DIAGNOSIS

As with many diseases, an integrated approach is required to diagnose leishmaniasis. First of all, one must remember that canine leishmaniasis does not necessarily result in clinical disease. While most animals living in an endemic area do carry the parasite, only part of them show clinical signs.

In addition, many dogs with leishmania may suffer from other diseases, mostly caused by Erlichia, Borrelia, Hepatozoon, etc. Hence the utmost importance of performing a differential diagnosis based on clinical and pathologic abnormalities. To this end, complete blood count, complete blood chemistry and urinalysis are indispensable.

In animals suspected of having leishmaniasis, clinical signs are the first indicators of the presence of the disease. Disease confirmation is usually obtained by means of indirect (serology) and direct (PCR or parasite visualization in bone marrow or lymph node cytology slides) methods.

Parasite isolation and proliferation from infected tissues is unpractical for rapid diagnosis. Indeed, these practices are considered to be less sensitive –that is, less able to detect positive animals– than PCR and serology. At any rate, direct visualization of Leishmania amastigotes in tissues such as lymph node or bone marrow is considered to be a diagnosis of leishmaniasis.

Indirect diagnostic methods include different techniques for antibody detection. The immunofluorescence antibody test (IFAT) is usually performed starting with promastigotes as antigens and using an anti-IgG fluorescein conjugate. Like any immunofluorescence technique, it is time consuming and requires some degree of experience to interpret the reaction visualized under a microscope.

In this sense, ELISA techniques are an alternative approach that brings together sensitivity and specificity with easy handling and easy interpretations. The fact that the reading is taken by an ELISA microplate reader leaves no room for subjective interpretations. In addition, the whole process can be fully automated, thereby further reducing the chance of human error. While positive serology is obtained in 88-100% of symptomatic animals, only a small number of asymptomatic dogs are positive. That is where serology becomes useful: however indirect it may be, it is the best diagnostic technique to assess the presence of the disease.

PCR (PROTEIN CHAIN REACTION). The sensitivity of this test depends on the number of copies of the fragment to be detected and amplified that can be found inside the parasite. PCRs that target genomic ribosomal DNA are less sensitive than those that target kinetoplastid DNA –which has around 10,000 copies per parasite. Quantitative PCR techniques (real.time PCR or qPCR) are more advanced techniques that allow detecting extremely low amounts of parasite as compared to conventional PCR techniques. PCR allows detecting infected animals, but not sick animals. Indeed, a good immune (cell-mediated) response means that the animals will not fall ill with leishmaniasis but will remain PCR positive.