The majority of the poison, once ingested by the rat, is accumulated in the liver and then excreted in the faeces with only a tiny amount in the urine. The main danger to pets comes from eating the bait directly or eating rodents that have eaten the bait. So your piggies will not get poisoned by rat bait simply by being exposed to rat urine....the risk is from the faeces.
Rat urine can however carry the infectious agent Leptospriosis which can be transmitted to humans and guinea pigs via cuts in the skin...normally by contact with infected water......and there is another pathogen, Yersinia pseudotuberculosis, found in the faeces, which can also infect both humans and piggies. Thankfully however transmission of both these organisms in a domestic situation is rare.
It is unlikely however that rats will pee and poo over the lawn - it's an open space and all they will be concerned about is scampering across it as quickly as possible to cover....where they can do their business safe from predators rolleyes
I live next to a stream and often see rats scampering across the garden (they have identifiable routes or "runs") to live in the banks of the stream- and and each year they have the audacity to try and nest in my compost bins. I bait every year in March/April with bromodialone (the Retokil second generation rat poison) and sometimes later in the year if one takes up residence under my shed or in my compost bin. I have also seen the odd one (usually a nursing mother) climb up the cage stands when the piggies are outside and nick food and hay from through the wire mesh.
When my piggies got ill last year - we spent an inordinate amount of time and money on diagnostic tests....and although we don;t know what's wrong with them....we DO know that despite their repeated epxosure to rats over the years, it's not leptospirosis, yersinia or bait poisoning! Clearly I have very healthy rats rolleyes.
So I don;t think you need to worry about grazing your piggies on the lawn, especially in view of all the rain we've had.
HTH
x
