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Acaricide Use and the Control of Theileria parva Infection
at the Wildlife-Livestock Disease Interface in Kenya

Adviser: Simon A. Levin

Josephine G. Walker

Ecology and Evolutionary Biology

“... on the last night of the 18-month journey, I was proud of myself for pulling it all together, and relieved that I still found my topic exciting.”


By the time I submitted my thesis for binding, hoping I’d found all the typos (I hadn’t), I realized I had been working on this one task for close to 18 months. A year and a half of thinking about my topic even when I was supposed to be doing other work. A year and a half, and somehow I was still working the night before it was due.

The start of this process came in the fall of my junior year, when the ecology and evolutionary biology (EEB) faculty all gave presentations on their work to the students in the department. Unfortunately for me, every one of them described incredibly interesting research. Deciding who to ask to be my adviser was therefore both difficult and a little awkward; I knew only that that I didn’t want to be told what to do, and that I wanted to study something awesome, preferably somewhere awesome.

Professor Simon Levin gave a guest lecture in one of my classes that fall, and I knew then what a privilege it would be to work with him. He is an incredibly accomplished scientist whose work spans a wide range of fields, from epidemiology and conservation to economics and social decision making. His work sparked my interest, and the variety of his projects inspired me at a time when I was afraid of getting stuck studying one organism or one gene forever. I came to him with a few ideas that I was enthusiastic about and we talked through them all, but it eventually became clear that I was most drawn to studying disease dynamics in animals.

It still amazes me what an enormous influence pathogens can have on the dynamics of an ecosystem. This is the basis of my interest in epidemiology and disease ecology. In the same course that Professor Levin guest lectured in, I learned that domesticated animals can spread diseases to wildlife. This can lead to disease outbreaks that threaten wild species, such as the rabies and distemper viruses that decimate already threatened lion and wild dog populations. Meanwhile, human management can have a critical effect on such diseases, such as in the early 20th century when vaccinating African cattle against rinderpest during a widespread epidemic allowed the wildebeest population to recover.

In the spring semester of my junior year, I studied abroad through the EEB department at a combination research center/cattle ranch in a wildlife-rich area of Kenya, where large-scale commercial ranchers and indigenous Masai cattle herders are continually challenged by the presence of a plethora of diseases, many of which afflict both their cattle and the wildlife. The area also is host to many ticks and tick-borne pathogens, which have historically been managed through intensive application of tick-killing chemicals to the cattle. Frequent dipping (literally) of the cattle into a vat of chemicals doesn’t seem like a great idea for a variety of reasons, but I wanted to investigate how these practices influence the dynamics of the disease. Because ticks that attempt to bite the cattle are killed, the presence of cattle should lead to lower tick population levels in grazed areas. These lower tick levels result in the reduced transmission of pathogens, which one might think would be good for the wildlife; however, reduced transmissions also could mean that instead of a steady state in which all animals are somewhat disease-resistant, there is a low incidence of disease punctuated by severe outbreaks in the susceptible animals.

After a semester abroad thinking about these phenomena, I came back to Princeton to write my spring junior paper introducing this topic and then traveled back to Kenya that summer to actually gather my data. The data collection process was varied: I followed herds of cattle all day in the equatorial sun collecting ticks from the ground, identifying dung, and measuring vegetation. I drew blood from the cows and pulled ticks off of them, although my initial plan to count every tick on multiple cows in each herd was scrapped when I realized that in some cases there were more than 100 ticks inside one ear on one animal. Back at the lab, I extracted DNA, ran polymerase chain reaction (PCR) tests to look for the organism that causes East Coast fever in the blood samples and in the ticks, and used a dichotomous key to try to match my tick samples to species.

One of my favorite things involved in working on my thesis was the combination of different methods that I had the chance to use: sampling from the environment, handling the cattle, identifying tick species, extracting DNA, and running PCR. I also wanted to do some theoretical modeling, but initially was bogged down with running statistical tests looking for patterns in the data I had collected. By the time I started coding my model, it was already the middle of the spring semester, but with help from one of Professor Levin’s graduate students I managed to pull together a model of disease transmission based on a series of linked differential equations. The model helped me predict what I couldn’t measure directly because I could not sample the wildlife that the diseases impact. Creating the model so close to the due date was certainly stressful, but on the last night of the 18-month journey, I was proud of myself for pulling it all together, and relieved that I still found my topic exciting.

Many times in the process, I discovered that some aspect of my project was totally infeasible and had to redraw my proposed plan. My initial plan was to study three different diseases transmitted by three different species of tick to many different species of wildlife, and I ended up looking at only one disease, one kind of tick, and one wild animal. In addition, my attempt to use PCR to find the pathogen in my samples was thwarted again and again. For every challenge, Professor Levin was supportive and encouraged me to tell him how I would deal with it. I really appreciated being in control of what I was working on, and learned the most from working through the challenges on my own before asking for help.

Balancing my thesis with other coursework, extracurriculars, friends, and sleep was sometimes difficult, but the fact that everyone else was going through the same process made it a great bonding experience. I grew much closer to my fellow EEB seniors over the course of the year as we talked through challenges and provided moral support to each other. In addition to other seniors, I drew on the help of many people to get this project done: At least four Princeton professors other than my adviser, professors from several other universities across the country, veterinarians, and a ton of grad students were willing to share their expertise to help me out. Working on such a lengthy, ambitious, and independent project obviously provided me with knowledge and experience. Perhaps more importantly, my thesis led me to develop a love of research and collaborative, multidisciplinary work. My plan is to treat this project as the first stage in a lifetime of useful, interesting, and fulfilling research.

Acaricide Use and the Control of Theileria parva Infection
at the Wildlife-Livestock Disease Interface in Kenya

Josephine G. Walker

Simon A. Levin

George M. Moffett Professor of Biology and
Professor of Ecology and Evolutionary Biology

“She developed an impressive project on her own, with occasional guidance from me and many others, a project that required fieldwork, lab work, modeling, and close interactions with the Masai.”

The senior thesis is the cornerstone of the undergraduate experience at Princeton, but it also is the most intensive and rewarding undergraduate teaching that most faculty ever experience. In a period of less than two years, the student and faculty adviser progress from casual acquaintances to partners in research, who part at graduation with a mixture of pride of accomplishment and sadness at the end of a journey. It is the ultimate formative experience for the student, a bridge to independence and the challenges that wait beyond graduation. The best students, like Josephine Walker, are fellow travelers in a mutual process of coeducation, leading the adviser into new and exciting areas of learning. I am at Princeton not only because I love to teach, but also because I love the learning that comes with that teaching. The part I enjoy most is one-on-one mentoring, of undergraduate and graduate students alike, where the learning curve is steepest for both the student and the mentor, and the rewards are greatest.

In ecology and evolutionary biology (EEB), the senior thesis experience really begins early in the junior year, and especially in the spring semester, when students work on papers that set the background for their summer research experience. The strongest theses are those, like Josephine’s, that involve a field project that will mature into a publishable piece of science. I always enjoy the transition. Students that I know initially only superficially, perhaps through a course I have taught, but more often through a brief conversation following my annual 15-minute presentation to all EEB juniors, come to discuss their ideas for research and how those might be complemented by my interests and expertise. A dialogue ensues, a contract is struck—and as work begins in earnest, we get to know each other much better and learn from each other. I don’t impose my own interests on students, but rather try to help them find and take possession of their own research problems. This is a fundamental aspect of my teaching philosophy, because I regard problem development as one of the most important parts of the research experience. For most of their undergraduate careers, students are told what is interesting, handed problems, and sent off to solve them. That is a place to start, but it provides inadequate preparation for research careers, much less for life. By the junior year, the best students have their own questions to solve, their own places they want to go intellectually. When they take ownership of the problems themselves, they are more motivated, and hence do more interesting work. I also have an ulterior motive—this broadens me, forces me to think about new problems as well, and gives me the chance to see these students at their intellectual best. Because this often means, as in the case of Josephine, that they will spend their summers doing fieldwork in places where I have no projects myself, a large part of my job is in helping them to make connections and find places where what they want to do can be supported.

In Josephine’s case, our first meeting occurred after I gave a guest lecture in one of her courses, and she came to speak with me. She didn’t know yet what she wanted to do, but I was impressed by the scope of her ambitions and ideas. It was the first of many meetings we had over the next 18 months, and I always looked forward to them. One of the crucial features of the EEB program is the opportunity to study abroad in our own field courses, and Josephine took advantage of that opportunity at the Mpala Research Centre in Kenya, which has had a unique partnership with Princeton for decades, and where Professor Daniel Rubenstein and other faculty regularly offer field courses for Princeton students. Josephine returned from that course inspired, impressed by how much of a problem animal diseases present to Masai tribesmen, and wanting to help develop scientifically better ways of clearing cattle and wildlife of the ticks that carry the agents for many of those diseases. She developed an impressive project on her own, with occasional guidance from me and many others, a project that required fieldwork, lab work, modeling, and close interactions with the Masai. As with any such project, there were lots of hurdles to overcome, samples that were lost or delayed, data that were less clear than one would hope, and the continual challenge of changing directions to match what was possible. Josephine overcame all of these to produce an outstanding thesis, which I fully expect to be published in the open research literature. 

The thesis work helped shape Josephine’s career development, which is building on her experiences toward a career in veterinary epidemiology. Currently, she has a Compton Fellowship to allow her to work with the Kenya Medical Research Institute in a partnership with the Centers for Disease Control and Prevention in Kenya to do localized human health surveillance, in order to check for zoonotic transfers of diseases such as Q fever, brucellosis, and salmonellosis from cattle to humans. The thesis has served its ultimate objective, providing a pathway to the next stage of an exciting life. 

I always have mixed emotions when seniors leave, especially after the intense experience of the last hectic stages of finishing the thesis, but nothing matches the reward of seeing them emerge knowing where they want to go next, and better prepared to find their way. Josephine is a model of the student for whom the thesis experience was created.