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Does My Donkey Have
Besnoitia? Judy
Marteniuk, DVM and Charles Mackenzie, BVSc FRCVS Michigan
State University What
is Besnoitia? Besnoitia is a protozoan parasite that
is found worldwide and can affect
many species of animals, such as cattle, sheep, goats, reindeer, equids,
rabbits, opossums, and rodents. Besnoitia bennetti is the species that
has been found in horses, donkeys and mules.
Other protozoan parasites in this family that you may recognize are Toxoplasma gondii (causes abortions in
multiple species, including humans) and
Sarcocystis neurona (causes Equine
Protozoal Myelitis - EPM). B.
bennetti
was first reported by Bennett in 1927, and 1933 in four horses in the
Sudan. Initially, the organism was
thought to be in the family Sarcocystis,
but was reclassified as Besnoitia in
1932. Over the years, sporadic reports
of B. bennetti have been
published. In 1955, Schultz and Thorburn
found tissue cysts in a horse in S. Africa. By 1960, Pol determined that
cutaneous besnoitiosis in horses was not able to affect cattle. Then, Bigalke in 1970 suggested that Besnoitia in horses, mules and donkeys
in S. Africa was a separate species. Van Heerden described the unique
ultrastructure for B. Bennetti in
1993. The tissue cysts are between 100-200 μm
in diameter and the organisms (slowly replicating bradyzoites) in the cysts are
8.7 X 1.9 μm. Interestingly, B. bennetti has been reported primarily in Africa and only more
recently in the United States. In the
U.S., sporadic recognition of the parasite has been made in Florida, Maryland,
Massachusetts, Michigan, Montana, Texas; it has also been reported in West
Virginia in the past year. Thus, it is probable that the infection has a much
wider distribution in the US than was first thought. What
are the signs of Besnoitia? Donkeys clinically affected with Besnoitia will have hair loss with
thickening and crusting of the skin.
This abnormal skin is most prominent over the face, ears, neck. perineal
area, and lower legs. Some donkeys are reported to have pruritus (itchiness)
while others show no signs of being irritated by the skin lesions. Most of the Michigan donkeys that we reported
on in 2005 did not appear to be itchy; however, one jack had a significant
degree of pruritus and hair loss..
Another prominent feature of this disease is the presence of “pinpoint”
nodules in the sclera, and mucous membranes of the lips, nostrils and vulva. In
the Michigan outbreak, the sclera nodules were close to the limbus and common
toward the medial canthus (inner corner) of the eye. Blood work on affected donkeys
varied. Some individuals had completely
normal blood work, while other donkeys were anemic, and had an increased white
blood cell count. Total protein and
globulins (proteins used to fight infection) also varied from normal to
increased. The severity of the clinical
signs was not consistently correlated with the changes in the blood. Interestingly, most of the donkeys in
the Michigan outbreak and in the literature seem to be female. This may be related to the fact that the
farms affected with Besnoitia were breeding farms. Also, affected donkeys tend to be the younger
members of the herd. The age breakdown
for the Michigan outbreak is presented below.
How
is Besnoitia diagnosed? Clinical presentation with the nodules
in the sclera, vulva, and lip are very characteristic for the disease. The loss
of hair and the thickening and crusting of the skin although present with Besnoitia, may be present in several
other skin problems. Therefore, a skin
biopsy of the affected areas is needed.
Histologically, characteristic cysts are seen in the dermis and
epidermis. Each cyst is packet with
hundreds of bradyzoite stage parasites. With the outbreak in Michigan, all
donkeys survived and recovered over a prolonged period of time. However, in the literature a necropsy of a
severely affected donkey is reported.
That donkey had hundreds of pinpoint white granular cysts (0.5 to 1.5
mm) in the epidermis, and dermis. These
cysts extended to the subcutaneous tissues and connective tissue under the
affected skin. Lymph nodes near affected
skin were also enlarged, as was the lymphoid tissue surrounding the
larynx. When the tissues were examined
under the microscope, lesions were seen in the thyroid, around the thyroid,
around the laryngeal connective tissues, in the lung, colon, kidneys and lymph
nodes. Interestingly, numerous free bradyzoites were seen in the skin crusts.
It is suspected that these free, infective organisms may be readily picked up
and transferred to other equids by biting insects. However, the route of
transmission of this protozoan has never been fully defines. Can
I treat my donkey for Besnoitia? There is no approved treatment for Besnoitia. The most common treatment
used for Besnoitia is prolonged
treatment with trimethoprim sulfa.
Treatment is often given continuously for several months, similar to
treating a horse for EPM. Others have
described an alternating treatment protocol where the donkey is treated for
several weeks, then not treated for several weeks. This protocol may be repeated a number of
times. Another treatment option may be
using Marquis (ponazuril) which is another drug licensed for EPM treatment. Currently, there is no information on the
efficacy of this drug for Besnoitia treatment. Interestingly, Besnoitia cysts can still be found in skin biopsies even when
treatment is extended over months and the donkey’s skin is almost normal in
appearance. It is currently unknown if
these cysts are cleared with time. Also, in the Michigan outbreak, donkeys that
were mild to moderately affected and did not receive prolonged treatment,
improved with time to the point that it was very difficult to find any
lesions. How
is Besnoitia spread? Currently, the lifecycle of Besnoitia in all hosts is poorly
understood. However, we do know that
the lifecycle is what is termed an “indirect lifecycle” meaning that the
parasite needs both a direct/definitive host and an indirect/intermediate host.
Many of the members of the Besnoitia family
are believed to have a member of the cat family as the definitive host. To
date, researchers have not been able to find or reproduce bradyzoites of B bennetti in mice, rabbits, gerbils or
cats. With this type of parasite, the most
common method of spread is when the direct host eats tissue from an infected,
intermediate host. However, there is also the possibility that the organism can
be spread from intermediate host to intermediate host ( directly between
donkeys - termed horizontal spread) by insects since the bradyzoites can be
found in sloughing, crusty skin debris. To further support the potential for
horizontal spread is the fact that Besnoitia has been shown to remain infective
for 50 hours in Culex spp
(mosquitos). Finally, even though understanding of
the lifecycle is incomplete, you can still implement preventive measures. Just
good sanitation and fly control will be beneficial in reducing the potential
for horizontal spread. Also, keeping
your grain bin covered will discourage wildlife from setting up residence in
your barn, thus reducing the risk of the definitive host depositing B. bennetti oocysts in the environment. Future
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