Bites and Stings: When to Prescribe an Antibiotic

Dr Caroline Ward Summarises Recommendations from Two NICE Antimicrobial Prescribing Guidelines for Insect Bites and Stings, and Human and Animal Bites

In 2020, NICE published two antimicrobial prescribing guidelines covering the management of insect bites and stings,¹ and human and animal bites.² The guidelines aim to optimise antibiotic use and reduce antibiotic resistance. This article summarises each of the guidelines, highlighting common themes and important differences in management. 

Insect Bites and Stings

Insect bites and stings are a common presentation in primary care. The true rate of insect bites and stings is unknown as most instances go unreported. One recent survey of GPs found that all respondents had managed patients with insect bites in the previous year, and that further investigations and referrals to secondary care were rare.³

Most insect bites and stings result in minor local reactions, which usually last from a few hours to several days.⁴ They usually present as a red lump, with or without an area of surrounding erythema, and are often associated with swelling and itching.¹ Bacterial infection can result, but is uncommon.¹ Treatment should normally be directed towards first aid (for example, removal of the sting/tick and ice or a cold compress applied) and symptomatic relief of the inflammatory reaction.¹

Rarely, large local skin reactions, or potentially life-threatening systemic reactions including anaphylaxis, can occur in some people.⁴

Assessment

Insect bites and stings usually result in an inflammatory reaction only, and infection is uncommon.¹ Antibiotics are not required for most insect bites and stings.¹ In practice, it can be challenging to differentiate between inflammation and infection as both may present with erythema and swelling.

Signs of inflammatory or allergic reaction (common) include:¹

• rapid onset
• red and itchy skin
• symptoms may last for up to 10 days.

Signs of possible infection (uncommon) include:⁵

• painful, rather than itchy, skin
• fever, malaise, nausea, shivering, or rigors
• inflamed regional lymph nodes or associated lymphangitis (tracking), characterised by red lines spreading proximally from the bite or sting.

Systemic reactions are rare. Signs of hypersensitivity reactions usually start within minutes and can develop rapidly into life-threatening anaphylaxis, with swelling of mucous membranes, airway compromise, tachycardia, hypotension, and collapse.⁶ Immediate treatment and transfer to hospital are required.

Lyme disease is a bacterial infection that can be transmitted after a bite from an infected tick and, if untreated, can lead to long-term complications, such as neurological symptoms and Lyme arthritis.⁷ Most tick bites do not transmit Lyme disease, and prompt, correct removal of the tick reduces the risk of transmission.⁷ Erythema migrans is a sign of early Lyme disease (see Figure 1). This rash usually appears at or near the site of a tick bite after 1–4 weeks and lasts for several weeks. It is not usually itchy, hot, or painful.⁷ Erythema migrans should always be treated with antibiotics (see Table 1).

Management

As most insect bites and stings are self-limiting, a community pharmacist can advise about self-care treatments. Antihistamines are commonly used; however, there is uncertainty about their effectiveness in the management of bites and stings,¹ so symptoms may persist despite correct use. Antihistamines that cause sedation may help night-time symptoms.¹

Patients should avoid scratching to reduce inflammation and the risk of secondary infection. They should be advised to seek medical help if their symptoms worsen rapidly or significantly at any time, or they become systemically unwell.¹

Table 1: Summary of Assessment and Management for Insect Bites and Stings^1,5,7,8

Referral

Referral to secondary care is rarely required;³ however, consider referring or seeking specialist advice for people with an insect bite or sting if:¹

• they are systemically unwell
• they are severely immunocompromised with symptoms or signs of an infection
• they have had a previous systemic allergic reaction to the same type of bite or sting
• they have been bitten or stung in the mouth, throat, or around the eyes
• the bite or sting has been caused by an unusual or exotic insect
• there is fever or persisting lesions associated with a bite or sting that occurred while they were outside the UK.

Refer people with an insect bite or sting to hospital if they have symptoms or signs suggesting a more serious illness or condition, such as a systemic allergic reaction or anaphylaxis.¹

Human and Animal Bites

Bite wounds can take a variety of forms, including lacerations, puncture wounds, and crush or degloving injuries. Different animals typically inflict different patterns of wounds.

Human bites are usually wounds inflicted by actual biting, or clenched-fist injuries sustained when a clenched fist hits a person’s teeth, often during a fight. The latter typically cause small wounds over the knuckle area of the hand.⁹

Dog bites usually involve puncture wounds from the canine teeth, while the other teeth bite and tear tissues, and cause structural damage.⁹

Cats have fine, sharp teeth and can inflict deep puncture wounds contaminated with saliva. Although cat bite puncture wounds may appear small, they can be deep and are capable of penetrating bone, joints, and tendons.⁹ Cat bites are approximately twice as likely to become infected as dog bites, with 30–50% of cat bites becoming infected compared with 5–25% of dog bites.¹⁰

Assessment

Assess the type and severity of the bite, including what animal caused it, the site and depth of the wound, and if there are any symptoms and/or signs of infection.² The circumstances under which the bite occurred should be identified and documented.⁹

Safeguarding

Be aware of potential safeguarding issues, particularly in children and vulnerable adults, and when assessing a human bite mark that has not been caused by a young child. An intercanine distance of more than 3 cm suggests an adult bite.¹¹ Consider the possibility of child neglect when assessing an animal bite inflicted on a child who may have been inadequately supervised.¹²

Blood-borne Infection Risk Assessment

The risk of tetanus, blood-borne virus infection, and rabies should be assessed.² An immunisation history should be taken, and appropriate treatment given, including prophylactic treatment and/or vaccination.¹¹

Tetanus can occur after an animal or human bite, especially in puncture wounds, those containing damaged or devitalised tissue, dirt, or foreign bodies, or from an animal that has been rooting in soil or lives in an agricultural setting.^11,13 In the UK, the risk of rabies is very low as there is no indigenous infection in animals, except in bats.⁹

Management

Antibiotic Prophylaxis

Not all bites require antibiotic prophylaxis. NICE’s recommendations are based on patient factors and wound site, and are species specific due to the known variation in infection rates from different animal bites. If the skin is unbroken, antibiotic prophylaxis is not required.² If a bite has broken the skin but not drawn blood, antibiotic prophylaxis is usually not required; however, it may be considered in some circumstances (see Table 2). If indicated, the duration of prophylaxis is 3 days only.²

People at high risk of a serious wound infection include those with co-morbidities, such as diabetes, immunosuppression, asplenia, or decompensated liver disease.²

High-risk areas for infection include the hands, feet, face, genitals, skin overlying cartilaginous structures, or an area of poor circulation.²

Table 2: Summary of Antibiotic Prophylaxis and Treatment for Human and Animal Bites²

Dog Bites

Antibiotic prophylaxis does not reduce the incidence of infection after a bite from a dog or other traditional pet, excluding cats. It should only be considered if:²

• there is significant, deep tissue damage
• the wound is visibly contaminated (e.g. by dirt)
• the bite is in a high-risk area for infection or in a person at high risk of a serious wound infection due to co-morbidities, as detailed above and in Table 2.

Cat Bites

Cat bites can result in a deep wound despite a small puncture. All patients with cat bites that have drawn blood should be offered antibiotic prophylaxis, and antibiotics should be considered if the wound has not drawn blood but could be deep.²

Human Bites

All human bite wounds that have drawn blood should be treated with antibiotic prophylaxis. Antibiotics should be considered for bites where the skin has been broken but not drawn blood if the patient is at high risk of a serious wound infection or the bite is in a high-risk area for infection, as detailed above and in Table 2.²

Antibiotic Treatment

Start empirical antibiotic treatment if any bite wound appears infected (for example, redness, swelling, induration, necrotic tissue, purulent discharge, pain, localised cellulitis, lymphangitis, lymphadenopathy, or fever). Treatment duration should be 5 days, and only extended to 7 days if not resolving after a review at 5 days.²

Seek specialist advice from a microbiologist for bites from a wild or exotic animal, including birds and non-traditional pets. Consider seeking specialist advice from a microbiologist for domestic animal bites, including farm animal bites, with which you are unfamiliar.²

Take a swab for microbiological testing to guide treatment if there is a discharge. Review antibiotic choice based on swab results and switch to a narrow-spectrum antibiotic, if possible.²

Reassessment

Reassess the bite if symptoms or signs of infection develop or worsen rapidly or significantly at any time, there is no improvement within 24–48 hours of starting treatment, or the person becomes systemically unwell. Be aware of pain that is out of proportion to the infection as this can be a sign of necrotising fasciitis, a rare but serious bacterial infection that affects the deep fascia and subcutaneous tissues.²

Referral

Consider referral or seeking specialist advice for patients with a human or animal bite if they:²

• have severe cellulitis, abscess, osteomyelitis, septic arthritis, necrotising fasciitis, or sepsis
• have a penetrating wound involving bones, joints, tendons, or vascular structures
• are systemically unwell
• have developed an infection after taking prophylactic antibiotics
• are unable to take, or have an infection that is not responding to, oral antibiotics. 

Summary

The NICE guidelines discussed in this article provide recommendations on antimicrobial prescribing for insect bites and stings, and human and animal bites. Insect bites and stings do not usually require antibiotic treatment. Signs of erythema and swelling are more often due to an inflammatory reaction than infection. Patients can usually be given information and safety-netting advice and directed to a community pharmacy for symptomatic treatment. By contrast, human and animal bites are at higher risk of infection, and antibiotic treatment or prophylaxis may be required. Cat bites are the most likely to become infected, with dog bites being at a relatively lower risk of infection.  

Dr Caroline Ward

GP and member of the NICE managing common infections advisory committee