Serum sickness is a syndrome that is characterized by skin rash, joint stiffness / 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 the causative agent. Antiserum, anti-venom, and insect stings trigger a true serum sickness. Serum sickness-like reactions (SSLRs) can look like classic serum sickness. They are typically less severe and occur following a reaction to a variety of drugs, vaccines, and some infections.

What Causes a Serum Sickness-like Reaction?

In a serum sickness-like reaction, the body’s immune system mistakenly identifies a substance (commonly an injectable medicine) as harmful. The result is an immune system response from the medication. It occurs again if you give the same medication.

Although the clinical signs and symptoms for classical serum sickness and serum sickness like reactions (SSLR) are the same, SSLR do not have a reproduceable immune response.

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. In fact, many times the SSLR develops after completing the antibiotic. It may develop in a child who never got an antibiotic at all. Therefore, SSLR is an immune response to the infection rather than the antibiotic.

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. SSLR typically start first 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. Due to knee or ankle pain, infants may refuse to walk or bear weight. Hives or welts may come and go for one to 12 weeks after all other symptoms resolve.

Serum sickness-like rash that 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 Serum Sickness-like Reactions may include a variety of medications as prescribed by your doctor. Label the medications your child is taking as allergies until allergy testing can be done at a later date. Increased skin sensitivity is common.

Keep the skin cool and dry. Heat and moisture can make the hives and itching continue or reoccur.

Take oatmeal baths one to two times a day to help relieve itching, inflammation, and help soothe the skin. With the water running, sprinkle one cup of oatmeal to lukewarm water. Soak for 10-15 minutes and then gently pat the skin dry.

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, and other potential triggers while the SSLR resolves, if possible.

Future Testing

Future testing to find any potential triggers 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 patient comfort level and the risks associated with ongoing avoidance.

When to Call the Doctor

If you think your child may have 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.