I often get asked about wild rats or those found outside and what to do with them if you plan to keep them, as there actually doesn't seem to be that much information about this on the internet...not reliable/tried and tested sources anyway.
I do 3 weeks in a separate airspace, with full showers/hair wash/clean clothes between visiting them and my own rats.
Panacur aka Fenbendazole dosed at 0.1ml per 100g of bodyweight, just once is enough to clear worms, I use the 10% liquid for worming dogs, as it tastes nice it seems! You can swap the liquid for rabbit granules sprinkled on wet food. Remember Ivermectin alone does not cover all internal parasites.
Stronghold aka Selamectin or just Ivermectin, though I am finding that lice from rats who live outside, are quite resistant to Ivermectin and it makes no difference to them at all, and twice I have had to repeat treatment with Selamectin instead.
For leptospirosis in animals, the first choice of antibiotics is penicillin-based ones, I use Synulox/co-amoxiclav/clavulanic acid & amoxicillin at normal dose for 3 weeks. I use the suspension/liquid form mushed in wet food, and it's never been refused, even with seriously neophobic rats. Dose at 10mg per kg of bodyweight, twice daily ideally. Some people use Doxycycline which will also work, but getting the rats to take it, can be difficult.
If you DO go down the testing route, testing can either be via blood test (0.5ml needed usually) or via PCR urine test, which is much cheaper and easier as it just involves the rat peeing in a clean container which should be jiggled gently and the urine sucked up in a clean syringe and then put in a little pot, and voila, can be tested quickly and often 'in-house' with many UK vets.
Leptospirosis in the fancy would be very VERY bad indeed and easy to spread, it's not worth the risk of leaving it to fate, when you can easily cover all bases and eliminate the chances of it.
I use Lactol to raise wild orphans, and it works fine, but others have used soya based formula with ok growth, but they lose all hair around the 4.5 week mark, there is obviously *something* slightly 'off' with soy based formulas and rats, so I always stick to lactol and recommend others to do the same. Babies start eating solid food rapidly (bread and lactol smushed up in a shallow dish or jam jar lid always works) and it's easily introduced, but I give a bottle of lactol hung on the cage until around 5 weeks old.
If orphans are reluctant to feed, you can add in a bit of natural yoghurt to the lactol and warm it a bit more, and that does the trick in most cases.
If you need to release them
You can legally release Rattus Norvegicus now, under the Wildlife and Countryside Act 1981 which was amended and updated in 2008.
(ii) Section 14 of the Wildlife and Countryside Act 1981 (WCA) (available at http://www.opsi.gov.uk/legislation/about_legislation.htm) prohibits the release into the wild any animal which is 1) of a kind that is not ordinarily resident in and is not a regular visitor to GB in a wild state or 2) is included in Part 1 of Schedule 9.
Currently (June 2008), the black rat (Rattus rattus), the fat/edible dormouse (Glis glis), the grey squirrel (Sciureus carolinensis) and the Mongolian gerbil (Meriones unguiculatus) are listed on Part 1 of Schedule 9 and therefore cannot be released, except under licence. Under the WCA there is no prohibition on the release of other species of rats or mice which are ordinarily resident in GB.
For example, the view of the Department for the Environment, Food and Rural Affairs (Defra) is that the release of brown/common rats(Rattus norvegicus) into the wild is not unlawful under the Wildlife and Countryside Act 1981 because, despite their non-native origins, they could be classed as 'ordinarily resident' in Great Britain as they are now well-established. However, this is not to say that it is acceptable or permissible to release them on other people’s property.
Also be aware that hand-reared wild rats may not survive when released, and if they are friendly, it is often best NOT to release.