Benign Animal Bacteria Can Be a Potent Human Pathogen

Above: Banded Mongoose. Source: Wikipedia.

Article: Medley S, Ponder M, Alexander KA (2020) Anthropogenic landscapes increase Campylobacter jejuni infections in urbanizing banded mongoose (Mungos mungo): A one health approach. PLOS Neglected Tropical Diseases 14(3): e0007888. https://doi.org/10.1371/journal.pntd.0007888

Introduction:

Wherever humans live, we’re part of the local ecosystem, along with all other living things. Within the ecosystem, space, food, and other resources are shared. Also shared are the ecosystem’s bacteria and other microorganisms, many of which can cause disease.

Increasing awareness of the interconnectedness of the health status of all of the living things in the ecosystem has led to a concept called One Health, which seeks to attain optimal health for humans, animals, and the environment in general.

One Health is particularly important in the study of zoonoses, which are diseases caused by a pathogen that can be transmitted from animals to humans. Examples of zoonoses include bubonic plague (including the Black Death), anthrax, Ebola, and of course, COVID-19.

Paper Background:

A recent paper published in PLOS Neglected Tropical Diseases describes scientists’ study of Campylobacter jejuni bacteria in Botswana within a One Health framework.

As described in the paper, the Campylobacter genus and the Campylobacter jejuni species (usually written as C. jejuni) in particular is a common cause of food-borne diarrhea and other gastric illness. Such illnesses are common in Botswana among both adults and children. In the part of Botswana where the study was conducted (shown below in Figure 1 from the paper), places of human habitation are in close proximity with the habitat of the banded mongoose (Mungos mungo in binomial nomenclature).

In the One Health context, as interactions and proximity between animals and humans increase, so do the chances of zoonoses. Each species within whose bodies a disease-causing bacteria can survive is called a reservoir.  In many cases, bacteria from one reservoir infects another by way of contaminated water sources, food crops, or surfaces.

Humans and domesticated animals (like pets) have long been known to be reservoirs of C. jejuni. To determine whether wild animals like banded mongooses are also C. jejuni reservoirs, the scientists collected mongoose stool samples from the local area, in particular near the dens where the mongooses live.  They also collected samples of the local water and soil. All of these samples, along with human stool samples, were tested for the presence of C. jejuni and other Campylobacter bacteria.

Overall Findings:

Nearly 25% of the human stool samples contained Campylobacter bacteria. C. jejuni was the most common species found, making up over 80% of all the Campylobacter bacteria identified in the human stool samples.

Among the mongoose, 56% of stool samples contained Campylobacter bacteria. Among those samples, 49% contained for C. jejuni, which was the most common species among the stool samples.

Moreover, C. jejuni was the only Campylobacter present in stool samples from each one of the troop habitat areas (the dots in the map in Figure 1).

The human and mongoose results are summarized in Figure 3 from the article, below. The soil and water samples showed very little presence of any Campylobacter species, with only positive sample.

Man-made vs. Natural Den Materials:

Because of the close proximity and overlap between areas of human habitation and areas of mongoose habitation, some mongoose troops build their dens out of building materials, debris, and other materials discarded by humans. An example of anthropogenic (man-made) versus natural dens is shown below.

Interestingly, C. jejuni was much more commonly present among mongoose troops that made use of anthropogenic den materials.

What This Means:

As discussed above, the soil and water samples showed very little presence of any Campylobacter species, with only positive sample. This means that neither humans nor the mongooses are currently showing signs of transmitting Campylobacter bacteria into the surrounding environment.

However, the results both humans and the mongoose are reservoirs of multiple species of Campylobacter bacteria, with C. jejuni being the most common. C. jejuni is also responsible for most of the Campylobacter-related illnesses in humans. Finally, the mongooses that seem to have the closest interaction with humans (anthropogenic den materials) also seem to more likely to have C. jejuni.

This means the most common Campylobacter species in humans, which also is the one that causes the most disease, has an additional reservoir in the banded mongoose. It was present in every mongoose troop. It is highly likely that humans and mongooses are transmitting Campylobacter bacteria to one another.

While being around humans (or at least man-made den materials) does seem to increase the likelihood of mongoose having C. jejuni, this bacteria does not seem to cause any health issues among the mongoose.  However, since the bacteria can move back and forth between humans and mongooses, being in close proximity to mongooses would increase the chances of disease in humans.

Interaction between mongooses and humans has increased (mainly via tourism and urbanization) over the past several years, and mongoose troops have grown increasingly comfortable around humans and man-made structures. Therefore, C. jejuni infections in humans connected to mongoose-borne bacteria seem likely to increase, and add to the already considerable number of human-to-human C. jejuni infections.