Canine visceral leishmaniasis (CVL) is a major zoonotic disease endemic in more than 70 countries in the world. It is present in regions of southern Europe, Africa, Asia, South and Central America and has recently emerged in the USA. CVL is also an important concern in non-endemic countries where imported disease constitutes a veterinary and public health problem. Dogs are the main reservoir for human visceral leishmaniasis and the disease is usually fatal if not treated in people and dogs. Phlebotomine sand flies are the vectors of Leishmania infantum, the causative agent of CVL in the Old World and for its New World synonym Leishmania chagasi. Seroprevalence rates found in studies carried out in the Mediterranean basin range between 10 and 37% of the dogs in endemic foci. Surveys employing the polymerase chain reaction (PCR) method for the detection of leishmanial DNA in canine tissues, or combining serology and DNA detection, have revealed even higher infection rates approaching 70% in some foci. It has been estimated based on seroprevalence studies from Italy, Spain, France and Portugal that 2.5 million dogs in these countries are infected. The number of infected dogs in South America is also estimated in millions with high infection rates in some areas of Brazil and Venezuela. A World Health Organization (WHO) report from 1999 indicates that there are approximately 500,000 new human cases of visceral leishmaniasis annually and the population at risk globally is 200 million people. Anthroponotic visceral leishmaniasis caused by L. donovani mainly in India and Sudan is responsible for a large part of the fatalities in people. However, CVL with the dog as a major reservoir for the parasite is the main form of the disease in other parts of the world including Brazil and the Mediterranean region. Visceral leishmaniasis caused by L. infantum in the Mediterranean basin was traditionally predominantly a disease of young children and the name of the causative agent of this disease reflects the predilection to infants. Malnutrition has been recognized as risk factor for infantile leishmaniasis, and may explain why this disease is more prevalent among children in poor countries as compared with affluent ones despite high prevalence rates in the dog populations. With the appearance of the AIDS epidemic, HIV patients are now the predominant risk group for human visceral leishmaniasis in Southern Europe. The co-infection of HIV and leishmaniasis reported from more than 33 countries where these infections geographically overlap and has been described as a "deadly gridlock" that does not respond well to therapy.
Transmission
Leishmania is a diphasic parasite that complete its life cycle in two hosts, a sand fly which harbors the flagellated extracellular promastigotes and a mammal where the intracellular amastigote parasite forms develop. Dogs are infected by Leishmania promastigotes deposited in the skin during the bites of infected female sand fly vectors. The promastigotes invade host cells and replicate as intracellular amastigotes. The disease incubation period prior to appearance of clinical signs may last months to years, during which the parasite disseminates from the skin throughout the host's body primarily to the hemolymphatic system organs.
Transmission of L. infantum through blood products has been reported in humans and also in dogs that received blood transfusions from infected donors in North America. Studies on human intravenous drug users sharing syringes in Spain have indicated that transmission of the infection 'through the needle' occurs. Although natural transmission of L. infantum takes place by the bite of sand flies, vertical in-utero transmission from dam to its offspring has been documented in a few reports. Direct transmission without involvement of a hematophageous vector has been suspected in some cases of infection in areas where vectors of the disease are apparently absent.
Pathogenesis
Population studies in Leishmania-endemic areas have shown that a proportion of the canine population develops a symptomatic disease, another fraction has persistent asymptomatic infection, while yet another fraction is resistant to the infection or intermittently resolves it without developing clinical signs. The immune responses mounted by dogs at the time of infection and thereafter appear to be most important factor in determining if they will develop a lasting infection and whether and when it will progress from an asymptomatic state into a symptomatic disease. Dogs that are able to resist infection and either resolve it and eliminate the parasite, or restrict the infection and remain constantly asymptomatic, have been termed "clinically resistant". Animals that are predisposed and will develop symptomatic disease are considered "susceptible".
Specific immune responses play a major role in susceptibility to infection. An experimental model of cutaneous leishmaniasis in mice infected with L. major has shown that a susceptible mice strain (BALB/c) which typically develops a T helper type 2 cellular response will succumb to infection with secretion of specific cytokines such as IL-4 and IL-10, and production of a significant antibody response,. Another mice strain (C3H) that responds with a different set of cytokines typical of a T helper type 1 response such as IFN-γ; IL-12, is resistant to infection. This general model appears to be only partially valid for CVL and human visceral disease where studies of the immune response have shown mixed Th-1 and Th-2 types of reaction. During the infection, dogs become increasingly immunosuppressed and develop decreased CD4+ lymphocyte counts and a decrease in the CD4+/CD8+ ratio. Moreover, it has been demonstrated that the infectiousness of dogs with leishmaniasis to sand flies increases with the decrease in CD4+ counts.
Immune-mediated mechanisms are responsible for much of the pathological findings in CVL. Circulating immune complexes and antinuclear antibodies have been detected in animals with CL. Glomerulonephritis associated with the deposition of immune complexes in the kidneys is a hallmark of CVL. Renal pathology is present, even if not manifested clinically, in the majority of dogs infected with this disease. Vasculitis induced by immune complexes which activate the complement cascade is an important pathological mechanism responsible for tissue necrosis and accountable for dermal, visceral and ocular lesions found in this disease.
Genetic Basis for Susceptibility and Resistance
Susceptibility or resistance to CVL is influenced by genetics. The presence of overt CVL among Ibizan hounds in the Balearic islands is lower than among other breeds and it has been shown that this breed mounts a predominantly cellular immune response against L. infantum. Other breeds that originate from regions that are not endemic for leishmaniasis such as the Boxer and German shepherd are overrepresented in CVL surveys. A study on the polymorphism of the canine NRAMP1 gene which encodes an iron transporter protein involved in the control of intraphagosomal replication of parasites and macrophage activation, has implied that susceptible dogs have mutations in this gene. A DLA class II DLA-DRB1 genotype, which is a dog major histocompatibility complex (MHC) class II allele has been linked to the risk of being infected in an endemic area in Brazil. Further studies on specific genes and whole genome comparisons among resistant and susceptible breeds might further elucidate the mechanisms responsible for susceptibility to CVL.
Clinical Findings
The typical history reported by owners of dogs with CVL includes the appearance of skin lesions, ocular abnormalities, or epistaxis. These are commonly accompanied by weight loss, exercise intolerance and lethargy. Dogs from all breeds can be infected with leishmaniasis, however, symptomatic infection is uncommon in young immature animals. The incubation period prior to the appearance of clinical signs may last 3 months up to several years. On physical examination, the main clinical signs associated with CVL are dermal lesions, lymphadenomegaly, splenomegaly, abnormal nails growth (onychogryphosis) and poor body condition. Additional findings include: epistaxis, renal failure, decreased appetite, polyuria and polydipsia, vomiting, melena and lameness. About 25% of the dogs with clinical leishmaniasis have ocular or periocular lesions including keratoconjunctivitis and uveitis. The dermal lesions associated with CVL include exfoliative dermatitis which can be generalized or localized over the face, ears and limbs. Nodular dermatitis has been reported and cutaneous ulceration is frequently found with bleeding from pinnal and other local ulceration sites. The most consistent serum biochemistry findings in dogs with clinical CVL are serum hyperproteinemia with hyperglobulinemia and hypoalbuminemia resulting in a decreased albumin/globulin ratio. Grossly elevated activities of liver enzymes or azotemia are found in only a minority of dogs with CVL. However, some degree of renal pathology is present in most dogs with CVL and subsequent renal failure due to immune-complex glomerulonephritis eventually develops and is believed to be the main cause of death in dogs with CVL. Epistaxis, ocular abnormalities or real failure may be the only presenting clinical findings in CVL and this disease should be considered among the differential diagnoses for these conditions in endemic areas or in dogs that have traveled or were imported from an endemic region.Marked hyperglobulinemia with no apparent cause in dogs from endemic regions should also be investigated for CVL.
Diagnosis
CVL is a good example of a disease in which infection does not equal clinical illness due to the high prevalence of asymptomatic infection. This makes CVL a diagnostic challenge for the veterinary practitioner, clinical pathologist and public health official in endemic countries as well as non-endemic regions where imported infection is a concern.
Leishmania amastigotes can be demonstrated by cytology from lymph nodes, spleen or bone marrow stained with Giemsa stain or a quick commercial stain. Detection of amastigotes by cytology is frequently unrewarding due to a low number of detectable parasites present even in dogs with a full blown clinical disease. Leishmania parasites may also be viewed in histopathologic formalin-fixed, paraffin-embedded biopsy sections of the skin or other infected organs. Definite identification of parasites within tissue macrophages may be difficult and an immunohistochemical staining method can be employed to detect or verify the presence of Leishmania in the tissue.
Various serological methods for the detection of anti-Leishmania antibodies have been developed. These include indirect immunofluorescence assays (IFA), enzyme-linked immunosorbent assay (ELISA), direct agglutination assays (DAT) and western blotting. A purified recombinant antigen for ELISA, rK39, has been used for detection of visceral leishmaniasis in humans and dogs. In general, good sensitivities and specificities are gained with these methods for the diagnosis of clinical CVL.
Detection of parasite-specific DNA in tissues by PCR allows sensitive and specific diagnosis. Several different assays with various target sequences using genomic or kinetoplast DNA (kDNA) have been developed for CVL. PCR can be performed on DNA extracted from tissues, blood or even from histopathologic specimens. Assays based on kDNA appear to be the most sensitive for direct detection in infected tissues.
Therapy
Anti-leishmanial treatment often achieves only temporary clinical improvement in dogs with leishmaniasis and it is frequently not associated with the elimination of the parasite. The main drugs used against canine leishmaniasis include the pentavalent antimony meglumine antimonite which selectively inhibits leishmanial glycolysis and fatty acid oxidation and allopurinol that acts by inhibiting protein translation through interfering with RNA synthesis. Amphotericin B which acts by binding to ergosterol in the parasite's cell membrane and altering its permeability is also used but it is highly nephrotoxic. New drugs are currently under evaluation for dogs.
Keeping and treating infected dogs presents a dilemma to owners, veterinarians and public health officials especially in areas where suitable vectors are found, because of the risk of transmission to people and pets in the community. Treated dogs often remain carriers of the disease, are infectious to sand flies and commonly experience clinical relapses. Owners must receive a thorough and realistic explanation about the disease, its zoonotic potential, the prognosis for their dog, and what should be expected from treatment.
Prevention
A commercial vaccine against CVL has recently been approved in Brazil and several vaccine candidates are under evaluation in Europe. The use of topical insecticides against CVL in collars or spot-on formulation has been shown to be effective in reducing disease transmission. Deltamethrin-impregnated collar significantly reduced the number of dog sand fly bites under experimental transmission and decreased infection transmission in field studies. In a study supported by the WHO in Iran, collaring of dogs in intervention and control villages significantly reduced the seroconversion rate in dogs and in children living in the intervention villages.
References
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