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An Urban Experience
H. Mejer1, S.M. Thamsborg2, J. Willesen3
1University of Copenhagen, Veterinary and Animal Sciences, Frederiksberg C, Denmark
2Univeristy of Copenhagen, Veterinary and Animal Sciences, Frederiksberg C, Denmark
3Unisversity of Copenhagen, Veterinary Clinical Sciences, Frederiksberg C, Denmark
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. vasorum (2) 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 time (3) and this may reflect that acquired immunity is relatively weak and not
a dominant factor in regulating moderate infection levels (4). The fact that young individuals are more commonly infected with A. vasorum than older dogs and foxes (5, 6) may in part be due to natural age resistance as juvenile foxes are more susceptible than adults (7). 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.
Improved diagnostics
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.

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