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Unseen Vector, Visible Rash


On a tropical vacation, a woman develops a severe pruritic rash after being bitten on the hand by an insect. Can you name the bug?

A 60-year-old woman on vacation at a resort in Punta Cana, Dominican Republic (DR) felt a bite on the back of her left hand, while walking in late afternoon. The next day, she noticed an extremely pruritic wheal-like rash on her left hand. Over the next 24-48 hours, bullae appeared on both arms and both lower legs.

Past medical history includes rheumatoid arthritis and hypothyroidism. Medications include: infliximab and levothyroxine.

After a one week holiday, she returned to the US and saw her primary care physician (PCP).

Skin lesions on the right forearm are shown in the Figure. Similar lesions were also on her left arm and both lower legs.

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Infliximab is an immunosuppressive that can cause hives, but this patient had been taking this drug for several years, so it was unlikely related.

The PCP diagnosed an allergic reaction to an unknown allergen and prescribed a methylprednisolone taper and topical steroids.

Skin lesions in travelers returning from tropical destinations are one of the medical problems seen most frequently, accounting for >10% of reported cases.1

Which insect is most likely resonsible for this patient's vacation-related rash?

A. Mosquito

B. Sand flea

C. Sand fly

D. Fire ant

E. Biting gnat, midge

For answer and discussion, please see next page>>


Answer: C. Sand fly 

An Unseen Fly

Sand flies, actually tiny winged insects and also known as “no-see-ems,” are widely disbursed in climates ranging from warm-temperate through subtropical to tropical. Mosquitoes and sand flies commonly live along the coast of Punta Cana, DR.

Sand flies appear at dusk and dawn in swarms. Many species bite most actively after sunset, but others bite after midnight. They may bite if disturbed (for example, if hikers brush against tree trunks, or other sand-fly resting places). In tropical climates, sand flies are active year round.

Sand flies are attracted to a range of hosts and respond to host odor cues such as CO2.2 The females fly onto an unsuspecting person and inject saliva into the skin to facilitate blood sucking. Saliva, an irritant, causes a hypersensitivity reaction. Sand fly bites may leave large, red itchy bumps that may turn into a rash. These bumps are frequently several times as itchy as mosquito bites, and tend to last longer.

The diagnosis of pruritic dermatoses depends on the location of the symptoms and the presence of skin lesions. Self-limited and localized pruritis suggests an allergic reaction to insect bites. Arthropod bites produce the clinical presentation of vesicles and bullae.

Nota bene

Why do physicians need to know the vector is a sand fly? Sand flies carry protozoa (Leishmania), bacteria (Bartonella bacilliformis) and viruses. Cutaneous leishmaniasis (CL) causes skin lesions, which can persist for months, or years. The skin lesions usually develop within several weeks or months after the exposure, but occasionally years later (for example, after trauma or immunosuppression). The CDC has information on CL diagnosis and treatment and advises clinicians to consider CL in people with chronic (nonhealing) skin lesions who have been in areas where leishmaniasis is found.3

There are 18 different Leishmania species that are pathogenic for humans. The only country where an indigenous focus of CL has been discovered within the last 20 years is the DR. Leishmania waltoni has been isolated from patients in the DR. Since 1975, human cases of diffuse CL have been reported from the provinces of El Seibo, La Altagracia and Sánchez Ramírez in the northeastern part of the country. Subclinical forms are frequently seen.4

More than 75% of CL cases diagnosed in US civilians have been acquired in Latin America, including popular tourist destinations such as Costa Rica.5

As the number of travelers to tropical destinations has risen, the number of skin illnesses has also increased.2 Pre-travel consultations should include advice on prophylaxis that focus on preventing insect bites, while considering the intended destination of the traveler.4


After one week of treatment, the rash remained only distally (on wrists and ankles). Hopefully, none of the sandflies that bit this patient carried Leishmania protozoa. Interestingly, there have been case reports of patients taking infliximab and corticosteroids developing CL. So, if any of the skin lesions do not heal, or become chronic, then there should be a high index of suspicion for CL and further testing done.

There is an adage in medicine “if you don’t think of it, you won’t diagnose it.” Another saying is “think of the worst (diagnosis) and hope for the best.”




1. Korzeniewski K, Juszczak D., et. al. Skin lesions in returning travelers. Int Marit Health, 2015; 66 (3): 173-80. https://www.ncbi.nlm.nih.gov/pubmed/?term=Int+Marit+Health%2C+2015%3B+66+(3)%3A+173-80.

2. http://www.cvbd.org/en/sand-fly-borne-diseases/about-sand-flies/sand-fly-feeding/host-seeking-behaviour/

3. https://www.cdc.gov/parasites/leishmaniasis/health_professionals/

4. http://www.who.int/leishmaniasis/resources/DOMINICAN_REPUBLIC.pdf

5. https://wwwnc.cdc.gov/travel/yellowbook/2016/infectious-diseases-related-to-travel/leishmaniasis-cutaneous

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