There are tiny little wormlike parasites living inside the bodies of more than one billion people all over the world. The beasties are called 'helminths' and they're not just found inside human bodies, but inside the bodies of animals as well. The trouble with helminth infections is they don't make a mess of things on their own; they also suppress the immune system, which makes an animal or human less able to fight off other types of infections. So if you get rid of the helminths, the immune system rebounds and other diseases are vanquished. At least, that's the idea.
But infections interact in complicated ways. And diseases don't just affect individuals, they affect populations of individuals: groups, flocks, schools and herds. And what's beneficial for one individual might actually, in some ways at least, make things worse for everybody else. That's the conclusion of new research published today in Science Magazine.
Among humans, helminth infections are often accompanied by other infectious diseases like tuberculosis (TB) or AIDS. As a result, some researchers and clinicians have proposed that taking out the helminths might make it easier to treat the other illnesses, at least in folks who suffer from the double-whammy of both infections.
Epidemiologists Vanessa O. Ezenwa and Anna E. Jolles from the University of Georgia and Oregon State University, respectively, turned to wild African buffalo to find out how treating them for their helminth infections might affect the infection and transmission of bovine tuberculosis (BTB).
Every six months over the course of four years, the research duo captured 216 female buffalo in South Africa's Kruger National Park. Of those, 108 were given treatment that attacks helminth parasites and helps to prevent future helminth infections, for a while at least. The remaining half were not provided with any treatment at all. At the beginning of the experiment, none of the buffalo had a BTB infection.
The researchers first verified that administering treatment (known as anthelmintic treatment) to the buffalo resulted in an increase in a part of the immune response called TH1 immunity. The TH1 response is known to be involved in the defence against tuberculosis, so Ezenwa and Jolles expected that the two groups would have different levels of BTB infection. But they were surprised to find that the two groups instead had roughly equivalent levels of infection!
Over the course of the study, 69 buffalo wound up with a BTB infection: 36 from the control group and 33 from the treated group.
Here's where the story takes a sharp turn to the left. While the treatment didn't seem to help the buffalo avoid BTB, it did help them survive it once they were infected. The BTB-positive buffalo that did not receive treatment were more likely to succumb to the disease and eventually die.
And since the treated buffalo lived longer after being infected with BTB, they had more opportunities to pass on the disease to others within their herd. What that means is that treating the buffalo for the helminth parasites had the opposite effect than was intended: it made the transmission of BTB even more likely, not less!
"At the individual level, the outcome of such an intervention is positive because it reduces BTB-induced host mortality ... and ... the progression or severity of other pathogens such as HIV and Streptococcus pneumoniae," write Ezenwa and Jolles. "At the population level, however, the consequences of intervention are negative because surviving ... individuals continue to spread the pathogen."
To put it into context, the average BTB-positive, untreated buffalo would have transmitted the infection to two others. But each BTB-positive, treated buffalo went on to infect some fifteen others, on average. While each individual undoubtedly reaped the rewards of a longer lifetime thanks to the treatment, the herds as a whole suffered.
Even now, large-scale programmes are being carried out to target helminth infections in humans around the globe. It's thought a decrease in other microbial infections that co-occur with helminths is an added benefit. But this study serves as a warning that the reality may prove more complex.
It turns out that administering an anthelmintic treatment to people could help on an individual basis, but could also exacerbate the transmission of pathogens within groups. "This is especially likely for chronic infections such as HIV/AIDS and TB, two human microbial diseases for which anthelmintic treatment strategies are being considered," the researchers warn. They add that more work is needed to see whether the patterns seen in African buffalo would apply to humans, and if so, how anthelmintic drugs can be combined with other treatments to help solve the problem.