The Bristol-based agency won the brief after a three-way pitch, and will provide strategic planning support to the charity's in-house team.
In particular, the agency will work to continue the charity's campaigns about pet health, following on from the PDSA's PAW Report earlier this year.
The report revealed that the welfare needs of ten million pets in the UK are not being properly met.
McCann Erickson Bristol MD Fraser Bradshaw said: 'We're coming in to help bring the PAW Report's statistics to life in a creative, engaging way, with social networks, traditional media and events.'
The agency will support the PDSA's three key PR objectives - to raise awareness of the charity, increase engagement with existing supporters and new audiences, and to contribute to the organisation's income.
PDSA head of PR James Puxty said: 'We were looking for external PR support from an agency that could assist us in developing campaigns to support the strategy and contribute towards its continued evolution.'
PDSA is keen to communicate trends it has found alarming. For example, more than one in three of the 25,000 UK dogs it has weighed in the past four years have been overweight. One in four cats also tipped the scales. It says half of pets will be obese by 2013.