By Barry C. Fox, M.D., University of Wisconsin
Let’s compare bacterial and virus pandemic potentials. When looking at which could be the source of the next pandemic, there are a few biological behaviors to consider—their speed of growth, their infectiousness, their speed of spreading, and how well they respond to medication, among other things. Let’s look at some bacterial and viral pathogens in this light to see the potential threat they hold.
Comparing Bacteria and Viruses
Let’s begin with bacterial pandemics. The only bacteria that can spread beyond 8 feet is tuberculosis. There are some multiple antibiotic resistant germs, including TB and E. coli, but the growth rate of bacteria—for example TB—is very slow.
Viruses are smaller than bacteria, have a more effective aerosol spray within 8 feet. Several can ride the air currents so they can be spread quickly. They replicate rapidly and mutate quickly. They do not respond to antibiotics, and the antivirals are limited. Let’s review their mode of transmission. Ebola and HIV are spread by direct contact with body fluids. Smallpox is spread by direct droplets, and airborne. Influenza is mostly limited to droplet transmission, with some contact. SARS and MERS have both droplet and airborne capacity. All of these viruses are transmitted human to human.
Learn more about respiratory infections.
Can we remove any germs from the list? First, let’s look at bacteria. What about bacterial spores like anthrax? We know they can travel long distance by air and are highly lethal when contracted. But, if they’re too efficient at killing their host, they are not likely to be propagated on a large scale. What about extensive drug resistant TB? It has good transmission ability by aerosol, but it grows too slowly and could be controlled by quarantine.
Why not cholera? It meets several of our criteria as a candidate. It jumped from Nepal to Haiti by the fecal-oral route. However, it’s not likely to take hold in developed countries due to modern medicine, vaccines, and improved sanitation conditions. What about plague? While extremely deadly, it still has vector-borne restrictions, unless we postulate a bioterrorist attack. The strain of E. coli that is resistant to all known antibiotics is spread by the fecal-oral route but mostly in developing countries. So, we will rule out the pandemic probabilities of anthrax, drug-resistant TB, cholera, plague, and drug-resistant E. coli.
Let’s review the viral scenarios. Ebola can be spread by close contact or blood. But it is so deadly that the spread is more contained. With a 3- to 21-day incubation period, quarantine measures may also be effective.
What about the coronaviruses? They are good candidates since they easily transmitted, have a modest mortality rate, and have no known effective anti-viral medications. But for coronaviruses, the incubation period is 4 to 7 days, so patients can be isolated and transmission interrupted.
Measles and mumps are unlikely to spread unless there is a significant mutation in childhood viruses that would render current vaccines ineffective. Smallpox could be a problem, but we’ll consider it less likely since there are no natural reservoirs in the world, we already have a vaccine, and we’re working on improving laboratory safety. What about HIV? Again, the developed countries have a better chance of stemming HIV infections with cutting edge diagnostics and drugs. This will most probably remain a deadly killer in developing countries.
This is a transcript from the video series An Introduction to Infectious Diseases. Watch it now, on Wondrium.
Influenza has already been implicated in four pandemics in the last 100 years. It has a short 48- to 72-hour incubation period, so you’ve already transmitted the virus before you know you’re sick. It’s also tough to implement quarantine with this short time window. Influenza makes people ill, but it usually doesn’t kill them, so people are carrying the virus at various stages of their illness and could infect many others. Influenza can mutate rapidly and undergo antigenic shift or drift.
Fortunately, so far, avian influenza H5N1 strain does not transmit easily between humans because certain throat cell receptors are missing. However, it had a 50 percent mortality in the thousand cases in the 20 countries it affected. Most of these cases were a result of direct contact with humans and infected poultry.
Learn more about why changes in viruses can have such a huge impact on disease prevalence.
The Effects of Influenza
Since influenza can mutate so rapidly, we also would need a new vaccine that would need to be produced quickly and made readily available worldwide. Finally, based on our experience with the 1918 flu pandemic, many of the deaths resulted from secondary infection from bacterial pneumonia.
We now have antibiotics, but the idea of co-pathogens playing an additional role in the pandemic must be considered. In conclusion, a strain of avian influenza or unknown emerging zoonotic virus are our most likely future culprits.
Common Questions about Potential Pandemic Pathogens
Viruses are smaller than bacteria, have a more effective aerosol spray within 8 feet. Several can ride the air currents so they can be quickly spread. They replicate rapidly and they mutate quickly. They do not respond to antibiotics, and the antivirals are limited.
Whether viral or bacterial, germs spread by a combination of methods probably are the most likely candidates for our next pandemic, with droplet and airborne being the most efficient combination.
Influenza has a short 48- to 72-hour incubation period, so it’s tough to implement quarantine with this short time window. Influenza makes people ill, but it usually doesn’t kill them, so people could infect many others. Influenza can also mutate rapidly and undergo antigenic shift or drift.