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Dog or Cat Parasites That Infect Humans

Stephen Kayes skayes at usouthal.edu
Mon Jan 31 12:06:00 EST 2005

There are several parasites that humans can acquire from dogs and cats. 
The most significant of these, IMHO is Toxocara. We can get T. canis
from dogs and T. cati from cats although the latter is less likely given
that cats are more fastidious in their personal habits than are dogs
(they cover or bury their droppings).
These two parasites are members of the ascarid family and related to
the human intestinal worm, Ascaris lumbricoides.  However, unlike
Ascaris, when Toxocara finds itself in a human host, it is unable to
complete its developmental life cycle and return to the digestive system
and mature into egg-laying adults.  The worms continue to reside in the
tissues of the body and the resulting condition is known as visceral
larva migrans.  Severity of this condition depends on the number of
larva infecting the human host and the location of the larva.  Some
people apparently produce immunological reactions that are more severe
to these organisms than do others.  
Salient to the diagnosis of this parasite is the fact that because the
worms do not mature into egg laying adults the classical O&P test is
essentially useless in making a diagnosis.  However, the immunoassay
known as T. canis-specific ELISA is extremely sensitive and specific for
this infection.  It measures the antibody levels made in response to the
organism.  T. canis may be one of the most immunogenic parasites known
and in fact was used in the first experiments to show that humans had
Th2 cells that responded to invasive parasites just they did in
laboratory mice.  Again, IMHO, T. canis may be one of the most
underdiagnosed parasites in North America.  According to the CDC 3 to 4%
of Americans may be infected and if you look deeper, 7 to 10% of
children are infected and if you look at ethnic groups in poor rural
areas of the South the prevalence approaches 30%.   Unbelievably, on the
Island of St. Lucia, one study indicated 86% seroprevalence.
The blood test is an effective way to diagnose a suspected case that is
indicated on clinical findings of cough, wheeze, elevated WBC and
significant eosinophilia.  Association with dogs, especially puppies is
a contributing factor.
Steve Kayes, Ph.D.
Stephen G. Kayes, Ph.D.
Cell Biology & Neuroscience
College of Medicine
University of South Alabama
Mobile, AL 36688-0002
(251) 460-6768 ofc
(251) 460-6771 fax
kayes at sungcg.usouthal.edu
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