Insect Bites and Stings (Mosquitos)

Insects bite by puncturing the skin with piercing mouthparts usually to feed (including mosquitos. An insect sting is defined by the use of another body part of the insect to pierce the skin and inject venom. Mosquitoes bites derive from females using a 6 needle- like mouthpiece; preferring exposed skin mainly seen in the summer months. Mosquitoes are vectors (which carry pathogens), therefore their bites can transmit diseases such as malaria (which are vector-borne)

Epidemiology

In the UK, insect stings are the second most common cause of anaphylaxis outside of medical settings [1]
One study looking at data recorded by GPs in England and Wales found that from 1999 to 2003; the highest incidence rate was in females aged 15 or older (7.0 per 100 000) and the lowest incidence rate being 3.2 per 100 000 in males aged 15 years or older. [2]
In areas where vector-borne diseases like malaria or dengue fever are prevalent, certain racial and ethnic groups might be more affected due to the regions they inhabit.
For example, African and South Asian populations are more exposed to malaria due to the geographical presence of the disease. [3]

Source: Skin For All

Insect (mosquito) bite located on the ankle on brown skin with brown/red discolouration

Source: Skin For All

Erythema and swelling due to insect bite on pale skin

Causes [1]

Insect feeding or a defence reaction

Mosquito eggs are laid in stagnant water (increased risk after floods/ tsunamis)

Pathophysiology [4]

Medical Student
- The mosquito penetrates the skin using its proboscis.
- The maxillae of the mouth contain serrated edges which cut through the skin while the mandible holds the tissue apart.
- The proboscis is used to probe through the skin in search of blood vessels. Once it inserts its labrum, it draws blood from the host.
- The mosquito injects saliva into the host's skin as it injects blood.
- Mosquito saliva contains a mixture of proteins, including anticoagulants, vasodilators and enzymes that help digest blood. These substances trigger an immune response due to it being recognised as foreign by the host.
- Histamine increases blood flow and the permeability of capillaries, allowing immune cells to reach the site of the bite.
- The release of histamine leads to inflammation, causing the characteristic redness, swelling, and itching associated with mosquito bites.
- Immediate response can manifest in wheal and flare formation (with surrounding redness)
- Delayed responses can be mediated by T cells which recognise the proteins in the mosquito saliva (this can occur hours to days later) leading to a more persistent presentation of swelling and itchiness.
- In some cases, especially in individuals with darker skin tones or those who scratch the bite excessively
- The bite may leave behind a scar or post-inflammatory hyperpigmentation, where the skin becomes darker in colour.Patients

Patients
- When a mosquito bites, it uses a needle-like mouthpart called a proboscis to pierce the skin.
- It has tiny, saw-like edges on part of its mouth that cut through the skin, while another part helps hold the skin open.
- The proboscis then searches for a blood vessel to draw blood from.
- As the mosquito feeds, it injects its saliva into your skin. This saliva contains various proteins, including ones that keep your blood from clotting and others that widen blood vessels to help the mosquito feed more easily.
- However, these proteins are foreign to your body, so your immune system reacts to them.
- In response, your body releases histamine, a chemical that increases blood flow and makes your blood vessels more permeable.
- This allows immune cells to reach the bite area, leading to the redness, swelling, and itching that are typical of mosquito bites.
- Immediately after the bite, you might notice a small, raised bump surrounded by redness.
- Sometimes, a delayed reaction occurs, where your body’s immune system takes hours or even days to fully respond to the proteins in the saliva, causing longer-lasting swelling and itching.
- For some people, especially those with darker skin tones or those who scratch the bite a lot, the bite might leave a scar or a dark spot on the skin after it heals.

Source: DermNet NZ

Wheal which can be caused by an insect bite on black skin

Source: DermNet NZ

Wheal caused by an insect bite on lighter skin

Risk factors [5]

Time period: Some are most active at dawn and dusk, while others bite throughout the night or even during the day.

Lifestyle factors: occupation and hiking which increases exposure to the regions which higher rates of tick infestation
Increased skin exposure
Carbon Dioxide: Mosquitoes are attracted to carbon dioxide (CO2 can be detected from over 100 feet away)
Blood type: Mosquitoes are particularly drawn to people with O blood type (anecdotal, no significant proof)

Presentations

There is some evidence to suggest that darker skin may have different reactions to bites, potentially leading to less noticeable or different types of bite marks compared to lighter skin.

However, this varies significantly among individuals.
Painless bite
Wheals and papules are round and have a central bite mark
Erythema (redness)
Pruritus (itchiness)
Oedema (swelling) [6]Skin-coloured, violet, or brown on darker skin
After the bites heal, they may leave brown marks on your skin [7]
Small blisters
Dark spots that look like bruises
In severe cases; Low-grade fever and swollen lymph nodes [8]

Investigations [9]

Clinical examination of appearance and history of symptoms

Investigations for severe reactions to insect bites or stings should be carried out in secondary care.

Differential diagnosis [10]

Trauma to the skin
Other stings from plants
Skin infection such as abscess, folliculitis or cellulitis.
Skin tumours such as basal cell carcinoma, squamous cell carcinoma, melanoma and Kaposi’s sarcoma
Contact dermatitis
Chicken pox
Urticaria

Source: Skin For All

Small blistering on the leg due to an insect bite on brown skin

Source: Skin For All

Diffused redness on pale skin with a clear sting location.

Management [11]

Medical student
- Clean the Area:
- Wash the bite area thoroughly with soap and water to reduce the risk of infection.
- Apply Cold Compress:
- Place a cloth soaked in cold water over the bite to reduce swelling and soothe the area.
- Pain Management:
- If the bite is painful or swollen, recommend an analgesic such as paracetamol or ibuprofen to alleviate discomfort.
- Reduce Swelling:
- Administer an oral antihistamine (such as chlorphenamine [sedating]) or topical corticosteroids (such as hydrocortisone 1%) to help reduce swelling and alleviate itching.
- Topical Treatment:
- Topical antipruritics (such as crotamitin), topical antihistamines and topical anaesthetics sold over-the-counter are of uncertain value in the treatment of insect bites and may themselves cause irritant skin reactions.
- Prevention:
  - General Precautions:

Wear long-sleeved clothing, trousers, and socks to cover exposed skin. Avoid bright or flowery prints, as these can attract insects.
Avoid walking barefoot or in sandals outside.
Refrain from using strongly scented products like perfumes, soaps, and shampoos.
Stay away from areas with a high risk of insect stings, such as places with food, drink, or rubbish bins. Be cautious when drinking from opaque containers outdoors.
Have insect nests near your home removed by professionals.
Keep approved insecticides for stinging insects on hand for emergencies, but note that insect repellents are ineffective against these insects.
If in Tick-Infested Areas:
Cover exposed skin with long sleeves and tuck trousers into socks. Light-colored clothing helps spot ticks more easily.
Use insect repellent containing DEET.
Regularly inspect skin, particularly children’s heads, scalps, and necks, for ticks. Also, check clothes and pets for ticks.

Patients
- Clean the Area:
- Wash the bite thoroughly with soap and water to prevent infection.
- Apply a Cold Compress:
- Place a cloth soaked in cold water on the bite to reduce swelling and soothe the area.
- Manage Pain:
- If the bite hurts or is swollen, take a pain reliever like paracetamol or ibuprofen to ease discomfort
- Reduce Swelling and Itching
- Take an oral antihistamine (like chlorphenamine) or apply a mild cream like hydrocortisone to reduce swelling and itching.
- Topical Treatments
- Be cautious with over-the-counter creams for itching or pain; some may not be very effective and could irritate your skin. [https://www.nhsinform.scot/illnesses-and-conditions/injuries/skin-injuries/insect-bites-and-stings/]

General Precautions:

Wear long sleeves, trousers, and socks to cover your skin. Avoid bright or floral prints that may attract insects.
Don’t walk outside barefoot or in sandals.
Avoid strong scents from perfumes, soaps, or shampoos.
Stay away from areas where insects are common, like near food, drinks, or trash bins. Be careful when drinking from containers you can't see into.
Have any insect nests near your home removed by professionals.
Keep insect sprays that are approved for stinging insects nearby, but remember that regular insect repellents won’t work on these insects.

If in Tick-Infested Areas:

Cover up with long sleeves and tuck your trousers into your socks. Light-colored clothing makes it easier to spot ticks.
Use a DEET-containing insect repellent.
Check your skin regularly, especially around the head, scalp, and neck, and remove any ticks you find. Also, check your clothes and pets for ticks.

Source: NHS

Small skin coloured wheal found on black skin due to an insect bite

Source: Skin For All

Circular erythema due to the insect bite on brown skin

Complications [6]

Scratching the bite can introduce bacteria into the skin, leading to secondary bacterial infections such as impetigo.
Some individuals may develop more severe allergic reactions to mosquito bites, known as "skeeter syndrome."
This condition involves intense swelling, pain, and sometimes systemic symptoms like fever.
Mosquitoes can transmit various pathogens through their bites, including malaria, dengue fever, Zika virus, and West Nile virus.
When a mosquito carrying a pathogen bites, the pathogen is introduced into the bloodstream along with the saliva, leading to infection.

Myths [12]

Mosquitos are only attracted to ‘sugary’ blood in people with conditions like diabetes
Mosquitoes transmit HIV
All mosquitoes bite humans
Citronella candles repel mosquitoes
Perfumes and lotions repel mosquitos

Questions to ask your doctor