Tropical rat mite dermatitis: case report and review.

Abstract

Six medical students inhabiting a centuries-old, rat-infested house in Lübeck, in northern Germany, were suffering from itching papules and seropapules. Prior to these patients' visit to our institute, their conditions had been diagnosed as pediculosis, scabies, or pulicosis and treated unsuccessfully with the antiparasitic agent lindane (0.3%). The final diagnosis, tropical rat mite dermatitis, was based on the identification of the arthropod Ornithonyssus bacoti, which has an unsegmented body with eight legs. No treatment was recommended, and the dermatitis disappeared within 2 weeks. Measures taken to prevent reinfestation included extermination of the rats and treatment of the rooms of the house with the acaricide benzyl benzoate. Because the mite O. bacoti spends a relatively short time on a host and penetrates the skin for feeding only, the application of an antiparasitic agent is not necessary. If indicated, treatment should be symptomatic.

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1465
Tropical Rat Mite Dermatitis: Case Report and Review
Peter M. Engel, J. Welzel, M. Maass, U. Schramm, and
H. H. Wolff
From the Institute of Medical Microbiology and Hygiene, Department of
Dermatology and Venerology, and Institute of Anatomy, Medical
University of Lu ¨beck, Lu ¨beck, Federal Republic of Germany
Six medical students inhabiting a centuries-old, rat-infested house in Lu ¨beck, in northern Ger-
many, were suffering from itching papules and seropapules. Prior to these patients’ visit to our
institute, their conditions had been diagnosed as pediculosis, scabies, or pulicosis and treated unsuc-
cessfully with the antiparasitic agent lindane (0.3%). The final diagnosis, tropical rat mite dermatitis,
was based on the identification of the arthropod Ornithonyssus bacoti, which has an unsegmented
body with eight legs. No treatment was recommended, and the dermatitis disappeared within 2
weeks. Measures taken to prevent reinfestation included extermination of the rats and treatment of
the rooms of the house with the acaricide benzyl benzoate. Because the mite O. bacoti spends a
relatively short time on a host and penetrates the skin for feeding only, the application of an
antiparasitic agent is not necessary. If indicated, treatment should be symptomatic.
The tropical rat mite is found in various parts of the world,
but its presence appears to be locally restricted. In most cases,
the mite is recognized only when it attacks humans. The unspe-
cific symptoms of the dermatitis caused by attacks of the mite
might be attributed to more well-known parasitic arthropods
or even to nonparasitic causes, with potentially unfavorable
effects on therapy.
to itching(figure 1). Primarylesions that had notbeen scratched
had no central punctum. On histologic examination, perivascu-
lar infiltrates with eosinophilic granulocytes, confined to the
upper corium, were observed.
No domestic animals were kept in the house, but uninvited
commensal rodents—rats—were heard during the night. Dur-
ing a visit to the location of the centuries-old house, extensive
work on the sewage disposal system in the street was noted.
Fortunately, specimens of parasites the patients had collected
from their bodies were available. First observations through a
light microscope allowed identification of a mite (Acari), a
small arthropod (õ1 mm) with an unsegmented body (the idio-
soma) carrying four pairs of legs and a tiny gnathosoma with
its mouthparts (figure 2A).
Identificationofthespeciesrequired reviewofpertinentliter-
ature [1] and preparation (protein denaturation by guanidine-
HCl solution) of the mite for further microscopic examination.
This examination revealed female adults of the species Orni-
thonyssus bacoti (first described by Hirst 1913; family Derma-
nyssidae, order Mesostigmata). Some of the anatomic features
characteristic of this mite are shown in figures 2A and 2B.
Careful consideration of the exact position, size, and morphol-
ogy of the indicated structures and even of the hairs (setae) is
essential for identification of the species.
A professional exterminator was contracted. In the days after
the rat extermination with poisoned bait began, the dermatitis
became so annoying that all inhabitants moved out of the house
temporarily. In the meantime, all rooms of the building were
repeatedly sprayed with a solution of 3% benzyl benzoate and
1% tannic acid (INNOV-ALL Pharma, Du ¨sseldorf, Germany);
they were subsequently renovated. After the students returned,
no more mites or cases of dermatitis were observed.
No drug treatment was recommended, and the dermatitis
disappeared within 2 weeks for five patients; the lesions on
one female patient developed into maculae within a month.
Case Report
Six medical students sharing a house in the historic center
of the seaport of Lu ¨beck, in northern Germany, had itching
papules developing in the daytime for several weeks. In prior
medical consultations, treatment with lindane (0.3%) and ben-
zyl benzoate (25%) emulsion (HERMAL; Reinbek, Germany)
was recommended because the papules were considered to be
due to pediculosis, scabies, or pulicosis. After several courses
of unsuccessful treatment, the students rejected further applica-
tion of the antiparasitic agent.
See article by Bonomo et al. and editorial by Mathisen on a
related topic in the September issue on pages 645–6 and
646–8, respectively.
Physical examination of the students revealed pale, red pap-
ules and seropapules, located individually or in groups predom-
inantly on the legs and arms but also in the area of the waist
and laterally on the trunk. Some papules were as large as peas,
and some were excoriated because of scratching in response
Received 13 November 1997; revised 4 August 1998.
Reprints or correspondence: Dr. Peter M. Engel, Institut fu ¨r Laborato-
riumsmedizin und Mikrobiologie, Dr.-Eden-Strasse 10, D-26133, Oldenberg,
Federal Republic of Germany.
Discussion
Mite attacks on humans attributable to rat mites were first
reported from Australia in 1913. In the same year, mites were
Clinical Infectious Diseases
? 1998 by the Infectious Diseases Society of America. All rights reserved.
1058–4838/98/2706–0022$03.00
1998;27:1465–9
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1466Engel et al.CID 1998;27 (December)
The larvae of the tropical rat mite were observed to move
slowly, making it rather improbable that they would reach a
host, starting from the place of oviposition. Thus, protonymphs
seem to be the first host-infesting stage. However, Fishman
has observed a larva attach itself to the skin of a host and molt
there into a blood-feeding protonymph [7].
Despite long periods living on different cotton rat hosts, the
meals of starving male mites were tiny in comparison with the
quantities of blood drawn by starving female mites [10]. After
a blood-meal, the female mite drops off its host. Two to 5 days
of digestion and successive maturation of eggs are followed
by 2–3 days of oviposition of one to 12 eggs [10]. The average
life span of the females is 6.5 months, whereas the life span
of the males ranges between 1.5 and 2.5 months [9].
Detailed information on the physiology of the feeding pro-
cess of O. bacoti was not obtainable. The two terminal chelic-
eral segments of an attacking mite cut through the skin to reach
a bloodvessel. Theproximal cheliceralsegments form atubular
functional unit for sucking blood from the vessel tapped. From
our micrographs, we estimate the inner diameter of this retract-
able pipe to be comparable with the inner diameter of a blood
capillary (6 mm).
Although Rattus norvegicus and Rattus rattus are known to
be the preferred hosts, infestation of 10 other rodent species
has been reported. The low host specificity might be the reason
why veterinary technicians were infested while handling labo-
ratory mice [3]. Although reports of infestation of several ro-
dent species caged for laboratory purposes (such as rats, mice,
gerbils, and guinea pigs) have been published, there have been
no reports of mite infestation of humans via these rodents when
the animals were kept as pets in households. Young children
sitting or lying on the floor or ground while playing and babies
asleep during the daytime are hosts easily accessible to the
mite. Within families, children have been reported to be the
only or most heavily mite-infested patients [5, 6].
The tropical rat mite can infest patients of all age groups,
and dermatitis may manifest itself on any part of the human
body. The skin lesions can be more numerous on the parts of
the body easy to reach by the mite or where parts of the clothing
in tight contact with the skin have formed an obstacle for the
mites. In one female patient, the condition was first restricted
to the pubic region and initially diagnosed and treated as an
infestation with Phthirus pubis [6]. An infestation including
the scalp has been reported only once [2].
The single or grouped bites of O. bacoti may manifest them-
selves with mild pain, followed by slight, reddish swellings
that can become erythematous papules of up to 4 mm [2] or
even 10 mm in diameter [3]. The papules may be excoriated
after scratching because of pruritus. Vesicles, nodules, or an
urticarial component have been seen rarely [3, 5]. In addition
to possible psychological effects, prolonged infestation may
have severe physical consequences; for instance, chronic infes-
tation for several months in an initially 3-week-old baby re-
sulted in retarded growth [5].
Figure 1.
tropical rat mite dermatitis.
Excoriated papules on the shins of a female patient with
collected in Egypt and described as a new species, Liponyssus
bacoti, or tropical rat mite. Bdellonyssus bacoti is another im-
portant synonym. This mite was identified in 1923 as a cause
of human dermatitis in the United States. In Europe, the tropical
rat mite was first detected in the German seaport of Hamburg
in 1931, a site indicating the mite was carried across the oceans
along with infested ship rats, probably in the times of sailing
vessels.
In recent decades, O. bacoti has been reported from all conti-
nents except the arctic and antarctic regions as a parasite of
wild, commensal, and laboratory rodents or of humans [2–7].
O. bacoti was also reported to exist among other mites in
samples of house dust in Egypt [8].
The life-cycle of O. bacoti is presented in table 1. The veloc-
ity of development and the degree of activity of the various
stages depend on the ambient temperature and the availability
of a host. The life-cycle can be completed within 10 days at a
temperature of 25?C. Parthenogenesis is facultative, and unfer-
tilized eggs have been observed to result in the development
of males [10].
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1467CID 1998;27 (December)Tropical Rat Mite Dermatitis
Figure 2.
A, venter of female (bar Å 0.5 mm). Arrows indicate, from top to
bottom: chelicera, pedipalp, sternal shield with three pairs of setae,
genital shield with genital setae, anus, and anal shield with three setae.
B, ventral view of gnathosoma of female (bar Å 0.1 mm). Arrows
indicate, from top to bottom: chelicera, trochanter of pedipalp with an
anteroventral keel, gnathosoma with four pairs of setae, and tritoster-
num. C, the more common obligate human itch mite Sarcoptes scabiei:
dorsal view of female (bar Å 0.1 mm).
A and B, the tropical rat mite Ornithonyssus bacoti (Hirst).
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1468Engel et al. CID 1998;27 (December)
Table 1.
Life-cycle of Ornithonyssus bacoti.
benzoate emulsion has been reported once [4]. Lindane and
benzyl benzoate are usually recommended for the treatment of
scabies and pediculosis pubis. In light of the adverse effects
of lindane [11] and benzyl benzoate, these drugs should be
used for well-diagnosed cases only, and never empirically. A
simple bath will immediately remove specimens of O. bacoti
from humans and domestic pets.
To prevent reinfestation, extermination of the rats and, if
possible, removal of their nests are essential. Treatment of
the environment with an acaricide such as benzyl benzoate is
necessary given that the tropical rat mite is able to survive
extended periods of starvation and to run long distances. Chem-
icals mainly known as insecticides, such as malathion [4],
methylcarbamate [3], and lindane [7], have been reported to
be effective. Fluoridated silica aerogel dust blown into spaces
behind walls and under floors has also been used to control
the tropical rat mite [12].
Characteristics of tropical rat mite dermatitis and other para-
sitoses that might be confused with the condition are presented
in table 2. Figure 2C shows the more common obligate human
itch mite (Sarcoptes scabiei) for comparison.
The tropical rat mite was claimed in 1931 to have transmitted
endemic (murine) typhus to humans [13]. Until now, no further
cases of transmission of disease to humans have been reported.
However, several infectious diseases, including murine typhus,
rickettsial pox [14], Q fever [15], tularemia [16], plague [17],
eastern equine encephalitis [18], epidemic hemorrhagic fever,
and coxsackievirus [19] and Langat virus (tick-borne encepha-
Development
(no. of h) at
20?C–25?C
No. of
legsStage Size (mm)Feeding
Egg
Larva
Protonymph
0.35 1 0.22
§Egg
F: 0.7 1 0.34,
M: 0.5 1 0.27
F: 0.7 1 0.34,
M: 0.5 1 0.27
F: 0.7–1.5,
M: 0.4–0.5
0
6
8
No
No
F: twice,
M: once*
No
28–54
30–40
48–52
Deutonymph824
Adult8F ú M 120 to first egg
NOTE.
* According to [9], frequency of feeding is related to the sex.
F Å female; M Å male.
The diagnosis of rat mite dermatitis requires identification
of the parasite, which is more likely to be found in the environ-
mentof itshost thanonthe hostitself. Microscopicexamination
of skin scrapings from pruritic papular eruptions on the backs
of hands and the neck region has been reported to reveal tropi-
cal rat mites [4]. However, we doubt skin scrapings are useful
for collection of O. bacoti.
Symptomatic relief has been achieved by treatment with
antihistaminic drugs [2] or glucocorticoids [7]. All these medi-
cations were used to prevent further scratching and unrest
caused by pruritus. Treatment with lindane was reported by
some authors [3, 6, 7], and topical application of a 25% benzyl
Table 2.
Characteristics of parasitoses that might be confused with dermatitis due to the tropical rat mite Ornithonyssus bacoti.
Specificity of
skin lesion or
reaction
Parasite, length (mm)
of female
Typical anatomic
locationMain host Infestation Antiparasitic measure
Dermanyssus gallinae,
õ1.5
BirdsTemporary, at night;
nidicole (parasite
living in or near
nest of its host)
Temporary, in
daytime; nidicole
Permanent, in
epidermis
None NoneEnvironmental: benzyl
benzoate, lindane
Ornithonyssus bacoti,
õ1.5
Sarcoptes scabiei, õ0.4
Rattus speciesNoneNoneEnvironmental: benzyl
benzoate, lindane
Benzyl benzoate,
lindane
Humans, varieties
found on
domestic
animals
Humans
Digital interspace,
armpit,
genitals, navel
region
None
Burrows
Pediculus humanus
corporis, õ4.0
Temporary; permanent
in clothes
None Disinfection of clothes
with pyrethrum
extract
Pyrethrum extract,
lindane
Insecticides for
environment
Pediculus humanus
capitis, õ2.7
Xenopsylla cheopis, õ2
Humans Permanent, in hairScalp None
Rattus speciesTemporary Clothed parts of
body
Multiple,
grouped; with
central
punctum
Ctenocephalides canis,
õ3.5
Pulex irritans, õ3
Dogs, cats. . .
Humans. . .
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1469CID 1998;27 (December)Tropical Rat Mite Dermatitis
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buch der speziellen Zoologie (begru ¨ndet von Kaestner A), Bd.I, Arthro-
poda. Stuttgart: G. Fischer Verlag, 1993:352–4.
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Liponyssus bacoti Hirst 1913, and investigation into its role as vector
of Litomosoides carinii to cotton rats and white rats, together with some
observation on infection in white rats. Ann Trop Med 1946;40:
228–54.
11. Brown S, Becher J, Brady W. Treatment of ectoparasitic infections: review
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of endemic typhus. JAMA 1931;97:1506–10.
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mental vector of rickettsial pox. Am J Trop Med 1948;28:697–705.
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(Dermanyssus gallinae) and the rat mite (Ornithonyssus bacoti) with
the causal agent of Q fever. Dakl Hald Nauk SSSR 1955;101:391–2.
16. Hopla CE. Experimental transmission of tularemia by the tropical rat mite.
Am J Trop Med 1951;31:768–82.
17. Yamada S. Observations on a house-infesting mite (Liponyssus nagayol)
which attacks human beings, rats and other domestic mammals, with
brief notes of experiments regarding the possibility of plague transmis-
sion by means of this mite. Far East A Trop Med Tr 1932;2:358–72.
18. Clark GM, Lutz AE, Fadness L. Observations on the ability of Haemoga-
masys liponyssoides and Ornithonyssus bacoti (Acarina, Gamasine) to
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107–12.
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as an experimental vector of coxsackie virus. Am J Trop Med Hyg
1952;1:982–6.
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litis virus complex) infections [20], have been transmitted by
O. bacoti under experimental conditions. Finally, these mites
have been reported to be the intermediate host of the filarial
nematode Litomosoides carinii of rodents [10].
In contrast to the experimental potential for disease transmis-
sion and to the information available in the literature, the epide-
miological importance of the temporary ectoparasite O. bacoti
as a vector of pathogens is very rarely documented in human-
case reports,possibly becauseof thedifficulty of identifyingthe
mite. We hope the information given here will draw attention to
this subject.
Acknowledgments
The authors are grateful to H. Manfeldt for valuable technical
assistance in performing electron microscopic studies and to Joa-
chim Bro ¨ker for his helpful advice.
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