Visceral Leishmaniasis: Successful Vaccine Trial In Dogs

Visceralleishmaniasis, which is the most severe form of the leishmaniases, hitsan annual total of 500 000 people, mostly in the developing countries.It is caused by the parasite Leishmania infantum. A flagellateprotozoan, it uses as vector an insect resembling a midge, the sandfly, colonizing the intestine and then the salivary glands. The femaleinsect feeds on mammals’ blood. It can thus pass the parasite on tohumans by a single bite. Once in the blood stream, L. infantum passesinto particular cells of the immune system, the macrophages. Theseeventually burst, releasing the parasites which move on to penetrateother cells. The infected subject suffers bouts of fever, anaemia,enlarged spleen and liver, and weight loss. In the absence oftreatment, these clinical signs usually announce a fatal outcome.

Thesand fly sucks blood from mammals other than humans. This is how, rightaround the Mediterranean rim, 5 million dogs, a proportion of from 1 to42 % depending on the area, are affected by visceral leishmaniasis.These animals are thus a reservoir for these parasites, whichcontinuously feed the mammal-sand fly-human cycle. In this context,development of a canine vaccine would help reduce the portion of theanimal population infected. The risks of transmission of the disease tohumans would in this way be indirectly reduced.

Up to now,several dog vaccines, mostly developed from whole dried parasites, haveproved not to be really effective. A team from the IRD Montpellierresearch centre, working with the Rocher veterinary clinic (La Garde,Var, France) and the biopharmaceutical firm Bio Véto Test (LaSeyne-Sur-Mer, Var), have recently produced and tested a new type oftreatment, composed solely of antigen proteins excreted by the parasite(1). The first trials indicate that this would completely and lastinglyprotect dogs against the disease.

Twelve out of 18 dogs includedin the study were treated with increasing doses of protein antigensexcreted by the parasite (that is 50, 100, 200 micrograms) made up to aformula with an adjuvant. The other six received no treatment. Twoinjections at an interval of three weeks resulted in infection of allthe animals with L. infantum. They were followed up for two years inorder to monitor the progress of the disease. The mixture of parasiteproteins proved to be especially effective, as 100% protection wasobtained for the doses of 100 micrograms (six immunized dogs out ofsix) and 200 micrograms (three out of three).

The researchersalso focused on the changes to the immune system brought on by thevaccination. Laboratory experiments showed that the effectiveness ofthe vaccine stems from the activation of certain cells of the immunesystem, the T lymphocytes of type Th1. These induce the infectedmacrophages to produce nitric oxide, highly toxic for cells. Thisprocess, which did not occur in the untreated dogs, thus enablesmacrophages to get rid of the parasites that are infecting them. Theanimal thus acquires long-term protection against visceralleishmaniasis.

Although this vaccine’s effectiveness has beenshown only on a limited number of animals, it is a further step towardsprotection of dogs against this disease. These results, confirmedindeed by the first, highly encouraging, data from a large-scaleclinical trial currently under way (phase III), are promising forefforts to reduce transmission of leishmaniasis to humans. They alsopoint to new lines of investigation for elaborating a possible humanvaccine. An integrated research project, involving several IRD groups(2), has just been set up in India, to work on this. It should lead toan assessment of the effectiveness of such a vaccine in humans.

(1) The finding of these proteins furthermore necessitated thedevelopment, achieved in 1992, of suitable culture media (patented),with the attached proteins removed. The media normally used ijn factcontain many protein-containing compounds (serumalbumins, albuminsetc.) which prevent specific isolation of the proteins excreted by theparasite. References : Lemesre J.L., Blanc M.P., Grébaut P., ZilberfardV. et Carrière V. (1994). Culture continue de formes amastigotes deLeishmanies en condition axénique. Réalisation du cycle évolutif invitro. Médecine et Armées, 22 (1), 99 and Merlen T, Sereno D, Brajon N.and Lemesre J.L. (1999). Leishmania Spp: completely defined mediumwithout serum and macromolecules (CDM/LP) for the continuous in vitrocultivation of infective promastigote forms. Am. J. Trop. Med. Hyg., 60(1), 41-50. Brevets : Lemesre J.L. (1993). "Procédé de culture in vitrode différents stades parasitaires obtenus et applications biologiques".Brevet français, FR n° 93 05779 ; Lemesre J.L. (1994)."Method for theculture in vitro of different stages of tissue parasites".Brevetinternational, PCT/FR N° 94/00577.

(2) The project, entitled «Study of the host and parasite factors determining the outcome ofvisceral leishmaniasis: application for prevention and treatment »,involves research IRD units UR 08 « Trypanosome pathogenesis » and 165« Genetics and evolution of infectious diseases », in conjunction withthe Institute of Medical Sciences, Banares Hindu University, Varanasi,India.