Vector-borne diseases are caused by a wide range of infectious agents including viruses, bacteria and parasites (protozoa and helminths) which are transmitted by a variety of arthropod vectors, such as ticks, lice, fleas and Diptera (in particular mosquitoes, phlebotomine sand flies and muscid flies).
Canine vector-borne pathogens and their associated diseases are important because:
- They may be highly pathogenic in dogs
- Their transmission is often unpredictable
- Their diagnosis and control are difficult
- Variable clinical signs can develop after long incubation periods, and these are rarely pathognomonic
- Animals may have persistent infections and thus act as reservoirs
- Several vector-borne diseases are important zoonoses, such as leishmaniosis, borreliosis, rickettsiosis, bartonellosis, anaplasmosis and dirofilariosis
Increased importation of pets comes hand-in-hand with expanding global parasite distribution, and it is increasing the risk of pets infected with exotic infectious agents entering the UK. Rapid recognition of the clinical signs, effective use of preventive treatments and screening tests in imported dogs are vital, both to improve prognostic outcomes for infected dogs and to limit parasite spread and zoonotic risk.
This series will discuss the detection and diagnosis of six important vector-borne diseases found in Europe, most of which have zoonotic implications. Leishmaniosis, dirofilariosis and borreliosis will be discussed in this first article, while the tick-borne babesiosis, ehrlichiosis and hepatozoonosis will be discussed in a follow-up article.
In Europe, canine leishmaniosis is predominantly caused by Leishmania infantum. The infection by L. infantum is typically transmitted by a specific group of phlebotomine vectors (more commonly known as sand flies) which represent the main risk of transmission; however, non-vectorial modalities have also been demonstrated. These modalities include venereal, vertical, dog-to-dog and blood transfusion. Dogs are the main reservoir of L. infantum infection, but cats can also be hosts. Canine leishmaniosis is endemic in southern Europe, and there is one study that demonstrated the presence of canine leishmaniosis in Romania (Dumitrache et al., 2016).
The clinical signs of this vector-borne disease are quite variable depending on immune responses and disease history, but they commonly include lymphadenomegaly, alopecia, dermatitis, hyperkeratosis, dermal ulcers, lameness, anorexia, weight loss, conjunctivitis and epistaxis. Mucosal lesions, osteolytic or osteoproliferative lesions, chronic colitis, splenomegaly and hepatomegaly are other possible signs.
Common clinicopathological abnormalities include mild to moderate non-regenerative anaemia and, less frequently, thrombocytopenia and leucopenia. Plasma protein changes with hyperglobulinaemia and hypoalbuminaemia are particularly common. Proteinuria and variable azotaemia with an increase in the urine protein/creatinine ratio, due essentially to immune-mediated glomerulonephritis, are present in some Leishmania-infected dogs and are considered an indicator of poor prognosis.
A pet positive for Leishmania infection may have other infections responsible for presenting clinical signs, such as Ehrlichia canis
Clinical signs may take months to years to develop, so foreign travel may not be recent. Infected pets may be subclinically infected and mixed infections with tick-borne pathogens are common, so a pet positive for Leishmania infection may have other infections responsible for presenting clinical signs, such as Ehrlichia canis.
Diagnosis is based on clinical signs and/or clinicopathological abnormalities that are compatible with this vector-borne disease and by confirmation of L. infantum infection using mainly serological and molecular techniques.
Serology is the most common first step allowing for the detection of a specific antibody response in dogs around 12 weeks after initial infection. In subclinical infections, this period may extend to years. Different laboratory-based methods have been used to detect anti-Leishmania antibodies; these include enzyme-linked immunosorbent assays (ELISAs, offered by NationWide Laboratories), indirect fluorescent antibody tests (IFAT) and western blots.
Diagnosis is based on clinical signs and/or clinicopathological abnormalities… and by confirmation of L. infantum infection using mainly serological and molecular techniques
Polymerase chain reactions (PCRs) have proven to be highly sensitive, especially if done on the following samples: lymph node aspirates, bone marrow, spleen, skin biopsies or conjunctival swabs. Blood samples for PCR are less sensitive, and the most sensitive technique is the real-time PCR. Direct diagnosis is possible by detecting the amastigote stages in Giemsa or Diff-Quik stained smears obtained from superficial lymph nodes, bone marrow aspirates or skin biopsies.
Dogs that have travelled to endemic areas and do not show clinical signs should be tested for L. infantum infection six months post-travel via quantitative serology.
Dirofilariosis – canine heartworm disease
Canine heartworm disease (HWD) is caused by infection with Dirofilaria immitis (ESDA, 2017; American Heartworm Society, 2020). Dogs with HWD harbour adult parasites in their pulmonary arteries. Microfilariae circulate in the blood and are taken up by mosquitoes. Approximately 15 days later, larvae become infective L3 that are introduced into a new host. Following several months of tissue migration, the parasites arrive in the pulmonary artery and begin to release microfilariae in the new host, beginning the cycle anew. D. immitis is endemic across southern Europe (Portugal, Spain, Italy and Greece), and in the Czech Republic, Slovenia, Romania and Bulgaria.
Coughing, tachypnoea, dyspnoea and exercise intolerance are the most common clinical signs seen in chronically infected dogs. Acute clinical signs are associated with thromboembolism, subsequent pulmonary hypertension and caval syndrome. Worm death can also lead to thromboembolism and anaphylaxis. The typically resulting acute clinical signs include sudden death, anorexia, weakness, dyspnoea, vomiting and, rarely, respiratory signs linked to pleural effusion.
Clinicopathological abnormalities during heartworm infection are non-specific, may not always be present and are commonly related to an inflammatory state
Clinicopathological abnormalities during heartworm infection are non-specific, may not always be present and are commonly related to an inflammatory state. Most common changes seen in infected dogs include leucocytosis, non-regenerative anaemia, eosinophilia and neutrophilia. More rarely thrombocytopenia can also be found, especially when disseminated intravascular coagulation is present. Changes in clinical biochemistry include azotaemia, a rise in liver enzymes and hyperbilirubinemia, whereas urinalysis may indicate proteinuria.
Diagnosis of HWD in dogs is based on the detection of microfilariae and/or the detection of circulating heartworm antigens.
Microfilariae may be detected on blood smear exams, but for more accurate results a concentration technique such as the modified Knott test should be used. The modified Knott test remains the preferred method for observing morphology and measuring body dimensions to differentiate D. immitis from non-pathogenic filarial species, such as Acanthocheilonema reconditum.
Diagnosis of HWD in dogs is based on the detection of microfilariae and/or the detection of circulating heartworm antigens
Antigen tests, including ELISAs and immunochromatographic tests, are available for detecting circulating heartworm antigens. These antigens are detectable only when the female heartworms develop to the adult stage, therefore antigen testing should not be carried out earlier than seven months after exposure to infection. Low worm burdens, with few or no adult females, can result in significantly decreased sensitivity of antigen tests.
Borreliosis – Lyme disease
There are currently 11 known species/genotypes of the Borrelia burgdorferi complex, which are spirochaetes that infect many mammals and birds. These vector-borne bacteria are transmitted by ticks of the genus Ixodes. Human infections are of major public health importance and although infections have been demonstrated in dogs they are not of major clinical importance. Humans as well as dogs acquire Borrelia infection when exposed to infected ticks, but there is no interdependency between dogs and humans in terms of transmission. Endemic areas of borreliosis are related to the distribution of their tick vectors. Lyme borreliosis is present all over Europe, except in extremely hot southern or cold northern areas.
Most Borrelia-infected dogs have subclinical status, and it is difficult to correlate naturally acquired B. burgdorferi infection with clinical signs, such as fever, lameness, myalgia and lethargy, in dogs. “Lyme arthropathy”, which is defined as lameness in one or more joints, has been described; puppies may be at higher risk of polyarthritis. The term “Lyme nephropathy” has been used for a syndrome of protein-losing immune complex nephropathy that occurs in 2 percent of seropositive dogs. However, renal invasion of this vector-borne spirochaete does not appear to be responsible for the histopathological and functional changes in the kidneys of affected dogs. It is estimated that less than 5 percent of dogs with suspected Lyme disease actually have the disease due to B. burgdorferi.
It is estimated that less than 5 percent of dogs with suspected Lyme disease actually have the disease due to B. burgdorferi
Some studies suggest that the detection of Borrelia by PCR may be difficult because B. burgdorferi invades the soft tissue of dogs after transmission by ticks and does not appear to be present in the bloodstream or urine of infected dogs. That is why the organism is rarely found in blood, urine, joint fluid or cerebral spinal fluid, but can be detected in skin and/or synoviae. Antibodies against Borrelia usually appear three to five weeks after infection and can be detected using several commercially available qualitative and quantitative immunochromatographic tests; however, positive results merely indicate exposure to the infection rather than existing disease.
It is very important that veterinary professionals recognise the clinical signs of vector-borne diseases and diagnose them early, given their morbidity and possible mortality. The growing trend of importation of rescue dogs from abroad means that it is increasingly likely that veterinary professionals will encounter exotic vector-borne diseases which present a health risk to individual dogs, members of the public and UK biosecurity as a whole. Veterinary professionals have a vital role in assessing imported dogs for evidence of vector-borne diseases and putting effective preventive measures in place.
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