In class the other day, we were discussing how viruses are obligate intracellular pathogens. This means that in order to be able to replicate, the virus HAS to express a viral spike that interacts with a cell’s surface receptor. This binding allows the virus to enter the cell. Once inside the cell, it can replicate and make new virions which go on to infect new cells.
So, with that in mind, when there is a virus that can only infect cats, such as Feline Leukemia virus, the ligand that the virus binds to is only found on cat cells. For viruses that can only infect humans, such as HIV, the ligand is only found on human cells.
What is interesting for the influenza virus, different strains are capable of infecting specific animal species. In class we discussed how the regular garden variety flu (the one that is beginning to circulate in the Northern Hemisphere) can easily bind to and infect human respiratory epithelium. The RGV (regular garden variety) flu is defined as a human flu. However, there are strains of flu that predominantly infect pigs (swine flu) and strains that infect birds (avian flu). There was just a report out about an outbreak of dog flu:
“This just spread like wildfire,” says veterinarian Michele Wright of Huebner Oaks Veterinary Hospital.
In fact, Wright was referring to a recent outbreak of canine influenza, a highly contagious respiratory disease of dogs that swept through a local clinic and ultimately spread to a large dog kennel.
This means the ligand that this strain of flu binds to is only found on the epithelium of dogs.
So how do pig and avian specific viruses become a problem for humans? Not ALL flu viruses are limited to the animals they infect. Humans and pigs share relatively similar respiratory and circulatory systems. Some RGV flu viruses are capable of binding and infecting both humans and pigs fairly easily. So when a swine flu and a RGV human flu infect the same animal, it can create a NEW swine flu virus that now can infect humans easily (by picking up the ability to bind to human cells or genetic reassortment). With the swine flu outbreak of 2009 , this was exactly what happened. A swine flu and a RGV flu infected a pig and genetic reassortment occurred. The virus normally that could only infect pigs, was now capable of binding easily to ligand on human cells. This was the swine flu outbreak. This swine flu was then identified as the H1N1 virus, which identifies different surface proteins involved in binding to cellular receptors.
Spoiler Alert: Interestingly, the movie Contagion showed genetic reassortment with a different kind of virus that involved pigs and bats.
So, back to the avian flu (also identified as H5N1 ). Currently the H5N1 flu is easily spread in bird populations, but not easily to humans. In most human cases of avian flu, the World Health Organization (WHO) has been careful to identify that humans that are infected with avian flu have been in close or direct contact with infected animals. An example from the WHO earlier this year for one victim of the avian flu:
had multiple exposures to sick and dead poultry between the second half of January and early February. A blood specimen collected at hospital on 12 February was transferred to Institut Pasteur du Cambodge on 22 February and tested positive by (polymerase chain reaction) PCR.
We have had two more alerts of individuals coming down with avian flu last week. In both cases, the victims had family that worked with poultry or their family had poultry that was infected. So, right now, the avian flu cannot easily spread to humans. What we are concern about is that a human that has been infected by direct contact with infected poultry COULD also become infected with a RGV flu. It wouldn’t take long for the RGV flu to give the ability to easily infect humans to the H5N1 flu.
And when that happens,what we are worried about is that the entire movie Contagion will become a reality.