Dose of Disease: Two’s a Party, Three’s a Co-infection

The coral hotel

Purple spot disease (Photo credit: Dr. Ernesto Weil)

Keeping corals alive in the lab requires a simulation of nature, which is a lot like trying to recreate a famous watercolor with your at-home paint set. You can get some of the basic elements, people can see what you were going for, but the original is expert territory.

I’m certainly willing to admit that nature is the expert and we are not. That is one of my biggest take-aways from keeping pieces of sea fans alive in the laboratory.

When I first attempted this feat last year, I learned a lot about the corals just from the process of keeping them alive during pilot experiments. They like light, but not too much. They like salt, but not too much. They do not like touching each other. They do not like stagnant water. They do not like green eggs and ham.

Last week, we collected small pieces of corals from the field, put them in tanks under high-powered lights, and kept them happy for about a week by providing circulation and fresh saltwater (pictured on the left, above). It’s like a weird hotel, but at the end of their stay, we hope that our guests can help us answer the questions we set out to address.

1)    How do corals respond when they’re infected with two pathogens instead of one?

2)    How do the two pathogens respond to this arrangement? Is one of them just along for the ride?

3)    Do the answers to questions #1 and #2 change if we look at one colony vs. another colony?   

4)    Are there differences in examining this relationship soon after it begins vs. 2 days later?

These are questions we need to go into the laboratory to answer. The controlled conditions in the laboratory allow us to keep all else constant while we change the number of pathogens and measure the response of the corals and the pathogens. The experiment compares what happens when the coral host is infected with the purple spots disease (pictured above on the right), the Aspergillus fungal pathogen of sea fans, and both together. 

The motivation for this experiment is the reality that life is not as simple as one pathogen infecting one host. Even though the classic disease ecology Venn diagram, shown below, portrays the relationship between one host, one pathogen, and the “environment” (as a blanket term), disease ecologists of course know that it’s more complicated. Indeed, my laboratory experiment in 2014 delved deeper into the “environment” component of this Venn diagram by exploring the effects of combined stressors. This year, I’m switching gear to further explore the “pathogen” component.

Figure from Burge et al. 2014: Annu. Rev. Mar. Sci. 2014. 6:1.1–1.29

While the field of disease ecology has primarily focused on single infections up to this point, recent studies have been adding to this foundation one step at a time. Research in other plants and animals, though very few marine organisms, has begun to investigate the multiple-pathogen scenario we call “co-infection”. Co-infections in African buffalo, for example, affect mortality rates and disease transmission. Especially interesting is the finding that treating the worm infections may actually exacerbate bovine tuberculosis, a bacterial infection! In the case of Lyme disease, the co-infection of the tick vectors with the pathogens for anaplasmosis and babesiosis has important public health implications: being bit by a co-infected tick matters.  

As with buffalo and ticks, co-infections may be very important for corals and coral reefs. Imagine that a coral is infected with one pathogen, and then suddenly beset by a second invader! Will it be better equipped to fight off that second pathogen since it was already in the midst of fighting off the first one? Or will its immune resources be stretched too thin, leading to heavily reduced reproduction, or even death? In either case, there may be a turn in the tide of the battle between the pathogens and the corals.

The results of our laboratory experiment will give us a glimpse of this battle, and we can hope the coral wins. The outcome of coral diseases, including co-infections, determines the health of reef ecosystems. After spending a week in the laboratory doing my best to mimic reef conditions, I appreciate the many moving pieces even more. There’s nothing quite like trying to recreate nature in a box that makes you realize how cool it is.