# Common Childhood Exanthems and Tick-Borne Illness

## Overview

Viral exanthems are among the most common presentations in pediatric primary care and emergency departments. Pattern recognition combined with clinical context enables diagnosis in most cases without laboratory testing. Tick-borne illnesses (Lyme disease, Rocky Mountain spotted fever) require high clinical suspicion, as delayed treatment can be life-threatening. The key skill is distinguishing benign viral exanthems from serious conditions requiring urgent intervention.

## Classic Viral Exanthems

| Exanthem | Pathogen | Key Rash Feature | Distribution | Distinguishing Clue |
|----------|----------|-----------------|-------------|-------------------|
| Measles | Paramyxovirus | Confluent maculopapular | Cephalocaudad spread | Koplik spots, 3 Cs (cough, coryza, conjunctivitis) |
| Rubella | Rubivirus | Fine maculopapular | Face then trunk | Postauricular lymphadenopathy |
| Varicella | VZV | Vesicles in crops ("dewdrop on rose petal") | Centripetal (trunk > extremities) | Lesions in all stages simultaneously |
| HFMD | Coxsackievirus A16/A6, EV-71 | Oral ulcers + vesicles | Palms, soles, buttocks | Painful oral lesions, febrile child |
| Roseola | HHV-6 | Maculopapular, pink | Trunk then spreads | Rash appears AS fever breaks |
| Erythema infectiosum | Parvovirus B19 | "Slapped cheek" then lacy/reticular | Face then trunk/extremities | Not contagious once rash appears |
| Scarlet fever | GAS (erythrogenic toxin) | Sandpaper texture, Pastia lines | Diffuse, spares face | Strawberry tongue, circumoral pallor |

### Measles (Rubeola)

Measles is caused by a paramyxovirus that is extremely contagious (R0 = 12-18) with airborne transmission. The prodrome lasts 3-5 days and consists of high fever (up to 40.6C), cough, coryza, and conjunctivitis (the "3 Cs"). Koplik spots are pathognomonic: white-blue papules on the buccal mucosa opposite the molars that appear 1-2 days before the rash. The rash is erythematous and maculopapular, starting on the face and hairline and spreading cephalocaudad. It becomes confluent, lasts 5-7 days, and is followed by desquamation. Complications include otitis media, pneumonia (the leading cause of measles death), encephalitis, and subacute sclerosing panencephalitis (SSPE), which develops years later. Treatment is supportive, with vitamin A recommended by the WHO to reduce mortality and complications, especially in malnourished or immunocompromised patients. Measles is a reportable disease requiring public health notification.

### Rubella (German Measles)

Rubella is caused by a rubivirus and produces a mild illness in children with low-grade fever, lymphadenopathy (postauricular, suboccipital), and a maculopapular rash starting on the face. The critical concern is congenital rubella syndrome, which is devastating when maternal infection occurs in the first trimester, causing cataracts, sensorineural deafness, cardiac defects (PDA, peripheral pulmonary stenosis), and blueberry muffin rash (dermal erythropoiesis). Rubella has been essentially eliminated in countries with robust MMR vaccination programs.

### Varicella (Chickenpox)

Varicella is caused by varicella-zoster virus (VZV), which is highly contagious via airborne and contact routes. The rash is pruritic, consisting of vesicles on an erythematous base ("dewdrop on rose petal") appearing in crops at various stages (macule, papule, vesicle, crust) with a centripetal distribution (trunk more than extremities). Complications include secondary bacterial skin infection (GAS, S. aureus), pneumonia, cerebellar ataxia, encephalitis, Reye syndrome (aspirin is contraindicated), and necrotizing fasciitis from GAS. Treatment is supportive for uncomplicated cases, with acyclovir reserved for high-risk patients (immunocompromised, adolescents, those with chronic skin or lung disease). The varicella vaccine has dramatically reduced incidence.

### Hand-Foot-and-Mouth Disease (HFMD)

HFMD is most classically caused by coxsackievirus A16, with coxsackievirus A6 producing a more severe variant and enterovirus 71 associated with CNS complications. The presentation includes fever, painful oral vesicles and ulcers (in the anterior mouth including the tongue, buccal mucosa, and palate), and a vesicular rash on the palms and soles that may involve the buttocks and perioral area. Coxsackie A6 causes more extensive, atypical distribution with larger vesicles and bullae, and onychomadesis (nail shedding) may occur weeks later. Enterovirus 71 is associated with brainstem encephalitis and neurogenic pulmonary edema, primarily in Asia. Treatment is supportive with pain management (magic mouthwash, cold fluids) and hydration monitoring.

### Roseola Infantum (Exanthem Subitum)

Roseola is caused by HHV-6 (most commonly) or HHV-7 and typically affects children aged 6 months to 2 years. It presents with high fever (often exceeding 40C) for 3-5 days, during which the child appears relatively well. The rash appears as the fever breaks, consisting of diffuse maculopapular ("rose-pink") lesions starting on the trunk and spreading to the face and extremities. The rash blanches, is non-pruritic, and lasts 1-2 days. The most important complication is febrile seizures, and roseola is the most common cause of febrile seizures in this age group. Diagnosis is clinical and no treatment is needed.

### Erythema Infectiosum (Fifth Disease)

Fifth disease is caused by parvovirus B19. It presents with low-grade fever followed by the characteristic "slapped cheek" appearance (bilateral facial erythema sparing the nose), then a lacy, reticular rash on the trunk and extremities. The rash may wax and wane for weeks with triggers such as sun exposure, heat, and bathing. Complications include transient aplastic crisis in sickle cell disease and other hemolytic anemias (since B19 targets erythroid progenitor cells), hydrops fetalis if maternal infection occurs in pregnancy, and arthropathy (more common in adults, especially women). Critically, the child is contagious before the rash appears; once the rash develops, the child is no longer infectious. Treatment is supportive.

<image>Visual comparison of classic childhood exanthems showing measles (confluent maculopapular rash with cephalocaudad spread, Koplik spots), varicella (vesicles in different stages on erythematous base, centripetal distribution), hand-foot-mouth disease (oral ulcers and palmar/plantar vesicles), roseola (diffuse pink macular rash appearing after fever breaks), and erythema infectiosum (slapped cheek appearance followed by lacy reticular rash)</image>

## Bacterial Exanthems

### Scarlet Fever

Scarlet fever is caused by Group A Streptococcus (GAS) producing erythrogenic exotoxin. It presents with fever, pharyngitis, and a diffuse erythematous sandpaper-textured rash that is accentuated in skin folds (Pastia lines, which are linear petechiae in the antecubital and inguinal creases). The tongue initially has a white coating ("white strawberry tongue") and then desquamates to a "red strawberry tongue." Circumoral pallor is characteristic. Desquamation begins 1-2 weeks after rash onset, especially on the fingertips and toes. Treatment is penicillin V or amoxicillin for 10 days, which prevents rheumatic fever.

### Staphylococcal Scalded Skin Syndrome (SSSS)

SSSS is caused by S. aureus producing exfoliative toxins (ETA/ETB). It presents with diffuse tender erythema followed by superficial desquamation, a positive Nikolsky sign (skin sloughing with gentle lateral pressure), and perioral crusting. It typically occurs in children under 5 years due to immature renal clearance of the toxin. The key distinction from toxic epidermal necrolysis (TEN) is that SSSS involves the superficial epidermis (granular layer), while TEN involves full-thickness epidermal necrosis. Treatment is antistaphylococcal antibiotics (nafcillin, cefazolin), wound care, and pain management.

## Tick-Borne Illnesses

### Lyme Disease

#### Epidemiology

Lyme disease is caused by Borrelia burgdorferi and transmitted by Ixodes ticks (deer ticks). It is endemic in the northeastern and upper midwestern US and mid-Atlantic states. The tick must be attached for 36 hours or more for transmission, with a partially engorged tick indicating increased risk.

#### Clinical Stages

Early localized disease develops days to weeks after the bite. Erythema migrans (EM) is an expanding annular erythematous lesion at the bite site with possible central clearing ("bull's eye"), measuring 5 cm or more in diameter, and is typically neither pruritic nor painful. EM is pathognomonic, and no laboratory confirmation is needed for a classic presentation. Systemic symptoms include fever, fatigue, headache, myalgia, and arthralgia.

Early disseminated disease develops weeks to months later and manifests as multiple EM lesions, facial nerve palsy (the most common cranial neuropathy, which may be bilateral in 25% of pediatric cases), carditis with AV block (first to third degree, though rare in children), and meningitis with lymphocytic pleocytosis.

Late disseminated disease develops months after infection and primarily presents as Lyme arthritis affecting large joints, especially the knee, with recurrent episodes and inflammatory joint fluid.

#### Diagnosis

Classic EM is a clinical diagnosis requiring no testing. Standard two-tier serology uses ELISA (sensitive) followed by Western blot (specific) if the ELISA is positive or equivocal. IgM Western blot requires 2 or more of 3 bands (for early disease under 4 weeks), and IgG Western blot requires 5 or more of 10 bands (for any stage). A modified two-tier approach uses ELISA followed by a second immunoassay (replacing Western blot, FDA-approved 2019). Testing pitfalls include the 2-3 week window period when antibodies may be negative, persistent seropositivity from prior infection, and false positives from cross-reactivity (positive ELISA with negative Western blot). CSF analysis with Lyme antibody index is used for suspected neuroborreliosis.

#### Treatment

Early localized disease (EM) is treated with doxycycline 4.4 mg/kg/day divided twice daily for 10-21 days (preferred for ages 8 and older) or amoxicillin 50 mg/kg/day three times daily for 14-21 days (for children under 8). Facial nerve palsy is treated with oral doxycycline or amoxicillin for 14-21 days (LP is indicated if meningeal signs are present). Lyme arthritis is treated with oral doxycycline or amoxicillin for 28 days, with IV ceftriaxone for refractory cases. Carditis or meningitis requires IV ceftriaxone for 14-21 days. Post-tick bite prophylaxis consists of a single dose of doxycycline 4.4 mg/kg (maximum 200 mg) within 72 hours if the criteria are met: Ixodes tick, attached 36 hours or more, endemic area, and doxycycline not contraindicated.

<image>Lyme disease clinical staging showing early localized disease (erythema migrans with bull's-eye pattern), early disseminated disease (multiple EM lesions, facial nerve palsy, carditis), and late disseminated disease (large joint arthritis), with two-tier diagnostic algorithm (ELISA screening followed by Western blot or second immunoassay confirmation) and treatment recommendations for each stage</image>

### Rocky Mountain Spotted Fever (RMSF)

RMSF is caused by Rickettsia rickettsii and transmitted by Dermacentor ticks (American dog tick, Rocky Mountain wood tick). Despite its name, it is most common in the southeastern and south-central US. Early symptoms are nonspecific: fever, severe headache, and myalgia. The rash appears on days 3-5, beginning on the wrists and ankles and spreading centrally (centripetal). It is initially macular and becomes petechial or purpuric. The rash may be absent in 10-15% of cases ("spotless" RMSF), so treatment should not be withheld while waiting for a rash. Without treatment, mortality is 20-25%; with early doxycycline, it falls to less than 5%. Complications include meningoencephalitis, DIC, renal failure, pulmonary edema, and gangrene.

Diagnosis is clinical because serology (IFA) is positive only after 7-10 days, making it useless for early diagnosis. Treatment must be empiric. Doxycycline is the treatment of choice for all ages, including children under 8 years, given at 2.2 mg/kg/dose twice daily (maximum 100 mg/dose) for 7-14 days, continuing at least 3 days after fever resolves. Treatment must not be delayed waiting for laboratory confirmation. Importantly, doxycycline does not cause dental staining in short courses, a fact confirmed by both the AAP and CDC; the old concern about tetracyclines and teeth does not apply to doxycycline at therapeutic doses for 21 days or less.

### Other Tick-Borne Diseases

Ehrlichiosis and anaplasmosis present with fever, headache, leukopenia, thrombocytopenia, and elevated liver enzymes, with morulae visible on blood smear; treatment is doxycycline. Babesiosis presents as a malaria-like illness with hemolytic anemia and thrombocytopenia, transmitted by Ixodes ticks; treatment is atovaquone plus azithromycin. Tularemia (Francisella tularensis) most commonly presents in the ulceroglandular form; treatment is gentamicin or doxycycline.

## Clinical Pearls

Roseola is the most common cause of febrile seizures because the very high fever (often exceeding 40C) before the rash appears is the trigger period. In parvovirus B19 infection, the child is contagious before the rash appears; by the time the slapped cheek rash develops, the child is no longer infectious and can attend school. Doxycycline is safe for children of all ages for tick-borne illnesses and should not be withheld for fear of dental staining, as short courses do not cause this. If RMSF is suspected, doxycycline should be started immediately because waiting for serology is dangerous and RMSF can be fatal within days. Erythema migrans of 5 cm or larger is diagnostic of Lyme disease, and no serologic testing is needed for classic EM; testing early may be falsely negative. Koplik spots are pathognomonic for measles and appear 1-2 days before the rash, potentially clinching the diagnosis before the exanthem develops.

## Key Controversy: Single-Dose Doxycycline for Post-Tick Bite Lyme Prophylaxis

IDSA recommends single-dose doxycycline (200 mg adult; 4.4 mg/kg child) within 72 hours of tick removal for prophylaxis in endemic areas if the tick was attached 36 hours or more. This recommendation is based on one RCT (Nadelman 2001) showing 87% risk reduction. Some experts argue this promotes antibiotic overuse and that the absolute risk reduction is small given the low baseline transmission risk. The counterargument is that doxycycline at a single dose is safe, inexpensive, and prevents a potentially significant illness. The AAP has endorsed short-course doxycycline for all ages, confirming that dental staining concern is unfounded for courses of 21 days or less. An alternative monitoring approach involves educating patients about EM rash recognition and treating promptly if symptoms develop rather than providing prophylaxis. In endemic areas, most clinicians offer prophylaxis for high-risk exposures through shared decision-making with families.

<image>Summary of tick-borne illness comparison table showing Lyme disease (Ixodes tick, erythema migrans, facial palsy, arthritis), RMSF (Dermacentor tick, centripetal petechial rash, potentially fatal), Ehrlichiosis (lone star/Ixodes tick, leukopenia, thrombocytopenia), and Babesiosis (Ixodes tick, hemolytic anemia), with treatment recommendations for each</image>

## References
- Wormser GP, et al. The Clinical Assessment, Treatment, and Prevention of Lyme Disease, Human Granulocytic Anaplasmosis, and Babesiosis (IDSA Guidelines). Clin Infect Dis. 2006;43(9):1089-1134.
- Todd SR, et al. No Visible Dental Staining in Children Treated with Doxycycline for Suspected Rocky Mountain Spotted Fever. J Pediatr. 2015;166(5):1246-1251.
- Nadelman RB, et al. Prophylaxis with Single-Dose Doxycycline for the Prevention of Lyme Disease. N Engl J Med. 2001;345(2):79-84.
- Cherry JD, et al. Feigin and Cherry's Textbook of Pediatric Infectious Diseases. 8th ed. Elsevier; 2019.
- American Academy of Pediatrics. Red Book: Report of the Committee on Infectious Diseases. 33rd ed. AAP; 2024.
- Helmick CG, et al. Rocky Mountain Spotted Fever: Clinical, Laboratory, and Epidemiological Features. J Infect Dis. 1984;150(4):480-488.