Worming might help reduce Toxocara parasite loads in domestic pets, but of course the use of veterinary drugs to kill parasites does have a serious knock-on effect on wildlife via the food web. E.g. Ivermectin, a commonly-used worming treatment for domestic grazing animals such as cattle and horses, persists in the animals' dung, where it then kills dung-feeding insects such as Dor beetles. This in turn seriously affects the Greater horseshoe bat, for which Dor beetles are an important prey item. It's a demonstration of an important ecological principle, essentially "What goes around, comes around."
There will always be feral dogs and cats transmitting Toxocara, not to mention wild species such as foxes which also carry the parasite. Research has shown that there are high levels of contamination of soil with Toxocara eggs anyway: 87% of soils tested in Germany, 66% in the UK, 42% in Japan. Cats are considered by some authorities to be less responsible for transmitting Toxocara to humans than dogs, partly because of cats' habit of burying their faeces, and partly because dogs are usually in more direct contact with humans. One of the most effective countermeasures against potential ingestion and infection by Toxocara is one of the simplest: thoroughly washing hands after outdoor activities such as gardening, playing etc. If pet owners were to pick up/clear away their dog or cat faeces and dispose of it properly via a contained composter/wormery system then that should reduce the pathological load of eggs in the soil in general, too.
I used to have a dog and I loved him; I own an elderly cat and I love him too... But I try to be realistic about his impact on wildlife and the environment. I guess it's all about balance. We have to accept that every decision we make about the pets we own, the food we eat, our lifestyles etc has an effect on the wider environment, and the ideal is to live as 'lightly' as possible in terms of our impact on that environment... Whether that be from carbon footprint, use of resources or conserving biodiversity. It's impossible for humans to live without affecting the countless other lifeforms around us, but we can be honest with ourselves about it and hopefully seek to reduce our impact as far as possible.
Toxocara worms are part of the environment just as much as butterflies and bees. But of course they're parasites and have the potential to cause us harm, so our response is to try to eradicate them, much as we do with the Plasmodium parasites which cause malaria. There is evidence that some parasites actually benefit human health - parasites are partially responsible for turning certain proteins in our immune system into the inflammatory cell destroyers they are now. Without them, humans would probably be vulnerable to a larger number of pathogens and suffer from a greater number of allergies (an idea known as the hygiene hypothesis). That said, infection with Toxocara is a potentially serious condition which (as is well-known) can result in blindness; although most people who contract it suffer minor or unnoticeable symptoms.
I can see that this topic is a potentially divisive one, especially given the average Brit's penchant for being a pet-lover, but I would agree with Cotham Marble's contention that any and all of our 'cherished activities and ownerships' shouldn't be immune from an analysis of the environmental impact that they have. We should be able to at least discuss these issues here without people getting upset, hopefully.
