Canine vector-borne diseases (CVBDs) are an emerging problem worldwide due to their frequency and morbidity and, in most cases, also to their zoonotic relevance, with dogs potentially serving as sentinels for human infection . CVBDs are caused by a diverse range of pathogens, mainly bacteria and parasites, which are transmitted to dogs by different arthropod vectors, particularly ticks and insects .
Nematode Dirofilaria immitis, bacteria Ehrlichia canis, Borrelia burgdorferi sensu lato, Anaplasma phagocytophilum and Anaplasma platys, and protozoan Leishmania infantum are among of the major vector-borne agents that can infect dogs .
D. immitis is transmitted by mosquitoes primarily from genera Culex, Aedes and Anopheles, and causes dirofilariosis or heartworm disease, a potentially fatal condition in dogs . Canine dirofilariosis is associated with a dry chronic cough, exercise intolerance, dyspnoea, weakness, weight loss, epistaxis, cyanosis and congestive heart failure . Dogs are the natural hosts, but infection may also occur in other canids and cats, and there is also a risk of zoonotic transmission . Human heartworm infections are relatively uncommon; nevertheless, D. immitis can cause pulmonary dirofilariosis in people with the occurrence of granulomas in the lungs [7, 8].
E. canis, a causative agent of acute or chronic canine monocytic ehrlichiosis, is transmitted by the brown dog tick, Rhipicephalus sanguineus. Dogs infected with E. canis present a spectrum of disease that ranges from subclinical infection to fatal illness . Clinical signs often include lethargy, anorexia, weight loss, hyperthermia, epistaxis and other haemorrhagic disorders, pale mucous membranes and lymph node enlargement . E. canis has a zoonotic potential as human infections have been reported from Venezuela .
B. burgdorferi s.l. spirochetes infect mammals, including dogs and human beings, and cause the so-called Lyme disease . In Europe, Ixodes ricinus ticks are important vectors of B. burgdorferi s.l. . Most people exposed to B. burgdorferi show mild non-specific symptomatology, but Lyme borreliosis can be a chronic debilitating disease in humans, with arthritis, skin changes and neurological or cardiac dysfunction . In contrast, relatively few infected dogs demonstrate clinical signs. However, canine borreliosis has been associated with lethargy, hyperthermia, anorexia, joint inflammation, lameness, lymphadenopathy and glomerulonephritis .
A. phagocytophilum, the agent of granulocytic anaplasmosis, is vectored in Europe by I. ricinus and can infect a wide range of domestic and wild vertebrate hosts, including rodents, horses, dogs and humans . Infection in dogs may be subclinical or result in a mild to severe acute illness, with lethargy, anorexia, hyperthermia, lameness and, occasionally, polydipsia, vomiting, diarrhoea and even neurologic signs . In human beings A. phagocytophilum induces a febrile syndrome associated with myalgia and headache, and is considered an emerging pathogen .
A. platys is a bacterium primarily of dogs that infects platelets and may cause canine infectious cyclic thrombocytopenia . Clinical signs include abnormalities such as lymphadenomegaly and pale mucous membranes, but canine infections with A. platys are mostly subclinical . Although its virulence is generally low, A. platys might play a role in co-infection with other vector-borne agents . The presumed vector of A. platys is R. sanguineus.
Dogs are the main reservoir of L. infantum, which is transmitted among canines and to humans by phlebotomine sand fly insects, Phlebotomus spp. in Europe . Canine leishmaniosis is a systemic chronic condition whose clinical manifestations usually include lymphadenopathy, dermatitis, alopecia, cutaneous ulceration, onychogryphosis, lameness, weight loss, cachexia, ocular lesions, epistaxis, anaemia and renal failure . A large majority of the infected dogs do not develop clinical signs but they may still be capable of transmitting the parasite to the vectors . In people, visceral leishmaniosis is the most severe clinical syndrome resulting from infections with L. infantum, and in Europe it is observed mainly in children and immunocompromised adults . Leishmaniosis due to L. infantum is a major zoonosis potentially fatal to dogs and humans, and infected dogs represent an important veterinary medical and public health problem .
Dogs can be sequentially or simultaneously infected with more than one vector-borne agent by being exposed to arthropods infected with a single pathogen species or to vector(s) concurrently infected with different organisms [2, 25]. Some arthropod species, particularly ticks, act as vectors of more than one agent and co-infection of individual arthropods can occur . Awareness of canine co-infections is an important clinical and diagnostic issue as they might induce more severe pathological effects than infections with either agent alone .
Diagnosis and screening are essential for the control of CVBDs, both at the individual and population levels, with detection methods including cytological examination of blood smears or other tissues, serology (for antibodies or antigens) and the polymerase chain reaction (PCR). Evidence of infection with or exposure to the causative agents of dirofilariosis, ehrlichiosis, borreliosis, anaplasmosis and leishmaniosis can be assessed via rapid in-clinic serological testing . Results of either single or co-infections must be interpreted in combination with data on the geographical origin, history of vector exposure and clinical status of dogs, along with other confirmatory tests .
Environmental changes, especially global warming, have an impact on the arthropods geographical distribution, abundance and vectorial capacity . Together with human and animal population dynamics, including the increased mobility of dogs, climatic changes may affect the occurrence and spread of CVBDs . Updated information on the epidemiology of infection and disease is required to map regional risk, identify new areas of endemicity and forecast CVBD scenarios .
D. immitis, E. canis, B. burgdorferi s. l., A. phagocytophilum, A. platys and L. infantum have been detected in Portugal in dogs and/or arthropods [31, 32]. Nevertheless, except for the latter , there is no comprehensive data available on the regional distribution and prevalence of these vector-borne agents at the countrywide level. The present study aimed at assessing the seroprevalence of infection with or exposure to D. immitis, E. canis, B. burgdorferi s. l., Anaplasma spp. and L. infantum in healthy and CVBD-suspect dogs in Portugal.