Ornithonyssus bacoti

Ornithonyssus bacoti bacoti is a hematophagous parasite commonly referred to as the tropical rat mite.^[1] They feed only on blood and serum from many hosts.^[2][3] They can be found and cause disease on rats and wild rodents most commonly, but also small mammals and humans when other hosts are scarce.^[4][5] Outbreaks tend to occur in older, less maintained buildings. The mite however can travel several hundred feet on its own if necessary to find a host and can survive for extended periods of time without a host. This along with the nonspecific dermatitis it causes can prevent accurate and fast diagnosis of rat mite dermatitis. The scarcity of reports, due in part to misdiagnosis and also the mildness of its symptoms, makes the disease seem less common than it is. The tropical rat mite can be found in both temperate and tropical regions or rather all continents except the arctic and Antarctic.^[6]

History

The tropical rat mite was first reported in Australia in 1913 in a human case report. 1923 marked the United States’ identification of it as a cause of dermatitis in humans. In 1931 it was discovered in Hamburg, Germany in a seaport.^[7][8]

Anatomy

The tropical rat mite is between .75 and 1.44 mm in length and is nonsegmented with chelicerae or mandibles which are suited to piercing.^[9][10] They have a sharp caudal apex of the scutum, an oval genital shield and a cranially positioned anus.^[11] These mites are capable of parthenogenic reproduction.^[12] After taking a blood meal they are static and yellow or dark red in color. However, before a blood meal they are more active and grey in color.^[13]

Life Cycle

Ornithonyssus bacoti has five life stages: egg, larva, protonymph, deutonymph, and adult. The only two stages that feed are the protonymph and the adult.^[14][15] Once they have fed, they either drop off the host to molt or lay up to a 100 eggs, respectively. An egg will take one and a half days to hatch into a larva which will then attach to a host and take one to two days to molt into a protonymph. A protonymph then molts into an adult. The whole life cycle takes anywhere from 7 to 16 days to complete. It takes a minimum of 13 days to go from egg to egg.^[16] The larva is the only stage that has three legs as opposed to four.^[17]

Disease

When the tropical rat mite bites its host, it causes an inflammatory reaction to its saliva and then pruritic nonspecific dermatitis. In children especially, vesicular or eczematous reactions sometimes occur. Secondary excoriations due to scratching are common.^[18] Papular urticaria has been suggested as developing in some people.^[19]
No human disease has been definitively found to be naturally vectored by this mite. Lab demonstrations have proved that the mite is at least capable of vectoring murine typhus, rickettsialpox, tularemia, plague, coxsackievirus and Q fever although it has not been known to do so outside the lab.^[20] There were some reports by Selmire and Dove and Ram et al. that the mite was capable of vectoring human typhus but these reports are not generally accepted.^[21]

Diagnosis

The bites tend to be grouped together due to the piecemeal feeding behavior of these mites.^[22][23] They can occur anywhere on the body though commonly on the neck, head, extremities, abdomen and chest. The papules are generally 1 to 4 mm in diameter and sometimes vesicles appear.^[24][25] The symptoms of this disease are common or at least mistakable for many things that should be checked for as well in differential diagnosis such as prurigo simplex, allergies, mosquito or other insect bites, bacterial folliculitis, dermatitis herpetiformis, scabies, and pediculosis.^[26][27][28] Identification of the mite itself is required for definitive diagnosis. This can be collected by laying cellophane tape or “Sticky cards” (cardboard cards with a glue surface) in the area believed to be infested with the mite.^[29]
A biopsy of the lesions only shows superficial and mid dermal perivascular infiltrates and occasionally eosinophils. This method is so nonspecific as to be inconclusive in diagnosis.^[30]

Treatment

Treatment of rat mite dermatitis is on a symptomatic basis. Generally oral antihistimines, topical corticosteroids, gamma benzene hexachloride, benzyl benzoate, or croamiton are used to provide relief.^{[31][32][33][34]} Some patients however require no treatment.^[35] In order to effectively rid the patient of dermatitis thorough eradication of the mite from pets and their environment is crucial. Acarides for the environment might be hydrocyanic acid, chlorophenothane (DDT), chlordane, permethrin, pyriproxyfen or lindane.^[36][37][38] To rid pets of the mite, selamectin is generally used in the form of Revolution, Stronghold or Pfizer.^[39] Extermination of rats and other wild rodents which are common hosts is also key. It takes several weeks to be sure that a proper eradication of these mites is complete because the mites are not only found mostly in the environment of the host instead of on the host itself but also have been found to survive up to 63 days without a blood meal.^[40]

See also

• List of mites associated with cutaneous reactions

References