Page 264 - parasitology for medical and clinical laboratoryprofessionals

244 CHAPTER 11

by the lone star tick Amblyomma americanum. This tick
was first postulated as a probable vector of the disease
in 1984, which at the time was called a “Lyme-like dis-
ease.” In the late 1990s it was recognized that the disease
had some similarities to Lyme disease but there were
separate and distinct differences also. One example of
a difference lies in the fact that Lyme disease transitions Source: Centers for Disease Control and Prevention (CDC)
through three distinct stages that have extremely serious
ramifications if untreated. Several studies have failed to
detect Borrelia burgdorferi, the causative agent of Lyme
disease in patients from the southeastern region of the
United States.
Some investigators have proposed that Southern FIGURE 11-18 Tularemia lesion on the hand, caused
Tick–associated Rash Illness may be caused by a related by the bacterium Francisella tularensis
bacterium called Borrelia lonestari, a spirochete that was
initially isolated in 2004 from a bacterial culture. But
this conclusion was shown to be inconsistent with later
cultures that showed that the spirochete has not been
detected in all cases of the illness as would be expected.
This has led some scientists to the conclusion that the
disease is not based on a bacterial pathogen. Because
viruses are more difficult to culture than bacterial ones, Source: Centers for Disease Control and Prevention (CDC)
as they require live cells in which to grow, perhaps a
breakthrough will occur in the near future and that will
provide a definite identity for an organism that is without
doubt the culprit.

TULAREMIA FIGURE 11-19 Photomicrograph of Francisella
tularensis bacteria using a methylene blue stain
Tularemia is a potentially deadly disease and occurs
naturally in the United States, because a ready reser-
voir of aquatic animals is available for maintaining the bacteria are intracellular pathogens that invade a type of
causative organism in large numbers. This is a disease large white cells called macrophages (large feeders). The
that has been widely known for many years and is also course of the disease involves the spread of the organ-
known variously as “rabbit fever,” “Deer fly fever,” and ism to multiple organ systems, including the lungs, liver,
sometimes as “Ohara’s fever.” Tularemia is a contagious spleen, and lymphatic system. The most clinically sig-
disease caused by a bacterial infection by an organism nificant of the subspecies is that of Type A F. tularensis
called Francisella tularensis (Figure 11-18). The or- tularensis, which is found in rabbit herds in North
ganism is a Gram-negative coccobacillus with several America and is highly virulent for both humans and do-
subspecies that are associated with varying degrees of mestic rabbits. F. tularensis is an intracellular bacterium,
virulence. which means that it is able to live in a similar manner of
The term Gram-negative coccobacillus means that a parasite that inhabits host cells. Bioterrorism using
the organisms stain a light pink color with Gram’s stain this bacterium would be quite simple, when dispersed
and indicates that the shape is intermediate between widely. F. tularensis is extremely infectious and can eas-
that of a round or coccal shape and of a rod or bacillus ily be disseminated by explosive military shells or bombs
(Figure 11-19). Little is known of the virulence of the and possesses the capacity to cause widespread illness
organism but evidence leads to the conclusion that the and death.