Canine leishmaniosis in South America

South America is a large continent located in the western hemisphere. Most of its landmass is situated within the tropical zone (which extends from the equator to the north and south parallels of 23°30'), which provides a very suitable environment for many kinds of arthropods (e.g., ticks, mosquitoes and phlebotomine sandflies) that can act as vectors of a number of pathogens. As a corollary, people living in South America are exposed to a number of arthropod-borne diseases, including malaria, leishmaniosis and dengue fever. Similarly, dogs are also affected by many arthropod-borne diseases, including ehrlichiosis, babesiosis, dirofilariosis and leishmaniosis [1, 2].

Canine leishmaniosis is widespread in South America and it is among the most important canine vector-borne diseases occurring in this region, mainly because of its major zoonotic relevance [1–4]. The present article provides an overview on key aspects related to canine leishmaniosis in South America, emphasising future research needs.

Leishmania infantum is the most important causative agent of canine visceral leishmaniosis in South America. Dogs have been regarded as the main reservoir hosts of L. infantum, which is a parasite of major zoonotic concern, particularly in Brazil where ~3500 cases of human visceral leishmaniosis are reported annually; about 10% of the cases have resulted in a fatal outcome [13]. Dogs infected by L. infantum can develop a life-threatening disease characterised by lymphadenomegaly, muscular atrophy, skin ulceration, weight loss and onychogryphosis (Figure 1). It is a common concept that all dogs with visceral leishmaniosis in South America are infected by L. infantum. However, in Venezuela, a strain characterised by isoenzyme analysis as L. colombiensis was isolated from a dog presenting visceral leishmaniosis [7]. In Brazil, two dogs diagnosed as having visceral leishmaniosis were actually infected by L. amazonensis [12]. These reports highlight the importance of using proper diagnostic tools to identify the species of Leishmania involved in each case of canine leishmaniosis irrespective of the clinical form.

Leishmania braziliensis is the main causative agent of cutaneous leishmaniosis in dogs in South America [10]. Most of the dogs infected by L. braziliensis live in rural areas and they may present single cutaneous or mucosal lesions (Figure 2) [14]. Dogs have been suspected to play a role in the domestic transmission cycle of L. braziliensis and L. peruviana in some areas of South America, but there is only circumstantial evidence supporting this hypothesis [10]. In fact, the role of dogs in the maintenance of these parasites is probably minor [3].

Hybrid Leishmania strains have also been isolated from dogs in South America. For example, L. braziliensis/L. peruviana and L. braziliensis/L. guyanensis hybrid strains have been isolated from dogs in Peru and Venezuela, respectively [10]. The hybrid strains have phenotypic and genotypic features of two Leishmania species, and it has been suggested that these hybrids might represent strains that originated directly from a common ancestor or that they might be the result of genetic exchange [15].

In certain areas, the enzootic transmission cycles of different Leishmania parasites might overlap and dogs might become co-infected. For instance, cases of co-infection by L. infantum and L. braziliensis in dogs have been reported in south-eastern Brazil [16, 17]. Co-infection by L. infantum and other trypanosomatids (e.g., Trypanosoma evansi) in dogs have also been reported [18]. For instance, a new species of Trypanosoma (namely Trypanosoma caninum) has recently been isolated from a dog co-infected with L. braziliensis in south-eastern Brazil [19]. Co-infections might be relevant in terms of diagnosis because of the possibility of serological cross-reactions among different Leishmania species [20] and with other related trypanosomatids [19, 21].

The primary mode of transmission of Leishmania parasites from dog to dog is through the bite of an infected phlebotomine sandfly. In South America, the vectors of Leishmania parasites belong to the genus Lutzomyia (Figure 3). Over 70 species of Lutzomyia have been suspected to be involved in the transmission of the eight Leishmania species known to infect dogs in South America [15, 22–29]. However, the only well-established vector of Leishmania parasites to dogs in South America is Lutzomyia longipalpis [24]. This phlebotomine sandfly is a proven vector of L. infantum, widespread in South America [23], and has an opportunist feeding behaviour. However, it is interesting to note that in some areas dogs are not the preferred source of blood of Lu. longipalpis. For instance, in a recent study carried out in central-western Brazil, it was found that Lu. longipalpis fed preferentially on birds, rodents, humans, opossums, oxen, horses and dogs, in decreasing order of importance [30]. In these areas, the role of dogs as reservoir hosts of L. infantum could be of minor relevance.

Molecular biology techniques, in particular PCR-based tools, have impacted many fields of parasitology, including the study of a number of parasites and their respective arthropod vectors. The development of PCR-based tools for the detection of Leishmania DNA in phlebotomine sandflies has increased the number of putative vectors of Leishmania parasites in South America, see, for example, [25–29]. However, the detection of Leishmania DNA in a given Lutzomyia species, the mere detection per se, does not necessarily mean vector competence. In fact, experimental transmission studies are needed to prove the role of a given phlebotomine sandfly species as a vector of Leishmania parasites, even though these studies might be expensive, time-consuming and require considerable expertise on phlebotomine sandfly rearing.

The absence of Lu. longipalpis in some areas where cases of canine leishmaniosis have been reported [31, 32] has suggested the participation of other phlebotomine sandfly species or the existence of secondary modes of transmission. Secondary modes of transmission that have been suggested in the literature include transplacental transmission [33], via blood transfusion [34], and venereal transmission [31, 35]. However, the relevance of alternative ways of transmission is unknown. In a similar way, fleas and ticks have long been regarded as putative vectors of L. infantum in Brazil [36–39], but an overwhelming proof that they are competent vectors of Leishmania parasites has never been provided.

Canine leishmaniosis is a widespread disease in South America (Figure 4). Infection by Leishmania parasites in dogs have been reported in all countries except Chile, Uruguay, Suriname, and Guyana (see Table 1). Cases of canine visceral leishmaniosis by L. infantum (zymodeme MON-1) have been diagnosed in French Guiana, although autochthonous transmission has not yet been demonstrated [31].

Leishmania braziliensis and L. infantum are the most widespread species infecting dogs in South America and their distribution is probably wider than it is actually conceived. In recent years, there has been a southward spreading of canine leishmaniosis caused by L. infantum in South America. For example, the disease has recently been diagnosed in previously free areas of southern Brazil [40] and northern Argentina [41].

For a long time, canine leishmaniosis was considered to be a disease confined to rural areas. Nowadays, the disease is well-established in large urbanised areas such as the metropolitan region of Belo Horizonte, south-eastern Brazil [42]. Many factors could favour the spreading of canine leishmaniosis in South America, including the movement of dogs between endemic and non-endemic areas [31] and changes in vector ecology. Lutzomyia longipalpis is widespread in South America [23] and it is adapted to colonise environments modified by man [24]. In the State of Pernambuco, north-eastern Brazil, sparse spots of modified Atlantic rainforest can be found in highly urbanised areas. These remnants of Atlantic rainforest are potentially inhabited by phlebotomine sandflies of many species [43], including Lu. longipalpis [44]. It means that the introduction of a Leishmania-infected dog into a non-endemic area where the potential vectors are present could result in the establishment of a new focus of disease. In fact, if the current tendency continues [40, 41], new foci of the disease should be expected to be detected in the future.

Most information on the prevalence of infection among dogs came from serological surveys conducted in Brazil, see, for example [45–48], and at a much lesser extent in other countries such as Argentina [49], Colombia [50] and Venezuela [51]. Although the prevalence of Leishmania spp. infection in dogs in South America can vary widely from region to region, and according to the diagnostic method used, it is usually over 25% [21, 47, 49, 51, 52] and might be as high as 75% in highly endemic foci [53]. However, it is difficult to estimate the overall prevalence of Leishmania infection in dogs in South America because of the limited amount of published data from some countries (e.g., Paraguay), the existence of methodological differences among studies (e.g., sample size and criteria of positivity) and the inherent limitations of serology (e.g., possibility of cross reactions).

An important epidemiological feature that has been observed in South America (and also in the Mediterranean basin) [54] is that the majority of the dogs infected by L. infantum are apparently healthy, exhibiting no visible clinical signs of visceral leishmaniosis. In some foci in Brazil, over 80% of the seropositive dogs might be clinically healthy [47, 55]. This information might be relevant because seropositive but apparently healthy dogs can also serve as a source of infection to phlebotomine sandflies [56, 57].

Studies attempting to assess the risk factors associated to infection in dogs in South America have been conducted mainly in Brazil. In some areas, there is a higher prevalence of anti-Leishmania antibodies among males when compared with females [47], but in others there has been no association between gender and seropositivity [48]. In some areas, there is a higher seroprevalence in young dogs (< 1 year) [47], whereas in other areas older dogs (1–6 years) are at a higher risk of infection [48]. This apparent disagreement among studies might reflect the local nature of canine leishmaniosis. The epidemiology of the disease varies widely among different regions and risk factors associated to infection in different disease foci might be difficult to predict. One important risk factor is the dog's lifestyle. For instance, guard dogs that are kept outside houses during the whole night are more exposed to sandfly bites and therefore are at a higher risk of infection as compared with companion dogs that are kept inside houses [48, 58]. In some rural areas, dogs (Figures 5 and 6) are highly exposed to phlebotomine sandflies, which can be found inside houses, in animal shelters and forested areas [59].

The risk factors underlying the outcome of infection by Leishmania parasites in dogs in South America are poorly understood. The majority of the dogs living in rural and suburban areas are mongrel dogs [47] and the susceptibility to L. infantum infection in these dogs has been shown to be associated with MHC class II polymorphism [60]. The relationship between nutritional status (which is a known risk factor for human visceral leishmaniosis) and the susceptibility to L. infantum infection in dogs should be further investigated.

Canine leishmaniosis is widespread in rural and urban areas in South America, although the factors associated with risk to Leishmania infection in dogs from this region are still poorly understood. Dogs are exposed to infection by a number of Leishmania species, which are potentially transmitted by different Lutzomyia species. Moreover, secondary modes of transmission might be involved and could be relevant for the establishment of new foci of canine leishmaniosis in non-endemic areas. Overall, this illustrates how complex is the epidemiology of canine leishmaniosis in South America and highlights the future research needs.

Little is known about the genetic relationship among the Leishmania parasites isolated from dogs, Lutzomyia sandflies and humans in many areas where canine leishmaniosis is endemic in South America. In this context, new attempts to isolate and characterise the species of Leishmania parasites circulating among dogs from urban and rural areas in different South American countries should be encouraged.

Despite of the long list of putative vectors, the only well-established vector of Leishmania parasites to dogs in South America is Lu. longipalpis. Indeed, dogs can serve as a source of Leishmania infection to different Lutzomyia species (e.g., Lutzomyia whitmani, Lutzomyia evansi and Lutzomyia youngi) [69–71]. However, it has yet to be proved that these Lutzomyia species are able to transmit the infection to a susceptible dog during a subsequent blood feeding.

In the same way, it is important to investigate the factors associated with risk to Leishmania infection in dogs, keeping in mind that these concepts cannot be generally extrapolated because canine leishmaniosis is a focal disease, whose epidemiology may vary widely from region to region. Some aspects (e.g., poor nutrition) might increase the risk of disease development, but so far this relationship has not been fully addressed in South American dogs. The factors dictating which dog will become sick (and when it will do so) should be addressed in future studies.

For some time, researchers working in South America have focused most of their efforts on canine leishmaniosis by L. infantum and L. braziliensis. Despite the inarguable importance of these two parasites, the study of canine leishmaniosis caused by other Leishmania species (e.g., L. amazonensis and L. colombiensis) would deserve more attention in the future. This constitutes a neglected issue that could provide new insights into the knowledge of the natural history of Leishmania parasites and the diseases they cause.