Pathogenic Organisms: Microscopic and Macroscopic

Match each picture with the phrase below that best describes it. The pathogenic organisms in these pictures might be microscopic or macroscopic, and they can be recovered from skin lesions or clothing by the patient and/or clinician. Answers and discussion appear on the following page.

1. May transmit tularemia to man.
2. Related to the organism that transmits the plague.
3. Skin necrosis follows a bite by this organism.
4. A mite is shown.

ANSWERS:1. A
2. B
3. D
4. C
 

Photo A depicts a non-engorged tick. Common pathogenic soft ticks include the wood tick (Dermacentor andersoni- pictured here), dog tick (Dermacentor variabilis), and Lone Star tick (Amblyomma americanum). Note the presence of 8 legs, which place the organism in the arachnid group (rather than with flies, beetles, and other organisms of the insect group, all of which have 6 legs). The causative organism of tularemia is transmitted to humans either by contact with rabbits or hares or by contact with deerflies and ticks that infest deer in the forest.

Photo B shows the cat flea, Ctenocephalides felis. Other common environmental fleas include the human flea (Pulex irritans), the dog flea (Ctenocephalides canis), the mouse flea (Leptopsylla segnis), and the rat flea (Xenopsylla cheopis).

Fleas are small brown insects with flat side-to-side contours and long hind legs. The latter facilitate their primary mode of travel, namely jumping. Fleas bite their natural or temporary host to secure a blood meal.

Fleabites vary in morphology from hemorrhagic macules to papules to urticarial plaques and bullae, depending on the degree of sensitivity of the host to flea salivary antigens. The rat flea transmits both plague and endemic typhus to man when it abandons a dead natural rodent host and seeks a blood meal from a human acting as an incidental host.

Photo C demonstrates 2 eggs and numerous scybala (fecal pellets) of the scabies mite with a remnant of a cutaneous burrow. Note that the eggs have not yet hatched. Finding eggs-or even scybala-from a skin scraping is as definitive for the diagnosis as finding intact adult organisms. These organisms are found during microscopic examination of skin scrapings obtained from inflammatory papules located in the finger web (a burrow).

Photo D shows the brown recluse spider, Loxosceles reclusa. This arachnid displays a characteristic violinshaped dark spot on the dorsal thorax, which accounts for the common term “fiddleback” spider.

There are actually some 20 related species, all of which may produce necrotic skin lesions several days to weeks following a bite. This spider resides in rarely disturbed portions of the human habitat, such as storerooms, cellars, and attics. It only bites in self-defense when its territory is intruded upon. The bite, initially painless, develops into exquisitely painful erythema and edema, followed by ischemic blister formation and, finally, into well-demarcated gangrene. Brown recluse spider bites may result in deep and slowly healing ulcerations.