Determining resolution of Angiostrongylus vasorum in dogs following anthelmintic treatment with an imidacloprid 10 per cent/moxidectin 2.5 per cent spot-on.

OBJECTIVES: To determine the time from treatment with a product containing imidacloprid 10 per cent/moxidectin 2.5 per cent spot-on (Advocate™), to dogs becoming negative for Angiostrongylus vasorum (A. vasorum). The authors hypothesised that most dogs would have resolution of A. vasorum within four weeks of treatment with Advocate™.
DESIGN: Prospective, non-randomised, prepost treatment study.
SETTING: Cases were enrolled from general practices along the southern coast of the United Kingdom. PARTICIPANTS: Nine dogs completed the study and were enrolled if A. vasorum had been diagnosed based on a positive commercially available, in-clinic, serological A. vasorum antigen test (Angio Detect®) or Baermann performed at an external laboratory or both.
INTERVENTIONS: The only treatment A. vasorum-positive dogs received was Advocate™ which was applied at the time of diagnosis and reapplied if necessary at four-weekly intervals until dogs tested negative by Angio Detect. PRIMARY OUTCOME MEASURES: Angio Detect® was performed and Advocate™ was reapplied at four-weekly intervals until dogs tested negative by this method. SECONDARY OUTCOME MEASURES: Baermann was also performed at four-weekly intervals until dogs tested negative by this method.
RESULTS: Application of Advocate™ was an effective treatment for A. vasorum infection in dogs and resulted in resolution of the infection, based on Angio Detect® testing and Baermann, within four weeks, in eight out of nine dogs. Post-treatment Angio Detect® testing was concordant with Baermann in seven of nine dogs.
CONCLUSIONS: Application of Advocate™ was an effective treatment for A. vasorum infection in dogs and resulted in resolution of the infection within four weeks in most dogs. Repeat Angio Detect® testing is recommended following treatment of A. vasorum to confirm resolution of the infection.

Introduction

Angiostrongylus vasorum (A. vasorum) is a nematode parasite with a diffuse distribution across the United Kingdom (UK) in addition to mainland Europe, Asia and Africa, as well as an endemic focus in Newfoundland, Canada.1 Appropriate diagnosis and treatment is generally associated with a good prognosis for recovery1 in dogs with A. vasorum infection; however, the disease can be fatal if left untreated. Mature parasites inhabit the right heart, pulmonary artery and pulmonary arterioles. The host’s inflammatory response against these parasites results in respiratory disease; however, A. vasorum can also cause haemostatic disorders1 and concern for infection sometimes becomes apparent if prolonged haemorrhage is detected at the time of surgery.

The underlying cause of bleeding is poorly understood but may be related to a consumptive coagulopathy.1 2 Traditional coagulation parameters, such as activated partial thromboplastin time (aPTT) and partial prothrombin time (PT), can remain within normal limits in many infected dogs,2 and a previous study found that thrombocytopenia was only present in 4.2 per cent of infected dogs at the time of diagnosis.3

Thromboelastography (TEG) is a haemostatic assay that measures the global properties of whole blood clot formation; in one study, 17 out of 18 A. vasorum-infected dogs presenting to a referral centre with signs of bleeding on physical examination had evidence of hypocoagulability on TEG.2 TEG samples must be processed within a few minutes of venepuncture, making this unavailable to the large majority of primary care veterinary practices.

If only traditional coagulation parameters and a complete blood count are assessed before surgery, dogs with haemostatic disorders secondary to A. vasorum may be overlooked. A decision as to when to pursue elective surgery following treatment of A. vasorum therefore has to be based on the results of other diagnostic tests, such as Baermann or serological testing.

The aims of this study were to determine the time from treatment with an imidacloprid 10 per cent/moxidectin 2.5 per cent spot-on (Advocate™ spot-on solution, Bayer, Reading, Berkshire) to dogs becoming negative for A. vasorum and to investigate whether there were any discrepancies between post-treatment Baermann and a commercially available, in-house, serological A. vasorum antigen test (Angio Detect® test, IDEXX Laboratories, Westbrook, Maine, USA) for detection of persistent infection following treatment.

Correctly determining when the infection has resolved will have important implications for practitioners including when to transition from therapeutic to preventative treatment and when to perform elective surgeries.

Methods

Dogs were recruited from primary care veterinary practices and were not referred. Clinical history, including signalment and presenting clinical signs, were obtained if available. A diagnosis of A. vasorum was obtained by means of either a positive Angio Detect® test or Baermann performed at an external laboratory, or both. The only treatment A. vasorum-positive dogs received was Advocate™ spot-on solution. Angio Detect® test was performed and Advocate™ was reapplied at four-weekly intervals until dogs tested negative by this method. Baermann was performed on a single faecal sample at four-weekly intervals until dogs tested negative by this method.

Nine dogs were diagnosed with A. vasorum between December 2013 and July 2015. Where recorded, breeds affected included Staffordshire bull terrier (1), lurcher (1), labrador retriever (1), English springer spaniel (1), bulldog (1), Manchester terrier (1) and crossbreed (1). The median age of dogs included in the study was 6 years 6 months (range 9 months to 10 years 7 months). Presenting clinical signs and clinicopathological findings were available for seven dogs. Two of seven dogs were asymptomatic and routine testing was performed before elective surgical procedures. Five dogs presented with clinicopathological abnormalities, including gingival haemorrhage (1), prolonged PT (1), prolonged aPTT (1), low fibrinogen (1), haematochezia (1), regenerative anaemia (1) and coughing (2) (see Table 1).

TABLE 1:

Results

Number	Breed	Age	Sex	Clinical signs and laboratory findings	Initial diagnostic method	AD at 4 weeks	B at 4 weeks	AD at 8 weeks	B at 8 weeks	AD at 12 weeks	B at 12 weeks
1	Lurcher	6 years	FN	Gingival haemorrhage, prolonged aPTT and PT, decreased fibrinogen	AD	Negative	Negative	NP	NP	NP	NP
2	Labrador retriever	8 years	MN	Acute haemorrhagic diarrhoea	B	Negative	Negative	NP	NP	NP	NP
3	Unknown	Unknown	Unknown	None	AD	Positive	Negative	Positive	NP	Negative	NP
4	English springer spaniel	9 years 7 months	ME	Regenerative anaemia (PCV 18%), thrombocytopenia, haematochezia	AD	Negative	Negative	NP	NP	NP	NP
5	Staffordshire bull terrier	9 years	FN	Intermittent cough for two months	AD	Negative	Negative	NP	NP	NP	NP
6	Unknown	Unknown	Unknown	Unknown	AD	Negative	Negative	NP	NP	NP	NP
7	Bulldog	2 years 10 months	FE	None	AD	Negative	Negative	NP	NP	NP	NP
8	Manchester terrier	10 years 7 months	MN	Coughing	AD; B (negative)	Negative	Negative	NP	NP	NP	NP
9	Crossbreed	9 months	MN	None	AD	Negative	Negative	NP	NP	NP	NP

AD, Angio Detect test; aPTT, activated partial thromboplastin time; B, Baermann; FE, female entire; FN, female neutered ME, male entire; MN, male neutered; NP, test not performed; PCV, packed cell volume; PT, prothrombin time.

A diagnosis of A. vasorum was based on positive Angio Detect® in eight dogs and Baermann in two dogs. Dog number 8 had an Angio Detect® and Baermann performed at the time of presentation; the latter was found to be negative at the time of diagnosis (see table 1).

Four weeks following diagnosis and first application of Advocate™, all nine dogs were negative on Baermann, eight dogs were negative on Angio Detect® testing and one dog was positive (dog number 3). This dog received a repeat application of Advocate™ four weeks postdiagnosis. The dog remained Angio Detect® -positive at 8 weeks (Baermann was not repeated) and received a third application of Advocate™; Angio Detect® test repeated at 12 weeks postdiagnosis was negative.

Discussion

Dog 8 with discrepant Angio Detect® and Baermann results at the time of diagnosis was felt likely to be truly infected, with negative Baermann results either reflecting low larval burden and/or intermittent larval shedding. A previous study has found the Angio Detect® test to have good sensitivity (84.3 per cent) and excellent specificity (100 per cent).4 The sensitivity of Baermann may be limited in cases of intermittent larval excretion, low parasite burden, testing during the prepatent period (although serological testing may also be negative during this time),5 operator error and/or poor sample quality, and a previous study suggested that serological tests resulted in more consistent results compared with this test.6

Advocate™ is licenced for treatment of A. vasorum with one or two doses administered 30 days apart and for prevention of A. vasorum when applied every four weeks; although the use of a third dose for treatment of infection in one of the dogs which remained positive on Angio Detect® at eight weeks was off label, this was done because there was evidence to suggest ongoing infection in this particular case. Twelve weeks post diagnosis, the dog was found to be negative on Angio Detect®. Despite proven efficacy of between 85.2 per cent and 100 per cent for a single moxidectin/imidacloprid spot-on in the treatment of experimental and natural A. vasorum infections,7 8 one study found that 9 of 10 experimentally infected dogs took between three to seven weeks to become negative on antigen testing following a single treatment with a moxidectin/imidacloprid spot-on and that 1 dog remained infected.4 Another study5 found larval excretion to have ceased and Baermann to be negative within three weeks of effective treatment of 16 experimentally infected dogs. In the dog with discordant Angio Detect® and Baermann results at four weeks post-treatment, these findings may have represented a true-negative Baermann result and a delay in the antigen test becoming negative (resulting in the Angio Detect® remaining positive) or treatment failure and either low or intermittent larval shedding resulting in a false-negative Baermann result. The later is supported by the finding of a second positive Angio Detect® result at eight weeks, following the second application of a moxidectin/imidacloprid spot-on to this dog, when a negative Angio Detect® result following effective treatment would have been expected.

Advocate™ is licenced for the treatment of A. vasorum in dogs in the UK; under the Cascade this would be one of the first-choice drugs for treatment of this condition. In addition, this medication has proven efficacy for this condition. Approval from an ethical review body was therefore not obtained for this study as primary care veterinary practices would likely have used this treatment protocol even without participation in the study and the case information was anonymised before being submitted to the study.

Conclusions

The study found that post-treatment, Angio Detect® testing was concordant with Baermann in seven of the nine dogs. In one of the remaining dogs, Angio Detect® testing was positive and Baermann was negative at diagnosis and in the other dog, this discrepancy was noted at four weeks post-treatment. In both of these dogs, the Baermann result was suspected to be false-negative.

The study also found that application of Advocate™ spot-on solution was an effective treatment for A. vasorum infection in dogs and resulted in resolution of the infection based on Angio Detect® testing and Baermann within four weeks, in eight out of nine dogs. One dog required three applications of Advocate™ before testing negative on Angio Detect®.