Renato L. Santos, DVM, MS, PhD
Departamento de Clínica e Cirurgia Veterinárias, Escola de Veterinária, Universidade Federal de Minas Gerais. Av. Antonio Carlos; Belo Horizonte, MG, Brazil
Leishmaniasis is caused by protozoa of the genus Leishmania. These parasites develop distinct forms during their life cycle. The flagellated and motile promastigote and paramastigote forms develop and multiply in the gut of phlebotomine sand fly vectors, whereas the intracellular non-motile amastigote form is found within macrophages of the vertebrate host.17,19 Leishmania is adapted to replicate in the sand fly gut, where it differentiates into infective metacyclic promastigotes.17 Metacyclic promastigotes transform into amastigotes soon after being internalized by a vertebrate host macrophage17,19, where it is capable of surviving and replicating.4,5
Approximately 30 different species of phlebotomine sand flies may act as biological vectors of Leishmania10, with two genera, namely Phlebotomus and Lutzomyia, which are the vectors of Leishmania in the Old and New World, respectively.12 Leishmaniasis is associated with variable clinical manifestations depending on the parasite and host species involved. In man, visceral, cutaneous and mucosal forms are clinically distinct.2 These syndromes are caused by a total of 21 different Leishmania species.10 Visceral leishmaniasis (VL), also known as kala-azar, is a major zoonotic disease caused by species grouped into the donovani complex, including L. donovani and L. infantum in the Old World and L. chagasi in New World.26
VL is endemic in tropical and subtropical areas and Southern Europe, including geographic areas ranging from rain forests to deserts, and rural or urban areas. This broad distribution of the disease is associated with the distribution of the sand fly vector.10 In developing countries, environmental changes have resulted in habitat destruction, favoring adaptation of the vector population. Therefore, VL that was originally restricted to rural and peri-urban areas has become a major public health issue in urban areas, where the dog is the most important reservoir for human VL.12
Dogs develop intense cutaneous parasitism, favoring infection of sand flies.31 Importantly, most dogs remain asymptomatic for long periods of time, which contribute to maintenance and transmission of the disease.12 Clinical manifestations of canine VL range from unapparent sub-clinical infections to a systemic disease, which is usually chronic, associated with fever, anemia, cachexia, cutaneous lesions, renal failure, hepatomegaly, splenomegaly and lymphadenopathy.21,22 Interestingly, a diverse range of atypical clinical manifestations have been reported in canine VL.8,29
Although vector-borne is the most common mode of transmission, other routes have been reported in humans, such as congenital and parenteral by blood transfusion, needle sharing or laboratory accident.15,30 Transmission by blood transfusion has also been documented in a dog23, and there is a documented case of transmission by packed RBC transfusion.14 Moreover, autochthonous cases of canine VL have been described in areas free of known invertebrate vectors.13,15 In one of these reports, a prevalence of 41% in a Foxhound kennel housing 112 dogs occurred in Northeastern US, where there is no known suitable biological vector.13 In these cases, exposure to an alternative insect vector, direct or vertical transmissions were considered possible routes of infection.13
In spite of a previous report of venereal transmission of human VL30, it has only recently been experimentally demonstrated by our group that venereal transmission of canine VL in the absence of the biological vector also occurs.27 Our previous studies have demonstrated that the L. chagasi has a tropism for the canine male genital system, particularly to the epididymis, prepuce and glans penis, resulting in inflammation of these organs and shedding of Leishmania in the semen.11 A very large proportion of naturally infected dogs shed Leishmania in the semen as assessed by PCR detection of Leishmania kDNA in semen samples, but shedding is often intermittent.27
Naturally infected male dogs usually do not develop gross lesions in the genital system. However, these dogs do develop microscopic changes, particularly in the epididymis, glans penis and prepuce, with a frequency that is significantly higher than in their Leishmania-negative cohort.11 These findings support the notion that L. chagasi has a tropism for the canine male genital system. There are several published reports of genital lesions associated with Leishmania infection in man6,7,16,25, although Leishmania-associated genital lesions in women have not been reported, with the exception of one case of sexual transmission.30 This tropism for the human male genital system parallels our findings in dogs.11,27
Although amastigotes are often present in the testis, there is not an increase in the frequency of orchitis in dogs with VL. Amastigotes are observed mostly within macrophages in the interstitium in association with a lympho-plasmacytic interstitial infiltrate with mild to moderate testicular degeneration.11
In contrast, VL in dogs is associated with a significant increase in the frequency of epididymitis, which is characterized by a lympho-histio-plasmacytic infiltrate that is more intense in symptomatic than asymptomatic dogs. Inflammatory infiltrate in the glans penis and prepuce of naturally infected dogs are predominantly histiocytic, with morphological features of a granuloma in some cases. Large numbers of Leishmania amastigotes were observed intracellularly in macrophages associated with these lesions.11 In contrast to male dogs, L. chagasi does not have a tropism for the female genital tract.28 These results support the notion that venereal transmission in dogs is likely to be unidirectional, preferably from infected dogs to susceptible bitches.27 Additional studies are needed to determine the source of infectious organisms during venereal transmission, i.e., whether Leishmania secreted in the semen or amastigotes from the male external genitalia are the source of infection. The traumatic copulatory behaviour of dogs would favour the prepuce and glans penis as a source of infection. Importantly, the recent study in which we demonstrated venereal transmission27 assessed parameters of infection such as seroconversion and PCR amplification of Leishmania kDNA, but not development of clinical disease. Therefore, it would be interesting to compare the infective potential of Leishmania transmitted through the vector-borne route or venereal route. Presumably vector born metacyclic promastigotes inoculated into the mammalian host through the bite of sand flies should be more infective than sexually transmitted amastigotes since the phlebotomine saliva contains several factors that favor infection.1,9
Although VL in bitches is not associated with genital lesions27, congenital transmission of Leishmania sp. has already been demonstrated in an experimentally infected Beagle whose infected fetuses were removed from the uterus by c-section thus preventing the possibility of transvaginal transmission.24 We have recently demonstrated the distribution of Leishmania in several organs of vertically infected canine fetuses (Pangrazio et al., unpublished data). Abortion associated with a necrotizing placentitis with large amounts of Leishmania amastigotes in the placenta but not in fetal organs was also reported13 as well as infection in newborn dogs.18 Congenital transmission has also been demonstrated in humans3,20 and in experimentally infected BALB/c mice.24 In spite of these clear indications of vertical transmission of VL in dogs, the frequency of placental transmission through the placenta remains unclear as well as a possible differences in transmission potential of symptomatic and asymptomatic bitches. We are currently conducting a study to address those questions. In conclusion, venereal and vertical transmission of canine VL in the absence of the biological vector should be taken into account for establishing eradication programs. The use of infected dogs for reproductive proposes should be avoided.
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