Babesia Species

BABESIA SPECIES
As noted, there are numerous species of Babesia
and, of those, four are known to be of concern
regarding transmission to humans. Following an
introduction to Babesia species that includes a
historical perspective and descriptions of the
most commonly found morphologic forms, two
of the most commonly encountered Babesia parasites
will be discussed.
Historical Perspective
Apicomplexan parasites belonging to the genus
Babesia are often seen infecting animals, wild and
domestic. Babesial organisms were first described
in the 1880s as being responsible for Texas cattle
fever or red water fever; this parasitic infection
almost decimated the cattle production industry.
However, in recent years, several species have demonstrated
an ability to cause illness in humans,
who are usually considered as an accidental host.
The two babesial organisms most commonly isolated
from clinical specimens are B. microti (Theileria
microti) and B. divergens; other species have
demonstrated an ability to cause disease, but are a
rarer occurrence. It is important to point out here
that some sources suggest that due to ribosomal
RNA comparisons B. microi fits more into a related
genus known as Theileria and thus now call it
Theileria microti. Until this change is universally
accepted in the parasitology community, the
current name of B. microti will be used in this text.
Parameter Description
Appearance Resembles a ring form
Does not contain Schüffner’s,
Ziemann’s, or Maurer’s dots
Ring characteristics
when stained
with Giemsa
Blue cytoplasmic circle connected
with or to red chromatin dot
Vacuole usually present
TABLE 6-6 Babesia Species
Trophozoite: Typical
Characteristics at a Glance
Morphology and Life Cycle Notes
The typical life history of each of these organisms
involves several morphologic forms. However,
for the purposes of this text, only the two forms
most commonly encountered in human specimens
will be discussed, the trophozoite and merozoite.
Other morphologic forms are responsible
for invading the RBCs, but are generally never
seen at the point of laboratory diagnosis.
Trophozoite. The trophozoite (Table 6-6)
develops after the sporozoite infects the red
blood cell. This form resembles the ring form of
Plasmodium infections. The typical ring, when
stained with Giemsa, consists of a blue cytoplasmic
circle connected with or to a red chromatin
dot, also referred to by some as a nucleus. The
space inside the ring is known as a vacuole. The
ring form is the most commonly seen diagnostic
feature of babesiosis and can be differentiated
from malarial organisms by the absence of
malarial pigments (hemozoin) and of Schüffner’s,
Ziemann’s, or Maurer’s dots (Table 6-7).
Merozoite. The merozoite develops within the
red blood cell as the trophozoite matures. The
merozoite resembles four trophozoites attached
together by their respective chromatin dots in the
Parameter Description
Appearance Resembles four trophozoites attached
by their respective chromatin dots
in the shape of a Maltese cross
TABLE 6-7 Babesia Species Merozoite:
Typical Characteristics at
a Glance
CHAPTER 6 Select Sporozoa: Plasmodium and Babesia 153
shape of a cross, often referred to as resembling a
Maltese cross. Merozoites undergo binary fission
in the human host to produce more sporozoites.
Babesiosis has a sexual and asexual phase in its
life cycle. The sexual phase occurs within its
vector, the tick, and the asexual phase occurs
within its host (e.g., mice, deer, cattle, dogs,
humans). It is generally transmitted through the
bite of an infected tick of the genus Ixodes. The
uninfected host must be in contact with the tick’s
saliva for 12 hours or longer before this parasite
can be transmitted. The infected tick transmits
sporozoites into the uninfected host. The sporozoites
invade the red blood cells and develop into
trophozoites. Multiple sporozoites can infect a
RBC, so multiple trophozoites can be seen within
the infected RBC. The trophozoites continue to
develop into merozoites. The merozoites mature
and develop into gametocytes inside their normal
animal host, but are not generally seen in the
accidental human host. In the human host, the
merozoites undergo binary fission to produce
more sporozoites; when the number of sporozoites
exceeds the red blood cell’s capacity, it ruptures,
releasing sporozoites to infect more red
blood cells. An ixodid tick bites an infected host
and the gametocytes travel to the gut, where they
unite to form an ookinete. The ookinete travels to
the salivary glands where sporogony—the process
of spore and sporozoite production via sexual
reproduction—takes place, resulting in numerous
sporozoites that can be transmitted to a new host.
because Giemsa is the recommended stain for all
blood films submitted for parasite study, the
specific morphologic discussion of Babesia is
based on the use of this stain. Thick and thin
blood films should be made and examined. Thick
blood smears serve as screening slides; thin blood
smears are used for differentiating Babesia from
Plasmodium spp. All blood films should be studied
under oil immersion. Careful and thorough screening
of all smears is crucial to ensure the correct
identification, reporting, and ultimately the proper
treatment of the organisms present. The timing of
blood collection for the study of Babesia is not
crucial to success in retrieving the Babesia parasites;
they have not shown periodicity, as have the
malarial organisms.
In addition to blood films, serologic tests and
PCR techniques for babesiosis are available.
These tests are generally best used for diagnosing
patients with a low parasitemia or in donor
blood supply screening and epidemiologic
studies. Serologic and PCR testing are also valuable
for the speciation of Babesia, because this is
a limitation of blood film tests. Representative
laboratory diagnostic methodologies are described
in Chapter 2 as well as within each individual
parasite discussion, as appropriate.
Quick Quiz! 6-16
Humans are an accidental host of Babesia species.
(Objective 6-6)
A. True
B. False
Laboratory Diagnosis
Giemsa-stained peripheral blood films are the
specimens of choice for the laboratory diagnosis
of babesiosis. Wright’s stain may also be used and
will result in an accurate diagnosis. However,
Quick Quiz! 6-17
The specimen of choice for the recovery of Babesia
is: (Objective 6-10)
A. Tissue
B. Cerebral spinal fluid (CSF)
C. Stool
D. Blood
Pathogenesis and Clinical Symptoms
of Babesia
The typical patient presenting with babesiosis
was exposed 1 to 4 weeks prior to the onset of
symptoms. Babesiosis is generally a self-limiting
infection. Its onset is usually gradual and characterized
by prodrome-like symptoms—fever,
154 CHAPTER 6 Select Sporozoa: Plasmodium and Babesia
Babesia Classification
Babesia species belongs to the phylum Apicomplexa,
class Aconoidasida, order Piroplasmida,
family Babesiidae. The Babesia species discussed
in this chapter occur in the blood, as indicated
in Figure 6-8.
Babesia microti
(baa”beez-ee’yuh/my”cr?-tee)
Common associated disease and condition
names: Presently, no common name exists.
Babesia divergens
( )
Common associated disease and condition
names: Presently, no common name exists.
Morphology
The morphologic features of B. microti and B.
divergens are described in the general notes concerning
babesiosis.
Laboratory Diagnosis
The laboratory diagnostic procedures for identifying
B. microti and B. divergens are described
in the general notes concerning babesiosis.
Life Cycle Notes
The life cycle of B. microti and B. divergens are
described in the general notes concerning
babesiosis.
Epidemiology
B. microti is commonly found in areas of southern
New England, such as Nantucket, Martha’s
Vineyard, Shelter Island, Long Island, and Connecticut.
It has also been isolated in clinical
specimens in patients in New Jersey, Wisconsin,
Missouri, Georgia, North Carolina, and Mexico.
The vector most commonly associated with the
transmission of B. microti is Ixodes dammini.
The principal reservoir host for this infection is
the white-footed mouse, Peromyscus leucopus.
B. divergens is commonly found in European
countries, particularly those in the former Yugoslavia,
Russia, Ireland, and Scotland. The vector
most commonly associated with the transmission
FIGURE 6-8 Parasite classification—Babesia species.
Phylum
Apicomplexa
Class
Aconoidasida
Order
Piroplasmida
Blood Species
Babesia microti
Babesia divergens
Quick Quiz! 6-18
Babesiosis is characterized by all the following except:
(Objective 6-8)
A. Trophozoites resembling the ring form seen in
Plasmodium infections
B. A mild to severe hemolytic anemia
C. Fever periodicity
D. None of the above
headache, chills, sweating, arthralgias, myalgias,
fatigue, and weakness. The fever shows no periodicity.
Hepatosplenomegaly and mild to severe
hemolytic anemia have been recorded. Elevated
bilirubin and transaminase levels have also been
demonstrated.
Babesiosis tends to be worse for the splenectomized
and immunocompromised patient. Rare
asymptomatic infections have also been recorded.
Infections tend to present in late summer to early
fall and generally correlate with the breeding
cycle of the ixodid tick. It is also not uncommon
to see a patient coinfected with Lyme disease and/
or human granulocytic ehrlichiosis.
CHAPTER 6 Select Sporozoa: Plasmodium and Babesia 155
of B. divergens is Ixodes ricinus. The principal
reservoir hosts are cattle and rabbits. B. divergens
has also been described in the Nantucket
area, primarily in the rabbits and birds of the
region.
Babesiosis has also been demonstrated to be a
transfusion-transmissible disease and has the
potential to be transmitted congenitally and by
the sharing of intravenous drug needles.
Clinical Symptoms
The clinical symptoms for B. microti and B.
divergens infections have been described earlier
(“Pathogenesis and Clinical Symptoms”). B.
divergens tends to be the more severe of the two
parasitic infections and is frequently fatal if
left untreated. B. microti tends to be rather
benign and self-limiting. Disease with either of
these organisms is often more severe for older
adult, immunosuppressed, and/or splenectomized
patients.
Treatment