Traditionally PRRS outbreaks were thought to be from replacement animals and tainted semen. New research has shown that the spreading of PRRS and other viruses can also be transmitted by area spread, in transport, by insects and by fomites. This study tries to evaluate some of the other sources of PRRS outbreaks where semen and replacement animals were negative. Over four years 35 PRRS outbreaks were looked at. Of the sites that had outbreaks one was in a sow herd, six in finishing sites and eight in boar studs. When an outbreak was detected blood samples were taken and tested by serology and PCR. The researchers then tried to find the source of the virus. Area spread was considered as a potential cause in pig dense areas and where pigs were transported near other sites. External transport was also evaluated, with special attention to removal of cull sows and slaughter pigs. Other potential sources of infection included mechanical vectors, insects, people, and suspected biosecurity breaches. Reviewing the data found that 83% of the PRRS outbreaks were a result of lateral transfer and 17% were from semen or replacement pigs. For the lateral transmissions, over half were from area spread. Transport due to the removal of cull sows, boars or slaughter and feeder pigs, was strongly suspected in 17% of the lateral infections. Insects were only suspected in one case while biosecurity breaches were suspected in two cases. Seven cases could not be determined and infections had a tendency to happen in colder months. Location of the barns was considered the most likely causes of lateral infection. This study provides insight into sources of PRRS infections other than by replacement animals and semen.