AssessmentsBites and stings: Clinical practice
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 37-years-old Caucasian HIV-positive man comes to the dermatologist because of a month-old lesion on medial arm. The lesion is an erythematous nodule 1.5 cm in diameter, with satellite minor red papules. It is firm, bleeds easily, and is tender on palpation. He works as a screenplay writer and owns a cat. He also complains of subcutaneous tender nodules on the distal medial end of his forearm. Blood cultures are positive for a bacteria. A biopsy is done and stained with modified Warthin-Starry staining, confirming the diagnosis. Which of the following best describes the typical histological characteristic of the patient’s condition?
Content Reviewers:Rishi Desai, MD, MPH
Bites and stings can be caused by a variety of animals, insects, snakes or yes, even humans!
The most common mammalian bites are from dogs, cats, and humans. And each bite transmits the polymicrobial flora that can be found in the mouth - including Staphylococcus aureus, group A Streptococcus species, Klebsiella, Enterococci, Prevotella and many more.
Dog bites can transmit a gram negative rod called Capnocytophaga canimorsus, which can cause gangrene at the bite site and can disseminate and cause sepsis, especially in asplenic individuals and those with liver disease.
Cat bites or scratches - particularly from kittens - can transmit Bartonella henselae, which causes cat scratch disease.
Finally, human bites can transmit Eikenella corrodens which is a gram negative anaerobe which can cause local and disseminated infection - including being a cause of culture-negative endocarditis - one of the HACEK organisms.
In general, bite wound infections cause local cellulitis and in some cases can lead to a local abscess and fevers.
Dog bites are the most common ones of all, and because dogs have strong jaws they often cause serious tissue injury, but infections aren’t that common.
In contrast, cats have long, thin teeth, that create deep puncture wounds, which very often get infected. Cat bites can even reach the underlying joint space or periosteum, leading to septic arthritis or osteomyelitis.
Like cat bites, human bites also have a high risk of infection.
Most human bites happen due to aggressive play between children, but can also happen among adults.
In a child, if the bite mark is over 2.5 centimeters in diameter, then it’s likely an adult bite, and should raise the suspicion of child abuse.
The most common human bite occurs when a clenched fist hits a person’s tooth, and that results in a laceration over the 3rd and 4th metacarpals. Because of how close the joint spaces and bones are, septic arthritis and osteomyelitis are potential complications.
Now if a bite from any mammal - dog, cat, human, or otherwise - appears infected, a gram stain and culture of the wound should be obtained before starting antibiotics.
If there are signs of systemic illness like fever or fatigue, then blood cultures should be taken.
Other labs include a CBC to look for leukocytosis and an ESR and CRP since they may be elevated.
In a deep bite wound an x-ray should be taken to look for foreign bodies like teeth, bone or soft tissue injury, or subcutaneous gas.
Management starts with controlling any active bleeding with direct pressure.
The wound should be irrigated under high pressure with normal saline or tap water, and any superficial dead tissue should be debrided.
The wound should be explored under local anesthesia to assess its depth and look for foreign bodies.
In general, primary wound closure is indicated for superficial dog bites, however, cat and human bites aren’t closed, and instead should close on their own by secondary intention due to the high risk of infection.
However, regardless of the biting mammal, a wound to the face should be closed because cosmesis is especially important.
Now, when a decision is made to close a mammal bite wound, four criteria must be fulfilled. It must be uninfected, less than 24 hours old, not located on the hand or foot, and the individual should have a healthy immune system.
Tetanus prophylaxis with tetanus toxoid and tetanus immunoglobulin should be considered.
If the individual has received less than 3 doses or their immunization status is unknown, then both the tetanus toxoid vaccine and immunoglobulin are given. However, if they have received 3 or more doses of the vaccine, then the immunoglobulin is not given, and the tetanus toxoid vaccine is given only if the last dose was 5 or more years ago.
Now, if the attack was from a wild or stray animal, then rabies prophylaxis with the rabies vaccine and immunoglobulin should be considered. If the individual was not previously vaccinated, then both the rabies vaccine and immunoglobulin are given. But if they were previously vaccinated, then only the rabies vaccine is given.
If the bite wound is infected, oral antibiotics like amoxicillin-clavulanic acid can be used to cover dog, cat, and human bites. Deeper or more serious wound infections may require IV antibiotics like piperacillin-tazobactam, and abscesses are incised and drained.
Next up are snakebites, which are particularly problematic in areas where access to healthcare and antivenom may be limited, such as parts of sub-Saharan Africa and Asia.
The two main venomous families are the Elapids, like corals, cobras, and mambas, and the Vipers, like rattlesnakes and copperheads.
In general, after a snake bite there are local changes at the bite site like erythema, swelling, and development of hemorrhagic bullae and tissue necrosis, as well as general symptoms like nausea, fatigue, and headache.
Elapids are neurotoxic snakes, and their venom acts by irreversibly binding to the acetylcholine receptor. Symptoms include a rapidly developing descending paralysis, resulting in diplopia, difficulty swallowing or talking, as well as limb or even respiratory muscle paralysis. In response, analgesics like acetaminophen, NSAIDs or opioids, as well as anti-venom should be given.
In contrast, Viper snakes causes direct cellular injury and are hemotoxic. Their venom alters the balance of the coagulation system, resulting in coagulopathy, which can cause gum bleeding, epistaxis, hematemesis, or hematochezia.
Additionally, venom tends to spread through the lymphatics, so if the bitten limb has tender lymphadenopathy, then you know that the venom is spreading beyond the bite site.
Laboratory testing may reveal thrombocytopenia or an elevated PT and a decreased fibrinogen.
Additionally, the venom may be myotoxic resulting in rhabdomyolysis. This leads to tender, weak muscles and dark red urine, and laboratory testing that shows an elevated CK level and an elevated urine myoglobin.
Treatment is to not suck out the venom, even though that’s still commonly believed. Instead the key is local wound care including cleaning the wound, analgesia with acetaminophen, NSAIDs or opioids, giving IV fluids, elevating the affected limb to prevent pooling of the venom, and giving a specific antivenom like CroFab.
Application of a tourniquet or ice have not shown to have any benefit in preventing dissemination of the venom.
Now, antivenom is usually derived from other animals like horse and sheep serum, so there is a risk of developing an allergic reaction to the antivenom.
Now, itsy bitsy spiders can also bite, and there’s usually a papule, pustule, or wheal, but sometimes they go unnoticed.
The brown recluse spider lives in the United States and parts of South America like Brazil and Chile, and causes a painless bite that slowly causes local erythema. Over time, there’s tissue necrosis and hemorrhagic blisters. In severe cases, there can be hemolysis, thrombocytopenia, and renal failure that develop within a few days of the bite.
Treatment includes analgesia with acetaminophen, NSAIDs or opioids and in severe cases, antivenom is given. Interestingly, the antibiotic dapsone helps alleviate the symptoms of a brown recluse spider bite, but dapsone shouldn’t be given to individuals with G6PD deficiency because it can trigger hemolysis.
Next up, is the infamous black widow spider that’s found all over the United States, and has a distinct red hourglass shape on abdomen.
The black widow spider releases alpha-latrotoxin, a toxin that causes a massive release of neurotransmitters like acetylcholine and norepinephrine. This causes a red blanching “target lesion” at the bite site, severe muscle spasms and cramping especially of the abdominal muscles, and can cause cardiac arrhythmias like atrial fibrillation. The severe abdominal cramping and pain can be mistaken for peritonitis or another surgical emergency like appendicitis.