Emerging Animal Diseases
A study was conducted that focused on two potential emerging diseases,
foot and mouth disease and bovine spongiform encephalopathy (BSE). Both
of these diseases could have a devastating impact on California and the
U.S. agriculture. The foot and mouth diseases study initially focused
on the economic impact of the disease to California's economy. The estimated
economic impact was $6 billion or possibly as much as $13 billion. These
figures are the latest projections of any government agency and have been
used by government and media reporters to quote the economic impact of
FMD on California. The BSE study focused on the impact of the disease
on livestock production and the potential losses associated with the disease
should it show up in California. The studies on both diseases have raised
the awareness of the impact of these diseases on California animal production
systems.
Trichomoniasis
Trichomoniasis is a parasitic disease of the reproductive tract of cattle
caused by Tritrichomonas foetus. The disease is manifested in cows
that fail to conceive, abort fetuses or have prolonged calving intervals.
The protozoal organisms are most readily transmitted in nature by bulls
from cow to cow. This is further complicated by cattle on the range that
may be bred by bulls from other herds thereby spreading infection widely.
The infection in a bull is usually mild or unapparent. The infection may
persist in cows so that subsequent breedings are associated with failed
conceptions. These are costly to cattleman as they are dependent on cows
producing calves annually for business profits. In the case of dairy cattle,
failure to conceive will lead to losses in milk production with a subsequent
loss of income. Trichomoniasis has been recognized as one of the top economic
diseases by beef cattle producers in California and the other Western
states. Efforts to control infection in herds has been associated with
purchasing and using breeding bulls free of infection, vaccinating cattle
and bulls, and lastly attempting treatment. Ideally, the best strategy
is to have herds free of infection and to maintain the herds as disease
free. This is very difficult with range cattle. Vaccines are equivocal
as they are sometimes reasonably effective in cows, but less effective
in bulls. Treatment is costly and very difficult to do with range cattle.
Issue
- Develop improved diagnostic technology to facilitate testing for excluding
the infections from herds.
- Develop a better understanding of the immune response necessary for
protective immunity and improved vaccines.
Research Findings
During the research efforts to develop improved diagnostic technology,
a new Trichomonad was identified. This new agent does not appear to cause
disease, however it has lead to false diagnosis by many laboratories.
Efforts are underway to develop reagents that will differentiate this
newly recognized trichomonad from the T. foetus. Relative to evaluation
of the immune response to T. foetus, the studies indicated that T. foetus
is able to escape detection of the immune system or to use the antibodies
of the immune system to protect it against destruction by the cow. These
factors are being further studied so that a different form of protective
immunity can be developed.
Impact
Recognition of the newly recognized trichomonad and the ability to differentiate
it from T. foetus is of considerable value for more effectively
identifying infected carriers of infection. This facilitates separating
out infected bulls from herds and certifying herds free of infection.
This management approach appears to be the best approach for controlling
infection at this time.
Cryptosporidiosis
Cryptosproridiosis is an important parasitic disease of animals and humans.
It is one of the over 350 zoonotic diseases that affects both animals
and humans. The protozoal parasite is most often transmitted through contaminated
water, but it can be transmitted by oral/fecal contact or contaminated
foods. The parasite localizes in the intestines where it causes a severe
diarrhea. Infection of immunocompromised individuals such as the aged,
young, or those with AIDS or on cancer chemotherapy are at risk and may
die from the infection. Some of the most explosive outbreaks of disease
reported in the U.S. have been associated with cryptosporidiosis. An outbreak
of the disease in Milwaukee, Wisconsin affected 400,000 people. The water
supply had been contaminated and the Center of Disease Control and Prevention
epidemiologists indicated that the source of the infectious agent was
dairy cattle wastewater runoff. The disease occurs throughout the U.S.
each year and livestock are thought to be the source of infection. The
usual chemical for destroying bacterial contaminates in water supplies,
chlorine, is not effective for killing the cryptosporidial parasites.
In California, many water districts that maintain reservoirs to hold surface
water for domestic water supplies, proposed fencing out cattle and horses
from range lands on the reservoir watersheds. The elimination of these
grazing lands would have a major negative impact on California rangelands
for both livestock production as well as the wildlife and human activities
in these areas.
Issue
- Determine whether cattle are a significant shedder of infectious
organisms into the environment and surrounding water supplies and; if
so,
- Develop management guidelines that can be used to minimize the risk
of cattle being a source of the agent in the immediate environment.
Research Findings
Evaluation of cattle for disseminating Cryptosporidia into the environment
indicated that adult cattle rarely shed the agent. The age of greatest
risk for contaminating the environment occurs within the first few weeks
of life and calves are seldom in areas where watersheds would be contaminated.
This is the case in Western U.S. Surprisingly, the animal with the highest
rate of shedding Cryptosporidia into the environment was the ground squirrel.
Recommendations
Newborn calves should not be in areas where there is a possibility of
contaminating the environment around watersheds during the first 8 weeks
after birth.
Impact
These research findings have made it possible to permit cattle to graze
on watersheds that are used for surface water storage for human consumption.
By permitting the cattle to graze on theses lands, the cattle have reduced
overgrowth of grass lands and reduced fire danger.
Bluetongue
Bluetongue is a viral disease of ruminants that is found in tropical and
temperate climates of the world. There are 24 serotypes of these viruses
worldwide that are transmitted by Culicoides flies. These biting flies
are the principle means of determining the distribution of bluetongue
worldwide. Furthermore the virus serotypes seem to be reasonably stable
within an ecosystem. Bluetongue disease causes its most serious damage
to sheep where the lesions lead to erosive and ulcerative lesions of the
mouth, damage to muscle and cardiac muscle, and extensive lameness along
with high body temperatures. Up to 30% of a flock may die from the infection.
Cattle appear to be the main species that are infected; however they rarely
show evidence of disease. Instead cattle have prolonged viremias permitting
insects to pick up the virus over longer periods of up to 150 days following
infection. There is also evidence that bluetongue viruses can be transmitted
through semen to susceptible cows. Because of these findings, bluetongue
is consider a List A disease by the Organization of International Epizootics
(O.I.E.), and as a rest of this listing, severe trade restrictions are
placed on countries that plan to export ruminants or ruminant germplasm
from countries that have the bluetongue infection to other countries.
Much of the information used to base the listing on is outdated and due
to a failure to understand the biology of the viruses and Culicoides vectors.
Issue
- Define the biology of bluetongue viruses, culicoides flies and the
ecosystems of these viruses in different parts of the world.
- Develop science based information for policy decisions that would
permit new approaches for international trade of ruminants and ruminant
germplasm.
Research Findings
Studies on cattle experimentally infected with bluetongue virus defined
that they are viremic for 50 days, but no longer than this. The culicoides
flies were able to pick up virus from the these infected cows for up to
30 to 45 days after infection and transmit the virus to other ruminants.
In studies comparing bluetongue viruses from China with those from the
U.S., it was clear from genetic analysis of the viruses that the serotype
distribution between the countries was different but the major factor
defining the distribution of the viruses was the selected gene segment
favoring the culicoides in their respective ecosystems.
Impact
New regulatory information has been drafted for consideration by the 125
member countries of the O.I.E. based on the scientific information derived
from the above research. The basis of the information clearly demonstrated
that cattle carry the infected virus for up to 50 days following infection,
rather than 150 days and that the amount (titer) of virus in blood that
culicoides flies can pick up is seldom more that 30 to 45 days. This information
can now be used to hold animals for considerably less time in quarantine
before shipment to foreign countries. Also, the information about the
confinement of bluetongue to different ecosystems based on the genetic
predisposition of the viruses to the particular culicoides flies of that
region/ecosystem will make a major difference in understanding the distribution
of these viruses in the world. This will eventually lead to eased trade
restrictions.
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