PMWS Transmission - Airborne Transmission

This report in the June 2008 issue of the newsletter from the EU PCVD Consortium demonstrates that Post-Weaning Mortality and Wasting Syndrome (PMWS) can be passed to healthy pigs from infected herds.

30 June 2008

4 minute read

To study whether PMWS is airborne, two identical containers were constructed as pig units (unit A and B) (Figure 3). The units were placed one meter apart and connected by pipes. By regulating the air pressure in the two units the amount of air transmitted from unit A to unit B could be controlled. In the experiments, 70% of the air from unit A was transmitted to unit B.

On another location five kilometres away, a third container (unit C) housed control pigs. Two separate studies were carried out. Pigs were obtained from two different herds in each of the two studies. For each study, one herd was a PMWS non-affected herd and one herd was PMWS-affected according to the EU definition. Herds included were matched on health status and all were infected with Porcine Reproductive and Respiratory Syndrome virus (PRRSV).

Unit A and B used to demonstrate airborne transmission of PMWS

Twenty-five (study II 31) PMWS-affected pigs from the PMWS-affected herd were mingled in container A with 25 healthy pig from PMWS non-affected herd. Half of the pigs in unit A and all the pigs in unit B (50 pigs) and C (30 pigs) originated from a PMWS non-affected herd. The other half of the pigs in unit A originated from a PMWS-affected herd.

All pigs with evident clinical symptoms of PMWS (wasting) were euthanased and necropsied. The study period in both studies was 69 days.

In the first study, 14 of the pigs from the PMWS-affected herd in unit A were necropsied. Nine of these pigs were diagnosed with PMWS according to the EU definition. Of the pigs in unit B died on day 2 after mingling due to diarrhoea and one pig was euthanased due to lameness 4 weeks after onset of the study. None of the pigs from the PMWS non-affected herd showed any signs of PMWS (wasting), in unit A nor in unit B and C.

In the second study, 12 of the pigs from the PMWS-affected herd in unit A were necropsied. Two of these pigs were diagnosed with PMWS. Nine of the pigs from the PMWS non-affected herd in unit A were necropsied. Three of these pigs were diagnosed with PMWS. Of the pigs in unit B receiving air from unit A, 20 were necropsied and 13 of these were diagnosed with PMWS. None of the pigs in unit C showed any signs of wasting.

In the second study, transmission of PMWS was demonstrated by close contact as well as by the airborne route. There are several possible explanations for the different outcome of the two experiments. There might be differences in the general infection profile of the herds supplying pigs for the studies or other unknown triggering factors.

In conclusion, this study showed that it is possible to transmit PMWS from pigs originating from PMWS-affected herds to pigs from a PMWS non-affected herd over short distance without direct contact.

Conclusions

PMWS seems to be transmitted from pig to pig both by close contact and by the airborne route. Therefore, pigs with clinical symptoms of PMWS should be removed from healthy pigs, mingling of pigs with PMWS and healthy pigs should be avoided and biosecurity measures should be taken to avoid spread of PMWS.

October 2008