The Role of Immunohistochemistry in the Confirmation of Rocky Mountain Spotted Fever


By Bahram Robert Oliai, M.D.


In this month’s Focus newsletter we are excited to introduce our new Rocky Mountain Spotted Fever (RMSF) immunohistochemical stain and briefly re-view its utility in the confirmation of this virulent dis-ease.  RMSF is a tick-borne disease occurring throughout much of the United States caused by the organism Rickettsia rickettsii.  In the Eastern and central regions it is transmitted by the American dog tick, Dermacen-tor variabilis (Fig. 1) while in the Western United States it is transmitted by the Rocky Mountain wood tick, Dermacentor andersoni (Fig. 1).  In 2005, an Arizona outbreak associated with the brown dog tick (Rhipicephalus sanguineus) was also reported.


Symptoms and signs are nonspecific, commonly in-cluding fever, headache, myalgia and the classic rash which is often macular to maculopapular and some-times petechial (Fig. 2).  Given the nonspecific pres-entation, one must keep the diagnosis in mind and question patients regarding possible exposure to ticks.  RMSF can result in severe illness and potentially death, with 5-10% of those infected dying and many more suffering severe sequelae (i.e. amputation, deaf-ness, and learning impairment).


The disease also seems to be on the rise and in 2004, 1,514 cases were reported, 4 times higher than ever previously reported in U.S. history!


— Figure 2. The classic maculopapular rash on the back of a patient with RMSF (from Google images).



— Figure 1, left – The Rocky Mountain Wood Tick (Dermacentor andersoni )  right – The American Dog Tick (Dermacentor vari-abilis )  (images from CDC website).



Fortunately, the disease can be quickly and effec-tively treated with doxycycline, and if suspected clini-cally, empiric therapy is recommended, given the great potential for an adverse outcome.  While the diagnosis is generally based on clinical fea-tures, several confirmatory tests are available.  Sero-logic studies have been utilized and a fourfold or more increase in antibody titer between paired serum specimens is considered diagnostic.  Unfortunately, serologic testing may not be helpful in the setting of active infection.  Polymerase chain reaction has also been used to con-firm infection, and although generally thought of as a very sensitive technique, there are some who suggest that it may not be particularly sensitive in this setting (this issue may be compounded by variability in tech-nique among labs performing the testing).  Additionally, molecular testing can be rather slow due to the necessity of batching specimens.  In an informal internet search for Rocky Mountain Spotted Fever PCR testing, turn around times range from 5-7 week days.


— Rocky Mountain Spotted Fever immunostains from hepatic tissue from an autopsy of an infected patient. The  immunoreactivity localizes to organisms within endothelial cells and Kupfer cells/histiocytes.



Culture of the organisms from infected tissue has also been used, however, this process is difficult and the organism is slow growing.  According to the CDC website culture may take weeks.  There is, however, a relatively sensitive (~70% over-all according to the CDC website, and perhaps a good bit more sensitive in experienced hands) and specific (for Rickettsial organisms) immunohistochemical stain for RMSF.  Through the generosity of Dr. David Walker of The University of Texas Medical Branch in Galveston, we have obtained this antibody, and thanks to our colleague Dr. Bob Cavagnolo who provided us with positive control tissue, we have suc-cessfully optimized the stain at ProPath.  In prelimi-nary testing with our fungal/bacterial control block, we have found it to be rather specific, cross reacting only with a few bacterial aggregates

(likely staphylo-coccal) which have a much different morphology than that of the immunopositive Rickettsia.  Addi-tionally, these bacteria would not be expected to lo-calize within endothelial and histiocytic cells as noted in our RMSF-infected positive control tissue.  The main benefit of the RMSF immunostain lies in its relative sensitivity and specificity (in the literature I reviewed immunostaining seemed superior to the other diagnostic modalities previously mentioned) and the speed (generally within 24 hours within receipt at our lab in uncomplicated cases) at which an infection can be confirmed.  We are now offering the Rocky Mountain Spotted Fever  antibody clinically and are very excited to provide this potentially useful immunostain to our clients with the quality service and turnaround time for which ProPath is famous.



I would like to thank Dr. David Walker of the University of Texas Medical Branch for his generosity in providing us with the Rocky Mountain Spotted Fever immunostain and my asso-ciate Dr. Bob Cavagnolo for providing us with positive con-trol material.



Demma LJ, Traeger MS, Nicholson WL et al.  Rocky Moun-tain Spotted Fever from an Unexpected Tick Vector in Ari-zona.  The New England Journal of Medicine.  2005; 353(6):  587-594.  Dumler JS and Walker DH.  Rocky Mountain Spotted Fever-Changing Ecology and Persisting Virulence.  The New Eng-land Journal of Medicine.  2005; 353(6):  551-553.  © 2008 ProPath®. All


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Date of last revision: July 2008.