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Memórias do Instituto Oswaldo Cruz
Fundação Oswaldo Cruz, Fiocruz
ISSN: 1678-8060 EISSN: 1678-8060
Vol. 102, Num. 5, 2007, pp. 541-547
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Memórias do Instituto Oswaldo Cruz, Vol. 102, No.
5,
2007, pp. 541-547
The leishmaniases - survival and expansion
in a changing world. A mini-review
Jeffrey Shaw
Departamento de Parasitologia, Instituto de Ciências Biomédicas, Av. Prof. Lineu Prestes 1374, Cidade Universitária, 05508-900 São Paulo, SP, Brasil
E-mail: jeffreyj@usp.br
Received 11 June 2007
Accepted 20 July 2007
Code Number: oc07086
The number of cases of visceral and cutaneous leishmaniasis is increasing globally at an alarming rate irrespective of the region and the leishmaniases are amongst the top emergent diseases in spite of control measures. In the present review attention is drawn to some of the reasons for this. The leishmaniases have expanded beyond their natural ecotopes due to the ecological chaos caused by man and this in turn affects the levels of his exposure to the vectors. Examples of how different phenomana (such as war, civilian migration, immuno-suppression caused by medication and viral infections, globalization of work and leisure and transmission outside endemic areas) contribute to the spread and increase of the disease are discussed.
Key words: cutaneous - visceral - leishmaniasis - war - social stress - travel - migration
The leishmanias have evolved over many millions of years under selective pressures that depended on natural ecological changes. These may have been rapid such as the havoc caused by storms, flooding, hurricanes, and volcanic eruptions that disrupted host and vector relationships. Or they could have been slower due to global climatic changes, such as the ice ages, in which vast amounts of water were locked away leading to drastic reductions in rainfall and the consequential extension of arid regions. Or even slower due to the movement of tectonic plates resulting in the separation of enormous land masses or the formation of gigantic mountain barriers. However, none of these situations approaches the speed and extent of the ecological damage wrought by man during the past 2000 years culminating in the devastation of enormous areas of natural habitats in periods as short as 50 years, global warming and the rapid movements between different ecotopes, at lightening speeds due to the transportation revolution. In the present paper I plan to use different examples to illustrate how the epidemiology of leishmaniasis has expanded into territories that are no longer related to natural phenomena but to man's own activities.
If we take a look at the recent history of the Leishmania there is are points in time at which man inadvertently domesticated some species, as he did other animals during the course of his social evolution. There seems to be no doubt that all Leishmania species have a zoonotic origin. Thus the first changes associated with man are the adaptation of L. (Leishmania) donovani and L. (L.) tropica to anthroponotic cycles, which in terms of the evolution of the genus must have been an extremely recent event. Some epidemiological observations indicate that in some regions L. (L.) tropica is still a zoonosis and experimental evidence (Kamhawi et al. 1995, Svobodova et al. 2003) suggests that rodents could be reservoirs.
During the last 20 years or so we have seen an increase in the number of cases of all forms of leishmaniasis throughout the world and this has led to it being considered by many experts as an emergent disease in some areas and in others as a re-emergent one (Ashford 2000). Attention has been drawn to many factors that contribute to the expansion of the leishmaniases and Dujardin (2006) considered that environmental changes, immune status, and treatment failure constituted the three major risk factors responsible for the emergence, remergence, and spread of the disease. There is no single reason for this and it varies from region to region. In some cases it is the stress and upheaval caused by civil war, such as in the Sudan (Ashford et al. 1992, Moszynski 2002) and in others it is uncontrolled migration to urban areas due to economic pressures (Dye & Williams 1993) or again war (Hewitt et al. 1998, Rowland et al. 1999). The number of cases in professionals has also been increasing in recent years, especially in members of the armed forces who are sent to endemic areas (Hepburn et al. 1993, Hyams et al. 1995). Air travel is now much cheaper and many thousands of holiday makers in searching for the sun or exciting places end up in endemic areas and contract leishmaniasis (Manfredi et al. 2001, Scope et al. 2003). There are also indications that global warming is changing the geographical distribution of vectors which could lead to transmission in hitherto non endemic regions (Kuhn 1999, Peterson & Shaw 2003).
A
POSSIBLE COMMON FACTOR INFLUENCING OVERALL INCREASES IN CASES OF
LEISHMANIASIS
Is there
a common group of factors in all the above events? In my opinion
there is and it is an infinite variety of different reservoir/man
vector contacts which modulate the level of immunity, determining
whether or not there will be clinical disease. This latter concept
is extremely important because there is a growing amount of evidence
indicating that parasites persist for life (le Fichoux et al. 1999,
Riera et al. 2004) and that due to some breakdown in the immune
system disease may result at a later date (Walton et al. 1973).
Sand fly biting rates may vary from extremely sporadic to almost
constant depending on the region and the habits of the individual
can also be important risk factors (Caldas et al. 2002). It is has
been shown experimentally in mice that the bites of uninfected sand
flies can infer protection (Kamhawi et al. 2000) against subsequent
infection. This suggests that high levels of sand fly bites in endemic
areas could be protective and result in asymptomatic infections
in man. Thus given the globalization of everything from commerce
to war it is quite conceivable that people from all walks of life
can find themselves in situations where their first experience to
sand fly bites could include infected ones. Examples of some such
situations are given below.
SOLDIERS
IN TRAINING AND WAR
Members
of the armed forces are perfect targets for the hungry infected
sand fly when they are sent to tropical regions. Most are residents
of countries or regions where leishmaniasis is not endemic. Brazilian
soldiers are sent on the jungle war training course near Manaus,
Amazonas and over the years many have become infected (de Oliveira
Guerra et al. 2003). There are, however, other records of similar
types of infections in such regions such as Belém, Pará
(Silveira et al. 2002) and Pernambuco (Brandão-Filho et al.
1998). The American Army's jungle training centre in Panama (Takafuji
et al. 1980, Sanchez et al. 1992) has similarly resulted in cases
of cutaneous leishmaniasis amongst troops for many years.
However,
soldiers are now becoming infected in war zones in different regions
of the world. There was an outbreak of zoonotic cutaneous leishmaniasis
in 36/80 Jordanian soldiers involved in a mine sweeping operation
after the Arab-Israel conflict (Jumaian et al. 1998). In the Desert
Storm war American personnel were infected with a parasite identified
as L. (L.) tropica that produced visceral symptoms (Magill
et al. 1994). Because of this all soldiers involved in the campaign
were banned as blood donors for one year. This raises important
questions related to blood donors and the risk of transmission via
blood transfusions. There is evidence that patients cure and that
their blood can become parasite positive again after 1 to 30 years
(Guevara et al. 1993, Delgado et al. 1996) as well as the reactivation
by immunosuppressive drugs (Scatena et al. 2003). Similarly the
fact that there is strong evidence that transmission of visceral
leishmaniasis (VL) amongst HIV positive drug users in Europe (Desjeux
& Alvar 2003) is by contaminated syringes again indicates the
danger of contaminated blood. These examples emphasize that there
must be asymptomatic cases of leishmaniasis in non-endemic areas
and at present blood banks do not screen for leishmanial infections.
CIVILIANS
IN WAR AND SOCIAL UNREST
There
are many examples of how war and social pressures can results in
outbreaks that reach epidemic levels. One of the more recent ones
of almost unimaginable proportions was the epidemic of VL in the
Southern Sudan, known as the Western Upper Nile (WUN) region (Seaman
et al. 1996). People found themselves in the middle of a civil war
which impeded diagnosis and treatment. In trying to escape they
moved to new areas where there was no food and this led to high
levels of malnutrition. It has been estimated that from 30 to 40
thousand people died in this epidemic (Davidson & Croft 1992)
and that it is still rampant (Moszynski 2002). One of the alarming
consequences of this besides the high death rate is that it has
been estimated that 50% of the treated cases develop post dermal
kala-azar (PKDL) which normally cures spontaneously (Zijlstra et
al. 2003). However, such cases could serve as reservoirs of infection.
Less
spectacular, but equally disturbing are the outbreaks in the poorer
slum regions of some Brazilian towns such as Santarém, Pará
and Terisinha, Piauí. Although the disease is endemic in
the rural areas of these regions it was not considered to be an
urban disease. There are, however, now records of VL in other cities
such as Belo Horizonte, Minas Gerais (Silva et al. 2001) and Araçatuba
, São Paulo (Camargo-Neves et al. 2001) The threat of the
urbanization of VL in many other regions of Brazil is real. The
reasons are unknown but it is most likely associated with an adaptation
of the principal vector, Lutzomyia longipalpis, to the urban
environment.
During
war civilians become fugitives in their own country or end up as
refugees in neighbouring countries where they live precariously.
The war in Afghanistan was no exception to this phenomena and the
result were epidemics of L. (L.) tropica in Kabul (Ashford
et al. 1992) and in refugee camps in Pakistan (Rowland et al. 1999).
IMMUNODEFICIENT
INDIVIDUALS
Another
scenario that we are presently witnessing is infections in immuno-suppressed
individuals. This suppression may be due to viral infections, such
as HIV, which have led to an increase in the number of recorded
cases of VL in countries where the disease was rare such as France,
Italy, Spain, and Portugal (Desjeux & Alvar 2003). Epidemiological
evidence indicates that within this group transmission is highest
amongst drug addicts due to contaminated syringes. There are ever
increasing numbers of organ transplants throughout the world whose
recipients are obliged to take immunosuppressive drugs for the rest
of their lives. The risks to such individuals are multiple. Clinical
leishmaniasis may result from the activation of occult infections
in patients undergoing organ transplants (Berenguer et al. 1998,
Morales et al. 2003), contaminated organs or higher levels of susceptibility
to natural transmissions.
Besides
exacerbating occult infections immunosuppression can alter the clinical
symptoms more commonly associated with a particular species of Leishmania.
For instance in HIV patients L. (Viannia) braziliensis can
cause visceral infections (Silva et al. 2002) or disseminated skin
lesions (Coura et al. 1987). Transmission levels may also increase
when the vector feeds on immuno-depressed individuals with high
parasitaemias (Molina et al. 2003).
GLOBILIZATION
AND BLOOD TRANSFUSION
The
globalization of world commerce has led to an unprecedented movement
of goods and individuals between countries and continents. Inevitably
this is associated with members of professional groups ranging from
company employees, who may or may not be accompanied by their families,
to servicemen and sometimes their families spending varying periods
of time in endemic leishmaniasis areas. Besides this there is also
an intense exchange of tourists in search of exciting holidays in
tropical countries. A literature search reveals many cases of leishmaniasis
associated with such events and it is not my intention to review
them in detail. However, I will use one or two examples to emphasize
how globalization is changing the distribution of the disease and
the dangers associated with this.
One
of the gravest clinical problems is the occurrence of leishmaniasis,
especially the visceral form, in regions where health workers do
not expect it to occur such as say Scotland. Many years ago there
was a case of a Scottish mother who after having a caesarean operation
began to loose weight accompanied by periods of fever. Miraculously
she was diagnosed as having VL but how could a women who had never
left Scotland have contracted the disease? A clinical inquiry revealed
that she had had a blood transfusion during surgery and they traced
the donor as being an Asian seaman who was in transit. Of 506 healthy
French blood donors 76 had a positive leishmanial serology in a
specific Western blot test and parasites were isolated from two
(le Fichoux et al. 1999).
The
above examples point to a serious problem transmission during blood
transfusion. There are only a few cases registered in the literature
but the danger is real and increasing. No blood bank screens for
leishmanial antibodies and even if it did the level of efficiency
of detecting healthy carriers with any of the existing tests is
to say the least dubious. In the absence of a suitable test one
measure is to eliminate any individual who has visited an endemic
area even during their holidays but what about those who live in
endemic areas? In Brazil there was a chance in the past that at
least some leishmaniasis carriers would be detected by the obligatory Trypanasoma cruzi screening test. However, the specificity
of the latter test has been improved to eliminate cross reactions
with such diseases as leishmaniasis! The question is how does one
deal with this problem which effects highly populated regions of
South America and the Mediterranean region? Treatment of the blood
with a preservative is a possibility that may have to be considered.
Of 1194
volunteer blood donors in Natal, Rio Grande de Norte, Brazil, 9%
were serologically positive for VL, increasing to 25% in a periurban
kala-azar focus (Luz et al. 1997). However, 37% of the multiply
transfused hemodialysis patients for the same city were also positive
whereas in Rio the VL antibody prevalence was 7% in the same patient
group. This strongly suggests that transmission is occurring via
blood transfusions.
One
can only conclude that many people are probably becoming infected
via blood transfusions. In countries, such as Brazil, where leishmaniasis
is endemic these cases would probably go unnoticed especially in
endemic areas.
DOGS
AND TRAVEL
Another
factor associated with travelling families is that when ever possible
they take their dogs with them and long periods of quarantine between
trips are no longer necessary with the advent of more efficient
vaccines for canine diseases and animal health passports. The incubation
period of canine visceral leishmaniasis (CVL) is variable and clinical
symptoms can appear after leaving the endemic zone. This means that
infectious dogs may go unnoticed for long periods and could serve
as sources of infection for sand flies and blood based products
or mechanical transmission.
In the
US there are both imported and autochthonous cases of CVL. Cases
that are considered to be autochthonous have been recorded in Oklahoma
in 1980; Kansas in 1982; Ohio in 1988; Michigan in 1989; and Texas
and Alabama in 1991. In a survey of 11,010 dogs from Hunt Club Kennels
performed between 200 and 2001 by CDC a total of 71 animals from
21 states had positive IFAT antibody titres. In this survey parasites
were isolated from dogs from the states of New York, Michigan, Virginia,
Kentucky, Alabama and Georgia and the Canadian State of Ontario
(Enserink 2000). So far there is no evidence of sand fly transmission
but they are present in Texas and some eastern states. It was thought
that sand flies did not occur in Upper New York state where the
most recent US outbreak of CVL occured. However, Lu. vexator was found (Ostfeld et al. 2004) to be abundant in the neighbourhood
of the Millbrook Kennel and must be considered as a potential vector.
Ever since the first recorded outbreak in 1980 in Oklahoma the methods
of transmission have been surrounded by mystery. Clearly vector
transmission must be considered especially as sand flies are known
to occur in the eastern states were many cases have been recorded.
Besides this though it is possible that different methods of mechanical
transmission may occur. Amongst those that must be considered are
biting, tattooing, contaminated needles and blood transfusion. Owens
et al. (2001) noted that 3/7 foxhounds who had received packed red
blood cells from seropositive dogs became infected. They concluded
that foxhounds should not be used as blood donors.
In the
recent out break in upper New York state infections were only found
in foxhounds and not in Beagles or Basset hounds from the same kennel.
The association of VL with foxhounds raises a number of points.
Firstly these dogs travel and could be exposed to sand fly bites
on such occasions. Besides this though they are gregarious and often
kennelled in large numbers that result in fighting. It could well
be that all these factors, as well as the stress of hunting, contribute
to them being the race in which most cases of VL have been recorded.
The problem is not limited to the US. A study in Holland (Slappendel
1988) showed that 92 dogs imported from the Mediterranean area were
infected but there were also two autochthonous cases. One of the
latter cases was the puppy of a bitch who had become infected in
Spain (Diaz-Espineira & Slappendel 1997). The review by (Slappendel
& Teske 1999) should be consulted for a fuller review of the
CVL records outside the endemic areas. All these observations raise
the question as to whether or not such transmission is occurring
in endemic areas? For instance many of the social characteristics
of foxhounds are similar to those of stray dog packs that are common
in endemic areas of Latin America.
Perhaps
no other country in the world has invested more money in attempting
to control CVL than Brazil. However, all their efforts have failed
for many different reasons but one of the most important lessons
that can be learnt from these failures is that the elimination of
infected dogs is neither effective nor socially acceptable. Of the
presently available methods those that kill sand flies that feed
on infected dogs, such as insecticide impregnated collars (Killick-Kendrick
et al. 1997), are socially the most acceptable and economically
feasible. Field trials in Iran (Gavgani et al. 2002) showed that
seroconversion rates for visceral leishmaniasis antibody was lower
in the inhabitants and dogs of the villages were collars had been
used. Observations are still required to define how long collars
must be used to eliminate transmission in an endemic area, the cost
and the effect of wild dogs. It must be remembered that there is
no point in beginning any program unless it is financially sustainable
and that a vaccine for CVL would almost certainly be more cost effective.
TRANSMISSION TO MAN OUTSIDE ENDEMIC REGIONS
On diagnosing an autochthonous case near Aachen,
Germany, Bogdan et al. (2001) reviewed the literature
and noted that there were sporadic cases in Central and
Northern France (Guilhon 1965) as well as in Switzerland
(Mazzi 1976) and Austria. The interesting point
raised by these cases is that it was impossible to determine
how these individuals had become infected. The
occurrence of infected dogs is not uncommon in these
areas (see section on Dogs and Travel) and some mechanical
method, such as contamination from infected
dog skin or urine, cannot be ruled out. Amastigotes have
been found (Riera & Valladares 1996) in the urine and
semen of experimentally infected dogs and it is conceivable
that both could be sources of contamination for other
animals including man himself. There is an embarrassing
record of horizontal transmission between a married
couple (Symmers 1960). Initially syphilis was suspected
but the husband said this was impossible since he
had had no sexual relationships outside his marriage. Unfortunately
his wife did not believe him and their marriage
ended in divorce in spite of the final diagnosis of
cutaneous leishmaniasis. In the case of VL it is possible
that transmission could occur during intercourse from
infected semen (Riera & Valladares 1996).
Vertical transmission in man appears to be rare. An
interesting case was recorded (Meinecke et al. 1999) in
a 15 month old boy whose mother, during her holidays,
had visited endemic visceral leishmaniasis areas of Europe
several times before her pregnancy. She had a positive
skin test and specific antibodies were demonstrated
in a Western blot reaction. She had never had any clinical
symptoms and the authors concluded that most probably
she had a sub-clinical infection that was reactivated
during her pregnancy and it was this that resulted in the
congenital transmission. The authors considered that this
method of transmission is rare and could only find eight
other published cases. However, these occurred in endemic
regions but in areas that were considered as unsuitable
for the vector.
However, sand fly transmission must always be considered,
even in non-endemic regions. In Switzerland
Phlebotomus perniciosus, a known VL vector, is common.
It also occurs in France as far north as the regions
of the Pays-de-Loire and the Ile-de-France, in which
Paris is situated, and P. mascittii has been found in the
department of Calvadas that borders the English Channel
(Rioux & Golvan 1969). More recently P. mascittii was found in the region Aachen region, Germany (Naucke& Pesson 2000). Although this species has never been
implicated as a vector it must be considered as a potential
one. It is quite possible that other sand flies, besides
the classical ones, could be vectors in certain situations.
Thus in Colombia it has been shown (Travi et al. 1990)
that Lu. evansi is an important VL vector in the absence
of Lu. longipalpis.
CHANGES
IN THE BEHAVIOR OF THE SAND FLY VECTORS
Transmission
of Leishmania by sand fly species other than those that are
associated with the feral enzootic cycle may occur. For instance
if an infected mammal enters an area where there is a susceptible
sand fly the latter will take over the function of the classical
vector. Volp nad Myskova (2007) reviewed this phenomenon and called
such sand flies permissive vectors. Such a sand fly in Brazil is Lu. longipalpis which is a potential vector of Leishmania such as L.(L.) amanzonensis and L.(V.) braziliensis. Lu. flaviscutellata has recently been found infected (Fouque
et al. 2007) with L.(V.) guyanensis and infections of L.(L.) amanzonensis have been found (Tolezano et al. 2007) in
dogs from a CVL region where the principal sand fly was Lu. longipalpis. The latter suggests that Lu. longipalpis may be transmitting
two Leishmania or that Lu. flaviscutellata is changing
is habits. What is emerging are infections in mammals and
sand flies that do not follow the well studied sylvatic enzootic
cycles. In the past such events happened and it is ironical that
the sand flies that became domesticated also became vectors of both
visceral and cutaneous parasites.
In Brazil
there are records of human cases of VL in areas considered to be
non-endemic that have baffled the public health authorities. Of
these two from Greater São Paulo (Iversson et al. 1979, 1982)
are particularly important because there was evidence of sub-clinical
infections in children from the same neighbourhood as one of the
sick children. Neither Lu. longipalpis nor infected dogs
were found but the authors considered that the epidemiological data
suggested some form of natural transmission. Very recently a naturally
infected cat was found (Savani et al. 2004) in Cotia which is close
to São Paulo city and so far Lu. longipalpis has not
been found. The absence of Lu. longipalpis suggests that
transmission may involve other sand fly species.
A general
picture is gradually emerging that suggests that transmission may
not necessarily be associated with the accepted vector species.
Further epidemiological and experimental data is needed to confirm
this in the areas where accepted vectors are absent but clinical
cases occur.
THE
FUTURE
It is
difficult to predict what will happen but the events reviewed in
this paper indicate that leishmaniasis is becoming a global disease.
There is now evidence that both vertical and horizontal transmission
by mechanical means are occurring for VL. Because of this the risk
of infection outside the endemic areas is increasing. However, the
possibility of sand fly transmission must always be considered.
The challenge is how to control so many different Leishmania that have enzootic cycles? The recent finding (Karunaweera et al.
2003) that the aetiological agent of cutaneous leishmaniasis in
Ceylon is L. (L.) donovani in the absence of the visceral
disease raises, yet again, the question of whether or not there
are animal reservoirs of those Leishmania that are classically
considered to have none.
Only
when we have a better understanding of the genetic diversity of
the Leishmania and the reservoirs involved in each species'
enzootic cycle will it be possible to evaluate which control method
or methods are most likely to succeed.
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