The Eradication of Infectious Diseases

Did you know that “Guinea worm disease,” also called dracunculiasis, is about to become the second infectious disease of humans to be fully eradicated by a disease control program? Humans become infected by the nematode that causes the disease when they drink unfiltered water that contains the intermediate host for the nematode: copepods. To reduce human infection rates, the Carter Center’s Guinea Worm Eradication Program has led an international effort to educate people about the importance of filtering their water. For instance, a simple straw containing a filter can prevent people from ingesting the copepods that transmit the nematode larvae. The control program has been very successful! Since 1986, the yearly number of reported Guinea worm cases has dropped by 99.99%! Just 126 cases were reported in 2014. We’re so close!

The first human disease to be fully eradicated was smallpox. If you don’t know much about smallpox, here’s the quick version: having smallpox was awful, and the disease was often fatal. But we couldn’t prevent smallpox transmission by giving people special straws with filters. The smallpox virus was transmitted directly between people, and the best way to stop transmission was to vaccinate a sufficient proportion of the population so that they would no longer be susceptible to the disease. In the late 1970s, the whole world hit that vaccination target, and smallpox was eradicated.

Also, here’s an interesting tidbit: recent work coming out of the Democratic Republic of the Congo suggests that during the time of the mass vaccination campaigns, the smallpox virus was actually working double duty by protecting people from the monkeypox virus. Since the 1980s, the incidence of monkeypox has increased dramatically in that region. People who live in forested regions are the most likely to become infected by monkeypox, because the virus is typically hosted by (you guessed it) monkeys (Rimoin et al. 2010). But it turns out that prior vaccination with the smallpox vaccine also reduces monkeypox infection risk. That means that people who weren’t born during the years of mass vaccination (i.e., young people) have a higher risk of becoming infected (Rimoin et al. 2010).

Finally, let’s talk about one more incredible eradication program, but this time let’s focus on a pathogen that didn’t infect humans. Rinderpest, or cattle plague, was a viral disease that infected both wildlife (e.g., wildebeest, buffalo) and cattle. The disease often caused high cattle mortality rates, which resulted in huge economic losses for humans. But after a massive vaccination program, rinderpest was officially declared as eradicated in 2011.

So, there you have it. We have almost eradicated Guinea worms via a long-term education program. And we have successfully eradicated two important viral diseases (smallpox and rinderpest) via massive vaccination programs. Next stop – HIV vaccine?

SuperVaccine

Vaccination Coverage and Herd Immunity

I’ve talked about vaccination and herd immunity on this blog before, but I think it’s important for me to emphasize how INCREDIBLY IMPORTANT it is to get vaccinated.  The importance of vaccinating most of the population is usually explained using mathematics, because scientists study the spread of pathogens by using mathematics.  But today, I’m going to try to explain it with cartoons and pictures, instead of math.

herdimmunity1

herdimmunity2

Without explaining the math, I’ll say that there are some “magic numbers” for vaccination.  These numbers are unique to each pathogen/disease.  For instance, for whooping cough, a disease that can make make babies very sick, the “magic number” is between 92 and 94.  That is, 92-94% of people must be vaccinated in order to prevent disease epidemics of whooping cough.  If that magic number – called the herd immunity threshold – is reached, babies are indirectly protected from whooping cough.  If not, you can expect outbreaks of whooping cough.

So, you might be wondering if there will be outbreaks of whooping cough where you live.  Check out this graphic that was published in Scientific American last year.  If your state’s bar is red – that is, if you live anywhere except Nebraska – you can expect epidemics of whooping cough in your state in the near future.  And while it looks like nobody will be seeing Mumps epidemics any time soon, you can expect to see Measles epidemics in many states.

vaccination coverage

At one point, we’d nearly eliminated whooping cough in the United States by vaccinating children with the DTP vaccine.  Here’s a graph from the CDC showing that after we started using the DTP vaccine around 1950, whooping cough (also called pertussis) almost completely disappeared.  But in the past decade or so, the number of cases reported each year has been increasing. That is likely due to a decline in the effectiveness of the newer pertussis vaccine, rather a decline in vaccination coverage.

pertussis