Over time, the common virus has become noticeably more pathogenic.

Circovirus, a virus that exists worldwide, has turned more dangerous as it has mutated and then combined with other pathogens, according to Purdue University researchers.

At this point, the big question is why a virus that has been known to infect swine for almost 40 years in North America suddenly started causing disease in young pigs and began mutating into more deadly forms.

Based on research at the Indiana Animal Disease Diagnostic Laboratory on the Purdue campus, the most recent mutation of the group of viruses known as porcine circoviruses can cause widespread acute illness. Other pathogens can combine with the virus to increase the fatality rate significantly.

“Our goal is to help the hog industry by understanding porcine circoviruses better,” says Roman Pogranichniy, a virologist at the Purdue School of Veterinary Medicine.

To learn how the mutated form of porcine circovirus type 2 (PCV2) turned deadly, Purdue scientists studied pigs exposed to a combination of PCV2 and bovine viral diarrhea (BVD) virus on the farm.

“We think that the new co-factors, including BVD virus-like pathogens and other swine viruses, work together with porcine circovirus to attack the animals' systems and become more virulent,” says Pogranichniy.

Studying virus-caused lesions and blood of PCV2-infected pigs helps researchers gain some understanding how the virus infects cells and causes diseases to become more deadly.

“Results of the study also indicated that the amount of the PCV2 virus found in the animals had a direct relationship to how sick the animals became,” he says. “There was a high correlation between the amount of PCV2 viral DNA in the lesions and the severity of the disease.”

In the last 10 years, porcine circoviruses have spread to nearly every corner of the globe where hogs are raised, but mortality rates are usually low.

But on farms where pigs are infected with other viruses besides porcine circovirus, the mortality rate has risen to 35% to 50%.

Scientists first identified one type of porcine circovirus in Europe in 1974. Further study indicated it had been present in pigs since 1969, but apparently didn't cause disease.

In 1991, a disease appeared in 6- to 11-week-old nursery age pigs in Europe in which pigs lost weight, developed lesions on their organs and often experienced respiratory problems, diarrhea and jaundice. This condition was called postweaning multisystemic wasting syndrome.

PCV2, which was first identified in 1996, caused lesions in the lymph tissue, kidney, liver and lungs. A more deadly form of the disease is now also found in older pigs.

PCV2 also produces other swine health problems, including abortions, pneumonia and systemic infection.

The newest mutation of the virus also causes enlargement of the spleen and fluid in the body cavity, lungs, abdomen and intestines.

Pogranichniy says further work is needed to learn more about the latest porcine circovirus mutation that includes lesions produced in the blood vessels.

Also, a more exact finding needs to be made of the role the BVD virus-like pathogen plays in the development of circovirus diseases.

The Purdue School of Veterinary Medicine, College of Agriculture and the National Pork Board have provided research funding.