This study is a preliminary investigation to verify the applicability of Raman spectroscopy as an alternative methodology for the diagnosis of canine leishmaniasis [27]. Raman Spectroscopy represents a versatile and fast method that allows us to distinguish between healthy and sick animals, also helping during therapeutic monitoring; in fact, Raman Spectroscopy is a useful tool providing data about motions in molecules and in pathological events, it is possible to observe changes, which exert their effects on molecular motions [39].
The results of the research did not entirely satisfy the hypothesis, although they are very interesting and encouraging, since the spectra obtained in the animals of each group are very homogeneous, practically overlapping them. The clear difference in intensity observed between the two spectra suggests the validity of Raman spectroscopy in the diagnosis of canine leishmaniasis, as recently proposed in studies conducted in mouse models infected with L. braziliensis [40].
In Fig. 3, it appears clear that, as disease occurs, an evident shift to the shortwave region is observed in spectral frequencies of the band centered at ~ 1250 cm− 1, corresponding to the region of leucine protein [30]. It is known that a critical point in the host-parasite interaction involves the attachment to and invasion of host macrophages. It occurs through the membrane coating formed by layers of glycolipids such as lipophosphoglycan (LPG), proteophosphoglycan (PPG), and proteins such as MSP/GP63 (major surface protein 63-kDa glycoprotein), PSA-2/GP46 (parasite surface antigen-2 or 46-kDa glycoprotein) and amastin surface proteins, which are expressed by amastigotes during their development [40]. The frequency shift in the region of leucine, observed by Raman spectroscopy in the serum of LG, could be related to the host-parasite interaction. Leucine is a constituent protein of the parasite surface antigen (PSA2) primary structure, which is one of the major antigens of the surface of the L. infantum parasite membrane. The most suggestive functional determinant in the PSA2 primary structure is the presence of leucine rich repeats (LRR). LRRs are primarily known to be involved in protein-protein and protein-glycolipids interactions, whereby the leishmania amastigote interact with host macrophages and resist to complement lysis [41, 42].
Observing the results depicted in Fig. 4, a subject is assessed as affected (positive) if the A1350/A1250 ratio tested marker values in greater than the 1.316 threshold value; otherwise the subject is diagnosed as NON-LD subject.
Moreover, some authors reported that LRRs are the main epitopes in L. infantum PSA during canine and human visceral leishmaniasis. They showed that the central region of PSA, formed exclusively by LRRs, has a very high antigenic capacity and it is recognized by 100% of the sera from the infected dogs and 40% of the human sera. Thus, the LRRs would be the immunogenic determinants of the PSA during natural leishmania infection, being the responsible of the host antibody response [43]. The modification of the Raman spectrum of amide III and in particular in the region leucine observed in sick subjects could be related to the mechanisms of parasite host interaction. The frequency shift in the region of phospholipids, observed by Raman spectroscopy in the serum of LG, seems also to confirm the key role of lipid metabolism in the pathogenesis of the disease [44–46]. In mammals, the host-Leishmania interaction involves the attachment and invasion of host macrophages, initially by metacyclic promastigotes and subsequently by amastigotes. In particular, it has been shown that the alteration of the pathways of lipid metabolism, the transfer, modification and accumulation of lipids during infection with Leishmania spp. are a key point in disease progression [46].
Results of the present study showed that Raman spectroscopy could be considered a diagnostic tool for detection of dogs affected by leishmaniasis. Accuracy, sensitivity and specificity of the proposed diagnostic test were determined using Receiver Operating Characteristic (ROC) curve and Youden index (Y), furnishing excellent results, which demonstrated the feasibility of Raman spectroscopy as diagnostic modality for rapid de-tection of LD subjects.
All the results obtained by using Raman Spectroscopy technique were confirmed by those obtained using the methodology reported in CLWG guidelines [26–27].
The few number of animals, however, does not allow us to formulate conclusive considerations and further investigations in dog cohorts will be needed, numerically significant, differentiated by clinical status and antibody titer and/or parasitic load (e.g., quantitative PCR).
Another limitation of this study is represented by the presence of only healthy and infected animals with L. infantum. Although the ability to distinguish between healthy and diseased animals is unequivocal, it cannot be excluded that the variation in the Raman spectra may be related to an organic / metabolic perturbation rather than to the specific disease.