On the long list of things that give me the heebie jeebies, parasitic worms are right up there with snake orgies (a uniquely Canadian experience) and house centipedes. Many, but not all, of these worms are intestinal parasites. They latch onto the wall of the intestine and slowly siphon away nutrients, leaving their unassuming hosts weak and malnourished. Left to their own devices, parasitic worms, or helminths, can survive for years inside a host. This is quite astonishing given that our bodies employ a complex immune system to hunt down and destroy invading pathogens.
The key to the helminths’ successful survival strategy is their ability to manipulate the host’s immune system. Our immune system is like a home surveillance system. Once an intruder trips it off, it blares sirens to alert the owners and sends out 911 calls for backup. Helminths are sneaky thieves that can break into our homes, disable the alarms, and steal our food. Because they’ve muted the alarm system, our bodies do not recognize that there is an intruder and the intruder becomes the unwanted houseguest that just won’t leave.
By dialing down our bodies’ immune response, helminth infections also restrict our ability to fight off bacterial and viral infections. This is a particularly important problem in developing countries where helminth infections are endemic and infectious diseases such as HIV and tuberculosis remain prevalent.
In a recent study published in the journal Science, Drs. Vanessa Ezenwa and Anna Jolles from the University of Georgia and Oregon State University showed that treating helminth infections has opposite effects on microbial infections in individuals and populations. The researchers captured and followed 216 wild female African buffalo in Kruger National Park, South Africa. Half of the buffalo received an anthelmintic drug to eliminate and control worm infections while the other half received no treatment.
The researchers wanted to know how treatment against parasitic worms would impact microbial infections, particularly tuberculosis caused by the bovine pathogen Mycobacterium bovis. At the beginning of the study, all the buffalo were tuberculosis-free. During the study, 69 buffalo became infected with tuberculosis, 36 from the untreated group and 33 from the treated group. While the rate of infection was similar between the two groups, mortality differed. In the untreated group, 11 of the M. bovis-infected animals died. In the group that received anthelmintic treatment, only 2 animals died. This means that the risk of mortality was roughly nine times higher in the untreated group compared to the treated group. The researchers also found that the animals that received anthelmintic treatment had a stronger immune response to M. bovis infection than the untreated animals, which could explain the differences in survival.
A key aspect of any infectious disease control program is managing disease transmission through a population. One way of measuring disease transmission is to look at the basic reproductive number, R0 (pronounced “R-nought”). As Kate Winslet explained in Contagion, R0 is a calculated prediction of how many new cases of disease are likely to be caused by one infected person. Many different things can affect the R0 of a disease – population size, rate of infection, mortality rate, length of infectious period, etc… When the researchers calculated the R0 for the two groups, they found that there was an almost 8-fold increase in the R0 for the anthelmintic treated group compared to the untreated group.
How can anthelmintic treatment simultaneously reduce individual mortality but worsen disease transmission through a population? Part of the reason is that once they become infected with M. bovis, buffalo can never completely clear the infection. So while anthelmintic treatment improves the survival of M. bovis-infected animals, these animals become life-long carriers of the disease and can continue to spread the bacteria to others. And since treatment does not reduce the risk of infection, this would lead to an overall increase in disease transmission.
In this study, Drs. Ezenwa and Jolles have shown that when it comes to microbial co-infections, anthelmintic treatments are a double-edged sword. The individual benefits of treatment need to be carefully weighed against the potential costs to the population, an important consideration given that anthelmintic treatments are currently being considered as strategies to help manage chronic infections like HIV/AIDS and tuberculosis.
Ezenwa, V., & Jolles, A. (2015). Opposite effects of anthelmintic treatment on microbial infection at individual versus population scales Science, 347 (6218), 175-177 DOI: 10.1126/science.1261714