Obligatory parasites need a host to complete their life cycle. Facultative
parasites can live freely and complete their life cycle without the need of a
host; only under certain conditions do they opportunistically enter the body
of the host and produce infection, e.g. Strongyloides worms or free-living
amoebae.
How can parasites develop resistance to antiparasitic treatment?
Parasiticides are commonly used to treat and control parasitic infections,
however they may exert selection pressure that may lead to resistance. Drug
resistance is relatively uncommon in companion animals due to wildlife reservoirs
of infection that are not exposed to drugs (e.g. cat fleas can feed on many
different hosts; the lungworm Angiostrongylus vasorum primarily infects
foxes). In the case of some parasites such as the roundworm of dogs, Toxocara
canis, encysted larval stages are present in muscle and are therefore protected
from anthelmintics in refugia. There have, however, been cases of drug
resistance in Dirofilaria immitis (heartworm) and Giardia spp. in North
America and evidence of drug resistance exists for intestinal worms in kennelled
dogs. Hence, it is important that veterinary professionals treating companion
animals for parasites do not become complacent when considering
drug resistance.
There are three general strategies by which parasites develop drug resistance.
Parasites can: (i) produce an enzyme that destroys the drug; (ii) alter the
molecular target so that it is no longer sensitive to the drug; or (iii) prevent
access to the target by actively pumping the drug out of the parasite. Once a
parasite has chanced upon an effective strategy, the newly acquired or mutated
genes that confer that resistance are frequently spread throughout the
parasite population.
The problem of drug resistance, at least for parasites of livestock, has been
exacerbated by human behaviour. Anthelmintics are also misused in agriculture,
where they are commonly employed as food additives to promote
the growth of healthy animals. For example, levamisole was commonly
added to cattle feed in Europe. Decisions as to when to employ parasiticides
are thus a balance between limiting resistance, while not compromising on
animal health and minimizing zoonotic risk.
