Epidemiology

 

Epizootiology of PCVD

PCVD was first identified in 1991, in Canada. Newly weaned pigs were primarily affected. The consequences of the disease were not severe at this stage. Since 1991 various versions of PCVD have been confirmed in Canada, the USA and Europe. The newer version of the disease seems a lot more severe and also affects older pigs. This form of PCVD was first identified in Europe and is often referred to as the European strain.
PCV associated diseases are now recognised all over the world. Most pig populations are serologically positive for PCV-2, although they do not necessarily show signs of disease. The Porcine Circovirus alone does not cause disease. Management practices, feeding and other viruses (e.g PRRSV) also play a part in disease development. It is also believed that part of the explanation as to why some pigs are more severely affected than others could be an inheritable natural resistance or an inheritable increased susceptibility to PCV-2. An increased susceptibility to developing PCVD’s has been seen in offspring from specific boar lines.

Transmission of PCV-2 and PCVD

The latest belief is that the virus is spread through nasal and fecal shedding. It also appears that it only takes a small oral dose of PCV-2 to infect pigs, minimizing the viral load is thus an important control factor.
 “The Control of Porcine Circovirus Diseases (PCVDs): Towards Improved Food Quality and Safety” research programme have carried out several studies.  

1. Study of PCV-2 dynamic.
   Statistically significant differences were found between PCVD cases and healthy pigs in all three types of samples (serum and nasal and rectal swabs). PCV-2 was detected in nasal secretions earlier than in the corresponding serum and faeces samples. Also, there was a tendency towards detection of PCV-2 in serum earlier than in faeces. The surprising finding, that PCV-2 nucleic acid was detected in nasal secretions weeks prior to detection in faeces and serum either indicates that nasal shedding of PCV-2 may play an important role in the route of infection or that the environment is heavily contaminated with PCV-2.
2. Transmission of PCVD/PMWS between pen-mates, between pens and over short distances.
   This study concluded that concluded that PMWS can be transmitted to healthy pigs after mingling with pigs from PMWS affected herds.
3. Airborne transmission of PCVD/PMWS.
   This study showed that it is possible to transmit PCVD from pigs originating from PCVD affected herds to pigs from herds without PCVD over short distances without direct contact. Subsequent transfer of PRRSV may, however, be a necessary co-factor in this set-up.

Conclusions of above studies

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. For more details on these studies see the June 2008 Newsletter of “The Control of Porcine Circovirus Diseases (PCVDs): Towards Improved Food Quality and Safety” research programme.