Feline Leishmaniasis: What Do We Know So Far?

Abstract

Feline leishmaniasis has a nonspecific clinical condition, like the clinical signs presented by dogs, including anorexia, lymphadenomegaly, dermatitis, emaciation, hyperthermia and atrophy of the temporal muscle. Cats have been identified as an alternative reservoir; however, felines are increasingly sought after as companion animals, further exposing man. Diagnosis in these animals is still considered a challenge, since most cats are asymptomatic. Although infected cats are less often sick than dogs, likely due to natural feline resistance, clinical illness has been linked to immunosuppressive coinfections with feline immunodeficiency virus and feline leukemia virus. The feline immune response to Leishmania infection has been poorly investigated. In cats, the same drugs prescribed for dogs are used in the treatment, namely pentavalent antimonials, allopurinol and miltefosine. Felines play an important role in the epidemiology of leishmaniasis, acting mainly as alternative reservoirs and accelerating the spread of the disease. Thus, there is a need for standardization of diagnostic and treatment methods, including thorough clinical evaluation. It is important to alert to the definition of public policies and to awaken feline owners, as well as the population in general, about care and prevention.

1. Introduction

Leishmaniasis is still one of the most neglected diseases in the world, mainly affecting developing countries. Approximately 350 million people are at risk of contracting the disease, and about two million new cases are reported annually. Annually, approximately 0.2 to 0.4 million cases of visceral leishmaniasis and 0.7 to 1.2 million cases of cutaneous leishmaniasis occur worldwide [1].

The interaction between humans and reservoir animals is an important factor in maintaining disease transmission. Although dogs are the preferred target of sandflies, in comparison to humans, studies on the susceptibility of the disease in other hosts help to elucidate more consolidated data on this zoonosis. Other important risk factors to be highlighted are the lack of basic sanitation and the urbanization process in rural areas, leading to an adequate environment for the maintenance of the vector, making the animals in these areas increasingly susceptible to infection [2].

In domestic cats, clinical disease has been associated with immunosuppressive co-infections with feline immunodeficiency virus (FIV) and feline leukemia virus (FeLV) [3]. These species are less often sick than dogs, probably due to their natural resistance. Due to the increasing number of cases of feline leishmaniasis lately in several endemic countries, the role of the domestic cat as a host of Leishmania has been discussed [4, 5]. Many researchers suggest the role of this species as an additional domestic reservoir [6] and/or a secondary host [7], alternative or accidental [5]. Studies carried out in Italy and Brazil showed evidence on the ability of the cat to transmit the protozoan to the vector, characterizing its behavior as a reservoir of visceral leishmaniasis [6, 8].

The feline immune response to infection has been poorly investigated. The most accepted hypothesis is that cats have natural immune resistance to arthropods and the microorganisms they transmit and that this response is controlled by genetic factors. It is possible that cats are less susceptible to the various immunomodulatory proteins contained in arthropod saliva, and cats generate protective and/or sterilizing immune responses to arthropod-borne pathogens more competently than other animals [9].

Here, we will discuss the role of domestic cats as reservoirs of leishmaniasis, as well as the immunopathological characteristics that help in the maintenance of the disease.

2. Epidemiological aspects

The etiological agents of leishmaniasis are protozoa of the genus Leishmania [10] in which four subgenera Leishmania, Viannia, Sauroleishmania and Mundinia are identified [11, 12, 13]. Recognition of the subgenus is performed by identifying the developmental site of the parasite in the vectors gut and by molecular characterization. Thus, the parasite is classified as subgenus Leishmania when they develop in the midgut and foregut and as subgenus Viannia when they develop in the midgut, foregut, and hindgut of the vector [11]. The subgenus Sauroleishmania is related to all reptile-infecting species of Leishmania. The subgenus Mundinia is the most recently created, in 2018, grouping Leishmania species that infect humans and animals, in which there is still a need for clarification about the vectors and hosts involved in its transmission cycles.

The parasite presents two main evolutive forms during its life cycle: amastigote and promastigote. The promastigote form has an elongated shape, with an external flagellum and it is found in the digestive tract of the insect vector. The amastigote is oval, with no external flagellum, and its survival depends on cells of the mammalian mononuclear phagocytic system [14]. Its biological cycle is heteroxenic, which depends on an intermediate host (invertebrate) and a definitive host (vertebrate) [15].

The transmission of leishmaniasis in felines is still poorly understood due to the few studies, but the most accepted hypothesis by researchers is the transmission by insect vectors, popularly known as sand flies [3]. The disease has vector transmission through the bite of sandflies, which in the New World belongs to the genus Lutzomyia and in the Old World to the genus Phlebotomus [16].

Only female sandflies have the hematophagous behavior that serves for the maturation of eggs in the reproductive period and become infected by taking a blood meal in a vertebrate host that develops cutaneous lesions or active parasitemia in visceral leishmaniasis. When feeding, the vector ends up ingesting infective forms of the parasite that transform into promastigotes in the midgut and reproduce in large quantities. After this process, they move to the salivary glands, where in the next feeding of the vector, they will be inoculated together with the saliva in another vertebrate host, continuing the cycle [16, 17].

Other forms of transmission already described in dogs, mice and humans, such as horizontal transmission, have not been proven in cats [18], but blood transfusion is considered a source of infection as it has already been proven in humans and in dogs [19].

The role of cats in the leishmaniasis cycle is still poorly defined, but it is known that cats are more resistant to manifesting the disease, and anti-leishmania treatments induce clinical but not parasitological cure. Once infected, the protozoan may be available to the vector, which can make them source of sandfly infection [20, 21].

It was reported that cats could be infected by at least seven species of the genus Leishmania: L. mexicana, Laccophilus venezuelensis, L. braziliensis, L. amazonensis, L. infantum [3], L. major and L. tropica [22]. Feline leishmaniasis is distributed in 13 countries (Figure 1).

Figure 1.

Cases of feline leishmaniasis recorded worldwide (Source: Created using the Canva app).

The first case of natural infection in a domestic feline by Leishmania spp. was reported in 1912 in Algeria, in an animal residing with a dog and a child with visceral leishmaniasis. After his death, amastigote forms of Leishmania spp. in the bone marrow of the animal confirmed the diagnosis [23].

In Brazil, the first report of leishmaniasis in felines was in 1939, in the state of Pará. A cat with lesions on the nose and ears was diagnosed with Leishmania spp. confirmed with cytological examination, but it was not possible to identify the species [24].

In Europe, the disease has been reported in cats in Italy [25], Spain [26], Portugal [27] and Greece [28]. In the Middle East, reports have identified the occurrence of leishmaniasis in domestic and stray cats in Turkey [22, 29], in Egypt [30] and in stray cats in Iran [31]. In Asia, the disease has been reported in cats of southeastern Thailand, in Songkhla and Satun provinces [32], in Surathani and Phangnga [33]. In North America, there were reports in the United States [34] and Mexico [35]. In South America, cases occurred in Venezuela [36] and in Brazil [37], which is currently the country with the highest number of studies on feline leishmaniasis [38].

The atypical manifestation of the disease was reported in Africa [39] in a feline with an ocular manifestation, unilateral uveitis, which was initially classified as exudative panuveitis. After 6 months, the disease progressed and ophthalmic enucleation was required, which allowed the diagnosis by visualization of amastigote forms of Leishmania spp. on histopathological examination.

In Brazil, the incidence of diagnoses using molecular and serological techniques is around 7 and 8%, identifying the species L. braziliensis, L. infantum and L. amazonensis as the most frequent [40, 41, 42]. Studies observed the occurrence of feline leishmaniasis in 12 states [43] of the country: Mato Grosso do Sul [40, 44], Mato Grosso [44, 45, 46], Maranhão [47], Minas Gerais [48, 49], Pará [41], Paraná [50], Paraíba [51], Pernambuco [52, 53], Santa Catarina [54], São Paulo [55, 56, 57], Rio de Janeiro [58, 59], and Rio Grande do Norte [60]. The species already identified in Brazil were L. braziliensis, L. amazonensis and L. infantum, the latter being the one responsible for most cases that occurs in felines [43].

3. Diagnosis

The diagnosis for the detection of leishmaniasis is based on clinical findings and the epidemiology of the disease; however, laboratory analyzes are considered important tools in the diagnosis of infection [43, 61]. Currently, several methods are used (Figure 2), and these are applied to both dogs and cats, such as evaluation of clinical signs and parasitological, serological, and molecular tests [20, 21, 38].

Figure 2.

Main methods of diagnosis to detect Leishmania in cats around the world.

Most cats are infected with L. infantum [51, 62]. In these cases, the predominant clinical signs are alopecia, crusts, nodules, ulcers and eye lesions, in addition to gingivostomatitis, hepatomegaly and lymphadenopathy [21]. The occurrence of signs such as fever, weight loss, stomatitis and enteritis, although rare, are also observed [55].

The diagnosis of feline leishmaniasis through parasitological analysis is based on the direct observation of the amastigote forms of Leishmania spp. The test can be used to visualize the parasite in aspirates from lymph nodes, spleen, liver, and bone marrow [63, 64]. In animals with skin lesions, the material obtained can be used to make smears or imprints on slides, histopathological and immunohistochemical analysis and isolation in culture medium [65]. The main advantage of parasitological tests in the diagnosis of leishmaniasis is their high specificity and low cost.

Among the serological (also called immunological) methods most used in Brazil, and in the world, for the diagnosis of disease in animals are the Indirect Immunofluorescence Reaction (IFAT) and the Enzyme Immunoadsorption Assay (ELISA) [66, 67].

In Brazil, the IFAT is widely used in epidemiological studies of endemic areas. The technique uses the intact parasite as an antigen, but there is no standardized IFAT method for felines, much less a universally accepted antibody titer value that corresponds to the infection; therefore, validated titers in dogs are also used for felines [29, 68]. The ELISA is based on the reaction of antibodies present in the sera with soluble and purified Leishmania antigens obtained from in vitro culture [69]. In general, it is considered more sensitive and less specific than IFAT [62, 64]. Diagnosis in felines, it demonstrates greater sensitivity in clinically affected animals [70].

Although parasitological and serological methods are widely practiced, the use of molecular techniques for the detection of Leishmania spp. has been increasing, highlighting conventional and quantitative PCR (qPCR). These are the most used techniques for the confirmatory diagnosis of feline leishmaniasis due to the high sensitivity in the direct detection and characterization of the parasites, and especially in cases of asymptomatic animals [19, 71, 72]. PCR allows the identification of the parasite through different types of samples, such as spleen, bone marrow, lymph nodes and peripheral blood [73, 74]. In Brazil, there are reports of variations in molecular results depending on the type of tissue used for analysis [42].

The diagnosis of feline leishmaniasis is considered complex, since the number of cases of asymptomatic animals is alarming. In addition, the variety of Leishmania species as well as the different existing clinical manifestations have made it even more difficult, increasing the risks of human infection. In this sense, there is a great need for more specific and accurate molecular techniques.

4. Immunopathology

The immune response plays a crucial role in the control of Leishmania infection. T cells modulate and guide the reaction of macrophages to the parasite through the production of cytokines, although there are some differences according to the host species [75, 76, 77]. Cats are less frequently affected by arthropod-borne diseases than dogs [9].

Although these animals do not show important differences between their immune systems, the lower prevalence of L. infantum infection, as well as the clinical manifestations of the disease in cats, may be due to differences in the innate and adaptive immune responses of felines [3, 78]. Cats from endemic areas of canine leishmaniasis were able to activate a cell-mediated adaptative immune response against L. infantum through the production of IFN-γ, after stimulation with soluble parasite antigen. This response was variably associated with antibody or blood PCR positivity, providing a better estimate of cat exposure to L. infantum [79]. A cross-sectional comparative study of the humoral and cell-mediated adaptive immune responses to L. infantum of naturally exposed dogs and cats was carried out in an endemic area (southern Spain) during the sand fly season. It was observed that all dogs and cats that produced IFN-γ had low negative or positive antibodies without parasitemia, whereas animals with high parasite loads and/or antibody levels did not produce IFN-γ after stimulation with soluble L. infantum antigens. It was concluded that there are similarities in dogs and cats for the immunopathogenesis of infection, and that cats can mount a specific Th1 immune response against L. infantum. However, these responses appear to have a lower level compared to dogs [80].

A comparative study on the activation of the complement system in different hosts after stimulation with L. infantum confirmed lower susceptibility of cats to infection due to lower activation of the classical and alternative pathways and higher activation of the lectin pathway due to lower deposition of C4b [81].

In two necropsied cats with multicentric, nodular and ulcerated skin lesions, the presence of Leishmania was observed in both cytological and histopathological analyses. In addition, the presence of Mott cells and multinucleated giant cells in the dermis, lymph node and spleen were described, suggesting the excessive production of immunoglobulins which is a finding in dogs with visceral leishmaniasis. In the same work, splenic, renal and hepatic amyloidosis was found, due to immunogenic amyloid, by the production of large amounts of antibody light chains and their fragments, which can be confirmed by the presence of Mott cells [51].

L. infantum infection in cats may also be associated with immunosuppressive diseases [82]. A study described the first case of disseminated viscerocutaneous feline leishmaniasis caused by L infantum, associated with an invasive squamous cell carcinoma (SCC), assuming that carcinogenesis is favored by the activation and function of macrophages and dendritic cells, which may allow clones of malignant cells to escape continuously from efficient immune destruction. In addition, they observed the development of a strong humoral response to the parasite, confirmed by the high titer found in the IFAT serology [26].

5. Treatment and control

A unique treatment for feline leishmaniasis is not yet reported, so the drugs currently available for dogs are the same as those prescribed for cats. The main drugs used in the treatment of this disease are pentavalent antimonials, allopurinol and miltefosine [83, 84, 85]. Currently, the use of allopurinol is the most common treatment against feline leishmaniasis [86, 87]. In addition, the use of drug association as a treatment for leishmaniasis has been proposed for some years.

For example, the combination of meglumine antimoniate with ketoconazole was successfully administered to a feline with skin lesions [88]. In addition, the association of allopurinol and N-methyl-glucamine antimoniate in the treatment of feline leishmaniasis showed satisfactory results; however, the appearance of adverse reactions was also reported [87, 89].

In general, the treatment of leishmaniasis is considered a great challenge due to its worldwide distribution, different species of parasites, wide variety of clinical forms and immune responses [90]. Although, in most cases, the available drugs promote the full clinical recovery of the animals, they do not cause the complete elimination of the parasite; therefore, proposing preventive measures against the disease is extremely necessary.

Currently, prevention methods such as vaccines, use of a collar, spot-on and spray with repellent properties against sandflies have been used against leishmaniasis in dogs [91, 92]. However, a recent study showed that a collar initially produced for the prevention of fleas and ticks also held promise against infection in cats [93]. According to the authors, the product was 75% effective in preventing infection. Additionally, it was well tolerated by the animals, with no systemic adverse reactions and a decrease in local skin reactions. As for vaccines, some countries already use dogs; however, none are available for felines [94, 95].

6. Conclusions

The role of cats in the leishmaniasis cycle is still poorly defined, with this species being more resistant to manifesting the disease than other animals. Although there are similarities in the immunopathogenesis of the infection in dogs and cats, the response in felines seems to be at a lower level when compared to canines. However, both present treatments promote complete clinical recovery but do not cause complete elimination of the parasite. Therefore, preventive measures against the disease are still the most effective way of controlling the parasitic cycle.

Acknowledgments

The authors would like to thank FUNADESP (Agency for the Development of Private Higher Education) for the T.A. Penha-Silva scholarship. Dr. Fernando Almeida-Souza is a post-doctoral research fellow and scholarship holder of CAPES, grant number 88887.363006/2019-00.

Conflict of interest

The authors declare no conflict of interest.