http://www.nature.com/news/2005/051121/full/051121-7.html
News
Published online: 23 November 2005; |
doi:10.1038/news051121-7
Most malaria affects 'unlucky few'
Some people are more susceptible because of their
smell or location.
Michael Hopkin
Most cases of malaria hit a relatively small
proportion of a given population, according to an
analysis of health data from sub-Sarahan Africa.
Identifying these high-risk people could lead to
better use of public-health resources, say the authors
of the study.
Just 20% of people in a given area account for 80% of
all infections, says a research group led by David
Smith of the US National Institutes of Health in
Bethesda, Maryland. The next problem is to work out
just who they are.
There are some obvious factors that make people more
susceptible, such as living in poor homes without
nets, close to stagnant ponds where mosquitoes tend to
breed, Smith says. But other factors, such as the
chemicals given off in breath, sweat and dirty linen
may also play a role that is not yet well understood.
"People often say, 'I'm one of those people that
mosquitoes love to bite'," Smith adds. "Now we've
shown that it's worth taking this biting pattern
seriously."
The analysis, which uses existing data on malaria
incidence in more than 90 populations of African
children, shows the extent of this person-to-person
variation. And the results, reported in Nature1, could
help to direct efforts to combat malaria, Smith
argues. "If we can find these people we can target
them," he says.
Weak links
Malaria currently infects some 300 million people a
year, mostly in sub-Saharan Africa. It has proven
difficult or impossible to distribute enough drugs,
vaccines and mosquito-proof bednets to everyone at
risk.
Smith argues that identifying the most vulnerable
should help to concentrate resources where they are
needed. But others argue that the best way to do this
is to target efforts at those who present themselves
with malaria, rather than searching for a mysterious
sub-section of the population who are most at risk.
The patterns spotted by Smith's group might be useful
in urban areas, says Charles Delacollette, a
coordinator in the World Health Organization's Roll
Back Malaria Department. Here, the at-risk 20% will
probably be identified as those living in poor
housing, close to known sites of mosquito breeding, he
suggests.
But in remote villages, he argues, the main problem is
simply getting supplies to the villagers at all, let
alone determining the 20% within each community who
are most at risk. "Those most at risk are those who
cannot access health services. This is where the major
problem is," he says.
Smith counters that the most effective protections
against malaria are expensive. So identifying the
people most in need of it will be important in
reducing the overall public-health burden of malaria,
he argues. "If we get these people right we'll be
protecting everyone else too."
References
Smith D. L., Dushoff J., Snow R. W.& Hay I..
Nature, 438. 492 - 495 (2005). | Article |
<http://www.nature.com/doifinder/10.1038/nature04024>
http://www.nature.com/news/2005/051121/full/051121-7.html
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Archive Number 20050327.0884
Published Date 27-MAR-2005
Subject PRO> Trypanosomiasis, foodborne - South
America (03)
TRYPANOSOMIASIS, FOODBORNE - SOUTH AMERICA (03)
***********************************************
A ProMED-mail post
<http://www.promedmail.org>
ProMED-mail is a program of the
International Society for Infectious Diseases
<http://www.isid.org>
Date: Sun 27 Mar 2005
From: Luiz Jacintho da Silva
<luiz_jacintho(a)uol.com.br>
Source: Fiocruz.br [translated from Portuguese by
Mod.LJS, edited]
<http://www.fiocruz.br/ccs/novidades/dez04/chagas_fer.htm>
Study confirms a different form of Chagas?
disease transmission
-----------------------------------------------
When one hears of the transmission of the
protozoan _Trypanosoma cruzi_, the 1st thing that
comes to the mind is the Reduviid bug vector.
This insect, when biting an individual,
eliminates parasite-contaminated feces that, as
the individual scratches himself, penetrates
through abraded skin or mucosa [intact]. This is
the most common route, but the occurrence of
certain outbreaks has led scientists to consider
the possibility of oral transmission of _T.cruzi_.
A study by the Goncalo Muniz Research Center, a
FIOCRUZ unit in Bahia, presents evidence that
this protozoan can indeed be transmitted by the
ingestion of contaminated food [or beverages]. On
Sun 20 Mar 2005, an episode in Santa Catarina
confirmed the results of the research.
The deaths of 3 [now 5] persons, and the report
of another 15 cases of the disease in the state,
have led health authorities in the state to
prohibit the sale of sugarcane juice. The
infection seems to have been transmitted by this
beverage.
In 1968, a member of a farming community in
Teotonia (Rio Grande do Sul) fell ill with a
severe febrile illness. No one suspected Chagas
disease, as there were no Reduviid bug vectors in
the local dwellings. After much investigation,
food borne transmission through vegetables served
in the common dining-room was accepted as the
cause. Possibly, marsupials had contaminated the
vegetables by droppings from their anal glands,
usually rich in _T.cruzi_.
Years later [1986], another outbreak occurred,
this time in Catole do Rocha (Paraiba). A large
wedding anniversary party had been held on a
farm. Days later, some of the guests fell ill
with a febrile illness, similar to the one in
Teotonia. Sugarcane juice and other foodstuffs or
beverages had been contaminated by animal
droppings. The couple that had celebrated their
50th anniversary eventually died as a consequence
of Chagas disease acquired at the party.
Recently, oral transmission of _T. cruzi_ has
been responsible for small outbreaks in the state
of Para. Entire families acquired Chagas disease
in the absence of insect vectors in their homes.
These insects live in palm trees and, attracted
by the lights, fall into the machinery used to
grind acai to obtain juice (much the way
sugarcane juice is obtained). The insects are
ground and contaminate the juice with _T. cruzi_,
transmitting the infection to those drinking it.
All of these cases in Rio Grande do Sul, Paraiba
and Para attracted the attention of scientists.
The question which was brought forward was why
didn't gastric juice destroy _T. cruzi_? The
physician Sonia Gomes Andrade from CPqGM, who
specializes in experimental pathology, formulated
an experiment:
- We introduced _T. cruzi_ through a tube
directly into the stomach of mice, she says. She
studied the strains isolated from cases in
Paraiba and Para.
- We observed that the parasite not only survived
gastric juice but was able to infect the mice.
They developed disease identical to the one
produced when mice are infected directly via the
bloodstream. The findings of the researcher
confirm that _T. cruzi_, when swallowed, can
cause infection through the digestive system.
- In the most common manner of infection,
scratching, _T. cruzi_ is able to penetrate the
skin. [It is easy to] imagine [that], when
someone ingests a large quantity of parasites,
the [infection] will be more severe, says Sonia.
The researcher stated that there is no reason to
stop eating vegetables or drinking sugarcane or
acai juice.
- The problem lies not with the kind of food but
with _T. cruzi_. These are mainly parasites of
biodem type III -- from wildlife -- that are able
to cause infection through the digestive system.
A biodem is a group of [genetically close]
_T.cruzi_ strains with similar characteristics.
Biodem type III strains are, as a rule, highly
pathogenic. In the experiment, a strain of this
type showed a higher capacity to cross the
gastric barrier and cause infection in mice than
strains from biodems I and II.
- It is significant to point out that biodem III
strains are associated with wildlife disease
cycles, causing outbreaks in places where
Reduviid bugs are not domiciled.
--
ProMED-mail
<promed(a)promedmail.org>
[It seems beyond doubt that _T. cruzi_ can infect
hosts, including humans, by the oral route, but
it still remains unclear how important this route
of transmission is for maintaining infections in
humans and causing outbreaks like the present
one. The mouse experiment cited above confirms
previous reports [cited in ProMED-mail,
Trypanosomiasis, foodborne - South America
20050324.0847; Hoft et al. Infect Immun
1996,64:3800-10] that _T. cruzi_ can indeed
infect the host through the gastric mucosa. The
key question is how long a time _T. cruzi_ can
survive in feces outside the triatomid bug, and
how long a time in different forms of fluids
after the triatomid bugs have been crushed. Oral
transmission, either from bug feces or from
crushed bugs, presumes that _T. cruzi_ can
survive for at least several hours, possibly
days, in juice. Information on this survival time
is key to determining whether the present
outbreak is a rare coincidence or just part of a
more widespread problem. - Mod.EP]
[see also:
Trypanosomiasis, foodborne - South America (02)
20050325.0870
Trypanosomiasis, foodborne - South America
20050324.0847
1998
----
Trypanosomiasis - Brazil: RFI 19980306.0426
1997
----
Chagas disease - Latin America 19970114.0066
Chagas disease vector (05) 19970118.0105
1996
----
Trypanosomes, New World, Symposium - Guyana 1996
19960830.1493]
...............ep/msp/lm
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