

Cause

Malaria is caused by a protoctist, Plasmodium. There are several species, which cause different types of malaria. In a person, the Plasmodium infects red blood cells and breeds inside them. Toxins are released when the Plasmodium burst out of the cells, causing fever.

The malaria parasite, plasmodium falciparum,
 infecting red blood cells (source National Geographic). 

Transmission

Plasmodium is transmitted in the saliva of female Anopheles mosquitoes, which inject saliva to prevent blood clotting when they feed on blood from a person. When a mosquito bites an infected person, Plasmodium is taken up into the mosquito's body and eventually reaches its salivary glands. The mosquito is said to be a vector for malaria.

A detailed study of the life cycle of the malarial parasite is not required in the syllabus. 

Prevention and control

Reducing the population of mosquitoes, for example by removing sources of water in which they can breed, or by releasing large numbers of sterile males, can reduce the transmission of malaria.

Preventing mosquitoes from biting people, for example by sleeping under a mosquito net, or by wearing long-sleeved clothing and insect repellant, can reduce the chances of a mosquito picking up Plasmodium from an infected person, or passing it to an uninfected person.

Prophylactic drugs (that is, drugs that prevent pathogens infecting and breeding in a person) can be taken. However, in many parts of the world Plasmodium has evolved resistance to some of these drugs.

3.Tuberculosis (TB) 

Cause

TB is caused by the bacterium Mycobacterium tuberculosis or (more rarely) Mycobacterium bovis.

Transmission

M. tuberculosis can enter the lungs in airborne droplets of liquid that are breathed in. This is more likely to happen in places where many people are living in crowded conditions.

Prevention and control

TB is most prevalent amongst people living in poor accommodation, or whose immune systems are not functioning well, perhaps because of malnutrition or infection with HIV (see below). Increasing standards of living and treating HIV infection can therefore help to reduce the incidence of TB.

Vaccination with the BCG vaccine confers immunity to TB in many people. New vaccines are being developed that it is hoped will be more effective.

Treatment of HIV with drug therapy reduces the risk that an HIV-positive person will get TB.

Treatment of TB with antibiotics can often completely cure the disease. However, this is not always the case because:

• there are now many strains of the M. tuberculosis bacterium that have evolved resistance to most of the antibiotics that are used;
• the bacteria reproduce inside body cells, where it is difficult for drugs to reach them;
• the drugs need to be taken over a long time period, which often requires a health worker checking that a person takes their drugs every day.

4. HIV/AIDS

Cause

AIDS is caused by the human immunodeficiency virus, HIV. This is a retrovirus, which contains RNA. The virus enters T-lymphocytes, where its RNA is used to make viral DNA which is incorporated into the T-lymphocytes' chromosomes. Usually nothing more happens for several years, but eventually multiple copies of the virus are made inside the T-lymphocytes, which are destroyed as the viruses break out and infect more cells. Eventually there are so few functioning T-lymphocytes that the person is no longer able to resist infection by other pathogens and develops one or more opportunistic diseases such as TB.

HIV. 

Transmission

HIVcan be passed from one person to another through:
• blood from one person entering that of another, for example by sharing hypodermic needles, or through blood transfusions
• exchange of fluids from the penis, vagina or anus
• across the placenta from mother to fetus, or in breast milk

Prevention and control 

All blood to be used in transfusions should be screened to ensure it does not contain HIV.

All hypodermic needles should be sterile and used only once, and disposed of carefully.

A person should avoid sexual activity with anyone whose HIV status they do not know. If everyone had only one partner, HIV could not be transmitted. Condoms, if properly used, can prevent the virus passing from one person to another during intercourse. If a person is diagnosed with HIV, all their sexual contacts should be traced and informed that they may have the virus.

The chance of HIV passing from an HIV-positive mother to her fetus is greatly reduced if the mother is treated with appropriate drugs. These drugs can also greatly increase the length of time between a person becoming infected with HIV and developing symptoms of AIDS, and can significantly prolong life.

HIV infection rates are especially high in sub-Saharan Africa. Many of these people are not able to receive treatment with effective drugs, generally for economic reasons.

Natasha Chisenga Simpasa, holding her six-week-old daughter, Mutale,
listens as a health worker explains proper dosing of Mutale’s prophylactic antibiotics
during a consultation. Ms. Simpasa is HIV-positive. (Source: UNICEF).

5. Smallpox

Cause

Smallpox is caused by the variola virus.

Variola virus.

Transmission

Transmission occurs by the inhalation of droplets of moisture containing the virus.

Prevention and control

Smallpox was a serious disease that was fatal in 20-60% of adults who caught it, and in an even higher percentage of infected children. It was eradicated by 1979, through a vaccination campaign coordinated by the World Health organization.

Smallpox was higly fatal. 

Read more: The Smallpox Virus and Its History

6. Measles 

Cause

Measles is caused by a morbillivirus.

Morbillivirus.

Transmission

Transmission occurs through the inhalation of droplets of moisture containing the virus. It is highly infectious, so that a high proportion of people who come into close contact with an infected person will also get the disease.

Prevention and control

Measles is a serious disease, which can cause death, especially in adults and in people who are not in good health, for example because they do not have access to a good diet. Vaccination is the best defence against measles. The vaccine is highly effective, especially if two doses are given. The people most likely to suffer from measles are therefore those who are malnourished and who live in areas where no vacclnatlon programme is in place.

A child with measles. (Source: CDC)

Global patterns of disease

Malaria is found in parts of the world where the Anopheles mosquito species that can act as vectors are found. This is mostly in tropical and subtropical regions where humidity is high.

TB is found in all countries of the world, including developed countries such as the USA and the United Kingdom. However, it is most common in areas where living conditions are poor and crowded, or where large numbers of people have HIV/AIDS.

Global eradication of infectious disease

The world Health Organization has helped to organise world-wide campaigns to eliminate the serious infectious diseases smallpox and poliomyelitis.

Smallpox has been successfully eradicated by vaccinating large numbers of children with weakened viruses similar to those that cause smallpox. This succeeded because the vaccine is highly effective. The programme involved the vaccination of all relatives and contacts of anyone who had the disease, called ring vaccination. The virus did not mutate, so the same vaccine could be used everywhere.

Diseases that have not been successfully eradicated include:

• Measles. This is partly because several successive doses of vaccine are required to produce immunity, especially in children who have weak immune systems because of poor diet or living conditions. The virus is very infective, so a very high percentage of people must be vaccinated to ensure a population is free of it. Booster vaccinations are also needed. This is difficult to achieve in places where infrastructure is poor.
• TB. The BCG vaccination gives only partial immunity, although new vaccines are now being developed which it Is hoped will be more effective. TB is difficult to treat because the bacteria live Inside body cells. Many strains have developed resistance to antibiotics.
• Malaria. No effective vaccine has yet been developed against Plasmodium. This is a eukaryotic organism, not a bacterium or virus, and is not affected by antibodies produced by B-lymphocytes or by T-lymphocytes.
• Cholera. This disease is caused by the bacterium Vibrio choterae. In the body, it lives and reproduces in the intestine, which is outside the body tissues and not easily reachable by lymphocytes or antibodies. Current cholera vaccines are ineffective, partly because injected vaccines do not readily reach the intestines. Oral vaccines are being developed, which are proving more effective. Vaccines are, of course, completely ineffective against any diseases that are not caused by pathogens, such as sickle cell anaemia.

Antibiotics 

An antibiotic is a substance that, when taken orally or by injection, kills bacteria but does not harm human cells. Antibiotics are not effective against viruses. Manyantibiotics are originally derived from fungi, but they can also be obtained from other organisms (for example, amphibian skin or plants) or synthesised in the laboratory.

Antibiotics act on structures or metabolic pathways that are found in bacteria but not in eukaryotic cells. The correct antibiotic must be chosen for a particular disease.

For example:
• penicillin prevents the synthesis of the links between peptidoglycan molecules in bacterial cell walls; when the bacteria take up water by osmosis, the cell wall is not strong enough to prevent them bursting.
• rifampicin (rifampin) inhibits an enzyme required for RNA synthesis in bacteria.
• tetracycline binds to bacterial ribosomes and inhibits protein synthesis.

Exposure to antibiotics exerts strong selection pressure on bacterial populations. Any bacterium that is resistant to the antibiotic - for example, because it synthesises an enzyme that can break down the antibiotic - has a selective advantage and is more likely to survive and reproduce successfully. The offspring will inherit the alleles that confer resistance. A whole population of resistant bacteria can therefore be produced. For this reason, it is important that antibiotics are only used when necessary. A person prescribed antibiotics should complete the course, as this increases the chances of eradicating all the disease-causing bacteria in the body.

Syllabus 2015 (a) define the term disease and explain the difference between an infectious disease and non-infectious diseases; (b) state names and types of causative organism of each of the following diseases: cholera, malaria, TB, HIV/AIDS, smallpox and measles (detailed knowledge of structure is not required. For smallpox (Variola) and measles (Morbillivirus) names of genus only is needed); (c) explain how cholera, measles, malaria, TB and HIV/AIDS are transmitted; (d) discuss the factors that need to be considered in the prevention and control of cholera, measles, malaria, TB and HIV/AIDS (a detailed study of the life cycle of the malarial parasite is not required) (an appreciation of social and biological factors and how economic factors can affect these should be included); (e) discuss the factors that influence the global patterns of distribution of malaria, TB and HIV/AIDS and assess the importance of these diseases worldwide; (f) outline the role of antibiotics in the treatment of bacterial infectious diseases (knowledge of specific antibiotics and their mode of action is not required);

Syllabus 2016  - 2018 10.1  Infectious diseases While many  infectious diseases have been successfully controlled in some parts  of the world, many people worldwide are still at risk of these diseases. a)   define  the term  disease and explain the difference between an infectious disease and a non-infectious disease (limited to sickle cell anaemia and lung cancer) b)   state the name and type  of causative organism (pathogen) of each  of the following diseases: cholera,  malaria, tuberculosis (TB), HIV/AIDS, smallpox  and measles (detailed  knowledge of structure is not required. For smallpox  (Variola) and measles (Morbillivirus) only the name of genus is needed) c)   explain how cholera,  measles, malaria, TB and HIV/AIDS are transmitted d)   discuss the biological, social and economic factors that  need to be considered in the prevention and control of cholera, measles, malaria, TB and HIV/AIDS (a detailed study  of the life cycle of the malarial parasite is not required) e)   discuss the factors that  influence the global patterns of distribution of malaria, TB and HIV/AIDS and assess the importance of these diseases worldwide 10.2  Antibiotics The ‘age of antibiotics’  began in the 1940s with the  availability of penicillin. With an increase in antibiotic  resistance is this age about  to come to an end? a)   outline  how penicillin acts  on bacteria and why antibiotics do not affect  viruses b)   explain in outline  how bacteria become resistant to antibiotics with reference to mutation and selection c)   discuss the consequences of antibiotic  resistance and the steps that  can be taken  to reduce its impact