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Infectious disease and microbiology – Mites (including chiggers)
Mites and chiggers are arthropods with four pairs of legs that infest humans, causing primarily cutaneous disease, but in some cases transmitting systemic infections such as scrub typhus.
The most important human mite is Sarcoptes scabiei, which causes scabies, while chiggers (larval forms of trombiculid mites) are responsible for intensely pruritic skin lesions and can transmit Orientia tsutsugamushi, the causative agent of scrub typhus.
Scabies has a worldwide distribution, with an estimated 300 million cases annually, and is more prevalent in tropical and resource-limited settings. Transmission occurs through close personal contact, including sexual contact, and less commonly via fomites. Household transmission rates can be high, especially in severe forms such as Norwegian (crusted) scabies, where mite burden is extensive.
Scrub typhus is endemic within the “tsutsugamushi triangle” (northern Japan to northern Australia and Afghanistan) and is commonly seen in rural areas, particularly among farmers and individuals exposed to vegetation.
The pathophysiology of scabies involves burrowing of mites into the skin, where they lay eggs. The resulting hypersensitivity reaction to the mites, eggs, and feces causes intense itching and inflammation.
In scrub typhus, the organism causes perivasculitis of small blood vessels, leading to systemic illness and potential multiorgan involvement.
Clinically, scabies presents with intensely pruritic papules and characteristic linear burrows, often found in the web spaces of fingers, wrists, and genital areas. Norwegian scabies, seen in immunocompromised individuals, presents with widespread crusted, hyperkeratotic lesions, which may resemble psoriasis and may not be itchy, leading to delayed diagnosis.
Chigger bites cause erythematous papules or pustules, commonly located around the ankles or waistline, appearing within hours of exposure.
Scrub typhus typically presents with fever, headache, and systemic symptoms, and is characterized by the presence of an eschar—a black necrotic scab—usually found in areas where clothing is tight, such as the groin or waist. Severe cases may progress to ARDS, renal failure, or hepatic dysfunction.
Diagnosis of scabies is primarily clinical, based on characteristic lesions and distribution.
Scrub typhus is diagnosed using serologic testing (IgM ELISA), with laboratory findings that may include thrombocytopenia, lymphocyte changes, and elevated liver enzymes. Chest imaging may be required if respiratory complications are suspected.
Treatment depends on the specific condition.
Scabies is treated with permethrin 5% cream, applied over the entire body and repeated after one week. Oral ivermectin is an alternative. Antihistamines may help relieve itching.
Norwegian scabies requires more aggressive and repeated therapy, often combining topical agents and systemic treatment.
Chigger bites are managed symptomatically with antihistamines or topical steroids.
Scrub typhus is treated with doxycycline, which is highly effective, while azithromycin is preferred in children and pregnant patients.
Preventive measures include good hygiene, contact precautions, and treatment of close contacts in scabies cases. For scrub typhus, protective clothing and insect repellents are essential in endemic areas.
The prognosis is generally excellent with treatment. However, untreated scrub typhus can have a mortality rate of 3–30%, especially when diagnosis is delayed.
Complications may include secondary bacterial skin infections in scabies.
In scrub typhus, severe complications include acute respiratory distress syndrome, renal failure, hepatitis, myocarditis, disseminated intravascular coagulation, meningoencephalitis, and hearing loss.
Mites and chiggers are arthropods with four pairs of legs that infest humans, causing primarily cutaneous disease, but in some cases transmitting systemic infections such as scrub typhus.
The most important human mite is Sarcoptes scabiei, which causes scabies, while chiggers (larval forms of trombiculid mites) are responsible for intensely pruritic skin lesions and can transmit Orientia tsutsugamushi, the causative agent of scrub typhus.
Scabies has a worldwide distribution, with an estimated 300 million cases annually, and is more prevalent in tropical and resource-limited settings. Transmission occurs through close personal contact, including sexual contact, and less commonly via fomites. Household transmission rates can be high, especially in severe forms such as Norwegian (crusted) scabies, where mite burden is extensive.
Scrub typhus is endemic within the “tsutsugamushi triangle” (northern Japan to northern Australia and Afghanistan) and is commonly seen in rural areas, particularly among farmers and individuals exposed to vegetation.
The pathophysiology of scabies involves burrowing of mites into the skin, where they lay eggs. The resulting hypersensitivity reaction to the mites, eggs, and feces causes intense itching and inflammation.
In scrub typhus, the organism causes perivasculitis of small blood vessels, leading to systemic illness and potential multiorgan involvement.
Clinically, scabies presents with intensely pruritic papules and characteristic linear burrows, often found in the web spaces of fingers, wrists, and genital areas. Norwegian scabies, seen in immunocompromised individuals, presents with widespread crusted, hyperkeratotic lesions, which may resemble psoriasis and may not be itchy, leading to delayed diagnosis.
Chigger bites cause erythematous papules or pustules, commonly located around the ankles or waistline, appearing within hours of exposure.
Scrub typhus typically presents with fever, headache, and systemic symptoms, and is characterized by the presence of an eschar—a black necrotic scab—usually found in areas where clothing is tight, such as the groin or waist. Severe cases may progress to ARDS, renal failure, or hepatic dysfunction.
Diagnosis of scabies is primarily clinical, based on characteristic lesions and distribution.
Scrub typhus is diagnosed using serologic testing (IgM ELISA), with laboratory findings that may include thrombocytopenia, lymphocyte changes, and elevated liver enzymes. Chest imaging may be required if respiratory complications are suspected.
Treatment depends on the specific condition.
Scabies is treated with permethrin 5% cream, applied over the entire body and repeated after one week. Oral ivermectin is an alternative. Antihistamines may help relieve itching.
Norwegian scabies requires more aggressive and repeated therapy, often combining topical agents and systemic treatment.
Chigger bites are managed symptomatically with antihistamines or topical steroids.
Scrub typhus is treated with doxycycline, which is highly effective, while azithromycin is preferred in children and pregnant patients.
Preventive measures include good hygiene, contact precautions, and treatment of close contacts in scabies cases. For scrub typhus, protective clothing and insect repellents are essential in endemic areas.
The prognosis is generally excellent with treatment. However, untreated scrub typhus can have a mortality rate of 3–30%, especially when diagnosis is delayed.
Complications may include secondary bacterial skin infections in scabies.
In scrub typhus, severe complications include acute respiratory distress syndrome, renal failure, hepatitis, myocarditis, disseminated intravascular coagulation, meningoencephalitis, and hearing loss.
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