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Serum Sickness-like Reaction (SSLR)

What Causes a Serum Sickness-Like Reaction?

Serum sickness is a syndrome that is characterized by skin rash, joint stiffness, joint pain, facial and extremity swelling, and fever. Sometimes vomiting or respiratory distress happen. It may be mistaken for anaphylaxis.

Serum sickness and serum sickness-like reactions (SSLRs) differ in what causes them. Some injected medications (such as Rituximab) trigger a true serum sickness. It occurs again if you give the same medication.

SSLRs can look like classic serum sickness. They typically occur after giving a variety of drugs, including antibiotics and vaccines. Many cases of SSLR occur in children who are not taking any medication at all. These children have had a recent viral infection.

Although the clinical signs and symptoms for classical serum sickness and SSLR are the same, SSLRs are not usually allergic reactions to medications. In fact, many times the SSLR develops after completing an antibiotic. Therefore, SSLR is likely an immune response to an infection. For example, if a child takes amoxicillin for an ear infection and develops SSLR, the child can likely take amoxicillin again in the future without problems.

What are the Symptoms of a Serum Sickness-Like Reaction?

Symptoms of SSLR develop seven to 14 days after the start of an infection. Examples are a respiratory infection or streptococcal pharyngitis (commonly known as strep throat). SSLR may last for several weeks. SSLRs typically start with itchy rashes that look like hives, either red- or skin-colored welts. They may change into a rash with bruised-looking centers. The rash will go away in five to seven days.

Additional symptoms may develop after the initial rash. This may include swelling of the face, hands, and / or feet. Fever, joint stiffness and pain, and / or vomiting may happen as well. Infants may refuse to walk or bear weight due to knee or ankle pain. Hives or welts may come and go for one to 12 weeks after all other symptoms resolve.

Rash evolves: SSLR may look like hives but turn purple after that. This is not dangerous.

Patients and families may be seen by a healthcare provider many days in a row because new symptoms appear. Symptoms of SSLR get worse several days after rash development. SSLR is not life threatening. It will go away.

How is a Serum Sickness-Like Reaction Treated?

Treatment for SSLR may include a variety of medications ordered by your doctor. Label the medications your child is taking as allergies until allergy testing can be done.

Increased skin sensitivity is common. To help with this:

  • Keep the skin cool and dry. Heat and moisture from warm showers or baths can make hives and itching worse.
  • Keep fingernails filed and trimmed short to prevent any scarring or opening of any raised areas if itching occurs.

Try not to use any new medications, vaccines or other potential triggers while the SSLR resolves, if possible.

Future Testing

Future testing to find any any true allergies to medications is recommended. Patients and families may feel nervous to expose their child to the same antibiotics the child was taking while experiencing an SSLR. Most reactions are not reproducible.

Testing involves a re-challenge by mouth, done six to 36 months after the initial SSLR. This depends on the child's comfort level and the risks associated with ongoing avoidance.

When to Call the Doctor

If you think your child may have an SSLR, call PATS (Penicillin Allergy Testing Service) at 513-517-7287 with any questions. You can also set up a telehealth same-day appointment with an allergy provider to manage allergic symptoms.

Last Updated 04/2023

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