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 macrophage^17,19, 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 man^6,7,16,25, 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 humans^3,20 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.

References

1.  Almeida MC, Vilhena V, Barral A, Barral-Netto M. (2003). Leishmanial infection: analysis of its first steps. Mem. Inst. Oswaldo Cruz, 98, 861-870.

2.  Ashford RW. (1997). The leishmaniases as model zoonoses. Ann. Trop. Med. Parasitol., 91, 693-701.

3.  Boehme CC, Hain U, Novosel A, Eichenlaub S, Fleischmann E, Löscher T. (2006). Congenital visceral leishmaniasis. Emerg. Infect. Dis., 12, 359-60.

4.  Bueno R, Carvalho Neta AV, Xavier MN, Oliveira RG, Diniz SA, Melo MN, Santos RL. (2009). cDNA sequencing and expression of Nramp1 (Slc11a1) in dogs phenotypically resistant or susceptible to visceral leishmaniasis. Vet. Immunol. Immunopathol., 127, 332-339.

5.  Bueno R, Mello MN, Menezes CAS, Dutra WO, Santos RL. (2005). Phenotypic, functional and quantitative characterization of canine peripheral blood monocyte-derived macrophages. Mem. Inst. Oswaldo Cruz, 100, 521-524.

6.  Cabello I, Caraballo A, Millan Y. (2002). Leishmaniasis in the genital area. Rev. Inst. Med. Trop. São Paulo, 44, 105-107.

7.  Cain C, Stone MS, Thieberg M, Wilson ME. (1994). Nonhealing genital ulcers. Arch. Dermatol., 130, 1311-1316.

8.  Carvalho Neta AV, Paixão TA, Silva FL, Santos RL. (2007). Pan-oftalmite em cão com leishmaniose visceral: relato de caso. Clin. Vet., 66, 52-56.

9.  Cavalcante RR, Pereira MH, Gontijo NF. (2003). Anti-complement activity in the saliva of phlebotomine sand flies and other haematophagous insects. Parasitol., 127, 87-93.

10. Desjeux P. (1996). Leishmaniasis. Public health aspects and control. Clin. Dermatol., 14, 417-423.

11. Diniz SA, Melo MS, Borges AM, Bueno R, Reis BP, Tafuri WL, Nascimento EF, Santos RL. (2005). Genital lesions associated with visceral leishmaniasis and shedding of Leishmania sp. in the semen of naturally infected dogs. Vet. Pathol., 42, 650-658.

12. Diniz SA, Silva FL, Carvalho Neta AV, Bueno R, Guerra RMSNC, Abreu-Silva AL, Santos RL. (2008). Animal reservoir for visceral leishmaniasis in densely populated urban areas. J. Infect. Developing Countries, 2, 24-33.

13. Gaskin AA, Schantz P, Jackson J, Birkenheuer A, Tomlinson L, Gramiccia M, Levy M, Steurer F, Kollmar E, Hegarty BC, Ahn A, Breitschwerdt EB. (2002). Visceral leishmaniasis in a New York foxhound kennel. J. Vet. Int. Med., 16, 34-44.

14. Giger U, Oakley DA, Owens SD, Schantz P. (2002). Leishmania donovani transmission by packed RBC transfusion to anemic dogs in the United States. Transfusion, 42, 381-383.

15. Harris MP. (1994). Suspected transmission of leishmaniasis. Vet. Rec., 135, 339.

16. Kapila K, Prakash M.B, Mehrota R, Vermar K. (1994). Testicular leishmaniasis in a boy with acute lymphoblastic leukemia. Acta Cytol., 38, 878-879.

17. Killick-Kendrick R. (1990). Phlebotomine vectors of the leishmaniases: a review. Med. Vet. Entomol., 4, 1-24.

18. Masucci M, De Majo M, Contarino RB, Borruto G, Vitale F, Pennisi MG. (2003). Canine leishmaniasis in the newborn puppy. Vet. Res. Commun., 27, Suppl 1, 771-774.

19. Mauel J. (1990). Macrophage-parasite interactions in Leishmania infections. J. Leukoc. Biol., 47, 187-193.

20. Meinecke CK, Schottelius J, Oskam L, Fleischer B. (1999). Congenital transmission of visceral leishmaniasis (Kala Azar) from an asymptomatic mother to her child. Pediatrics, 104, e65.

21. Moreno J, Alvar J. (2002). Canine leishmaniasis: epidemiological risk and the experimental model. Trends Parasitol., 18, 399-405.

22. Oliveira GG, Santoro F, Sadigursky M. (1993). The subclinical form of experimental visceral leishmaniasis in dogs. Mem. Inst. Oswaldo Cruz, 88, 243-248.

23. Owens SD, Oakley DA, Marryott K, Hatchett W, Walton R, Nolan TJ, Newton A, Steurer F, Schantz P, Giger U. (2001). Transmission of visceral leishmaniasis through blood transfusions from infected English foxhounds to anemic dogs. J. Am. Vet. Med. Assoc., 219, 1076-1083.

24. Rosypal AC, Troy GC, Zajac AM, Frank G, Lindsay DS. (2005). Transplacental transmission of a North American isolate of Leishmania infantum in an experimentally infected beagle. J. Parasitol., 91, 970-972.

25. Schubach A, Cuzzi-Maya T, Gonçalves-Costa SC, Pirmez C, Oliveira-Neto MP. (1998). Leishmaniasis of glans penis. J. Europ. Acad. Dermatol. Venereol., 10, 226-228.

26. Shaw JJ. (1994). Taxonomy of the genus Leishmania: present and future trends and their implications. Mem. Inst. Olwaldo Cruz, 89, 471-478.

27. Silva FL, Oliveira RG, Silva TMA, Xavier MN, Nascimento EF, Santos RL. (2009). Venereal transmission of canine visceral leishmaniasis. Vet. Parasitol., 160, 55-59.

28. Silva FL, Rodrigues AA, Rego IO, Santos RL, Oliveira RG, Silva TM, Xavier MN, Nascimento EF, Santos RL. (2008). Genital lesions and distribution of amastigotes in bitches naturally infected with Leishmania chagasi. Vet. Parasitol., 151, 86-90.

29. Souza AI, Juliano RS, Gomes TS, Diniz SA, Borges AM, Tafuri WL, Santos RL. (2005). Osteolytic osteomyelitis associated with visceral leishmaniasis in a dog. Vet. Parasitol., 129, 51-54.

30. Symmers WS. (1960). Leishmaniasis acquired by contagion: a case of marital infection in Britain. Lancet, 16, 127-132.

31. Tafuri WL, Santos RL, Arantes RM, Gonçalves R, Melo MN, Michalick MS, Tafuri WL. (2004). An alternative immunohistochemical method for detecting Leishmania amastigotes in paraffin-embedded canine tissues. J. Immunol. Methods, 292, 17-23.