4.10 Travel-Related Illness, Vector-Borne Disease & Outbreaks

1. Fever in the Returning Traveler

The first task is triage: which febrile travelers have a potentially fatal, rapidly progressive illness, and which have a self-limited one?

Two questions dominate the history:

1. Where, exactly, and when? Match the incubation period to the exposure window. An illness beginning more than 21 days after return excludes dengue, chikungunya and most viral hemorrhagic fevers, but not malaria, typhoid, tuberculosis, viral hepatitis or leishmaniasis.
2. What exposures? Freshwater swimming (schistosomiasis, leptospirosis), unpasteurized dairy (brucellosis), undercooked meat, animal and arthropod bites, sexual contact, healthcare exposure, and whether malaria chemoprophylaxis was actually taken and completed.

Incubation periods that narrow the differential

Malaria appears in every window. That is the point.

2. Malaria

Fever in anyone returning from an endemic area is malaria until excluded. Chemoprophylaxis reduces but does not eliminate risk, and non-adherence is common.

Diagnosis: thick and thin blood smears. The thick smear detects parasites; the thin smear speciates and quantifies parasitemia. A single negative smear does not exclude malaria — repeat every 12 to 24 hours for a total of three sets. Rapid antigen tests are useful adjuncts.

Species matters:

• Plasmodium falciparum — causes nearly all severe disease and death. High parasitemia, cerebral malaria, acute kidney injury, acute respiratory distress syndrome, hypoglycemia, severe anemia and acidosis. Short incubation, usually within a month.
• P. vivax and P. ovale — form dormant hepatic hypnozoites and relapse months later. Eradication requires primaquine or tafenoquine, which mandates G6PD testing first because these drugs cause severe hemolysis in deficient patients.
• P. malariae — chronic low-grade parasitemia, associated with nephrotic syndrome.
• P. knowlesi — Southeast Asia, can be rapidly severe.

Severe malaria — impaired consciousness, seizures, acidosis, hypoglycemia, renal failure, pulmonary edema, parasitemia above 5%, or jaundice with organ dysfunction — is treated with intravenous artesunate. Uncomplicated falciparum malaria is treated with an artemisinin-based combination or atovaquone-proguanil.

3. Enteric Fever (Typhoid)

Salmonella Typhi and Paratyphi, most often from South Asia. The classic picture is stepwise fever rising over days with relative bradycardia, abdominal pain, and constipation preceding diarrhea; rose spots are uncommon and evanescent. Leukopenia with a normal or low white count in a febrile patient is a useful clue. Blood cultures are positive in roughly half; bone marrow culture has higher yield. Fluoroquinolone resistance is now widespread in South Asia, so ceftriaxone or azithromycin is used empirically.

4. Dengue, Chikungunya and Zika

All three are Aedes-borne, overlap geographically, and present with fever and rash.

In dengue, deterioration occurs as the fever breaks, not at its peak — the critical phase is marked by plasma leakage, rising hematocrit and falling platelets. NSAIDs and aspirin should be avoided because of bleeding risk; supportive fluid management is the treatment.

5. Tick-Borne and Rickettsial Disease

The blueprint lists rickettsioses and ehrlichioses and vector-borne diseases under specific causative organisms.

The rule that generates the most exam points: treat empirically with doxycycline. Rocky Mountain spotted fever mortality rises steeply with each day of delayed therapy, and serology is negative during the first week — the window in which treatment matters most. Waiting for confirmation is the wrong answer. Doxycycline is recommended even in young children and is used in pregnancy when the alternative is untreated rickettsial disease.

Other points that discriminate: the absence of a rash does not exclude rickettsial illness (roughly 10% of Rocky Mountain spotted fever is spotless, and ehrlichiosis is usually rashless). Babesiosis coinfection should be considered in a Lyme patient with hemolysis, and is treated with atovaquone plus azithromycin rather than doxycycline.

6. Other Specific Organisms Named in the Blueprint

• Prion disease — Creutzfeldt-Jakob disease presents with rapidly progressive dementia, myoclonus and ataxia; MRI shows cortical ribboning and basal ganglia hyperintensity, EEG may show periodic sharp waves, and cerebrospinal fluid RT-QuIC is the most useful assay. There is no treatment.
• Brucellosis — undulant fever, sacroiliitis, unpasteurized dairy exposure; treated with doxycycline plus rifampin or an aminoglycoside.
• Leptospirosis — freshwater exposure; conjunctival suffusion without exudate is the classic sign; Weil disease adds jaundice and renal failure.
• Q fever — Coxiella burnetii, from parturient livestock; causes pneumonia, hepatitis and culture-negative endocarditis.
• Strongyloidiasis — chronic autoinfection can persist for decades; corticosteroids precipitate hyperinfection syndrome, so screening is required before immunosuppression in anyone with relevant exposure history.

7. Infectious Disease Outbreaks

A distinct blueprint subsection. The internist has three responsibilities in an outbreak:

1. Recognize the pattern. Clustering in place, time or exposure — several patients from one facility, event or food source.
2. Institute correct isolation immediately, before the pathogen is confirmed. Airborne precautions with a negative-pressure room for suspected tuberculosis, measles or varicella; droplet for influenza and meningococcus; contact for C. difficile and multidrug-resistant organisms. Standard precautions apply to every patient at all times.
3. Notify public health. Reporting is a legal obligation and is the mechanism by which contact tracing and source identification occur. Suspected bioterrorism agents — anthrax, plague, tularemia, botulism, smallpox, viral hemorrhagic fevers — require immediate notification.

Travel-related prevention is the counterpart: pre-travel counseling includes destination-specific vaccination, malaria chemoprophylaxis matched to regional resistance patterns, food and water precautions, insect-bite avoidance, and standby therapy for travelers diarrhea.