An Introduction to Medical Entomology
For educational purposes.
(Parasites of Birds, Mammals & Insects)
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Species in this Superfamily of wholly parasitic mites do not possess tracheae and most of the external characteristics are reduced or absent. Their entire life is spent on their hosts, where they may infest hair, skin, or tissues. The sexes differ in appearance and structure. Due to the restricted number of diagnostic characters the group is very difficult to classify. The adults have eight legs like ticks and thus are not insects. They differ from ticks by an absence of teeth on the mouthparts hypostome and by having bristles or setae on their bodies and legs.
GENERAL APPEARANCE (See Photo).
Females are very tiny at barely 1/2 mm They are of pale color and rounded. There are many small spines on the dorsum and several lines across the top and bottom of the body. The four pairs of legs are short and cylindrical consisting of five ringed segments (Service 2008). The anterior two pairs of legs terminate in short pedicels with suckers at the tips. The posterior two leg pairs in females do not have suckers, ending instead with bristles. The head is not clearly defined, but thick short palps and chelicerae of mouthparts extend forward from the body.
The mite females excavate below the skin's surface where it is thin as around wrists, fingers, feet, etc. However, most are found on the hands and wrists and sometimes they also infest the head region of their hosts. Once under the skin females create winding tunnels and feed on liquids produced by the dermal cells. About 1-3 eggs are laid daily in the tunnels, which then require about four days to hatch with small larvae that have only six legs. The larvae migrate to the skin's surface where most perish, but the survivors seek out a hair follicle where they moult and develop into a "protonymph) with eight legs. A "tritonymph" is produced after about 3-4 days. Service (2008) noted that female nymphs are much larger than male nymphs. About 3 days later the tritonymph moults to produce either a male or female adult. Mated females then increase their size and begin their penetration into skin. Males, on the other hand, are much smaller and wander about the skin surface and construct short dens for refuge. The total life cycle usually requires less than two weeks, and females may live up to six weeks on their hosts, but perish in a few days without hosts.
Skin diseases, known as Scabies, Acariasis, Sarcoptic Itch, etc., are produced in humans and animals. Some of the Sarcoptes spp. actually inhabit tunnels underneath the skin. These mites may pass their entire lives on their hosts. Infestations among hosts are acquired by contact. It has been estimated that over 300 million cases of Scabies occur annually worldwide.
One family, Sarcoptidae, and genus, Sarcoptes, is of principal importance for humans. Sarcoptes scabiei is known as the "Human Itch Mite," of "Norwegian Itch" as it is sometimes called. Females that are larger than males have the dorsal part of the body marked with distinctive parallel lines. The mites locate in the upper layers of epidermis especially around the groin and more sequestered areas. Mature females that bore directly into skin where they remain concealed for a while construct egg tunnels. Enlarging the excavation and laying eggs follow this. Eggs hatch in 3-4 days and the larvae leave the tunnel for the skin surface where they enter hair follicles. Molting occurs in 2-3 days followed by two nymphal stages. Nymphs construct narrow tunnels where mating occurs. The life cycle varies from 8-15 days at room temperature. Adult longevity is 3-5 weeks.
A person may acquire 50 or more mites at any given time, and any infections that develop are not obvious for several weeks. Following an attack there are at first few symptoms. Gradually as one becomes sensitized an intense itching ensues, which is especially intense at night. Infections are more likely the more one scratches the infested areas.
Acquisition of mites is through close contact with infested persons or their clothing. Avoidance of infested areas is preferred, but if infected one should seek medical attention from a physician, for current products available for treatment.
There are also species (eg., Psoroptes communis and Notoedres cati itch mites attacking animals that do not tunnel bur rather possess suckers for exterior attachment to the skin. Humans only become affected from close contact with infested animals, such as cats and rats.
The attention of a medical physician is advised for this group of mites, as medicinal treatment is usually required. Prevention involves the usual precautions of cleanliness and limiting contact with infected surfaces, animals and people. However, Service (2008) recommended that during epidemics clothing and bedding should be dry cleaned or washed in 50-deg. Centigrade water. (Also See: Scabies)
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Arlian, L. G. & M. S. Morgan. 2000. Serum antibody to Sarcoptes scabiei and house dust mite prior to and during infestation with S. scabiei. Vet.
Arlian, L. G., Morgan MS, Neal J.S. 2003. Modulation of cytokine expression in human keratinocytes and fibroblasts by extracts of scabies
mites. Am J Trop. Med Hyg. 69: 652656
Arlian, L. G., M. S. Morgan, & J. S. Neal. 2004. Extracts of scabies mites (Sarcoptidae: Sarcoptes scabiei) modulate cytokine expression by
human peripheral blood mononuclear cells and dendritic cells. J Med Entomol. 41: 6973.
Arlian, L. G., M. S. Morgan, C. M. Rapp & D. L. Vyszenski-Moher. 1996. The development of protective immunity in canine scabies.
Vet. Parasitol. 62: 133142.
Arlian, L. G., C. M. Rapp, B. L. Stemmer, M. S. Morgan & P. F. Moore. 1977. Characterization of lymphocyte subtypes in scabietic skin
lesions of naοve and sensitized dogs. Vet. Parastitol. 68: 347358.
Arlian, L. G., C. M. Rapp, D. L. Vyszenski-Moher & M. S. Morgan. 1994. Sarcoptes scabiei: Histopathological changes associated with
acquisition and expression of host immunity to scabies. Exp. Parasitol. 78: 5163.
Kemp, D. J, S. F. Walton, P. Harumal, & B. J. Currie . 2002. The scourge of scabies. Biologist. 49: 1924.
Matheson, R. 1950. Medical Entomology. Comstock Publ. Co, Inc. 610 p.
Service, M. 2008. Medical Entomology For Students. Cambridge Univ. Press. 289 p
Maxwell, S. S., T. A. Stoklasek, Y. Dash, J, R., Macaluso & S. K. Wikel. 2005. Tick modulation of the in-vitro expression of adhesion molecules by
skin-derived endothelial cells. Annals Trop Med Parasitol. 99: 661672.
Orkin, M, H. Maibach, L. C. Parish & L. M. Schwartzman. 1977. Scabies and pediculosis. Lippincott Co., Philadelphia. p. 203.
Stemmer BL, Arlian L. G., M. S. Morgan, C. M. Rapp & P. F. Moore. 1996. Characterization of antigen presenting cells and T-cells in progressive
scabiatic skin lesions. Vet Parasitol. 67: 247258.