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Parasite effects on populations
1) Can Parasites wipe out entire populations?
- entire species
2) Can Parasites affect life cycles?
3) Mate selection?
4) Drive other mechanisms?
A. Bubonic Plague In Renaissance Europe
Bubonic plague (Yersinia pestis) had been absent from Western
Europe for nearly a millenium when it appeared in 1348. Up to two
thirds of the population of many of the major European cities
succumbed to the plague in the first two years. Government, trade
and commerce virtually came to a halt.
There was hardly a generation which did not experience a local,
regional or pan-European epidemic for the next two hundred years.
There was virtually no aspect of European society that was not
affected by the coming of plague and by its duration.
At the most basic level, recurrent plague tended to skim off
significant portions of the children born between infestations of
plague, dampening economic and demographic growth in most
parts of Europe until the late seventeenth century.
DISASTER STRIKES
Estimated population of Europe from 1000 to 1352.
1000
1100
1200
1300
1347
1352
38 million
48 million
59 million
70 million
75 million
50 million
25 million people died in just under five years
between 1347 and 1352.
India: 1898-1908: > 500,000 died
What Conditions contributed to this situation
Large urban populations-Very congested housing conditions
Poor hygiene: water / garbage-Very high rat population
Bacteria entered susceptible rat population-Massive die off
Fleas abandoned ship-People in early stages moved away from
area- took infection with them
Ring-a-ring-a-roses,
A pocket full of posies;
Ashes, Ashes, (A-tishoo, A-tishoo)
We all fall down
Many have associated the poem with the Great Plague in England in 1665, or
with earlier outbreaks of the Black Death in England. Interpreters of the rhyme
before the Second World War make no mention of this.
By 1951, however, it seems to have become well established as an explanation
for the form of the rhyme that had become standard in the United Kingdom. "The
invariable sneezing and falling down in modern English versions have given
would-be origin finders the opportunity to say that the rhyme dates back to the
Great Plague. A rosy rash, they allege, was a symptom of the plague, posies of
herbs were carried as protection and to ward off the smell of the disease.
Sneezing and coughing was a final fatal symptom, and 'all fall down' was exactly
what happened." The line Ashes, Ashes in alternative versions of the rhyme is
claimed to refer to cremation of the bodies, the burning of victims' houses, or
blackening of their skin.
In its various forms, the interpretation has entered into popular culture and has
been used elsewhere to make oblique reference to the plague.
For centuries, plague represented disaster for those living
in Asia, Africa and Europe, where, it has been said,
populations were so affected that sometimes there were
not enough people left alive to bury the dead.
Because the cause of plague was unknown, outbreaks
contributed to massive panic in cities and countries. The
disease was believed to be delivered upon the people by
the displeasure of the gods, by other supernatural powers
or, by heavenly disturbance. Innocent groups of people
were blamed for spreading plague and were persecuted
by the panicked masses.
Could such a panic occur today?
1990s: A disease outbreak occurred in
India. The rumour leaked out it was
plague. Much movement of people
away from the focus, and out of India.
All international flights leaving India
were met in N. America by health
officials. In the US many people
quarantined. Most international
airports not equipped for these issues.
No imports or exports into the region,
no tourism, no commerce.
Estimated cost $ 4-5 billion
How can we reduce parasite/disease problems in
humans and animals?
Should we?
Can we?
Where are “new” diseases and Parasites coming from?
A detailed map highlighting the world's hotspots for emerging infectious
diseases (EIDs) has been released. It uses data spanning 65 years and shows the
majority of these new diseases come from wildlife.
Conservation efforts that reduce conflicts between humans and animals could limit
future outbreaks. 60% of EID events were caused by "non-human animal" sources,
and 71% of these outbreaks were "caused by pathogens with a wildlife source".
PARASITE CONTROL
In the last 50 years the number of human cases of ascaris
has gone up by 300 % from 400 million to 1200 million. The
increase in hookworm disease has gone up 100 %, from
600 million in 1947 to 1200 million in 1997.
The other major change in parasitic disease in the last 50
years is an increase in opportunistic infections associated
with immunosuppression and in particular AIDS, with
parasites like Cryptosporidium, Toxoplasma, and
Pneumocystis becoming life-threatening diseases
What do we do?
1) Nothing
2) Rational Intervention
3) Mass drug delivery
4) Vaccines
5) Take Bisc-318 and become paranoid
6) ????
Control Strategies
Ideally control methods should be integrated with knowledge of weak
points of the parasites life cycle.
Intuitively it might seem that parasites with a direct life cycle might be
easier to control than parasites with intermediate hosts, since only the
definitive host and the external environment need to be considered.
Safe sewage disposal will give satisfactory control of fecally transmitted
parasites of man, e.g. Ascaris.
There are advantages with regard to control when dealing with one host
life cycles, there is, however, an important disadvantage in that control
of one host life cycles usually requires the co-operation of the infected
individual or the co-operation of the owner of infected animals.
Co-operation from people even if they are likely to benefit directly can
be difficult, so in practice parasites with heteroxenous life cycles may
prove easier to control.
Who should do the controlling? Countries? UN?
Successful malaria control was done by professional spray teams who treated the
inside of houses with DDT, without any direct involvement of the infected population.
Onchocerca control campaign in West Africa has been due to professional teams
treating streams and rivers with insecticide to kill the vector.
Parasites with a direct life cycle usually have no capacity to multiply outside of their
definitive host. Parasites with intermediate hosts often undergo asexual reproduction
in the intermediate host, increasing biotic potential = difficult control
For the control of digeneans such as schistosomes, prevention of fecal
contamination of snail habitats has to be almost perfect, since a single infected snail
can shed thousands of cercariae.
Protozoa usually multiply within their definitive host, helminths do not. So low levels
of helminth infection can be tolerated, but low levels of protozoan infections can build
up to critical levels.
Prevention of Environmental Contamination
Antiparasite drugs are the most common method of parasite control and by
reducing the number of adult parasites, this reduces environmental
contamination.
Maximize benefits- prevent reinfection. This can be done by timing the
treatment to coincide with a period when parasite transmission is low.
Treatment can be followed by chemoprophylaxis to prevent reinfection if a
suitable drug is available.
Sanitation has complemented drug treatment in almost all campaigns against
fecally transmitted parasites. This is effective, provided the systems are
maintained in working order.
Destruction of Free-Living Stages
Protozoan cysts, helminth eggs and infective larvae are often extremely resistant
to toxic chemicals, so disinfecting areas is not very practical, unless you have
animals in concrete pens which can be steam cleaned.
The destruction of nematode larvae on pasture is not very practicable, grazing
with a non-host species helps i.e. alternating horses and sheep, liming fields will
kill some larvae, the alternative is to plough up the field and start again.
A broad range of microbial agents attack nematode eggs and larvae.
In biologically rich soils there are a variety of fungi that will attack parasite eggs.
There are also a range of invertebrates which will eat and destroy parasite larvae,
these include free-living nematodes, annelids and molluscs. Dung beetles that
bury dung can also act as useful control agents. Anaerobic digestion kills eggs in
sewage as does UV treatment.
Destruction of Intermediate Hosts and Vectors
This has often proved a very successful method of control, not least because it
can be carried out by specialist teams and does not rely on public co-operation.
Environmental manipulation can eliminate habitats suitable for survival. Before
the development of modern insecticides this was one of the major forms of
control, for example, mosquito control. Remove breeding sites by drainage, land
reclamation projects, removal of vegetation overhanging water, speeding up
water flow in canals and periodic drainage and drying out of canals.
A variety of chemicals have been used to kill the molluscan host of
schistosomiasis including CuSO4 and CaCN and more effective modern
compounds such as niclosamide. Mollusciciding can be quite effective providing
the water body is small.
Destruction of Reservoir Hosts
Most leishmania infections of man are zoonoses, i.e. they are caught from
animals that are the natural hosts.
One approach to their control is destruction of the reservoir host. In the former
USSR large scale destruction of gerbils has succeeded in providing control of
Leishmania near towns and villages.
Destruction of free-ranging dogs is important in the control of leishmaniasis in
China and also in the elimination of hydatid disease in Iceland. At one time the
control of African trypanosomiasis was centred on the systematic destruction of
wild reservoir hosts, particularly antelope. This did not work particularly well and
was abandoned.
What about Foot and Mouth Disease in North America?
Prevention of Infection
Since parasite control is rarely 100% effective, prevention of infection represents
an important tactic.
Many infective stages gain entry in drinking water, Cryptosporidium, Giardia,
Dracunculus etc. These can be controlled by safe water supplies, the problem is
that installation of water supplies is expensive and they have to be maintained.
Meat inspection of carcasses for cysticerci of Taenia solium and T. saginata is one
of the oldest public health measures (dates back to 1400). In areas where
infection is common carcasses are also inspected for Trichinella spiralis larvae.
Thorough cooking or freezing will kill the infective stages of most parasites.
Wearing shoes, thus stopping the larvae burrowing through the skin can prevent
hookworm infections.
Window screens, repellents and bed nets prevent mosquito bites. Ointments have
been produced to try and prevent infection by hookworm larvae and by
schistosome cercariae. Avoiding schistosome-contaminated water is another
major factor as is education and a general increase in the standard of living.
Integrated Control
Integrated control is an overall approach, using a number of different tactics
employed strategically based on a thorough knowledge of the life cycle and
ecology of the parasite. It avoids over-dependence on a single method, and
recognizes that no single method is likely to be 100 % effective, but in
combination, complete control may be achieved.
Most control programmes are based on chemotherapy- drugs to rid the host of
the infection or to prevent entry and establishment by parasite
Who pays to develop these drugs?
Is this drug development altruistic by drug companies?
Why is effective parasite chemotherapy so important ?
In absence of vaccines chemotherapy is the most cost-effective
treatment method
Potential Problems:
1. Re-infection in endemic areas
Few drugs 100% effective
Drug Active against only a few stages
Parasite resistant to Drug
Drug cannot reach migrating parasite
2. Some drugs expensive
3. Serious side effects
4. Cannot be given orally
Drug treatment exerts selection pressure on Parasite population
Resistance due to:a)
b)
Presence/absence of single gene:
Result of high & frequent exposure to drug killing almost 100% of
population
Polygenetic (combination of genes)
Drug treatments rarely 100% effective
Drug treatment causes increase in frequency of resistant alleles in treated
population
Drug resistance: protozoa v helminths
Protozoa
Long history of resistance
Coccidiosis
Malaria
Helminths
Overall less widespread resistance
(intensive usage does lead to resistance)
Protozoa replicate more rapidly than parasitic worms
Anti-protozoan drugs used more extensively worldwide
Why develop parasite vaccines?
Part of a control programme
Advantages over drugs and disease prevention strategies
History of success in other diseases:
Diphtheria, Tetanus, Measles, Mumps, Rubella, Polio,
Hepatitis B, Yellow fever
1. understand life–cycle of parasite
→ find best target stage
2. understand immune mechanisms stimulated by parasite
→ humoral /cellular response ?
3. Selected antigens must produce a protective response
→ not stimulate non-protective mechanisms
Vaccine must stimulate a good response
Good level of protection without boosting
→ using a simple delivery system
Currently very strong efforts to develop vaccines for the parasites that
cause malaria (sporozoite, merozoite, gametocyte)
Schistosomiasis, Hydatid disease, Leishmania
What do all these intervention strategies try to do?
R0 represents the average number of secondary infections
generated by one primary case in a susceptible population
Can be used to estimate the level of immunization or
behavioural change required to control an epidemic
R0
=
βH
α+µ+b
β =transmission rate, H= number of hosts
α = parasite induced host mortality (a measure of virulence)
µ= parasite mortality rate within the host, b=natural host
death
A detailed map highlighting the world's hotspots for emerging infectious
diseases (EIDs) has been released. It uses data spanning 65 years and shows the
majority of these new diseases come from wildlife.
Conservation efforts that reduce conflicts between humans and animals could limit
future outbreaks. 60% of EID events were caused by "non-human animal" sources,
and 71% of these outbreaks were "caused by pathogens with a wildlife source".
What do we do with enzootics?
Bury or burn
Climate linked to plague increase
Up to 3,000 cases of plague are
reported each year
Climatic changes could lead to
more outbreaks of bubonic plague
among human populations, a study
suggests.
Nepal's government has been
accused of "inaction and apathy"
after it emerged that 10 out of 250
endangered elephants are
suffering from tuberculosis (TB).
Malaysia mystery virus unmasked The
killer virus which killed at least 100 people in
Malaysia and led to the slaughter of
thousands of pigs has been identified.
Malaysian medical authorities initially blamed
a mosquito-borne disease called Japanese
encephalitis for the deaths.
The Nipah virus was the cause of the mystery
illness. It is thought to be a brand new disease that
has crossed only recently from animals to people.
In a separate report, doctors in Singapore say the
Nipah virus was also responsible for an outbreak of
fever and drowsiness among abattoir workers there
in March this year.
Blame game amid Rio dengue crisis
In one of Rio's poorest shanty towns, city fire
officers make their way through narrow polluted
lanes, in what is the front line of the fight against
dengue.
Their job is to treat the stagnant water and cover
exposed tanks that are a breeding ground for the
mosquito that spreads potentially fatal dengue virus.
More than 60 people have died in the state of Rio
de Janeiro so far this year, and more than 60 other
deaths are being investigated. More than 40,000
people have been treated for dengue.
For the people who live in the Community of
Coroado, especially the children, the risks are all too
obvious. A large container filled with rubbish sits by
the road, while there are large pools of water, as
well as exposed drains and sewage.
A detailed map highlighting the world's hotspots for emerging infectious
diseases (EIDs) has been released. It uses data spanning 65 years and shows the
majority of these new diseases come from wildlife.
Conservation efforts that reduce conflicts between humans and animals could limit
future outbreaks. 60% of EID events were caused by "non-human animal" sources,
and 71% of these outbreaks were "caused by pathogens with a wildlife source".
Avian flu
SARS
Human flu
Swine flu
MERS
What is Next?
New death from SARS-like virus in Saudi: WHO 2013
GENEVA -- Another person suffering from a SARS-like virus has died in Saudi
Arabia, the World Health Organization said Thursday, bringing the worldwide
number of fatalities from the mystery illness to seven.
The Saudi health ministry had informed the U.N.'s health body that the patient had
been hospitalized on Jan. 29 and had died on Feb. 10, WHO said in a statement.
A laboratory had confirmed on Feb. 18 that the person had died from the so-called
novel coronavirus, or NCoV, it added.
This brings to 13 the number of cases of the virus that have so far been reported
to the WHO since it was first detected in the middle of last year, with six previous
fatalities — three in Saudi Arabia, two in Jordan and one in Britain.
The news comes just days after a person suffering from the virus died in hospital
in central England on Sunday.
That patient, who had a pre-existing health condition, was one of three people in
the same family with the virus, which appeared to have been caught by one of the
family members during a recent visit to the Middle East and Pakistan.
First known deaths from H7N9 bird flu in China
BEIJING – Two Shanghai men have died from a lesser-known type of bird flu in the
first known human deaths from the strain, and Chinese authorities said it wasn’t
clear how they were infected but there was no evidence of human-to-human
transmission. A third person, a woman in the nearby province of Anhui, also
contracted the H7N9 strain and was in critical condition.
H7N9 bird flu is considered a low pathogenic strain that cannot easily be contracted
by humans. The overwhelming majority of human deaths from bird flu have been
caused by the more virulent H5N1, which decimated poultry stocks across Asia in
2003.
The World Health Organization is “closely monitoring the situation. There is no
evidence of human-to-human transmission, and transmission of the virus appears to
be inefficient, therefore the risk to public health would appear to be low.
The 87-year-old victim became ill on Feb. 19 and died on Feb 27. The other man,
27, became ill on Feb. 27 and died on March 4. Scientists have been closely
monitoring the H5N1 strain of the virus, fearing that it could mutate into a form that
spreads easily among people, potentially sparking a pandemic. So far, most human
cases have been connected to contact with infected birds.
What would you do?
SALMON, Idaho (Reuters) - A federal judge on Monday denied a request from
environmentalists to halt the execution of buffalo at Yellowstone National Park, a
ruling that clears the way for scores of buffalo to be shipped to slaughter.
More than 500 buffalo, or bison, have been corralled in recent weeks in the park
after their search for food caused them to wander from the park into Montana,
where ranchers fear the bison will transmit brucellosis to cows. The disease can
cause cows to miscarry their young.
Federal and state agencies that oversee the nation's last purebred herd of wild
bison planned to kill straying bison when conservation groups sued to stop the
slaughter. Government officials have been testing the penned bison for exposure
to brucellosis as they awaited the court's decision. By Friday, testing had
revealed 137 of the 513 corralled buffalo had the antibodies for the bacterial
disease. The park plans to kill the 137 and any other bison that have been
exposed to brucellosis.
Environmental groups said Monday they plan to appeal the ruling by U.S. District
Judge Charles C. Lovell. There are some 3,700 bison in Yellowstone.
How do you stop transmission from bison to cattle and vice versa?
Hoof and Mouth Disease
Mad Cow disease
Avian malaria
Duck
Plague
Amphibian decline
and deformations
Plague
Chronic Wasting
Disease
Bovine tuberculosis
500 people are infected with Ebola virus in the Congo. What do you do?
50,000 people are infected with P. falciparum in Uganda in the midst of
a civil war. What do you do?
50,000 people are disabled (physically and mentally) in BC. What do
you do?
Populations are increasing everywhere, creating conditions in which
disease can flourish. Should some groups not be able to have children?
Should any regulations be in place to reduce family size?
Is any individual worth more than another?