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ZOONOSES
Zoonoses can be transmitted in different ways. Direct transmis-
sion refers to the common way that we humans transmit disease
among ourselves: aerosolized particles or body fluids. In the case of
zoonoses, this method involves three components: the infectious
agent, the animal reservoir, and the human who acquires the disease.
There are relatively few animal infectious diseases that are transmitted
this way. Among the more common are rabies, tularemia, Q-Fever
and brucellosis. Some ringworm fungi are common to humans and
animals, but there seems to be some conflict about who had them
first, particularly since some have been found thriving in soil.
Indirectly transmitted zoonotic diseases require these three factors
as well as a fourth, the vector. The vector can be water, soil, feces, or the incidence of rabies in much of the western world. According to
various arthropods, such as mosquitoes, biting flies (tabanids) or
ticks. Included among these conditions are malaria, plague, Lyme
disease, and the various encephalitis viruses. Water-borne diseases
include Giardiaisis (beaver fever) and cryoptosporidosis. My reading
indicated that conditions which include transmission of toxins
through animals, such as might occur if we eat fish contaminated by
mercury, are not generally thought of as zoonoses.
There isn’t time or space here to discuss the etiology, presentation,
course and treatment of all of these conditions, but I thought it might
be interesting to go over a few that have been around for a while and
look at them from both the human medicine and veterinary medi-
cine standpoints.
RABIES
Rabies is definitely endemic in Texas, and each year members of
several species, both domestic and wild, are diagnosed as rabid. Here
in Texas the animal most commonly found to be rabid is the skunk,
but nationwide raccoons seem to lead the list. Pet vaccination, re-
quired quarantine of domestic pets after a biting incident, and the
use of oral vaccines distributed in baits for coyotes, foxes and rac-
coons have significantly decreased the prevalence of this nearly uni-
formly fatal neurologic disease. The use of oral vaccine-containing
baits started in 1995, and the latest statistic I could find indicated
that 47,683,000 doses had been spread by air drops, over 664,100
square miles of Texas: this is an interesting bit of information from
the Texas State Department of State Health Services (DSHS), since
Texas has only 268,820 square miles!
Compulsory immunization of dogs and cats has greatly decreased
the World Health Organization, each year more than 60,000 people
die from rabies worldwide. In 2012 in the United States, 6,162 cases
of rabies were diagnosed in animals; 92 percent of these were in
wildlife. There was one human case that year.
Below is an illustration from the DSHS website which demon-
strates the effectiveness of the oral vaccines. There are two genetic
variants of the virus, and two different baits have been used to attack
the variants: as you can see, the incidence of animals diagnosed with
these viruses has diminished greatly. There is also an oral rabies vac-
cine that is effective in raccoons and coyotes, but it does not seem to
have been used in Texas.
TUBERCULOSIS
Bovine tuberculosis, caused by M. bovis, has been largely elimi-
nated in the United States through the efforts of the veterinary pro-
fession, the dairy industry and the federal government.
Tuberculin-based skin testing of cattle, which can evolve into “test
and slaughter” pro-
grams, have been
around since the
early 20th century,
and are supple-
mented by carcass
inspection at
slaughterhouses.
When an animal
tests positive, it is
slaughtered and the
herd is quarantined
and tested some
more. A carcass
that is found to har-
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