Summary
Leishmania donovani is the causative agent of historically
anthroponotic visceral leishmaniasis (VL) on the Indian subcontinent
(ISC). L. donovani is transmitted by the sand fly speciesPhlebotomus argentipes . Our collaborative group and others have
shown that sand flies trapped outside in endemic villages have fed on
cattle and dogs in addition to people. Domestic animals are reservoirs
for L. donovani complex spp., particularly L. infantum , in
other endemic areas. Multiple studies using quantitative PCR or
serological detection methods have demonstrated that goats, cattle, rats
and dogs were diagnostically positive for L. donovani infection
or exposure in eastern Africa, Bangladesh, Nepal and India. There is a
limited understanding of the extent to which L. donovaniinfection of domestic animals drives transmission to other animals or
humans on the ISC. Evidence from other vector-borne disease elimination
strategies indicated that emerging infections in domestic species
hindered eradication. The predominant lesson learned from these other
situations is that non-human reservoirs must be identified, controlled
and/or prevented. Massive efforts are underway for VL elimination on the
Indian subcontinent. Despite these herculean efforts, residual VL
incidence persists. The specter of an animal reservoir complicating
elimination efforts haunts the final push toward full VL control. Better
understanding of L. donovani transmission on the Indian
subcontinent and rigorous consideration of how non-human reservoirs
alter VL ecology are critical to sustain elimination goals.
Introduction
Leishmaniases are a spectrum of sand fly-borne infectious diseases of
humans spanning from localized cutaneous lesions to systemic disease.
The causative agents of Leishmaniases are more than 20 species of the
protozoan parasite genus Leishmania (Trypanosomatida :Trypanosomatidae ) (Burza et al., 2018, Espinosa et al., 2018).Leishmania spp. are often characterized as ”Old World”; found in
Eurasia, Africa and the Mediterranean basin, or ”New World”; found in
the Americas, based on their geographical distribution (McMahon‐Pratt
and Alexander, 2004, Reithinger et al., 2007). Old World Visceral
Leishmaniasis (VL) is caused by L. donovani and L.
infantum . New World VL is primarily caused by L. infantum(Marcili et al., 2014, Shaw, 2006, Monge-Maillo and López-Vélez, 2013).
The epidemiology of leishmaniasis is based on ecological interactions
between human and reservoir systems, parasite and sand fly species,
regional characteristics of endemic areas, present and past
susceptibility of human and animal populations and parasite and human
behavior (Ready, 2008, Oryan and Akbari, 2016, Patz et al., 2000, Ready,
2013, Lukeš et al., 2007). The Leishmania life cycle is directly
associated with ecological variables in rural or peri-domestic
environments that shelter sand fly niches and reservoir hosts in
conjunction with human living areas (Daszak et al., 2001). Leishmaniases
are found across tropical, equatorial areas, with warm and rainy
climates abetting reproduction of Phlebotomines (Fischer et al.,
2011). Human activities such as migration, deforestation, unorganized
urbanization or shifts in infection vulnerability caused by
immunosuppression and malnutrition are reflected in significant
increases in human susceptibility to leishmaniasis (Alvar et al., 2008,
Desjeux, 2004, Thakur, 2000).
Anthroponotic spread of Leishmania and elimination
strategy of VL on the ISC.
The World Health Organization listed leishmaniasis in category I,
Emerging or Uncontrolled Diseases. Although leishmaniasis is endemic in
over 98 countries and territories, eco-epidemiological hotspots for
visceral leishmaniasis (VL) occur in East Africa, the Indian
sub-continent (ISC), and Brazil. More than 50% of global VL incidence
has been reported in India, Nepal and Bangladesh (Burza et al., 2018,
WHO, 2020). VL is caused by L. donovani complex spp. protozoan
parasites and manifest epidemiologically by two types of etiologies(Figure 1) : Anthroponotic visceral leishmaniasis (AVL), where
parasites are transmitted by sand fly bites from human to human and
occurs mainly on the ISC and in East Africa; and zoonotic visceral
leishmaniasis (ZVL), which is transmitted from infected animals to
humans, with domestic dogs as the primary reservoir host and occurring
in Mediterranean countries, the Middle East, Asia, and South America(Figure 2) (Desjeux, 2004, Esch and Petersen, 2013). Despite
these geographic separations, parasites within the L. donovanicomplex have a high level of genetic similarity, estimated to be greater
than 99% (Franssen et al., 2020). This indicates that there are not
likely to be DNA-encoded reasons for host-preference differences betweenL. donovani and L. infantum . Instead, theLeishmania life cycle is directly impacted by ecological
variables in rural or peridomicile environments that shelter sand fly
niches and reservoir hosts in proximity to humans. Leishmania
donovani infection on the ISC has traditionally been recognized as an
anthroponotic disease with a continuous cycle of transmission in and
around human dwelling inhabited by vector sand flies.(Stauch et al.,
2011, Malaviya et al., 2011). Active VL patients are thought to be the
primary reservoir for AVL on the ISC (Figure 3) , based on the
clustering of cases around households with past VL history (Barnett et
al., 2005, Bern et al., 2005, Das et al., 2016).
National programs to eliminate VL were initiated by the governments of
India, Bangladesh, and Nepal in 2005 as a major public health problem on
the ISC. The goal of these programs was to reduce the incidence of VL at
the sub-district or district level to less than 1 VL case per 10,000
population per annum by the 2015 (WHO, 2005). A key strategy of the VL
elimination agenda has been systematic Indoor Residual Spraying (IRS),
targeting sand flies feeding on people within their homes, asPhlebotomus argentipes is the only recognized L. donovaniinsect vector on the ISC. This serves as one of the prongs in a
three-pronged approach; in addition to early active case identification,
and comprehensive treatment of clinical cases to eliminate VL (Huda et
al., 2012, Mondal et al., 2009). Although IRS campaigns have made
considerable progress in reducing VL in India, Nepal, and Bangladesh,
they fell short of the 2015 elimination target and are not on track to
meet the extended deadline set for 2020 (Programme, 2017, Rijal et al.,
2019). It is possible that these shortcomings are influenced by zoonotic
sources of transmission, which to date have not been considered in
intervention strategies on the ISC.
Challenges of elimination and possible infection sources on
the ISC.
An assumption behind the elimination campaign is that only symptomatic
human VL and post kala-azar dermal leishmaniasis (PKDL) cases are
infectious to P. argentipes on the ISC (Figure 3).
However, this hypothesis has not been completely evaluated, and is
challenged by the possible existence of latent, asymptomatic human
reservoirs of the parasite and potential animal reservoirs (Stauch et
al., 2011, Molina et al., 2020, Singh et al., 2014). Asymptomatic VL
cases are much more common than clinical cases, as shown in highly
endemic villages in India and Nepal (Ostyn et al., 2011, Gidwani et al.,
2011). Because of this high frequency, the role of asymptomatic
individuals in VL transmission should be assessed to effectively
identify human reservoirs in South Asia (Bern et al., 2010).
Additionally, co-infection with HIV is a significant threat to VL
management. Molina et al . used xenodiagnosis as a tool for
evaluating VL infection in HIV+ patients and concluded that early-stage
asymptomatic VL patients with HIV infection were infectious to sand
flies (Molina et al., 2020). Further, transmission studies suggest PKDL
patients are a significant parasite source for sand flies, particularly
in immunocompromised people (Kamhawi and Serafim, 2017). PKDL is a form
of VL sequelae where papular or macular dermal lesions occur that can be
extensively parasitized and/or arise in response to immune reactions
against persistent dermal parasites in treated VL patients (Zijlstra et
al., 2017, Mukhopadhyay et al., 2014). As another reservoir of
infection, PKDL patients represent a major health risk for disease
resurgence in that skin parasite load is a strong predictor of positive
xenodiagnosis (Mondal et al., 2019, Molina et al., 2017) (Figure 3).
Risk factors for VL on the ISC include migration, proximities of
households to animal shelters, and crowded conditions (Custodio et al.,
2012, Bern et al., 2010, Ranjan et al., 2005). Livestock density in
India is regarded as a threat variable for increasing human VL incidence
(Barnett et al., 2005). The phenomenon of multiple frustrated feeding
attempts seen by Leishmania -infected sand flies means a vector
may feed on several nearby hosts including livestock (Bern et al., 2005,
Courtenay et al., 2017). Indeed, the presence of cattle has been shown
to attract or sustain higher sand fly densities, thus indirectly
contributing to human infection through human-sand fly exposure. The
prevalence of outdoor P. argentipes combined with the inclination
of ISC residents to sleep outdoors during peak sand fly season correlate
with an increased risk of VL (Perry et al., 2013, Picado et al., 2012).
In Chad, epizootics in domestic animals aggravated Guinea worm
eradication efforts and indicated that dogs may have been a source of
worms acquired by humans (Thiele et al., 2018, Eberhard et al., 2014).
The presence of an animal reservoir raised the question of whether these
animals were the culprits hampering eradication efforts (Galán-Puchades,
2017, Callaway, 2016). While it is widely known that L. donovaniinfection is transmitted between people, however, there has been little
attention paid to the possibility of infections of domestic animals on
the ISC despite genetically similar Leishmania species having a
known zoonotic transmission ecology, (Esteva et al., 2017, Quinnell and
Courtenay, 2009). Mathematical models are an important tool to
understand disease transmission dynamics and assess elimination
viability. Nevertheless, most models are limited by data gaps of
critical biological parameters such as duration of immunity,
infectiousness of different disease stages, comorbidities leading to
recrudescent disease and transmission etc. (Esteva et al., 2017, Coffeng
et al., 2019). A recent xenodiagnosis study on asymptomatic VL subjects
in the Mediterranean basin (Molina et al., 2020) and India (Singh et
al., 2020b) indicated serologically positive asymptomatic people were
not the cause for ongoing transmission (Figure 3) , suggesting
that there could be an alternative non-human host sustaining
transmission. These models conclude that xenodiagnosis experiments and
more clinical data are necessary to predict the role of asymptomatic
infection, PKDL, and domestic animals as infection reservoirs on the ISC
(Tiwary et al., 2017, Le Rutte et al., 2016, Singh et al., 2020a).
Role for domestic animals in the current ecology of disease.
Reservoir hosts are an integral part of the Leishmania parasite
life cycle. The pathogen – vector – reservoir relationship is complex
and reveals regional and temporal variation (Raymond et al., 2003).
Several studies suggest that wild, domestic and synanthropic mammals
including rock hyraxes, rats, foxes, pigs, cats can be infected and dogs
have been identified as reservoir host for Leishmania (Akhoundi
et al., 2016, Palatnik-de-Sousa and Day, 2011, Rohousova et al., 2015,
Dereure et al., 2000). A reservoir species is characterized as one or
more epidemiologically bound populations where a pathogen can be
permanently established as an infection and from which an infecting
organism is transmitted to the target population (Haydon et al., 2002).
Identification of mammalian Leishmania reservoirs has occurred by
demonstrating that the mammal could harbor parasites in its blood
(parasitemia) through culture and more recently through PCR/qPCR, while
in many cases, their role as a reservoir has not been confirmed(Table 1) . Mammalian reservoirs are required for transmission
of zoonotic and rural/sylvatic
infections to sustain the life cycle of multiple Leishmaniaspecies (Esteva et al., 2017). VL is assumed to have only an
anthroponotic transmission cycle on the ISC since there is still no
direct evidence of non-human mammalian hosts infectious to the sand fly
vector (Stauch et al., 2011, Singh et al., 2010). Phlebotomus
argentipes , the principle vector of VL in India, aggregates in and
around animal housing, which offers a favorable, stable micro-climate
near a blood meal and may alter disease dynamics by altering vector/host
contact rates (Keeling, 1999). Several research studies indicate that
IRS has altered P. argentipes feeding behavior from endophilic to
exophilic and sand flies are most likely to feed on outdoor livestock in
areas where IRS occurs (Cameron et al., 2016). Alternative blood sources
for exophilic P. argentipes may include cattle, goats, dogs, or
rodents (Poche et al., 2018) (Figure 1). Importantly, PCR
evidence of L. donovani parasitemia has been found in cattle,
buffalo, and goats (Bhattarai et al., 2010).
The previous conclusion that there is no animal reservoirs for L.
donovani on the ISC is brought into question by observations of
anti-Leishmania specific antibodies and Leishmaniaquantitative PCR positivity detected from domestic animal blood(Table 1) . Dogs have been incriminated as a bridge
between the sylvatic cycle of Leishmania and humans.
Investigators believe dogs were responsible for the zoonotic cycle ofLeishmania (Courtenay et al., 2002b, Courtenay et al., 2002a). In
Brazil dogs are the primary reservoir host for L. infantum (Esch
et al., 2012), a visceralizing Leishmania species genetically
very similar to L. donovani (Courtenay et al., 1994, Blackwell et
al., 2009, Akhavan et al., 2010). Canine Leishmania infection in
VL-endemic areas of Bangladesh and Sudan. Another recent study in India
found L. donovani DNA in the blood of local dogs, goats and cows,
indicating that these are potential animal reservoirs of VL(Table 1) . Rodents and dogs are both classical reservoir
species for other L. donovani complex spp. Other less traditional
reservoirs caused outbreaks of L. infantum within certain foci
(Molina et al., 2012, Ruiz-Fons et al., 2013). Importantly, natural
zoonotic infections with L. donovani have been identified in
dogs, other domestic animals and rodents in other areas outside the ISC(Figure 2) . A study of the emergence of VL in Middle Eastern
and Mediterranean countries indicated that high prevalence of infected
dogs, in the presence of a competent vector species, led to the onset of
human disease (Hamarsheh et al., 2012, Nasereddin et al., 2005). Besides
being the primary reservoir for zoonotic VL in many endemic regions, the
presence of infected dogs was a critical risk factor predisposing humans
to infection in these locations (Gavgani et al., 2002). The epidemiology
of vector-mediated transmission from dogs and other non-traditional
reservoir species on the ISC requires closer inspection to evaluate
their potential role. The totality of L. donovani reservoirs
needs to be more widely understood to protect L. donovanielimination goals (Cameron et al., 2016).
Conclusion
To effectively control VL on the ISC, a more global understanding of its
epidemiology and regional transmission dynamics is needed. Management of
potential domestic and sylvatic zoonoses would involve implementation of
a more complex elimination strategy than exists under the current
framework, which considers humans as the only reservoir of ISC L.
donovani . Previously only considered anthroponotic, L. donovanihas recently been shown to have non-human mammalian hosts by molecular
and serological studies in many countries. As domestic mammals are a
common documented source of sand fly blood meals, the contributions of
livestock as parasite hosts or reservoirs for L. donovani remains
an open question for additional investigation. Xenodiagnosis is the most
practical way to assess transmissibility of parasites from a potential
host to sand flies, and can confirm the role of these populations in the
emerging non-human ecology of L. donovani . These
host-transmission insights are critical to reinforce models forecasting
VL outbreaks on the ISC and worldwide.
This article should serve as a catalyst for epidemiologists,
entomologists, veterinarians, physicians, parasitologists and
immunologists to come together and evaluate critical interactions
between sand flies, mammalian hosts and L. donovani leading to
transmission on the ISC. The main question is not how domestic animals
become infected with L. donovani but rather: to what extent doesL. donovani circulate in domestic species after being fed upon by
infected P. argentipes ? Instead of performing costly active
surveillance to look for the lack of clinical VL cases, we should
instead consider how can we best monitor transmission (or lack thereof)
within the whole potential L. donovani ecosystem. Infection
outcome is determined by parasite virulence as well as by host genetic
predisposition and immune/health status of the host. Infection can be
controlled by removing exposure to the parasite through insecticides or
reducing the parasite burden through chemo and/or immunotherapy (Baneth
et al., 2008). Surveillance is complicated by the long incubation period
of L . donovani in mammals, leading to periods of latency,
when infection can be difficult to detect to prevent transmission.
Interdisciplinary collaboration and broadly based ecological
surveillance of all potential P. argentipes blood meal sources
and xenodiagnosis studies will be required for some time to rule out
zoonotic contributions and achieve L. donovani transmission
eradication goals on the Indian Subcontinent.