infectious diseases

Infectious diseases

are diseases caused by an organism called pathogen.
Disease is an illness or disorder of the body or mind that leads to poor health, each is associated with set of signs and symptoms.
People who lack symptoms but are still spreaders of pathogens are carriers. Difficult to trace.
Transmission cycle – way in which a pathogen passes from one host to another.
Control methods attempt to break transmission cycles by removing the conditions that favour the spread of the pathogen. It is only possible when the cause of disease and the method of transmission is known and understood. Vaccination is a major control measure for many infectious diseases, it works by making people immune so that the pathogen do not live, reproduce and spread to others within them.

Endemic – a disease that is always in a population, species found in a certain area and nowhere else.

 

Cholera

Appeared in 1992 but restricted to Southeast Asia. Disease is on the increase, cases go unreported to not disrupt travel, tourism, trade.
Cause by the bacterium vibrio cholerae. Water born so occurs where there is restricted access to proper sanitation, clean water and food is contaminated. Transmits through faeces which contaminate the water.
Bacteria has to pass through stomach and survive the acidic content. It reaches the walls of small intestines, multiplies and releases a toxin called choleragen. It disrupts the functions of the epithelium lining the intestine, salts and water leave the blood. Causes severe diarrhoea and fatal fluid loss. Diagnosed ny microscopical analysis of faeces. Incubation period – 2 hours to 5 days.
Treatment – easy to treat if in time. Solution of salts and glucose given intravenously to rehydrate the body. If they can drink they are given oral rehydration therapy. Glucose is effective because it is absorbed in the blood and takes ions with it. Important to assure that patient’s fluid intake equals the loss and maintain osmotic balance of blood and tissue fluids.
Preventing – sewage treatment, provision of clean piped water chlorinated to kill bacteria.

 

Malaria

Has been on the increase since 1970, affects tropical countries. Caused by the protoctist Plasmodium falciparum. Female Anopheles mosquito acts as a vector. It feeds on human blood to get the protein they need to develop their eggs. When they bite and infected person with Plasmodium falciparum, they tame some of the pathogen’s gametes with the blood. Male and female gametes fuse in the mosquito’s salivary glands and move to the salivary glands. When the mosquito bites and uninfected person, she injects an anticoagulant from the salivary glands that prevents blood from clotting so that it flows from host into the mosquito. The infected stages pass into the human blood together with the anticoagulant. The parasites enter the red blood cells where they multiply.
It can also be transmitted during blood transfusion and when unsterile needles are re-used, as well as can pass across the placenta from mother to fetus.
A vector is an organism which carries a disease from one person or animal to a human.
Mainly affects tropics and subtropics. Site of action of pathogen is liver, red blood cells and brain.
Incubation period is a week to a year. Symptoms are fever, anemia, nausea, headaches, muscle pain, shivering, sweating. It is diagnosed by microscopical examination of blood with the help of dipstick test for malaria antigens in blood.
If people are continually re-infected by different strains of malaria they become immune. Immunity only lasts as long as people are in contact with the disease.
Epidemics in countries when malaria is not endemic (regularly found among particular people or in a certain area) can be very serious, more dangerous during and after rainy season.
Treatment
Anti-malaria drugs such as quinine and chloroquine are used to treat infected people. They are also used as prophylactic (preventative) drugs, stopping an infection occurring if person is bitten. They are taken before, during and after. Chloroquine inhibits protein synthesis and prevents parasites from spreading in the body. Proguanil inhibits the sexual reproduction of Plasmodium inside the biting mosquito.
Where antimalarial drugs are used widely used, there are strains of drug resistant Plasmodium therefore drug is no longer effective against pathogen. New drug is used, mefloquine but it has unpleasant side effects and is expensive. Resistance to it also developed. Antibiotic doxycycline used as prophylactic is used to treat the infections of P. falciparum.
Tourists visiting the tropics parts are at great risk of contracting malaria. Doctors in developed countries often misdiagnose it with influenza because of similar initial symptoms. Settled immigrants visiting their relatives don’t take prophylactic drugs because they don’t realize they lost the immunity.
Preventing

• Reduce nr of mosquitos
• Avoid being bitten by mosquitos
• Use drugs to prevent the parasite infecting people

Mosquitoes lay their eggs in water, larvae hatch and develop in water but come to surface for air. Oil can be spread over water surfaces to make it impossible for mosquito larvae to breath. Marshes can be drained and vegetation cleared.
Two biological control measures

• Stocking ponds, irrigation and drainage ditches and other permanents bodies of water with fish which feed on mosquito larvae
• Spraying a preparation containing the bacterium Bacillus thuringiensis which kills mosquito larvae but is not toxic for other forms of life

Impossible to completely eradicate breeding sites because mosquitos will lay eggs in any small puddle, especially in the rainy season. Best protection is to avoid being bitten, sleep with nets soaked in insecticide on windows and use insect repellents.
Worldwide control
In 1950s the WHO coordinated a worldwide eradication programme. Malaria was cleared from some countries but the programme was not generally successful. Plasmodium became resistant to the drug used to control it. Mosquitoes became resistant to insecticides used at that time. Programme was expensive and unpopular. People living in area where malaria was temporarily eradicated lost their immunity and suffered. Programme would’ve been more successful if tackled more sensitively and with help from locals.
Reasons for worldwide concerns over spread of malaria

• Increase in drug resistant forms of Plasmodium
• Increase in nr of cases caused by p. falciparum , severe and fatal
• Difficulty in developing vaccines against malaria
• Increase in nr of epidemics because of climatic and environmental changes that favour spread of mosquitoes
• Migration of people from area where malaria is endemic, economical and political reasons

Malaria is still one of the world’s biggest threat to health. 40% of the world’s population lives in areas at risk of malaria. Between 2000 and 2011 control measure achieved decrease in 25% across the world and 33% in the WHO’s african region.
Control methods now concentrate on working to improve diagnosis, supply of effective drugs and promotion of appropriate methods to prevent transmission. Simple dip stick test means diagnosis can be done quickly without the need of laboratory.
Factors that may lead to improvements in the control of malaria

• Use of modern techniques in gene sequencing and drug design
• Development of vaccines targeted against different stages of the parasite’s life cycle
• Renewed international will to donate towards removal of malaria in poor parts of the world

 

AIDS (acquired immune deficiency syndrome)

It is caused by the human immunodeficiency virus (HIV)
Outer envelope contains two glycoproteins. Protein core contains genetic material (RNA) and two enzymes. Reverse transcriptase uses the RNA as a template to produce DNA once the virus is inside the host cell.
HIV is a retrovirus, genetic material is RNA not DNA. Once inside the a host cell, the viral RNA is converted back to DNA by the enzyme reverse transcriptase. Virus infects and destroys the cells of the body’s immune system so that their nr gradually decreases. The T helper cells controls the immune system’s response to infection. When the nr of cell decreases the body is unable to defend itself against infection, allowing pathogens to cause a variety of opportunistic infections (takes advantage of the body’s weak immune system). AIDS is not a disease, it is a collection of the opportunistic diseases associated with immunodeficiency caused by HIV infections. SInce HIV is an infective agent, AIDS is called acquired to be not confused with inherited.
Transmission
Spreads by intimate human contact, virus is unable to survive outside the human body. Transmission only possible by direct exchange of body fluids. Spreads mostly through sexual intercourse, blood donation and sharing needles. Also transmitted through placenta from mother to child or mixing of blood during birth.
Initial epidemic amongst homosexuals practicing anal intercourse and had many partners. Mucous lining of rectum is not as thick as the one of the vagina and there is less lubrication, so as a result the rectal lining is easily damaged during intercourse, the virus can pass from semen to blood.
Having multiple partners allows the virus to spread more widely.
HIV is a slow virus, after infection there may be no symptoms until years later, often flu like symptoms. Infections which opportunistically develop to create AIDS tend to the characteristic to region.
Global distribution worldwide especially in sub-Saharan Africa and South-East Asia. Methods of diagnosis is testing blood, saliva or urine for presence of antibodies produced against HIV.
Treating HIV/AIDS
There is no cure for AIDS and no vaccine for HIV. Drug therapy can dramatically slow down the onset of AIDS. The drugs are expensive and have a variety of side-effects. I combined, two or more drugs that prevent replication of virus inside host cell can prolong life but cannot offer cure.
Preventing HIV/AIDS
Virus changes its surface proteins which makes it hard for the body’s immune system to recognize, makes development of vaccine difficult. Condoms, femidoms and dental dams, forms a barrier between bodyfluids, reducing chance of virus transmission. Drug injecting users are advised to give up their habit, stop sharing needles or take the drugs in other forms.
Contact tracing
Person diagnosed with HIV is willing and able to identify who s/he put at risk of infection, these people will be given and HIV test which identifies the presence of antibodies to HIV. Blood collected from blood donors is screened for HIV and heat treated to kill any viruses. In low-income countries not all donated blood is tested, puts person receiving transfusion at risk of infection. Widespread testing is not expensive but aren’t popular because of infringement of personal freedom. In developed countries HIV testing is encouraged in homosexuals, prostitutes and drug users. In Africa and south-east Asia testing is not feasible because of difficulty to reach majority of population and organising testing. People here find out they have HIv when the symptoms of AIDS develop. Infected lymphocytes are found in breast milk, in developed countries mothers are advised to not breastfeed their babies despite the availability of drugs. In contrast to low and middle income countries they are advised to breastfeed as protection against other diseases and lack of clean water outweigh the risk of transmitting HIV.

Tuberculosis

Caused by two bacteria – either mycobacterium tuberculosis or mycobacterium bovis. The are pathogens living inside the human cells, particularly in lungs which is the site of infection but bacteria can spread throughout the body and infect the bone tissue. In some, TB develops quickly while in other it may remain inactive many years. If inactive, infection doesn’t spread to others however can later become activated when people are weakened by other diseases. People with tb have persistent cough, cells release hormone-like compounds with cause fever and suppress appetite, people often lose weight and look emaciated. TB is the first opportunistic infection to strike HIV positive people.
Transmission of TB
When people with Tb cough or sneeze, bacteria are carried in the air in tiny droplets of liquid. Transmission occurs when uninfected people inhale the droplets. Spreads most rapidly where people live in overcrowded conditions, who sleep close together in large numbers, primarily attacks homeless people living in poor, substandard housing with low immunity; malnutrition or HIV positive are also particularly vulnerable. HIV pandemic has been followed closely by a TB pandemic. Spread of mycobacterium bovis occurs in cattle and spreads to humans in meat and milk.  Incubation period few weeks to several years.
TB decreases in the 50s because of improvements of housing and diet. Antibiotic streptomycin was introduced in the 40s and hastened the decrease on incidents. Once thought to be eradicated, Tb is on the increase. High rates across the developing world, in Russia and surrounding countries; cities with population of immigrants from countries where TB is common. Increase due to

• Some strains of TB bacteria are resistant to drugs
• The HIV/AIDS pandemic
• Poor housing in inner cities and homelessness
• Breakdown of TB control programmes, partial treatment increases chance of drug resistance

Treatment of TB
Mucus and pus are collected for analysis., identification can be done quickly with microscopy. If confirmed, patients should be isolated in the most infectious stages, especially if infected with drug resistant strain. Treatment involves several drugs to ensure bacteria is killed; it is long because it takes a lot of time to kill the bacteria so that none remain to continue the infection, they are slow growing and not very sensitive to drugs. Many people don’t complete their treatment because they think if they feel better they are cured, they may harbour drug-resistant bacteria and spread it to others.
The WHO promotes a scheme to ensure patients complete their course of drugs. DOTS involves health workers making sure the patients take their medicine regularly. Drugs such as Isoniazid or Rifampicin are used.
Penicillin has no effect on M.tuberculosis because the thick cell wall of this bacterium is not very permeable and it has a gene that codes for an enzyme that catalyses the breakdown of penicillin.

Drug-resistant TB
Strains were identified in the 50s. Antibiotics act as selective agents killing drug-sensitive strains and leaving the drug-resistant ones. Drug resistance occurs as a result if mutation in the bacterial DNA. It id a random event in frequency of one in every thousand bacteria. If multiple drugs are used at once, the chance of mutation is reduces. MDR-TB now exists and it is resistant to Isoniazid and Rifampicin. XDR-TB also emerged which is resistant to first line drugs and others which are less potent and more expensive. Preventing TB
Contact tracing and subsequent testing are essential parts of controlling. Contacts screened for symptoms of TB but diagnosis can take up to two weeks. Vaccine available is BCG to protect children from getting the disease. Cattle are routinely tested for TB and destroyed if found infected. TB bacteria are killed when milk is pasteurised.

 

Measles
Caused by a virus which enters the body and multiplies inside the cells in the upper respiratory tract (nasal cavity and trachea). No symptoms for 8-14 days after initial infection, after that  a rash appears and a fever develops. Symptoms are runny nose, cough, red and watery eyes (conjunctivitis), small white spots inside the cheeks. Treatment involves bed rest and medicine to lower fever, no specific medicine for measles. In about 10 days the disease clears up and there are usually no complications.  If there are complications, they can be serious such as pneumonia, ear and sinus infection, brain damage and convulsions. Fatal cases amongst malnourished children living in overcrowded conditions. It is a serious disease and a major cause of death. Infant deaths in developing countries is high and can also be responsible for childhood blindness. One of the most contagious disease,  when infected person coughs or sneezes they release many droplets containing virus particles. If these are inhaled by a person who has no immunity to disease it is almost inevitable that they will become infected. Rarely affects infants under 8 months as they have antibodies from their mother that crossed the placenta. Introduction of vaccine made the disease to fall steeply in the early 60s.
Antibiotics
A drug that kills or stops growth of bacteria without harming the cell of the infected organism. Derived from living organisms but often made more effective by various chemical processes. Antimicrobial drugs such as isoniazid used for treatment of TB is synthetically made (in a laboratory).
How it works
Interferes with aspects of growth or metabolism of the target bacterium including

• Synthesis of bacterial cell wall
• Activity of protein in cell surface membrane
• Enzyme action
• DNA/protein synthesis

Bacterial cell walls are made of peptidoglycans which are molecules containing peptides (chains of amino acids) and sugars. They are held together by cross links between them. Penicillin prevents the synthesis of cross-links between the peptidoglycan polymers in the cell walls of bacteria by inhibiting the enzyme that build there cross links; therefore it is only active while the bacteria are growing.
When newly formed bacteria are growing, the enzymes autolysins make little holes in the cell wall which allows the wall to stretch and for new peptidoglycan chains to link together. Penicillin prevents peptidoglycan chains to link uo, but the autolysins keep making new holes. Cell wall becomes weaker and weaker. Bacteria live in watery environments and take up water by osmosis. When they are weakened, cell walls cannot withstand the pressure exerted on them by the cell contents and the cell bursts. It doesn’t affect human cells because they don’t have cell walls. Antibiotics are not effective against viruses because they don’t have cells and no cell walls. When a virus replicates it uses the host’s cell mechanism for transcription and translation. Eukaryotic cells have proteins that are different than the bacteria so they are unaffected by antibiotics.

 

Antibiotic resistance

Proteins in membranes of some species of bacteria can inactivate antibiotics, bacterial membranes also have proteins that pump out antibiotics if they enter the cytoplasm. In some cases antibiotics simply cannot bind to the intended site of action.
Bacteria that are sensitive to an antibiotic are called susceptible, they may become resistant if they gain a gene coding for a protein that protects them from the antibiotic.
Soil bacteria have many resistance mechanisms as they live in an environment where there are many molecules that interfere with their metabolism.
Pathogenic bacteria became resistant to penicillin because they have acquired the gene that code for β-lactamase (penicillinase) enzyme.
Antibiotic resistance can arise when an existing gene within the bacterial genome changes spontaneously to give rise to a nucleotide sequence that codes for a slightly different protein. This change in DNA is a mutation. If bacteria survives the antibiotic, the next time there is an infection with this strain of bacteria, the antibiotic will not be effective.
Bacteria have only one copy of each gene since they have a single loop of double stranded DNA. This results in the mutant gene to have an immediate effect on any bacterium possessing it. Bacteria reproduce asexually by binary fission, the DNA in the bacterial chromosome is replicated and the cell divides into two which each daughter cell receiving a copy of the chromosome.
This method of spreading the antibiotic resistance is called vertical transmission:

1. Parent cell gains resistance
2. Bacterial chromosome and plasmid replicate
3. Cell division happens  
4. Each daughter cell receive a copy of the plasmid and are resistant

Genes for antibiotic resistance often occur in plasmids which are small loops of double stranded DNA. They are frequently transferred from one bacterium to the other even between different species. This happens during conjugation when a tube forms between two bacteria to allow the movement of DNA. During conjugation, plasmids are transferred from the donor bacterium to recipient.
This method of spread of antibiotic resistance is called horizontal transmission.

1. Cell gains resistance
2. Conjugation happens where a tube form between two bacterias
3. A single DNA strand of the plasmid is transferred
4. Each bacterium synthesizes a complementary strand, both are resistant

The more misuse of antibiotics, the greater the selection pressure exerted on bacteria to evolve resistance. Resistance may first occur in non pathogenic bacteriums and then passed to a pathogenic species. Multiple resistance – carrying resistance genes for several different antibiotics. Major problem in hospitals and prisons.
Reducing nr of circumstances in which bacteria develops resistance to antibiotics

• Using antibiotics only when appropriate and necessary
• Reducing nr of countries where they are sold without a doctor’s prescription
• Avoiding use of wide spectrum antibiotics
• Making sure they course of medication is complete
• Making sure patients don’t keep unused antibiotics for self-medication in the future
• Changing the type of antibiotics prescribed for certain diseases so tht that not the same one is always prescribed
• Avoiding using antibiotics in farming in order to prevent rather than cure