The clinical signs of leishmaniasis are directly related to the immune response of the infected dog and we can account the disease into four stages based on serological status, clinical signs, laboratory findings and type of therapy and prognosis for each stage [
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15]. In susceptible animals, organisms can spread from skin to the local lymph node, spleen and bone marrow in a few hours [
15]. In resistant dogs, the parasite remains restricted to the skin and draining lymph node [
15]. According these considerations the first-choice samples should be used for PCR are: bone marrow, lymph node, spleen, skin and conjunctival swabs and others, however samples of blood, buffy coat and urine are considered less sensitive [
3]. In parallel, the study of
LEVs of prokaryotes and eukaryotes has aroused considerable interest in the scientific community, due to the possible potential for the development of diagnostic and therapeutic methodologies [
1]. Despite advances in
Leishmania studies, the selective mechanisms of
LEVs are still poorly understood, there is no consensus on the differential characterization or ultrasensitive detection of their specific subtypes, biomarkers or their biogenesis and how this knowledge can be effective for faster diagnosis and prevention [
1]. Actually, the diagnostic methods for canine leishmaniasis include: parasitological (cytology/histology; immunohistochemistry and culture); molecular (conventional, nested and real-time PCR, considered the most sensitive technique) and serological quantitative (IFAT and ELISA) and quantitative (rapid tests) [
3]. Increased transmission of the parasite and higher incidence of disease, as well as the emergence and re-emergence of this disease recorded in recent years may be related to many factors, among which we highlight the socio-economic conditions, climate and environmental change, closer contact between pets and wild ecosystems and the parasite resistance to drugs and insecticides vectors in use [
1]. Concurrently with the spread of leishmaniasis recently studies reveal that dogs are at risk of acquiring coinfection with emerging zoonotic parasites [
1]. In that sense, the most popular pet animals worldwide, such as dogs and cats can become infected by parasites [
1]. Companion animals are important agents of different parasitic species, these infections can cause many complications with risk of high morbidity, including weight loss, anemia, and low immune resistance [
25,
26,
27]. In turn, these conditions can lead to secondary infections and even death of these animals [
27]. Many parasite species potentially threaten canine and feline health, while some dog and cat parasites are strictly associated to these animals, some of them may also infect humans causing zoonoses [
27,
28]. Protozoa and helminths are two major groups of organisms acting as etiologic agents of animal and human diseases with variable severity, especially for immunocompromised hosts resistance [
28,
29,
30,
31]. Parasitic infections caused by protozoa and helminths infect predominantly puppies, kittens, geriatric, chronically sick or immune-compromised animals and perhaps pregnant animals [
32]. Older dogs and cats are mainly immune after previous infections and seldom show symptoms, however, may still be a source of transmission of infection [
25,
26,
27,
28,
29,
30,
31,
32,
33]. Companion animals living in crowded conditions and poor sanitation or with access to the outdoors, may have a high risk of direct transmission of protozoan infections, for example the following:
Giardia sp.,
Trichomonas sp.,
Cryptosporidium sp. and
Cystoisospora sp. [
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35]. Dogs and cats may contact rodents or ingest raw meat are in risk to acquire infections caused by cyst-forming coccidia, i.e.,
Neospora sp.,
Hammondia sp.,
Toxoplasma sp. and
Sarcocystis sp. [
25,
26,
27,
28,
29,
30,
31,
32]. Dogs and cats can also be infected with others gender and parasites species, like
Ancylostoma sp.
Angiostrongylus sp. [
36,
37,
38]. Emerging infectious diseases may increase frequently in some regions, either due to increased importation of infected vertebrate hosts or by the establishment of pathogens and their vectors in previously non-endemic areas [
39]. Cases of zoonotic infections such as leishmaniosis, babesiosis and dirofilariasis was detected in some non-endemic regions, mainly due to the expansion of the parasitic transmission area and their increasing occurrence in wildlife, which act as reservoir or hosts of their complex cycles [
38,
39,
40,
41,
42]. Leishmaniasis affects humans and domestic companion animals and wild animals worldwide, involving reservoir and hosts such as rodents, marsupials, edentates, monkeys and wild canids [
40,
41,
42,
43]. More recent surveys reported from several countries in the world indicate that zoonotic parasites (
Leishmania sp.,
Babesia sp.,
Toxoplasma sp.,
Neospora sp. and
Dirofilaria sp.) are associated with infections and coinfections in companion animals [
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38]. Moreover, parasitic infections represent a serious public health threat, particularly in developing countries like Brazil where many unwanted animals are simply abandoned without clinical diagnosis to suffer and die on the streets of rural areas, but also in suburbs and large urban centers suffer deficient sanitation services [
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39,
40,
41,
42,
43,
44,
45] (
Tables S1 and S2—Supplementary Contents 2) [
1]. However individual case reports can add new parameters for the accuracy of diagnosis, to confirm the coinfection and the range of differential diagnoses - or if there the animal remains healthy or develops a mild, self-limiting illness [
15]. Considering the canine leishmaniasis, these hardy dogs mount a weak antibody response, but a strong and effective Th1 response may have low antibody titers but produce IFN-γ in response to antigens parasitic, generate type I granuloma, mount a strong response of hypersensitivity of the late type, and eventually destroy the parasites [
15]. The resistance to
Leishmania has a strong genetic component; for example, dogs of the Podengo breed Ibicenco (Ibizan Hounds - antique hunter of rabbits) appear to be resistant to this parasite. There is also an association between resistance and certain MHC class II haplotypes, as well as certain Slc11a1 (Nramp) alleles in dogs [
46]. Thus, the Ibizan Hounds may be an interesting canine model for the investigation of protective anti-
Leishmania immune response [
46]. Results of recent research show relevant differences between the cytokine serum profile and the data published for other canine breeds, and several genetic fixed variants in genes related to immune response, regulation of immune system, and genes encode cytokines and its receptors [
47]. The most relevant genes that present such fixed polymorphisms were IFNG and IL6R [
47]. Other variants with frequencies equal or above 0.7 were found in the genes ARHGAP18, DAPK1, GNAI2, MITF, IL12RB1, LTBP1, SCL28A3, SCL35D2, PTPN22, CIITA, THEMIS, CD180 [
47]. Epigenetic regulatory genes as HEY2, L3MBTL3 show also intronic polymorphisms [
47]. Future studies will can reinforce why the regulation of immune response is different in the Ibizan hound dogs compared to other breeds [
47]. By other side, some dogs develop severe and generalized nodular dermatitis, lymphadenitis granulomatous, splenomegaly and hepatomegaly, exhibiting activation of polyclonal (occasionally monoclonal) B lymphocytes involving all four classes of IgG, as well as hypergammaglobulinemia, and develop lesions associated with hypersensitivity types II and III [
15]. Additionally, excessive production of immunoglobulin can lead to the development of an immune-mediated hemolytic anemia, thrombocytopenia and the production of antinuclear antibodies [
15]. The chronic deposition of Immune complexes can result in glomerulonephritis, uveitis, and synovitis, leading to failure renal and death [
15]. The significant elevation of ant histone antibodies is a feature of some dogs with glomerulonephritis associated with leishmaniasis. There is a positive correlation between the levels of these ant histone autoantibodies and the protein/creatinine ratio once that antibodies increase the likelihood of the development of glomerulonephritis [
15]. But, despite their antigenicity, parasitic protozoa manage to survive in their host using multiple evasion mechanisms acquired over many millions of years of co-evolution [
1,
2,
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15]. Following the reproducible results for lipid bodies of
Leishmania labeled with BODIPY™ 493/503 we believe that the crucial 72 hours period has showed clear decrease of lipids released. This regression is in accord with Zhang (2021) in the way that amastigotes acquire most of their lipids from the host although they retain some capacity for de novo synthesis, differently of promastigotes that rely on de novo synthesis to produce the majority of their lipids including glycerophospholipids, sterols and sphingolipids [
48].