Secrets of the Woods: Acorns, Biodiversity, and Lyme Disease
When Rick Ostfeld wants to understand Lyme disease, he drops to his hands and knees and counts acorns. While other scientists remain in their laboratories, dissecting the intricate molecular workings of the Lyme disease bacterium, Dr. Ostfeld heads to the woods. It is there, he will tell you, that we need to look if we want to understand why and when outbreaks occur.
"The approach to most infectious diseases is to identify their causes and then develop vaccines or drugs to knock them out. With Lyme disease, we wanted to take a step backwards and ask, 'How does nature affect whether people get sick, and can we take clues from nature to help us reduce the incidence of disease?'" Through his studies, Dr. Ostfeld has begun to answer these questions.
The bacterium behind Lyme disease, Borrelia burgdorferi, is a spiral-shaped microbe transmitted to people through the bite of a tick. Ticks are not hatched carrying Borrelia, however, and must acquire the bacterium from somewhere before passing it to humans. Enter the lowly white-footed mouse. The tick is a generalist, slurping the blood of many species of birds, mammals, and reptiles, but it prefers the blood of a mouse. Once tick larvae drink from a Borrelia -infected mouse, they continue to harbor the bacterium throughout their lives, passing it to people during future blood meals.
Dr. Ostfeld, an ecologist with the Institute of Ecosystem Studies in Millbrook, N.Y., wants to understand the relationships between ticks, mice, and people and use this information to develop better ways of controlling Lyme disease. With the aid of NIAID funding, his laboratory has discovered two factors that likely have major effects on the number of Lyme-transmitting ticks in the wild: acorn abundance and biodiversity.
Nuts about Lyme Disease
Acorns are an important food source for many forest animals, including mice. Oak forests typically vary in the number of acorns that fall to the ground each year, also known as mast. Mouse populations vary with the size of the mast crop, and large crops in the fall lead to a booming mouse population the following summer. Dr. Ostfeld suspected that such a boom, which sometimes results in 100 times more mice than observed in "off" years, would increase the number of infected ticks by providing tick larvae with more opportunities to pick up Borrelia during feeding. These larvae then develop into infected nymphs and, ultimately, adult ticks.
To test his hypothesis, Dr. Ostfeld began to monitor patches of forest in the New York woods. In addition, he added acorns to selected sites to reduce the possibility that other environmental factors might coincidentally affect both acorn numbers and mouse populations. His studies, entering their eighth year, have confirmed his suspicions. "When acorns are high, we get lots of mice the next year and lots of infected tick nymphs the year after," says Dr. Ostfeld. "This occurs when acorn numbers are naturally high or when we add them ourselves."
Because nymphs are the key tick stage involved in transmitting Lyme disease to people, the researchers hope to use this information to predict when and where outbreaks are most likely to occur, and to do so two years in advance. Such information would be helpful for many reasons, from indicating when vaccinations might best prevent Lyme disease to pinpointing regions as selective targets for chemical or biological tick control.
A Full House Beats Two of a Kind
Dr. Ostfeld, who began his career studying the population and evolutionary biology of small mammals at the University of California at Berkeley, uses that background to investigate yet another area where the organization of the local ecosystem might have significant effects on Lyme disease incidence.
Because tick larvae can feed on many different animals, Dr. Ostfeld's team wants to understand how the presence of different food sources can increase or decrease the number of Borrelia -infected ticks. "Every species has a unique reservoir competence, or ability to carry Borrelia and transmit it to feeding ticks," he explains. "We want to know if having more species present might affect Lyme disease incidence, and if so which species have the greatest impact."
Dr. Ostfeld's studies suggest that two seemingly unrelated topics -- biodiversity and human disease -- might be more closely intertwined than previously thought, at least in the case of Lyme disease. After trapping and counting animals that coexist with the white-footed mouse, his team reached the following conclusion: the greater the relative abundance of non-mouse hosts, the lower the percentage of Borrelia -infected ticks.
Ecologists have known for years that when forested landscapes are carved up, biodiversity decreases. In small forest fragments, the white-footed mouse population skyrockets, probably due to the loss of their predators and competitors. Meanwhile, the absence of diversity removes the natural checks on Borrelia transmission provided by other animal species. As a result, Dr. Ostfeld's group sees a dramatic rise in infected nymphs. "The ticks have nothing but mice to feed on, but they have a lot of mice."
The Search Continues
Researchers have much more work to do as they attempt to understand the intricate relationships between bacteria, ticks, small animals, and people. They are focusing on a few species, including the opossum, that are poor at transmitting Borrelia to ticks but that host hundreds of them. Each host animal has its own unique properties, from food sources to reproductive habits to environmental sensitivities, which together determine their roles in influencing Lyme-disease risk to people. Understanding the roles of all the host species that may be important to Lyme disease risk is a prospect that promises years of additional research.
Deer, too, can affect Lyme disease incidence, drawn into oak forests by good mast crops just as adult ticks begin feeding. The adult ticks feast on the deer and leave behind the larvae that begin the cycle transmission cycle anew the following summer.
The seemingly endless possibilities leave Dr. Ostfeld with mixed emotions. "Some days you're very excited, others you're depressed. There is so much to do you know you'll never run out of questions to answer, but you'll never get a chance to answer them all."
While biomedical specialists continue to advance our ability to diagnose, treat, and vaccinate against Lyme disease, ecologists like Dr. Ostfeld continue to pursue the possibility of predicting high-risk times and places. "The lesson is clear," says Ostfeld. "Oak trees, mouse populations, forest destruction, all can influence risk. Our challenge is to figure out how much of this enormous complexity we need to understand."