The causative agent of canine leishmaniosis and (human) infantile leishmaniasis, Leishmania infantum, is a protozoan parasite of the family Trypanosomatida transmitted by female phlebotomine vectors (sand flies) (Moreno and Alvar, 2002; Muniesa et al., 2016; Santarém et al., 2020). Other species within the genus (eg L. donovani and L. major) have also been occasionally diagnosed as causing disease (Kasabalis et al., 2020). Similarly, alternative routes of transmission between domestic dogs have been demonstrated including transplacental, sexual and direct dog-to-dog spread (Kasabalis et al., 2020; Montoya et al., 2020). Canid species including the domestic dog are the main reservoir hosts, but hares and humans can also act as maintenance reservoirs for the parasite.
Canine leishmaniosis is endemic throughout the Mediterranean basin and the southern states of the USA, as well as the continents of South America and Africa (Maia et al., 2009).
With increasing numbers of pet dogs travelling abroad or being rescued to homes in the UK from countries where the parasite and vector are endemic, sporadic cases are appearing across the UK more frequently than ever before. These cases can be challenging, particularly for clinicians who may not have come across such cases before, to suspect, diagnose and manage effectively. Due to the proven zoonotic risk it is essential that leishmaniosis cases are handled appropriately and the following article aims to provide an overview of the condition, the choices for diagnosis and currently available treatment options in the UK.
The canine response to infection
The parasite L. infantum is an obligate intracellular parasite and so, when it is inoculated from a sand fly bite into a naïve host, it preferentially infects cells and particularly those of the myeloid lineage (monocytes, macrophages and dendritic cells), triggering a cell-mediated immune response (Barbiéri, 2006; Reis et al., 2010; Toepp and Petersen, 2020). Phagocytosis of the parasite by neutrophils and macrophages occurs rapidly (Toepp and Peterson, 2020). Once engulfed within the phagolysosome the parasite is either destroyed by the production of oxygen radicals or persists to replicate by engaging antioxidant mechanisms such as superoxide dismutase production (Barbiéri, 2006; Reis et al., 2010; Toepp and Petersen, 2020). Engaged innate cells produce and secrete pro-inflammatory cytokines, chiefly TNF-α, IFN-γ and IL-8, which leads to the recruitment and activation of more phagocytes as well as CD4+ Type 1 T-helper (Th1) cells. At this point of the infection, the number of parasites is well controlled by the host immune response and the patient will show no overt clinical signs (subclinical infection) (Barbiéri, 2006; Reis et al., 2010; Toepp and Petersen, 2020).
Over time, and this can be many years, there is a switch from the early pro-inflammatory response to a more regulated response due to prolonged antigenic stimulation (Toepp and Peterson, 2020). Here, the cytokine IL-10 is increasingly expressed (from initially very low or undetectable levels) which has a negative feedback effect on Th1 cell proliferation and so reduces IFN-γ production and subsequent macrophage activation (Barbiéri, 2006; Reis et al., 2010; Toepp and Petersen, 2020). This state of immune exhaustion is what leads to the development of clinical disease and occurs in about 5 to 10 percent of infected dogs, though the rate is higher in susceptible breeds, such as Foxhounds, due to genetically inherited immune defects (Barbiéri, 2006; Reis et al., 2010; Toepp and Petersen, 2020).
The role of B-cells in subclinical infections is predominately phagocytosis and antigen presentation; what little antibody production occurs allows greater immune-mediated pathogen destruction such as the opsonisation of parasites (Barbiéri, 2006; Reis et al., 2010; Toepp and Petersen, 2020). Once immune exhaustion is reached there is more widespread antibody production and a class switch to IgG. These antibodies form immune complexes with Leishmania antigen and C3 complement proteins that can deposit within small blood vessels such as those in the kidney and can lead to organ failure (Gizzarelli et al., 2020).
Diagnosis and clinical signs
Understanding what is known about the immunology of the host response to L. infantum by dogs in different stages of the infection processes can help guide clinicians in their choice and interpretation of appropriate diagnostic tests.
In a subclinically infected dog it would be expected that quantitative antibody measurement in serum samples, for example by enzyme-linked immunosorbent assay (ELISA) or immunofluorescence antibody tests (IFAT, the reference test in most European countries), will demonstrate low levels of circulating specific immunoglobulins (Reis et al., 2010; Muniesa et al., 2016). Since 2015 these tests have typically used recombinant antigen K39 which was shown to have a 100 percent sensitivity and specificity when evaluated on 122 dogs (Reiter-Owona et al., 2016). At this stage, the parasite itself will usually be detectable in whole blood by quantitative PCR due to its presence within circulating monocytes (Muniesa et al., 2016; Marcelino et al., 2020). As the disease progresses to the clinical phase, antibody titres rise and this is quantifiable in serum samples from patients. Cytological or histopathological examination of lesions can also be extremely helpful in establishing a diagnosis by demonstrating a compatible inflammatory pattern and/or parasite amastigotes (Muniesa et al., 2016; Marcelino et al., 2020).
In practice, the highly variable clinical manifestations and relative rarity of the disease complicates the diagnosis (Montoya et al., 2020). Cutaneous leishmaniosis has a common form which presents with skin lesions, observable in over 80 percent of the clinically affected animals. Potential signs are hyperkeratosis, alopecia, papules, pustules, nodules, erosion and ulceration. Peripheral lymphadenomegaly is also commonly found. Visceral leishmaniosis will cause a range of signs, typically systemic and affecting multiple organs (eg weight loss, lethargy, vomiting and diarrhoea or epistaxis). At the latter stages of disease, kidney dysfunction is almost ubiquitous (96 percent in one study (Giapitzoglou et al., 2020) due to the deposition of immune complexes and complement activation. This results in the typical findings associated with chronic kidney disease: polyuria, polydipsia, azotaemia and elevated SDMA concentrations in serum.
When disease is suspected, a thorough history and clinical examination is crucial. Because dogs can be asymptomatic for years, a complete travel history is particularly key.
Treatment and prognosis
Once confirmed, the prognosis varies from fair to very poor based on the severity of the clinical signs present (Kasabalis et al., 2020). Signs of chronic kidney disease is a major negative prognostic factor (Montoya et al., 2020).
The results of the most recently published prospective randomised blinded controlled treatment trial found that meglumine antimoniate (100mg/kg subcutaneously every 24 hours for 28 days) was superior to aminosidine (15mg/ kg subcutaneously every 24 hours for 28 days) with respect to the proportion of dogs ending the trial free from clinical signs (Kasabalis et al., 2020). Both groups received concurrent allopurinol at 10mg/kg orally every 12 hours for the full six-month duration of the study (Kasabalis et al., 2020). These combinations were reasonably safe and usually lead to clinical remission, amelioration of clinicopathological abnormalities and a reduction of parasitic load (Kasabalis et al., 2020). Notably, IRIS Stage 3 and 4 dogs were excluded from enrolment as they are known to respond poorly to treatment. A lack of clinical response, as well as a relapse whilst on treatment or shortly after, results in a more guarded prognosis (Montoya et al., 2020).
Suspecting and preventing cases
It is advisable to offer testing to any animal imported, especially dogs from endemic countries. A positive result needs to be discussed but does not mean the animal will necessarily ever show clinical signs. As a general rule, any dog presented with peripheral lymphadenomegaly and skin lesions and having travelled in endemic regions should be tested.
Prevention of infection is critically important. Convincing clients to protect their pets against an unfamiliar disease will always be more challenging. However, the severity of the clinical signs and implications of treatment should be explained, as well as the zoonotic potential. Any owner enquiring about a pet passport should be asked what part of Europe they are planning to visit. Some basic information about the disease and vector should be provided, and prevention discussed. Sand fly repellent collars are available and a vaccine is licensed for use in the UK (Anon., 2020).
Feline infections with L. infantum are considered rare compared to canine infections; however, recent studies have shown that they may be common in endemic areas (Maia et al., 2008; Trainor et al., 2010). Currently available guidelines from the Advisory Board on Cat Diseases were published in 2018 (ABCD, 2020). Cats manifest clinical signs similar to dogs with cutaneous lesions being most common (Maia et al., 2008; Trainor et al., 2010). The process of diagnosis is the same for cats as for dogs though treatment doses are the oral administration of allopurinol at 10 to 20mg/kg every 12 or 24 hours for six months and meglumine antimoniate administered subcutaneously at 50mg/kg every 24 hours for 28 days (Maia et al., 2008; Trainor et al., 2010; ABCD, 2020). Long-term follow-up data and clinical trials in cats are lacking. Preventative ectoparasiticides effective against sand flies and vaccines are licensed for dogs only; however, flumethrin collars were effective in reducing the incidence of feline Leishmania infection in one studied endemic area (ABCD, 2020).