Affected piglets in herd A and B were clustered i.e. some litters or pens were affected whereas others were not, although pigs of adjacent pens could have direct nose-nose contact. This supports earlier findings that horizontal transmission in swine is not very effective. This could also be an indication that transmission of EMCV between pigs is more effective via feces than via nasal secretions. The infection pattern on these herds (i.e. clustering) however, could be explained by multiple introductions because of the many infected pens and the low basic reproduction ratio (R0) between pigs within pens described by Kluivers et al. (2006). The most convenient cause of multiple introductions in this case would be rodents. The low R0 could also explain the die out of EMCV problems in herd A and B after implementing a more stringent rodent control. Since infected pigs are able to excrete the virus, feces may play a role in transmission. Contact with feces on herd A and B is minimal due to fully slatted floors. Manure coming up through the slatted floor would consequently be a potential risk for infection but Maurice et al. (2007) found this event to be protective (OR = 0.11) against EMCV infection. The authors suggested this protective effect was observed because pigs acquired immunity after they had contact with a low dose of EMCV in manure. This seems indeed controversial as feces and consequently manure could also be considered as sources of infection. Alternatively, the flow of manure up through the slatted floor could have resulted in a decrease in rodents (unable to walk on crusts in the manure pit, drowning) and thereby protected against EMCV. On herd A, piglets of 3 days old were already affected which supports the hypothesis of horizontal transmission because they didn't use drinking nipples or feeding troughs yet. Horizontal transmission between pigs remains a concern, therefore it is recommended to separate different age groups, limit movements of animals, apply a good quarantine management and general biosecurity principles. Further research on transmission patterns is warranted.
Typical EMCV lesions were not found during the initial stages of the problems and/or were very subtle on herd A. This is uncommon in an EMCV outbreak and results in a more difficult diagnosis. In other cases where lesions are commonly present, diagnosis is rather straightforward. Possibly the disease progressed that quickly so that no macroscopically visible lesions developed. It was thereby important not to exclude EMCV from the differential diagnosis. As a consequence, practitioners should clinically examine as many pigs as possible to avoid missing out lesions on necropsy. White foci of necrosis on the myocardium are pathognomonic. If no typical lesions are observed or in order to establish a conclusive diagnosis, the practitioner should submit piglets or hearts to a diagnostic laboratory in order to demonstrate the virus via identification tests mentioned in the background section. These tests may differ among laboratories. In this case real time PCR was performed. After several real time PCR tests turned out negative due to quality of the test, the technique used by the veterinary and agrochemical research centre, was optimized with success.
Supplementation with vit E and Se failed to reduce EMCV problems probably because the vit E and Se levels in standard feed were sufficient. After the change to drinking water from the public supply in herd A no problems occurred in the following batches. It is not known whether both were related, as switching back to ground and rain water three months later, no clinical EMCV problems were observed. Practicing a stand-empty period for a few days in the compartments following cleaning and disinfection could reduce the presence of EMCV in the environment because farrowing unit 1-2 in herd A, which was consistently left depopulated for 5-6 days, did not show problems.