Canine tick-borne diseases in Europe

In the first part of this series on canine vector-borne diseases in Europe, we looked at leishmaniosis, dirofilariosis and borreliosis. In this follow-up article, we will be looking at three vector-borne diseases that are transmitted to dogs by hard ticks. These are babesiosis, and ehrlichiosis and hepatozoonosis for which Rhipicephalus sanguineus is the vector.

With the recent increase in importation of pets and a widening global parasite distribution, the rapid recognition and effective use of preventative treatments and screening in imported dogs is essential in limiting parasite spread and lowering zoonotic risk.

Babesiosis

Babesia spp. are haemoprotozoa that are transmitted by hard ticks and exclusively infect erythrocytes. Babesia canis is endemic in northern Spain, Portugal, France, the Netherlands and Italy and is present focally in central and eastern Europe up to the Baltic region, as associated with the distribution of the Dermacentor species of ticks. At least one endemic focus has also been identified in the UK. Besides B. canis (a large Babesia species), small Babesia spp. such as Babesia gibsoni can sporadically occur in Europe.

Clinical signs

A Babesia infection can lead to immune-mediated haemolytic anaemia in dogs. This anaemia is most commonly acute, and dogs present with pale mucous membranes, icterus, fever and hepatosplenomegaly. Associated depression and anorexia may be present, as well as dark brown urine associated with haemoglobinuria. Concurrent thrombocytopenia may be present with petechiation on gums, spontaneous bleeding or bruising.

Infection with Babesia is often lifelong and relapses of clinical disease are common

Travel history will often be present for acute infections but may have occurred months or years previously as infection with Babesia is often lifelong and relapses of clinical disease are common. Anaplasma platys is a cause of cyclic thrombocytopenia in dogs, so this should be considered as a differential diagnosis in travelled dogs suffering from recurrent bouts of thrombocytopenia.

Diagnosis

A diagnosis of acute babesiosis can be confirmed with high sensitivity by examination of thin blood smears (Giemsa stain or Diff-Quik) to detect large or small Babesia spp; freshly prepared blood smears can be used. For B. canis, peripheral capillary blood taken from the ear pinna or the tip of the tail may yield a higher number of parasitised cells. A rapid diagnosis of the acute disease is therefore possible when a sick animal is first presented.

B. canis are large, piriform organisms found singularly or in pairs in erythrocytes. B. gibsoni are generally single, rounded intracellular organisms, but can occasionally be seen as four linked organisms (forming a “Maltese cross” shape) within single red cells. Parasitaemia is normally low in B. gibsoni-infected dogs. The diagnosis of Babesia gibsoni carrier dogs is a challenge under clinical settings due to very low and often intermittent parasitaemia. Specific antibodies can only be detected from two weeks after the first infection. Acute infections will therefore be missed if relying on serology for diagnosis.

The diagnosis of Babesia gibsoni carrier dogs is a challenge under clinical settings due to very low and often intermittent parasitaemia

Species- or subspecies-specific PCRs (including real-time PCRs) have been described and are being increasingly used in routine laboratory diagnosis. The sensitivity of the PCR has proven to be higher than blood smear examination, especially for the diagnosis of chronically infected dogs, but false negative results cannot be completely excluded. Identification of species and subspecies of this tick-borne disease can be important in terms of treatment options and prognosis.

Ehrlichiosis

Ehrlichia are Gram-negative, obligate intracellular bacteria. In Europe, Ehrlichia canis is the aetiological agent of canine monocytic ehrlichiosis (CME). This tick-borne pathogen mainly infects lymphocytes and monocytes wherein the typical microscopically visible microcolonies (morulae) develop. The main host of E. canis is the dog, though other canids can serve as reservoirs of infection. The geographical distribution of E. canis generally corresponds to the distribution of its vector, R. sanguineus. Countries with reported infections in dogs are France, Italy, Portugal, Greece, Switzerland, Germany, the UK and Spain.

Clinical signs

During the acute phase of CME, which lasts around one to three weeks, dogs show apathy, weakness, lethargy, anorexia, dyspnoea, fever, lymphadenopathy, splenomegaly, weight loss and vomiting. The clinical signs associated with the haematological abnormalities of this tick-borne disease are pale mucous membranes, petechiae, ecchymoses, epistaxis, prolonged bleeding during oestrus, haematuria and melaena. The clinicopathological abnormalities include thrombocytopenia, leucopenia and mild to moderate normocytic, normochromic and non-regenerative anaemia.

In the subclinical phase, which may last for weeks or months, Ehrlichia-positive dogs appear clinically normal. Typical laboratory abnormalities include anaemia, thrombocytopenia and hypergammaglobulinaemia.

In the subclinical phase, which may last for weeks or months, Ehrlichia-positive dogs appear clinically normal

Chronic CME is characterised by a very complex clinical picture. Noticeable signs are weakness, apathy, sustained weight loss, fever, lymphadenopathy, splenomegaly, peripheral oedema in the hindlimbs and scrotum, pale mucous membranes, a predisposition for bleeding with haemorrhages in the skin and mucous membranes, mucopurulent ocular and nasal discharges, epistaxis and haematuria. In addition, interstitial pneumonia with dyspnoea, renal dysfunction, glomerulonephritis, arthritis, polymyositis and lameness may occur, though this is less common.

Diagnosis

The diagnosis of Ehrlichia infections in dogs is generally based on the combination of a thorough anamnesis to assess the possibility of an exposure to ticks, the assessment of clinical signs, laboratory abnormalities (CBC and biochemical profile), serology and/or PCR.

Morphological diagnosis

A morphological diagnosis is confirmed if the microscopic examination of blood smears reveals morulae in lymphocytes and/or monocytes. During the course of CME, morulae are rarely found. Lymphocytes and monocytes (4 percent in the acute phase) are infected but not granulocytes. In order to increase diagnostic sensitivity, buffy coat smears or lymph node aspirates should be performed.

Serology

Serology is highly sensitive and specific to detect exposure to the parasite. Seroconversion may occur one to four weeks after exposure to infection, thus dogs with acute infections can be serologically negative. Quantitative serology is useful in the case of E. canis infection where a four-fold increase in test titres taken two weeks apart is indicative of active infection. PCR is also highly sensitive and specific to confirm infection. A positive PCR result generally confirms the presence of infection; however, a negative result does not exclude the presence of an Ehrlichia infection.

Hepatozoonosis

The host range of Hepatozoon canis in mammalian carnivores other than the domestic dog has not been clarified. H. canis,or Hepatozoon spp. morphologically resembling H. canis, has been reported from several wild canine species, from domestic and wild felines and from other carnivorous species including the red fox. Infection with this tick-borne disease has been reported in the domestic dog from southern European countries including Greece, Italy, France, Spain, Portugal, Croatia, Kosovo, Albania and Bulgaria. The primary vector of H. canis is the brown dog tick, R. sanguineus. Dogs become infected with H. canis when they ingest a tick rather than via a tick bite, unlike most other tick-borne infections (Sykes and Greene, 2011).

Dogs become infected with H. canis when they ingest a tick rather than via a tick bite, unlike most other tick-borne infections

Clinical signs

A variety of clinical presentations is associated with H. canis infection. These range in severity from an incidental haematological finding in an apparently healthy dog to a debilitating and life-threatening illness.

A low level of H. canis parasitaemia with gamonts found in less than 5 percent of neutrophils is the most common presentation of H. canis infection. It is usually associated with an asymptomatic to mild disease. A high level of parasitaemia, sometimes approaching 100 percent of neutrophils, with a leucocytosis is usually associated with severe disease. High parasitaemia rates are frequently found concurrently with extreme leucocytosis, reaching as high as 150,000 leucocytes per microlitre of blood. Dogs with high parasite numbers suffer mainly from fever, lethargy and weight loss.

Concurrent infections involving H. canis and other canine pathogens are common. Some of the reported co-infecting organisms, including Ehrlichia canis and Babesia canis, are transmitted by the same tick vector, R. sanguineus, and are likely to be found in dogs with tick infections in areas where these diseases are endemic in the dog population.

Anaemia is the most common haematological abnormality in H. canis infection and has been reported in the majority of cases

Anaemia is the most common haematological abnormality in H. canis infection and has been reported in the majority of cases. The anaemia is, in most cases, normocytic, normochromic and occasionally regenerative. The leucocyte count is usually in the reference limits when parasitaemia is low and is elevated in dogs with high parasitaemia. Thrombocytopenia is present in approximately one third of dogs with H. canis infection and is, in some cases, associated with concurrent canine ehrlichiosis.

Serum chemistry abnormalities include hyperproteinaemia with hyperglobulinaemia and hypoalbuminaemia and increased creatine kinase and alkaline phosphatase activities.

Diagnosis

Blood smear examination carries a high sensitivity and specificity for H. canis detection. Examination of buffy coat smears is more sensitive than routine blood smears; however, PCR methods are even more accurate.

Conclusion

As with other vector-borne diseases, it is very important that veterinary professionals recognise the clinical signs of tick-borne diseases and diagnose them early. With the increasing number of imported pets and the growing interest in importing rescue dogs from abroad, it is increasingly likely that veterinary professionals will encounter exotic vector-borne diseases. These not only present a health risk to individual dogs, but to members of the public and to UK biosecurity. As such, veterinary professionals have a vital role in not only assessing imported dogs for tick-borne diseases but putting effective preventive measures in place as well.