Prevalence of Angiostrongylus vasorum in southern Belgium, a coprological and serological survey

Originally, three practices were chosen for a pilot study because of previously diagnosed canine angiostrongylosis in the region. Additionally, 14 other practices in southern Belgian were enrolled in the study based on the interest they showed to test and deliver samples in order to explore whether more cases could be diagnosed. The implementation of this type of survey relies heavily on the willingness and rigour of the different participants, namely the veterinary practices. This may be considered as a potential bias. However, canine practices were selected in the five different provinces of southern Belgium (Wallonia) where a large number of both symptomatic and control dogs were sampled. Therefore, the present survey, the first large scale epidemiological study conducted in Belgium, should be considered as a pilot study aiming to give a fairly good estimation of the impact of angiostrongylosis in the canine population of southern Belgium.

For practical reasons (ease of use and rapidity) the 979 canine sera were screened using an in-clinic test (Angio Detect™) which is able to detect specific A. vasorum circulating antigen as early as nine weeks post-experimental inoculation. This test has a good sensitivity (84.6 %) for clinically affected dogs, and a very high specificity (100 %) regarding different lungworms [17]. In total 46 dogs (4.7 %) had specific circulating antigen which indicates that A. vasorum infections is well established in the canine southern Belgian population. This is in agreement with recently published data dealing with single case reports [11, 13] or small clinical series [12]. Large-scale seroepidemiological surveys were previously performed in Europe using ELISAs, showing antigen detection in 0.5 to 2.17 % of the study populations [18–20, 24–26]. Among Belgium’s neighbouring countries, in Germany, 4003 randomly selected canine sera were collected from western federal states and tested by Schnyder et al. [18] using Ab and Ag ELISAs, the latter test being able to detect A. vasorum specific antigen as early as seven weeks post-infection [27]. A total of 20 sera (0.5 %) were antigen positive in that study. The comparison of this seroprevalence with the results of the control group presented in our study (3.6 %) confirms that the investigated area, which has a border with North Rhine Westphal and Rhineland-Palatinate must be very suitable for the completion of A. vasorum life-cycle [10], particularly considering that the rapid in-clinic assay is recommended for testing clinically affected dogs and less sensitive than the ELISAs that have been used in the mentioned study from Germany. In symptomatic dogs the proportion of antigen positive dogs was even higher (8.6 %). The majority of these dogs had cardio-pulmonary symptoms (cough, dyspnoea, exercise intolerance) which are by far the most commonly observed clinical signs in dogs with canine angiostrongylosis [9]. Coagulopathies and neurological disorders, which were only reported in one seropositive dog during the present study, are reported in canine angiostrongylosis but represent a small proportion of the patients [7].

Other epidemiological studies based on coproscopy confirmed that asymptomatic dogs can excrete L1 and thus contribute to the dissemination of the parasite in the environment. For example, in UK, Morgan et al. [28] found A. vasorum L1 in the faeces of 15 % and 2 % of symptomatic and asymptomatic dogs, respectively. Barutzki Schaper [29] in Germany isolated L1 in 6 % of symptomatic dogs (n?=?810) versus 0.1 % in a survey conducted earlier by the same authors in asymptomatic individuals [30]. It is not known whether these asymptomatic carriers may or may not develop clinical signs later on, as the majority of these dogs may have been treated with an anthelmintic drug.

The larval migration-technique was difficult to implement during this study: a total of 47 stool samples were obtained and in some cases, samples from only one day of collection were provided by the owners, or the stools had dried up. This might explain at least partly the high level of discrepancy observed between the serological assay (positivity) and the Baermann technique (negativity). Additional explanations could be linked to the technical characteristics of the in-clinic test which requires reading after 15 min sharp (and resulting in false positive results if reading is performed later), and the potential of non-reported anti-parasitic treatment by the owner (resulting in negative coproscopy while serology may still be positive) [27]. Indeed, in experimentally infected dogs, patency started between 7–8 weeks after infection, approximately at the same time of antigen detection (while antibody detection may start as soon as three weeks after infection), with the difference that dogs, once diagnosed seropositive, remain positive unless treated [27], in opposition to Baermann examination, where intermittent larval shedding can be observed [31]. The potential role of the immune response that inhibits the production of L1 as described in canine dirofilariosis [32] could also be hypothesised. Nevertheless, some discrepancies were not explained by the above hypotheses and some dogs positive with Angio Detect™ were not confirmed by in house ELISAs (Ag and/or Ab).

Interestingly, L1 of C. vulpis were observed in nine and one symptomatic and control dogs, respectively confirming the presence of this nematode in Belgium, as recently reported [33]. Consequently, this parasitic infection must be included in the differential diagnosis of pulmonary conditions. It is noteworthy that in five of these C. vulpis infected dogs the Angio Detect™ assay resulted in a negative result, while a mixed infection in these dogs may have been missed. This confirmed the high specificity of the test as previously demonstrated by Schnyder at al. [20] towards C. vulpis and other canine nematodes. In one symptomatic Angio Detect™ negative dog, L1 of both A. vasorum and C. vulpis were observed, whereas the sample was found to be positive with Ab ELISA. A possible explanation could be the production of immune complexes, as shown for Dirofilaria immitis, which could block Ag detection on commercially available tests [34, 35].