The indoor cat: do the endoparasites knock at the door?

Cats are extremely popular in Europe, with 106 million kept as pets in 2019 (https://www.statista.com/statistics/453880/pet-population-europe-by-animal/). Although there is a growing trend towards a permanent indoor housing, European owners typically provide outdoor access to their cats. Data from a very recent survey indicate that the 59% of owned cats in Europe conduces an indoor-outdoor lifestyle (Foreman-Worsley et al., 2021 Animals 11:253). Lifestyle is crucial in putting cats at risk of infections and/or infestations with internal and external parasites. Outdoor access is associated with higher exposure to endoparasites such as Toxocara cati, Ancylostomatidae, Dypilidium caninum, Aelurostrongylus abstrusus, Troglostrongylus brevior, Dirofilaria immitis (Levy et al., 2017 J. Am. Vet. Med. Assoc. 250:873-880; Chalkowski et al., 2019 Biol. Lett. 15:20180840; Traversa et al., 2019 Acta. Trop. 193: 227-235; https://www.esccap.org/page/GL1+Worm+Control+in+Dogs+and+Cats/25/). However, also cats with no or limited outdoor access are at risk of infection with helminths or even ectoparasites (Coati et al., 2003 Par. Res. 3:146-7; Beugnet et al., 2014 Parasit. Vectors 7:291; Diakou et al., 2017 Par. Res. 116:3429-35; Traversa et al., 2019 Acta. Trop. 193: 227-235; Mirò et al. 2020 Parasit. Vectors 13:101), thus the misconception that indoor housing ensures a complete parasitological protection should be debunked. In fact, cats may become infected i) ingesting parasitic infective stages introduced mechanically into the domestic environment by the owners (e.g. dirty shoes), ii) preying intermediate or paratenic hosts (e.g. rodents, lizards, snails/slugs, earthworms) that find their way in the domestic environment (e.g. houses, terraces, gardens), iii) when fed with raw food by owners (e.g. BARF diet) (Papadopulos et al., 2020 Hell. J. Comp. Anim. 9:2), iv) when bitten by mosquitoes transmitting vector-borne helminths, e.g. D. immitis (Montoya-Alonso 2014 et al., Parasit. Vectors 7:506). Moreover, numerous owned cats are usually adopted from shelters or are found in the street and, if not properly parasitologically examined and dewormed before re-housing, they continue to harbor parasites, some of which have zoonotic potential. Finally, there is evidence that owners of indoor cats tend to a less accurate parasite management than owners of outdoor cats (Mirò et al. 2020 Parasit. Vectors 13:101). Therefore, adequate control strategies against feline endoparasitoses are of crucial relevance also for cats living indoor.