One Health: animals and environment in diagnosis

A forty-six-year-old man seeks care for the third time in five weeks. He has a fever that comes and goes, night sweats, fatigue and back pain. In the two previous visits, he was diagnosed with a viral infection and advised to rest. The medical record lists age, sex, allergies and medications in use, but does not say what he does for work. At the third visit, the doctor asks, and he says he tends a herd of goats and helps with the animals' deliveries. The scene is fictional and was written for teaching. This text uses this picture as a clinical reasoning scenario: why exposure to animals, work, water and food changes the list of hypotheses, and what the idea of One Health adds to the consultation.

NEXO is a daily medical diagnosis game: one clinical case a day, up to six progressive clues, and ICD-10 proximity feedback across five levels.

The text is not a guide to diagnosing or treating zoonoses. Each of these diseases has its own tests, management and notification rules, defined by local guidelines. The interest here is reasoning: how a question about the world outside the patient's body can change the chapter in which the hypothesis sits. If you play the case of the day in NEXO, you will recognize the move: a clue about exposure can take the guess somewhere else.

Key points

• Most emerging infectious diseases originate in animals, and many come from wild animals.
• Occupational history is recorded far less often than age and sex, and it is there that many zoonoses appear.
• Brucellosis is one of the most common zoonoses in the world and affects mostly those who work with animals.
• Rabies is almost always fatal once symptoms appear, and the decision to give prophylaxis after an exposure is made before any certainty.
• One Health is the idea that human health, animal health and the environment are linked and call for joint work across fields.

Why One Health is a reasoning scenario

The World Health Organization describes One Health as an integrated approach that seeks to balance the health of people, animals and ecosystems, and keeps a page on the topic. Zinsstag and colleagues report that, despite this integrated view, much of the thinking and action in human and animal health still works in separate fields (Zinsstag and colleagues, 2011).

For the clinician, this separation has a practical effect. The consultation usually looks at the patient's body and the history of symptoms, and leaves out the place where he works, the animals he lives with, the water he uses and what he eats. Many infections only enter the list of hypotheses when these questions are asked.

What the scene teaches

In the opening scene, the patient went through two consultations without anyone asking about work. The most common explanation, a viral infection, was accepted even with the fever persisting for weeks. The post on signs, symptoms and history shows how the patient's history goes beyond the list of symptoms.

A fever that lasts too long

A fever that persists for weeks is, by itself, a clue that the initial explanation may not be enough. The reasoning that accepts "viral infection" at the first visit needs to be reviewed when the picture does not follow the expected course. This review is the natural moment to broaden the history and ask about exposures that have not yet been explored.

Where emerging diseases come from

Jones and colleagues analyzed 335 events of emerging infectious diseases between 1940 and 2004 and found that 60.3 in every 100 were zoonoses, of which 71.8 in every 100 originated in wild animals (Jones and colleagues, 2008). The authors conclude that surveillance resources are poorly distributed, concentrated in countries where the next emerging disease is less likely to appear.

Pathogens that pass between species

Taylor and colleagues reviewed the literature and identified 1415 species of infectious agents that cause disease in people, of which 868 are zoonotic (Taylor and colleagues, 2001). In the same study, 132 of the agents associated with emerging diseases were zoonotic, and zoonotic pathogens were twice as likely to be linked to emerging diseases.

An origin that often goes unnoticed

An infection of animal origin does not always reach the consultation with signs that bring the animal to mind. It often appears as fever, fatigue, body aches or a common respiratory picture. It is the exposure history, not the appearance of the picture, that sets these infections apart from the most frequent causes of the same symptoms. That is why the question has to be asked before the hypothesis exists.

What this means in the consultation

These numbers show that the question about animals is not an exotic detail. It touches a large share of human infections. The post on pretest probability in diagnosis explains how the frequency of a disease in the reference population weighs on the interpretation of clues. For people who work with animals, the reference population is different, and so is the probability of a zoonosis.

The missing question: what do you do for work?

Politi and colleagues reviewed 2050 medical records at a teaching center and found age and sex recorded in about 99 out of every 100 patients, against only 27.8 out of every 100 with an occupational history (Politi and colleagues, 2004). The authors conclude that medical education needs to close this gap.

Questions that open up the differential

A few short questions change the course of the consultation:

• What do you do for work, and what exactly do you do day to day?
• Do you have contact with animals, at work or at home, live or slaughtered?
• Have you drunk raw milk or eaten cheese made from unpasteurized milk?
• Have you had contact with floodwater, river water or lake water?
• Have you traveled recently, and what did you do during the trip?
• Have you been bitten, scratched or licked by any animal?

How to ask without stretching the consultation

The most common objection to the exposure history is time. In practice, two or three open questions are usually enough to know whether it is worth going deeper. "What do you do for work?" and "Do you have contact with animals?" take seconds. When the answer opens a clue, the rest of the conversation gains focus, and the consultation may even get shorter, because the reasoning stops circling around generic explanations. Recording the answer in the medical record also helps whoever sees the patient later, who will not need to ask the question again.

Occupation as a clue

Occupation is not just a social data point. It tells you what the patient is exposed to every day, and it often explains a picture that seemed to have no cause. The post on differential diagnosis shows how a new clue can reorganize the list of hypotheses.

Brucellosis: the zoonosis that hides behind fever

Pappas and colleagues describe brucellosis as the most common zoonosis in the world, with a map that has changed a great deal in recent decades, with areas that controlled the disease and new foci in other regions (Pappas and colleagues, 2006). Franco and colleagues describe the disease as very common, but often neglected, in developing countries (Franco and colleagues, 2007).

Who is exposed

Pereira and colleagues gathered 63 studies on occupational brucellosis and identified five groups with the most exposure: farm workers, slaughterhouse workers, veterinarians and assistants, laboratory professionals and hunters (Pereira and colleagues, 2020). In the meta-analysis of the same study, animal breeders, laboratory professionals and slaughterhouse workers had a 3.47 times higher chance of infection than people with no contact with the sources.

The sources of exposure

The same authors point to direct contact with animal fluids, failure to use protective equipment, accidental exposure to live attenuated animal vaccines and noncompliance with biosafety rules as the main risk factors (Pereira and colleagues, 2020). They also report that lack of knowledge about the disease among the most exposed professionals increases the chance of infection.

The laboratory also needs to know

When there is a possibility of zoonosis, reporting it to the laboratory is part of the reasoning. The information helps choose the right test and protects whoever handles the sample, since laboratory professionals are among the exposed groups described by Pereira and colleagues (Pereira and colleagues, 2020).

Rabies: deciding before certainty

Fooks and colleagues describe rabies as a disease in which dog bites account for more than 99 in every 100 human infections and which, once symptoms appear, is almost always fatal (Fooks and colleagues, 2017). The authors explain that timely post-exposure prophylaxis can prevent the disease and involves proper wound care, rabies immunoglobulin and vaccine.

The size of the problem

Hampson and colleagues estimated that dog-transmitted rabies causes about 59 thousand human deaths per year worldwide (Hampson and colleagues, 2015). The authors conclude that joint investments by the medical and veterinary sectors could greatly reduce this burden.

A decision under uncertainty

The decision to give prophylaxis after a bite is a clear example of a decision under uncertainty. The doctor almost never knows whether the animal was infected, and the cost of erring on the low side is much greater than the cost of erring on the high side. The WHO technical report on rabies advises starting prophylaxis immediately when local surveillance is absent or limited, without waiting for the animal observation period to end, and offers immediate prophylaxis to bitten children (WHO, 2018). The indications follow the WHO recommendations, gathered in the 2018 position paper on rabies vaccines, and each country's rules. The post on clinical decision-making shows how to weigh the cost of each type of error.

Erring on the right side

In this decision, clinical reasoning moves away from the search for the most likely hypothesis and toward the question of which error would be worse. A patient who receives prophylaxis without having been infected goes through injections and follow-up visits. An infected patient who does not receive prophylaxis may die. This asymmetry justifies acting on a low probability, and the conversation with the patient must make this clear, so that the patient completes the schedule even when the animal looks healthy. The post on diagnostic uncertainty shows how to decide well when information is still incomplete, and why the best decision does not always depend on reducing all uncertainty before acting.

Travelers and migrants

Gautret and colleagues reviewed the records of 23 people with rabies who crossed a border between infection and diagnosis, and found that migrants from endemic areas and their descendants accounted for two thirds of the total (Gautret and colleagues, 2020). In the same study, 74 in every 100 patients did not receive prophylaxis before symptoms began. Asking about animal bites during travel or visits to family is part of the history.

Avian influenza: the red eye of people who work with animals

Rolfes and colleagues described 70 human infections with avian influenza A(H5N1) in the United States between 2024 and 2025: 41 in people exposed to dairy cows and 24 in people exposed to commercial poultry (Rolfes and colleagues, 2025). In the same study, 62 patients reported red eyes, most had a mild illness and there was no person-to-person transmission.

The clue is in the work

The authors point to occupational exposure to infected animals as a risk factor for infection (Rolfes and colleagues, 2025). A red eye, on its own, usually leads to common hypotheses. In a dairy farm or poultry farm worker during an outbreak in animals, the same clue calls for another question.

Other exposures, the same logic

The same reasoning applies to other zoonoses that are not the focus of this text, such as leptospirosis, linked to contact with contaminated water. In all of them, the decisive clue is usually in the history, not in the physical examination. The right question about exposure makes the hypothesis appear, and tests come afterward to confirm or rule it out.

When the clinician and the veterinarian talk

Cases like this show why information about outbreaks in animals matters to the clinician. Knowing that there is an outbreak in herds or farms in the region changes the probability of an infection that would otherwise not even make the list. The reverse also holds: an unusual human case can be the first sign of a problem in animals.

Exposures and questions: a table for the consultation

The table below summarizes, in a didactic way, some exposures and the questions that help bring them into the consultation. It simplifies a complex field and does not replace local guidelines.

Exposure	Example of linked zoonosis	Question that helps
Working with cattle, goats or sheep	Brucellosis	What do you do at work, and do you have contact with births or slaughter?
Raw milk and dairy products	Brucellosis	Do you consume unpasteurized milk or cheese?
Animal bite or scratch	Rabies	Were you bitten, scratched or licked on a wound, here or while traveling?
Dairy farm or poultry farm during an outbreak	Avian influenza	Do you work with cows or poultry, and is there disease in the animals?
Flood, river or lake water	Leptospirosis	Have you had contact with standing or flood water?
Laboratory or veterinary clinic	Several	Do you handle samples or sick animals?

Working with veterinary medicine and the environment

The WHO, together with other international agencies, published a joint One Health action plan for the period from 2022 to 2026. Destoumieux-Garzón and colleagues argue that the success of this approach depends on breaking down the barriers between human medicine, veterinary medicine and environmental sciences (Destoumieux-Garzón and colleagues, 2018).

What changes in practice

For the clinician, One Health does not require changing profession. It requires remembering that the patient lives in an environment, lives alongside animals and works somewhere, and that this information may be linked to the diagnosis. It also requires knowing whom to notify about a suspected case, so that human and animal surveillance services can act.

Back to the scene

In the opening scene, the question about work changes the course of the consultation. The doctor starts to consider a zoonosis linked to the herd, informs the laboratory about the hypothesis and thinks about who else may have been exposed: other people who help with births, the family that consumes the milk, the herd itself. If the hypothesis is confirmed, the case is no longer just one patient's and starts to involve the surveillance service and animal health. It is this path, from the patient to the common source, that the idea of One Health tries to make routine.

Notification as part of the reasoning

Many zoonoses are notifiable. Notifying is not just an administrative duty: it is the way to link the individual case to a possible common source, in animals or in the environment. The case the clinician sees may be the first of a series.

The biases that leave exposure out

The absence of the question about exposure is rarely a lack of knowledge. It usually comes from consultation habits and predictable biases:

1. Anchoring on the first explanation, such as "a viral infection", even when the picture does not follow the expected course.
2. Premature closure, when the list of hypotheses stops growing too early.
3. A history centered on symptoms, which does not ask about work, animals, water and food.
4. The idea that zoonoses are too rare to enter the differential.
5. The separation between areas, which leaves the clinician unaware of outbreaks in animals.

Croskerry describes anchoring and premature closure among the most frequent cognitive errors in diagnosis (Croskerry, 2003). The post about the diagnostic pause shows how a short moment of checking helps to notice what was left out.

How NEXO helps train this reasoning

In NEXO, the case of the day is fictional, with a real clinical presentation and up to six progressive clues. You propose a diagnosis by searching for the ICD-10 code and receive proximity feedback in five levels, which shows whether the guess stayed at the exact code, in the same category, in the same group, in the same chapter or in another chapter. This format connects with the theme of this text: a clue about exposure can move the guess from a nonspecific picture to the chapter of infectious and parasitic diseases.

To use the case of the day as training, try three habits. Before each guess, ask what exposure could explain the picture. When a clue about work, travel or animals appears, ask how it changes the list. After the result, note which question about exposure would have anticipated the diagnosis.

Three exercises with cases

Ask about work. In each case, imagine what the patient's occupation would be and how it would change the differential.

Change the setting. Reread a case imagining that the patient lives in a rural area, with animals, and see which hypotheses enter the list.

Think about who else. Faced with a zoonosis, ask who else was exposed to the same source, people or animals.

The post about learning based on clinical cases shows how cases help fix patterns that rarely appear in daily practice.

The limits on using the game are described on the NEXO economy page. If you research the teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information on curation and on creators, and institutions can learn about the proposal on the partnerships page.

Frequently asked questions

What is One Health?

It is the approach that treats the health of people, animals and the environment as linked to one another. It calls for joint work among medicine, veterinary science and environmental sciences.

What is a zoonosis?

It is an infection that can pass between animals and people. Many emerging infectious diseases are zoonoses, and several originate in wild animals.

Why ask about the patient's occupation?

Because work tells you what the patient is exposed to every day. Rural workers, slaughterhouse workers, laboratory workers and veterinarians, for example, have greater exposure to some zoonoses.

What should be done after an animal bite?

Seek care right away. Post-exposure prophylaxis given in time prevents rabies and includes care of the wound, the vaccine and, when indicated, immunoglobulin, according to WHO recommendations and local rules.

Does avian influenza pass from person to person?

In the human cases described in the United States between 2024 and 2025, transmission between people was not detected, and exposure to infected animals was the risk factor.

What does the clinician gain from One Health?

A more complete history and a more accurate list of hypotheses. Also a link with surveillance services, which turn an isolated case into information useful to the community.

Final checklist

• I asked what the patient does for work and what they do day to day.
• I asked about contact with animals, raw milk, water and travel.
• I considered zoonoses when the picture did not follow the expected course.
• Faced with a bite or scratch, I thought about rabies prophylaxis without waiting for certainty.
• I informed the laboratory about the hypothesis of zoonosis.
• I remembered notification and who else may have been exposed.

Want to practice reasoning with progressive clues? Play the case of the day.

References

1. Croskerry P. The importance of cognitive errors in diagnosis and strategies to minimize them. Acad Med. 2003. PMID 12915363.
2. Destoumieux-Garzón D, Mavingui P, Boetsch G, et al. The One Health concept: 10 years old and a long road ahead. Front Vet Sci. 2018. PMID 29484301.
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13. World Health Organization. One Health. Topic page.
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