Porcine circovirus is a virus that infects pigs, and as of 2026 one of the smallest viruses known. Its particles are about 17 nanometers across, and a nanometer is a billionth of a meter. That width is the diameter of the tiny ball of protein that holds the virus's DNA.

Size

Each particle is a shell of protein, called a capsid, with no outer envelope. The capsid is built from 60 copies of a single protein, grouped into 12 clusters of five. That arrangement gives it the symmetry of an icosahedron, a shape with 20 triangular faces. The shell is about 2.5 nanometers thick.

Published measurements range from about 15 to 25 nanometers. In 2019, researchers who purified the virus measured particles about 17 nanometers across in electron microscope images. A 2003 study built a three-dimensional model from images of frozen particles and measured about 20.5 nanometers, out to the tips of the slightly raised protein clusters. The International Committee on Taxonomy of Viruses (ICTV) gives 15–25 nanometers for porcine circoviruses and a related virus of parrots.

Size comparisons

A hepatitis B virus, about 42 nanometers across, is about 2.5 times as wide as a porcine circovirus and about 15 times its volume.
A porcine circovirus is about three and a half times as wide as a cell membrane's phospholipid bilayer is thick, and about 8.5 times as wide as the 2-nanometer DNA double helix.
About 5,000 porcine circoviruses side by side would span the width of a human hair, which is 80,000–100,000 nanometers wide.
If a porcine circovirus were enlarged to the height of a person 1.7 meters (5 feet 7 inches) tall, a human hair would be 8–10 kilometers (5–6 miles) wide.

DNA and name

Inside the capsid is a single loop of DNA with only one strand, 1,759 bases long in the type called PCV1 and 1,768 bases in PCV2. The circular DNA gives the virus its name: "circo" comes from "circular".

The DNA holds two main genes. One makes a protein needed to start copying the DNA, and the other makes the capsid protein. As of 2026, the ICTV describes the genomes of PCV1 and PCV2 as the smallest of any virus shown to multiply on its own in mammal cells.

Disease in pigs

Natural infections appear to be limited to pigs, including wild boars, and the virus spreads mostly when pigs swallow traces of infected droppings. PCV1 has not been linked to any disease. PCV2 is one of the main causes of a wasting disease in piglets 8–16 weeks old, with weight loss, breathing trouble, jaundice, swollen lymph nodes and diarrhea. The disease can be controlled by vaccinating pigs.

The particles are hard to destroy. PCV1 survives acid at pH 3, heating to 70°C (158°F) for 15 minutes, and solvents such as chloroform.

Sources

Genus: Circovirus. International Committee on Taxonomy of Viruses, ICTV Report.
Conformational Changes and Nuclear Entry of Porcine Circovirus without Disassembly. Wang et al., Journal of Virology, 2019.
Comparison of the Structures of Three Circoviruses: Chicken Anemia Virus, Porcine Circovirus Type 2, and Beak and Feather Disease Virus. Crowther et al., Journal of Virology, 2003.
The 2.3-Angstrom Structure of Porcine Circovirus 2. Khayat et al., Journal of Virology, 2011.
Hepatitis B Virus Biology. Seeger and Mason, Microbiology and Molecular Biology Reviews, 2000.
Structure of Lipid Bilayers. Nagle and Tristram-Nagle, Biochimica et Biophysica Acta, 2000.
DNA Structure and Sequencing. In Biology 2e, 2nd edition. OpenStax, 2018.
Just How Small Is "Nano"?. National Nanotechnology Initiative.

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