The feline lungworm, Aelurostrongylus abstrusus and the French heartworm, Angiostrongylus vasorum of dogs and other canids are closely related taxonomically and share features in biology and epidemiology. In Europe, A. vasorum has spread in the last two decades and has received considerable attention due to its high pathogenicity whereas it is only recently that A. abstrusus has been recognised as an important differential diagnosis for feline respiratory disorders.
New Transmission Routes
Both parasites have an indirect lifecycle requiring development of first stage larvae (L1), excreted by the cat or dog, to L3 in an intermediate gastropod host (snails and slugs). The intermediate host may be ingested either as direct or accidental (when attached to e.g., grass) prey. Both cats and dogs are believed to also be infected by other pathways but their relative importance is not known. Recent results suggest that L3 may be taken up directly from the environment as live and dead gastropods may release L3 to their surroundings. Lastly, rodents, birds, amphibians, and reptiles that feed on gastropods may harbour A. abstrusus L3 and can serve as paratenic (transport) hosts when consumed as prey.1 Apart from frogs it was recently shown that birds may also be a potential paratenic host for A. vasorum2 and it is possible that the spectrum is as varied as for A. abstrusus.
New Findings in Infection Biology
The overall indications are that both parasitic infections may be chronic in nature and that reinfection can occur. A recent study in foxes inoculated 1, 2, or 3 times with A. vasorum over a period of several weeks thus showed an accumulated worm burden with time3 and this may reflect that acquired immunity is relatively weak and not a dominant factor in regulating moderate infection levels4. The fact that young individuals are more commonly infected with A. vasorum than older dogs and foxes5,6 may in part be due to natural age resistance as juvenile foxes are more susceptible than adults7. Experimental infection studies in cats indicate that some level of immunity or parasite inhibition may be triggered in a few individuals.1,8 If this is common among naturally infected cats is not known as we do not yet understand the relative exposure levels but natural infections are seen in all age groups.1 It is possible that natural exposure tends to be sufficiently low and sporadic so as not to elicit a strong acquired immune response and/or that A. abstrusus may modulate host immune responses.
Changes in Epidemiology
In Europe, A. vasorum was formerly considered to occur in endemic foci but surveys indicate that the parasite is spreading from these foci and that it is becoming regionally endemic in both dogs and foxes in a way that cannot be explained only by increased awareness and better diagnostics.5,9 Relocation and migration of infected individuals (especially foxes) is probably a primary contributing factor. This may potentially have coincided with climate changes resulting in conditions that have favoured the intermediate hosts as A. vasorum transmission is influenced by season which has been suggested to reflect seasonal differences in gastropod populations.5 Similarly, A. abstrusus appears to be an emerging infection present throughout most of Europe,1 but more studies are still needed to better map changes in spatio-temporal distribution.
Both parasites are traditionally diagnosed in faecal samples using a Baermann technique that allows collection of a clean L1 suspension. This may take up to 24 hours, but sensitivity can easily be increased by examining a larger sample if material is available. Direct faecal smears are much quicker for clinical cases with high excretion levels, but the sensitivity is lower and faecal debris can make it difficult to distinguish the tail morphology needed for identification. Faecal flotation techniques may have a high sensitivity, but morphological features may quickly be distorted due to the osmotic pressure of the flotation fluid. All three approaches are simple and allow direct identification of the parasites but expertise is required as L1 of other parasites or soil nematodes may be present. A correct diagnosis is particularly important for dogs as treatment of heavy infections of A. vasorum may lead to health complications. For cats it is currently more of academic importance as the treatment is not dependent on the species present. In dogs, faecal diagnostics is further complicated by coprophagia, as many dogs regularly consume faeces from other animals such as deer that harbour the morphologically similar lungworm, Muellerius spp., and cats. It is our recent experience that live A. abstrusus L1 can occasionally be isolated from dog faeces during routine diagnostics.
Larval excretion of both parasites is highly variable over time in chronically infected animals6-8 and it is, therefore, recommended that faecal samples are collected for three consecutive days to increase the likelihood of detecting an infection6. However, this may still not be enough to compensate for irregular or periodic cessation of faecal excretion.8 Another general weakness of all larval detection methods is that they are only effective after onset of patency (e.g., larval production). However, a similar problem is faced when using either an ELISA or the commercial Angio Detect™ point of care test to detect circulating A. vasorum antigens from current infections as it can take several weeks before the antigens can be detected.10 On the plus side, both techniques are highly specific and the Angio Detect™ offers an easy applicable diagnostic option in the clinic. Blood parameters and diagnostic imaging1,6 can offer supportive evidence of parasite infections and their severity, but are not specific and it should always be attempted to positively identify a given parasite.
Options for Control and Prevention
Both parasites can be treated successfully with anthelmintics but repeated treatment may be required for safe and effective cure.1,6 However, care should be taken as, for example, dead A. vasorum may elicit adverse host immune responses and supportive treatment is sometimes provided with enforced rest.6 Control of gastropods is difficult and prevention should include collection of dog faeces to reduce environmental contamination. Unfortunately, this is not practical for cats and may have little effect in areas with infected wildlife and stray/feral cats and dogs. Lastly, it is generally difficult to prevent dogs and particularly cats from consuming intermediate or paratenic hosts. For animals known to be particularly at risk, preventive anthelmintic treatment may, therefore, be necessary.
The European Scientific Counsel Companion Animal Parasites (ESCCAP) is currently revising guidelines and recommendations on significance, monitoring, prevention, and control of major parasites in cats and dogs with the latest specialist knowledge for release in 2017 (www.esccap.org/). The material aims to give a comprehensive overview for easy use in the clinic and demonstration to clients.
Both A. abstrusus and A. vasorum are emerging infections that need to be countered by prompt action by local vets and continued research on transmission dynamics and infection biology supported by improved diagnostics to help predict how the infections may spread and to support treatment decisions.
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