Season 4, Episode 4: The Kiss

David and Carlos are sent to Iron River, Michigan, to investigate what caused several people to suddenly develop PML, a non-contagious disease in which parts of the brain are destroyed. The NorBAC team traces back the origin of the outbreak to a new type of cytomegalovirus that was spread to the Iron River patients from their dogs. Meanwhile, Wes is suffering from depression as a side effect of his interferon treatment for HCV and HIV infection. After a failed suicide attempt, he is taken off interferon, but now his only chance of survival is an experimental treatment.

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CAN YOU GET PML FROM A DOG CYTOMEGALOVIRUS?

What is PML?

PML, short for Progressive Multifocal Leukoencephalopathy, is a disease in which the myelin layer of the brain's neurons - sometimes called the "white matter" - is destroyed. Myelin helps speed up the electric signal in nerve cells, and without it these cells don't work anymore. PML patients have various symptoms, depending on which areas of the brain are affected, but many become uncoordinated and very weak, and usually die within several months.

Image provided courtesy of AXS Studio Inc and Shaftesbury Films.

In PML patients, JC virus causes necrosis (dark patches) of the brain's white matter.

 

PML is caused by a virus called the JC virus, named after the initials of the first person in which the virus was found. The JC virus is very common: it's found in 70-90% of the population! Obviously, not everyone who is infected with this virus gets PML. Only people with weakened immune systems (for example, AIDS patients) are at risk of developing PML.

What is cytomegalovirus?

Human cytomegalovirus, or CMV, is a common type of herpesvirus. Most people have been infected with CMV at some point in their life, but like the JC virus it usually doesn't cause any symptoms. The cytomegalovirus is transmitted by close contact with any bodily fluids of an infected individual.

In people with a weakened immune system, CMV can cause fever, pneumonia, liver infections, and affect several other organs. It does not cause PML. The drug valganciclovir is often used to treat CMV infections in AIDS patients. This is the drug that David recommends to treat the Iron River patients in this episode.

In addition to humans, CMV has been found in several other species, including mice and rhesus monkeys, but the viruses are very host-specific - they can only infect one species. Dogs have not been found to carry CMV, but even if they did, it's not likely that a canine CMV would be able to infect humans.

What are canine transmissible venereal tumours?

In this episode, the NorBAC team speculates that canine CMV had spread among some of the dogs in Iron River through a type of cancer they all had. Dogs can indeed spread cancer cells to each other: Canine transmissible venereal tumours are usually passed from dog to dog by sexual contact, but can also be spread to a dog's mouth by licking. This disease is very unique, because the cancer cells themselves - not a virus or other pathogen - are transferred between dogs. Only dogs can be infected by this type of cancer - it's not a threat to humans.

The cells probably originated from a wolf several hundred years ago. How did the first dog or wolf get this type of cancer? Scientists believe that it might originally have been caused by a virus, in the same way that humans can get cervical cancer from the Human Papilloma Virus, but there isn't currently an active virus in canine transmissible venereal tumours.

So?

It's not possible to get PML from a dog cytomegalovirus: PML is not caused by a cytomegalovirus, but by an unrelated virus, the JC virus. Both the JC virus and CMV only cause serious diseases in people with a weakened immune system. Furthermore, dogs don't have CMV, and even if a dog CMV were to be discovered one day, it is highly unlikely that it would infect humans, as all cytomegaloviruses are host-specific.

That being said, there are other viruses that can be transferred between species. For example, bird flu and SARS were each found in other species before the viruses spread to humans. Initially, researchers thought that the SARS virus was transferred to humans from civet cats in China, but more recent evidence suggest that it actually came from bats.

DID YOU KNOW?

Dr. Cove says he developed and tested an HCV cure using a toll-like receptor activator, which might help Wes. Unfortunately, he left out some important details in his lab book, and he dies before Rachel can ask him for more information.

Image provided courtesy of AXS Studio Inc and Shaftesbury Films.

Artist's rendition of HCV binding to a toll-like receptor on an engineered red blood cell.

Toll-like receptors are proteins attached to the outside of a cell. They recognize molecules that are usually found in typical pathogens (bacteria, viruses) but not in the human body itself. The receptor sends a message to other molecules inside the cell to activate an immune response that will then fight the incoming pathogen. There are more than ten different toll-like receptors in mammals, and each recognizes a particular pathogen or groups of pathogens. Viruses, such as HCV, are recognized by toll-like receptor type 3.

-- Eva Amsen

Want to read and learn more?

Find out more about Progressive Multifocal Leukoencephalopathy, click here:
www.ninds.nih.gov/disorders/pml/pml.htm

To look at a fact sheet about (human) CMV, follow this link:
www.cdc.gov/cmv/facts.htm

To learn about the origin of canine transmissible venereal tumours, visit:
www.the-scientist.com/news/display/24286/

Read about the transfer of the SARS virus from bats to people here:
www.cbc.ca/technology/story/2008/02/19/sars-bats.html

Visit Wikipedia to read more about toll-like receptors:
en.wikipedia.org/wiki/Toll-like_receptor