Understanding disease as a major ecological force

Working as an environmental reporter for more than 30 years, I’ve covered hundreds of topics — from sewage-treatment plants to killer whales. I’ve learned a great deal through the years, but I’m always striving to learn more about the environment, and I enjoy sharing new information with others.

Mist from the breath of killer whales is collected at the end of a long pole then tested for dozens of different types of bacteria. Photo: Pete Schroeder
Mist from the breath of killer whales is collected at the end of a long pole then tested for dozens of different types of bacteria. // Photo: Pete Schroeder

Recently, I found myself immersed in a fascinating subject that I knew almost nothing about, at least from a scientific perspective. What I learned in my reporting was enough to alter my thinking about the ecological forces that shape our world.

I’m talking about the role of disease, a force that can decimate populations, affect predator-prey relationships and disrupt social communities. So many animal diseases overlap with human diseases that we can no longer consider ourselves separate.

As Joe Gaydos of SeaDoc Society told me, “The crazy thing about disease is that it isn’t really on people’s radar. It is a smoldering factor in our environment, but one that can break out at any time.”

My recent stories featured potential diseases in killer whales, herring and salmon, animals that are related to each other through the food web. I wrote the stories for the Encyclopedia of Puget Sound, which is managed by the Puget Sound Institute, where I work part-time. The stories were reprinted in the Kitsap Sun, where I spent most of my career as an environmental reporter.

I owe a debt to Joe Gaydos and the other folks mentioned in my stories for helping me grasp the significance of disease in all animals, including humans. Scientists who understand the complexities of disease are now coordinating with the Puget Sound Partnership to increase awareness among other scientists and among people who live in the region.

It was only a couple years ago that sea star wasting disease burst into the news with unappealing pictures of melting sea stars that were losing their limbs and turning to mush. Review entries in Water Ways, Jan. 20, 2015, Nov. 22, 2014, and June 17, 2014. It is amazing how quickly the disease decimated the sea star population and altered tidal and subtidal ecosystems in many areas along the West Coast.

Disease does not need to cause death directly to affect individual animals. In my stories, I showed how diseases in herring and steelhead might make them more susceptible to predation, which can have the same end result.

Pollution may be affecting the immune systems of many marine animals and making them more susceptible to disease. Changing water temperatures, influenced by climate change, can have a similar effect.

The field of disease ecology is far from new, but I believe we will be hearing more about it, as growing evidence suggests that disease could be playing a major role in shaping populations. It is a fascinating subject when you learn about how disease organisms spread from one animal to another or cross over into other species.

For example, disease pathogens can be divided into two modes of transmission. “Density-dependent” pathogens tend to spread when the host population gets too crowded. If a threshold density is not reached, the disease tends to die out. “Frequency-dependent” pathogens tend to spread when the percentage of infected animals is high, regardless of density.

When germs are spread by coughing or sneezing, disease will spread more quickly when the individuals are close together (density-dependent). Sexually transmitted diseases are more likely to spread when more individuals are infected (frequency-dependent). Many diseases are a combination of the two, depending on conditions.

Interestingly, pathogens that are the most dangerous to a population are mostly the ones with intermediate virulence. That’s because highly virulent pathogens are likely to kill the host before the disease can spread to others. Low virulence will result in almost 100 percent survival.

The Centers for Disease Control and Prevention now recognizes that the health of humans is connected to the health of animals and the environment. The CDC program is called One Health. Research is rapidly expanding into zoonotic diseases, which are those than can pass from animals to humans.

By thinking of connections between humans and animals, new diseases can sometimes be identified before they create a major outbreak in humans. In other cases, protection of humans can involve treatment in animals.

One example is Rift Valley fever in East Africa, as reported by the CDC. The viral disease, spread by mosquitoes, can kill livestock — including sheep, goats and cattle. It can also cause serious problems in humans, including blindness and brain swelling.

While there is no vaccine against RVF for humans, researchers were able to develop a vaccine for livestock. Treating livestock prevented transmission to mosquitoes and thus reduced disease in humans.

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