Questions on infectious disease: transmission and control

Question 1

Tuberculosis is spreading in a low-income country, and cases that have stopped responding to the usual antibiotics are appearing. Discuss the biological, social and economic measures a control programme should use.

Accepted Answer

B1 biological: the BCG vaccine gives partial protection, strongest against the severe childhood forms, and is worth using even though it is imperfect B1 biological: active cases are treated with a combination of antibiotics for six months or more, because the bacterium grows slowly and shelters inside macrophages, and a treated patient stops coughing viable bacteria within weeks; pasteurising milk is biological too, heating it enough to denature the proteins of M. bovis and close that route B1 the resistant cases arise because a patient who feels well at two months and stops has spared the least susceptible bacteria, mutants present by chance from the start, which then reproduce; supervised courses, with a health worker watching each dose swallowed, are the answer B1 social: better ventilation and less crowded housing cut the droplet dose, contact tracing finds the undiagnosed cases still seeding the chain, and testing herds and slaughtering infected animals closes the M. bovis route at its source B1 economic: supervision, drugs, housing and pasteurisation all cost money, and the populations that need them most can afford them least, so a judgement is needed on where limited funds do most good, with the argument that money spent returns as the productivity of adults who can work and children who can learn

Question 2

Explain how the evasion mechanisms of Mycobacterium tuberculosis and HIV support the theory of an evolutionary race between pathogens and their hosts.

Accepted Answer

B1 M. tuberculosis stops lysosomes fusing with the phagosome inside a macrophage and has a thick waxy wall, so it survives instead of being hydrolysed B1 HIV infects and destroys T-helper cells, so B cells and T-killer cells are not activated B1 HIV mutates rapidly, so its surface antigens change and antibodies and memory cells made against an earlier form no longer bind B1 each mechanism defeats a particular host defence, so it must have been selected because pathogens carrying it survived that defence and were transmitted B1 the host's defences are themselves selected by pathogens, so each side exerts a selection pressure on the other and both keep evolving, which is the evolutionary race

Question 3

Describe how malaria is transmitted from an infected person to an uninfected person.

Accepted Answer

B1 the parasite cannot pass directly from person to person: it needs a vector, the female Anopheles mosquito, female because only females take blood meals B1 biting an infected person, she takes up the parasite's sexual stages with the blood B1 these develop in her gut and move to her salivary glands over one to two weeks, after which the infective stages pass into a person in her saliva each time she feeds B1 in that person the parasite multiplies first in the liver and then in the red blood cells, whose synchronised bursting gives the waves of fever

Question 4

Explain why antiretroviral drugs control an HIV infection without curing it, and explain why there is still no vaccine against HIV.

Accepted Answer

B1 the drugs hold replication so low that the immune system stays intact and AIDS never develops, and a person with an undetectable viral load does not transmit the virus sexually B1 the virus integrates its genome into the T-helper cells themselves, so it is never cleared and returns if the drugs are stopped B1 its surface antigens mutate faster within a single patient than those of influenza do worldwide, so antibodies and memory cells raised against one version do not recognise the next B1 it also infects the T-helper cells that any vaccinated response would have to depend on, so the virus destroys the very cells that would coordinate the response a vaccine provokes

Question 5

A camp built after a flood has one water source, no latrines and has just reported its first cases of cholera. Suggest why cholera spreads so quickly under these conditions, and suggest one measure that would break the chain of transmission.

Accepted Answer

B1 cholera takes the faecal-oral route: Vibrio cholerae leaves an infected person in faeces and enters the next by mouth, so with no latrines sewage reaches the one water source that serves thousands of people B1 the disease accelerates its own spread, because the watery diarrhoea produced by the toxin, litres of it a day, is loaded with the bacterium B1 many infected people are carriers with mild symptoms or none who still shed the bacterium, so the visible cases are only a fraction of the sources B1 separating sewage from drinking water, by providing latrines and a treated supply, removes the route itself and so stops transmission regardless of how many people are already infected; oral vaccine can blunt the outbreak meanwhile

Question 6

Compare why a measles vaccine can give lifelong protection with why a malaria vaccine, developed decades later, still gives only partial protection.

Accepted Answer

B1 measles virus has stable surface antigens, so a single set of antibodies and memory cells remains effective for life, whereas Plasmodium moves through liver, blood and mosquito stages that each present different antigens B1 a measles vaccine only has to match one stable set of antigens, whereas a malaria vaccine effective against one Plasmodium stage's antigens does not protect against the antigens of a different stage B1 measles vaccination has decades of use behind it against a virus that has not changed its antigens in that time, whereas a malaria vaccine is recent and must contend with a parasite whose antigens differ at every life stage B1 this variation between stages, on top of ordinary mutation, is why protection from a malaria vaccine is only partial, unlike the measles vaccine

Question 7

Compare the transmission of Mycobacterium tuberculosis with the transmission of Mycobacterium bovis, and compare the control measure that targets each route.

Accepted Answer

B1 M. tuberculosis travels in droplets coughed out by a person with active disease, whereas M. bovis travels in unpasteurised milk and through close contact with infected cattle B1 M. tuberculosis is controlled by measures such as ventilation, reduced crowding and supervised antibiotic treatment, which act on person-to-person spread B1 M. bovis is controlled by testing herds and slaughtering infected animals and by pasteurising milk, which act on the animal-to-person route instead B1 so tuberculosis needs two different sets of control measures, medical and social for one route and agricultural for the other

Question 8

A country wants to reduce the number of new HIV infections. Suggest three different measures it could introduce, each acting on a different route or stage of transmission.

Accepted Answer

B1 screening donated blood for HIV antibodies (and, in fourth-generation tests, for the p24 protein) prevents transmission through transfusion B1 needle exchange programmes supply sterile equipment to people who inject drugs, preventing transmission through shared needles B1 giving antiretroviral drugs to pregnant women who are HIV positive cuts mother-to-child transmission from roughly one in four to under one in a hundred B1 widespread antiretroviral treatment also reduces onward sexual transmission, because a person with a sustained undetectable viral load does not transmit the virus, so treatment functions as prevention as well as therapy

Question 9

DDT can still be sprayed on the inside walls of houses to control malaria where no safe, effective and affordable alternative exists. Discuss the ethical issues raised by this use of DDT.

Accepted Answer

B1 it kills the mosquitoes that rest on walls after feeding and so reduces malaria, which kills many people, children in particular B1 DDT persists in the environment and accumulates along food chains, harming other organisms, and may have long-term effects on human health B1 the risk is borne by the people in the sprayed houses and their environment, so their informed consent and involvement in the decision matter B1 countries without malaria banned DDT, while countries with it bear the disease, which raises the question of who has the right to decide

Question 10

A new method of controlling malaria has been developed. Describe how the scientific community would validate the method before it is recommended for wide use.

Accepted Answer

B1 the method is tested in large field trials, often randomising whole villages to receive the method or continue with existing measures, and comparing the number of cases B1 the results are published only after peer review, in which other scientists check the methods, results and conclusions B1 the trials are repeated in other places and seasons, and systematic reviews pool the results of all comparable trials B1 expert committees, such as those of the World Health Organization, review the evidence before recommending the method, and its effectiveness is monitored afterwards, for example for resistance

Question 11

Name the pathogen that causes cholera, name the four species of Plasmodium that cause malaria, and name the two species of Mycobacterium that cause tuberculosis.

Accepted Answer

B1 cholera is caused by the bacterium Vibrio cholerae B1 malaria is caused by Plasmodium falciparum, P. vivax, P. ovale and P. malariae B1 tuberculosis is caused by Mycobacterium tuberculosis and by Mycobacterium bovis, whose usual hosts are cattle

Question 12

Explain why tuberculosis spreads readily through overcrowded, poorly ventilated housing.

Accepted Answer

B1 a person with untreated active TB coughs M. tuberculosis into shared air in droplets, which other people inhale B1 infection is a matter of dose and time, so hours spent in the same unventilated room night after night deliver enough bacteria to establish an infection B1 crowding shortens the distance a droplet has to travel and puts more people in that air, and the malnutrition that accompanies poor housing weakens the immune response that would otherwise end the infection early

Question 13

In one district there are 4000 pregnancies a year in women infected with HIV. Untreated, 25 per cent of these result in transmission to the child; with antiretroviral treatment in pregnancy the figure falls to 0.8 per cent. Calculate the number of infections prevented each year if every pregnancy is treated, and calculate that as a percentage reduction.

Accepted Answer

M1 untreated infections are 25 per cent of 4000 = 1000, and treated infections are 0.8 per cent of 4000 = 32 A1 1000 − 32 = 968 infections prevented each year A1 968 ÷ 1000 × 100 = 96.8 per cent reduction

Question 14

In 1854, John Snow mapped cholera deaths in Soho, found them clustered around the Broad Street pump and had its handle removed, although the bacterium responsible was not identified for another thirty years. Explain what this shows about controlling an infectious disease.

Accepted Answer

B1 Snow had found the transmission route, water from the contaminated pump, even without knowing what pathogen it carried or how it caused disease B1 removing that one water source broke the route between the source of infection and new hosts, so it could cause no further cases B1 this shows that a disease can be controlled effectively by interrupting its transmission route, without needing to identify the pathogen or have a cure for it

Question 15

Artemisinin is given to treat malaria in combination with a second antimalarial drug, rather than on its own. Explain why.

Accepted Answer

B1 resistance mutations to any one drug arise by chance and are already present in a small number of parasites before the drug is used B1 a parasite resistant to artemisinin is still likely to be susceptible to the second drug, and vice versa, since the two act differently B1 using both together means a parasite has to carry resistance mutations to both drugs at once to survive treatment, which is far less likely, so resistant lines are much slower to establish than with either drug alone

Question 16

A course of antibiotics for tuberculosis is biological, finishing the course is social, and being able to afford time off work for clinic visits is economic. Suggest why a control programme needs measures from all three categories rather than the biological measure alone.

Accepted Answer

B1 a biological measure such as a drug or a vaccine only works if people can access it and are able to use it as intended B1 a patient who cannot afford to miss work for clinic visits may stop an antibiotic course early even though the drug itself is effective, which is an economic barrier to a biological measure succeeding B1 the three categories depend on each other: a drug or a vaccine is a biological object, but whether people use it correctly is social, and whether they can obtain or afford it at all is economic, so a programme relying on only one axis is incomplete

Question 17

Untreated tuberculosis can remain dormant inside macrophages for decades before causing illness. Explain how a later untreated HIV infection can cause this dormant tuberculosis to become active disease.

Accepted Answer

B1 in most people infected with M. tuberculosis, the bacterium survives inside macrophages, walled into granulomas by the immune response, without causing active disease B1 keeping the bacterium contained in this dormant state depends on continued immune surveillance, including T-helper cells co-ordinating the response B1 an untreated HIV infection destroys T-helper cells over years, so the immune system can no longer keep the dormant tuberculosis contained, and it can become active disease

Question 18

The allele for sickle-cell haemoglobin is common in parts of Africa where malaria has been endemic for a long time, and rare elsewhere. Explain how this is evidence that hosts, as well as pathogens, evolve in the evolutionary race.

Accepted Answer

B1 people with one copy of the allele are partly protected against severe malaria caused by Plasmodium falciparum B1 where malaria is endemic they are more likely to survive and reproduce, passing the allele on, so its frequency rises B1 the pathogen has acted as a selection pressure on the human population and changed its allele frequencies, which is evolution on the host's side

Question 19

State the general category of transmission route by which HIV passes between people, and state the general category by which tuberculosis passes between people.

Accepted Answer

B1 HIV is transmitted through direct contact with infected body fluids B1 tuberculosis is transmitted by droplets, breathed out in a cough or sneeze and inhaled by another person