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An
Overview of Canine Leishmania Infections
Leishmaniasis
is an infectious disease transmitted by sand flies and caused by various
species of Leishmania. The parasites can infect both
humans and canines, and the resulting condition is known as visceral
leishmaniasis. The disease is particularly common in tropical and subtropical
areas with significant sand fly populations. The domestic dog seems
to be the main reservoir for human visceral leishmaniasis, rendering disease control
that much more vital. Unfortunately efforts to control leishmaniasis
in dogs have been largely unsuccessful, though as more research is being
done, progress is being and will continue to be made.
How might my dog become infected?
The natural infection cycle requires involvement of an insect
vector – the sand fly. Female sand flies contract the Leishmania
parasite as they feed on an infected animal and ingest the intracellular,
non-flagellated form of parasite. Once ingested, the parasite transforms
into a flagellated
form, called promastigote. The promastigotes attach themselves to the
gut wall of the sand fly and proceed to multiply and differentiate into infective promastiogotes. When the sand fly feeds on a new host, the promastigotes
may be injected into the animal’s skin. Once within
their new host, the promastigotes lose their flagella and multiply inside certain types of white blood cells, the macrophages, as
amastigotes.
This initial
multiplication occurs at or near the site of infection on the skin.
The parasites will then spread from the skin to the bone
marrow, spleen, and liver. Once in these organs the parasites cause
a chronic, debilitating, and potentially fatal disease.
How is visceral leishmaniasis diagnosed?
The list
of symptoms associated with leishmania infections in dogs is long. The
most frequently reported complaints from owners of sick dogs include
skin lesions, progressive weight loss, decreased appetite and an intolerance
of exercise. Other common indicators of visceral leishmaniasis include
depression, ocular changes, epistaxis,
excessive thirst and urine production, diarrhea, vomiting, sneezing,
coughing, and lameness. This is however by no means an exhaustive list
– leishmaniasis affects so many parts of the body that the variety
of clinical signs is nearly boundless. It should be noted that Leishmania
infection should only be suspected in areas where sand flies are present
as they are the major vector of infection.
Definitive diagnosis of visceral leishmaniasis
is typically done by laboratory tests. Most commonly, serologic methods
such as IFAT, ELISA
and western blotting
are used to detect anti-Leishmania antibodies. If the infection is at an advanced stage, it may be possible
to isolate the Leishmania organism directly. Your veterinarian will
utilize the appropriate diagnostic technique(s) should it become necessary.
What are the treatment options for dogs diagnosed as having
visceral leishmaniasis?
Canine
cases of leishmaniasis are unfortunately much more resistant to treatment
than are human infections. For this reason a full parasitological cure
in dogs is rarely achieved. Drugs such as pentavalent
antimonials are frequently used to treat symptoms, and
significant clinical improvement often occurs initially. Long term,
however, relapses are common and the situation is complicated by the
fact that frequent, long term drug use can both be harmful to the dog
and create drug resistant microorganisms.
What are the public health concerns associated with visceral
leishmaniasis?
Humans
can and do become infected with the Leishmania bacteria. More
than 90 percent of the world’s human leishmaniasis cases occur
in India, Bangladesh, Nepal, Sudan, Brazil, and the countries along the Mediterranean Basin. Affected people have
an irregular fever, anemia, enlarged
spleen and liver, abnormal blood counts, and a high risk of mortality
if untreated. Humans with normal immune function are at a minimal risk
of infection, but infants, the elderly, and individuals with HIV or
AIDS are more vulnerable. Thankfully, humans respond much better and
more thoroughly to treatment than do dogs.
What steps can I take to minimize the risk of infection for
both my dog and myself?
It should
first be noted that if you live in the United States or Canada, the
chances of either you or your dog contracting leishmaniasis are
low. Only if you live in southern Texas might the risk be significant although leishmaniasis has recently been declared endmic in the US after a CDC survey that found positive dogs in 21 states. The sand fly distribution in the US is largely uknown.
If you do live in an area where Leishmania is
endemic, or if you have traveled or will soon be traveling to such an
area, certain steps may be taken to minimize the risk of exposure.
As far
as dogs are concerned, if you live in an area at high risk, blood screening
should be done on a regular basis. This allows for an infection to be
caught and treated early and avoid the risk of spreading the infection. General health status of your companion should
be kept high, as lowered immune response greatly increases the chance
of infection. Preventative collars that repel the sand fly are commercially
available and have been used with some success. It is well known that
sand flies are most active between dusk and dawn. If possible, time
outdoors should be limited during these hours.
As humans
are also infected only by the bite of a sand fly, avoidance of these
insects is the key to prevention. Staying inside between dusk and dawn
can reduce the risk of getting bitten. Insect repellent containing DEET
should be used at all hours of the day, and homes should be well-screened
so that flies are less likely to gain entrance to the house.
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Additional
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