Perspective
Evolution of the Zebra
When you hear hoof beats, you need to consider all ungulates.
By William Stauffer, M.D.
A 15-year-old female presents to your clinic complaining of “red urine” a few days earlier. She is currently asymptomatic. Her only laboratory abnormality is a urinalysis that reveals 20 to 50 RBCs. What is her likely diagnosis?
If her surname is Anderson and she has never left Minnesota, she might have a urinary tract infection, glomerulonephritis, or even idiopathic hematuria from her menstrual cycle. But what if her last name is Abdiraman and she moved to the United States three years ago from a refugee camp in Kenya? In that case, the most likely diagnosis would be schistosomiasis hemabobium. Never observed among people who’ve spent their entire lives in the United States, schistosomiasis is common among those who’ve recently arrived from East Africa (Sudan and Somalia, for example), with the infection rate in this population exceeding 40 percent.1,2 Given this epidemiology, asymptomatic hematuria in East African populations indicates schistosomiasis until proven otherwise. For a young woman who has spent her entire life in Minnesota, a diagnosis of schistosomiasis would be unheard of. For a young Somali woman, it’s one of the first diagnoses that ought to come to mind if she complains of blood in her urine. But would the average clinician know to consider this “zebra”?
In teaching medical students how to generate a differential diagnosis, we have passed down a pearl of wisdom that, in effect, encourages physicians not to think of such possibilities: “When you hear hoof beats, think horses, not zebras.” In other words, when a patient presents with a constellation of symptoms, base your decision-making on disease prevalence patterns in the community in which you are practicing, as the diagnosis is more likely to reflect a common disease, or a common disease with an unusual presentation, than a rare one.
Zebra Migration
Although the underlying premise of the hoof beats phrase still holds true, perhaps in the modern world, we need to re-examine what we consider rare. Like the animals they’re named after, medical zebras really have never been rare, they simply existed in far-away places.
Even when humans traveled by ship and train—slow modes by today’s standards, we saw transmission of infections such as smallpox and plague with devastating consequences. Today, with nearly a billion people crossing international boundaries annually, conditions that were once confined to a geographic region are showing up everywhere. Each of us carries 10 times the number of microbes as we have cells in our body. So when we board a jet in the heart of the Brazilian rainforest and arrive at the Minneapolis/St. Paul airport 15 hours later, we bring with us hoards of hitchhiking microorganisms that we can continue to spread to places where they’ve never before been seen.
Many of these organisms find new ecologic niches in these new settings. For example, in the 1990s, fluoroquinolone (FQ)-resistant sexually transmitted infections were rarely considered relevant by clinicians in Minnesota even though there were reports of FQ-resistant Neisseria gonorrhoeae infection in Asia. With the ever-increasing mass movement of people, it was not surprising when resistant N. gonorrhoeae appeared in Hawaii in the late 1990s and landed in San Diego in 2001. Shortly after, it turned up in Massachusetts, posing a challenge for physicians and their patients who needed treatment. Other diseases once considered rare that we now regularly talk about in our clinical practice include West Nile virus, virulent Clostridium dificile, and SARS. With competent mosquito vectors in the United States, what will be next? Will chikungunya virus become as common here as West Nile?
In addition to those migrating infections, other infections (for example, schistosomiasis and neurocystercercosis), intestinal parasites, and medical conditions such as cancer predispositions and hematologic disorders (for example, thalessemia) that exist only in their host are being seen in parts of the world where they haven’t been seen before. The resulting diseases, although historically rare in the United States, will often present in hospitals and clinics that care for immigrants. With recent immigrants constituting nearly 12 percent of the U.S. population (and more than 50 percent in some cities such as Miami), clinicians in Minnesota shouldn’t be surprised to encounter schistosomiasis in a young patient who spent time in East Africa.
Rare or Relocated?
Last year, when I stood on the front porch of our home in East Africa and heard hoof beats, it was likely a donkey (a punda in Swahili) passing by or our neighbor’s cow. If I ventured onto the Rift plain at the bottom of “our” hill, the hoof beats would have belonged to a zebra (punda milia—or “striped donkey”) or perhaps an impala. In today’s world of clinical medicine, every time you enter a patient’s room, you need to think about where you are and where your patient may have come from or traveled to. You may be practicing in a clinic in St. Cloud, but your patient may be native to an urban center in Asia or have recently returned from the Great Rift Valley in Africa. Depending on the wanderings of the individual sitting in front of you, the hoof beats you hear may be the sound of tuberculosis, strongyloides, cystercercosis, leishmaniasis, stomach or esophogeal carcinoma, hepatoma, thalassemia, or some other “ungulate.” If you don’t consider the zebra and don’t know the difference between an impala and an antelope, you will fail that patient.
Our medical training system must begin to assist clinicians in differentiating truly rare diseases from those that are common and simply being relocated. In the past, we may have considered infections or various health conditions in patients from other regions of the world as “zebras.” But we have to keep in mind that what is unusual here may be commonplace elsewhere. For that reason, we must broaden our perspective in order to best care for our patients. We must think globally, even though we work locally.
Although you may not see any rare species in clinic often, if you keep your mind and your eyes open, and bone up on global health, you may one day spot a Grant’s gazelle. MM
William Stauffer is an assistant professor in the department of medicine at the University of Minnesota who serves as a technical advisor to the Centers for Disease Control and Prevention’s Division of Global Migration and Quarantine. He is also director of a two-month course for physicians and medical trainees on global health and tropical and immigrant medicine (www.globalhealth.umn.edu) and director of a free monthly seminar on global health that takes place at Shriners Hospital for Children during the academic year (www.tropical.umn.edu).
References
1. Miller JM, Boyd HA, Ostrowski SR, et al. Malaria, intestinal parasites, and schistosomiasis among Barawan Somali refugees resettling to the United States: a strategy to reduce morbidity and decrease the risk of imported infections. Am J Trop Med Hyg. 2000;62(1):115-21.
2. Posey DL, Blackburn BG, Weinberg M, et al. High prevalence and presumptive treatment of schistosomiasis and strogyloidiasis among American refugees. Clin Infect Dis. 2007;45(10):1310-5.