Irish Farmer's Journal:
Tick-borne disease on Irish farms - 23-04-2011
Mícheál Casey from the Department of Agriculture's Regional Veterinary
Laboratory service, outlines the common tick-borne diseases affecting Irish
livestock
Ticks are blood-sucking parasitic members of the Arachnidae - the same
class of eight-legged arthropods as spiders. Diseases transmitted by ticks are
a major cause of economic loss, disease and deaths in farmed animals
worldwide.
Although there is only one species of tick that affects Irish livestock -
the 'castor bean tick', Ixodes ricinus - it can act as a vector for a range
of diseases. All references to ticks in this article refer to this tick.
Ticks have some fairly precise environmental requirements, especially when
they leave the relative shelter of the base of the vegetation. They find a
new host by 'questing', where they climb to the tips of the vegetation and
grab onto any animal (or person) that passes.
They need mild and moist conditions for questing, which are provided in
late spring and in autumn in a typical Irish year, resulting in clearly
defined spring and autumn peaks in tick-borne diseases.
On some farms the ticks have become adapted to one or the other season,
while on other farms both peaks are seen.
Tick-borne fever
This disease is caused by a bacterium (Ehrlichia phagocytophila) and is
normally mild and transient. Although this is not commonly diagnosed, it is
probably the most important tick-borne disease in Ireland.
Firstly, it is very common; so common, in fact, that most herds have a
high level of resistance and most infection occurs in young and bought-in
animals. As the name suggests, animals run a temperature for a couple of days,
lose their appetite, they may cough a little and milk yield of cows drops
significantly.
The reason for the significance of tick-borne fever is the brief but
severe immunosuppression that accompanies infection with the organism. Affected
animals are very susceptible to other infections at the time of infection,
and vaccines for other diseases that are administered at the time of
tick-borne fever infection will not take effect.
Furthermore, if the tick that infects the animal is also carrying one of
the other tick-borne diseases, then infection is more likely and the ensuing
disease may be more severe.
Tick-borne fever is a hidden but important factor in every other
tick-borne disease.
Babesiosis -'Redwater'
This parasite, Babesia divergens, is carried by ticks and is capable of
being transmitted from one generation of tick to the next, so a reservoir of
infection can be maintained on pasture even when no livestock have grazed
that pasture for several years.
Once inoculated into the bloodstream, the organism replicates rapidly in
red blood cells, which are ruptured as each generation of the parasite
emerges. Animals run a high temperature which then falls rapidly, often below
normal, as the disease progresses. Affected animals become dull, lose their
appetite, become slow and may have difficulty standing or walking as the
disease progresses. The oxygen-carrying haemoglobin is released from the
ruptured red blood cells and passes through the kidneys and out in the urine,
giving it a characteristic reddish brown colour and giving the disease its
common name - 'redwater'. The heart races as the body tries to compensate for
the loss of circulating blood cells. Deaths can occur due to heart failure,
kidney failure or anaemia, and blood transfusions may be required in the
treatment of the most severely affected cases.
Drugs that prevent multiplication of the parasite are administered, but it
is the effects of disease that are the most difficult to treat - anaemia,
dehydration (and associated constipation).
One unique feature of redwater is the 'reverse age immunity' phenomenon.
Calves are resistant to the disease until they are about six months of age.
After that, the resistance an animal has to redwater in later life will
depend on whether they were exposed to the disease as calves. Animals that
have no resistance tend to develop a very severe form of the disease, and many
farms routinely protect bought-in animals with a drug that gives
protection for about four weeks. If this is given just before peak tick activity,
there is a good chance that the animal will be bitten, infected and develop
resistance without getting the disease, while protected by the drug.
Redwater seems to be decreasing in incidence and in severity in recent
years. Partly, this is due to improved pasture management, which eliminates
tick habitat. It also seems likely that the widespread use of Ivermectin-type
products may have had an impact on tick productivity.
Tick pyaemia
This is a disease of young lambs, which is caused by a common skin
bacterium, Staphylococcus aureus. These bacteria are inoculated from the skin
surface by the tick as it bites and get into the bloodstream causing
septicaemia (blood poisoning). While the lamb's blood carries the bacteria around the
body, the tick will frequently be infecting the animal with tick-borne
fever, which results in the bacteria 'seeding' the internal organs and
tissues, especially the liver and the joints. A second septicaemia, often fatal,
may occur at this stage. Affected lambs become slow and stiff and will die
if untreated.
Q fever
Q fever is caused by bacterium, Coxiella burnetti, and is very similar to
tick-borne fever. Little is known about Q fever in Irish farm animals,
largely because of the unavailability of diagnostic tests. It is known to occur
here, and is tick-transmitted. It is likely to be behind some abortion
outbreaks in sheep and cattle and may have an immunosuppressive role in a wide
range of diseases
Lyme disease
This bacterial tick-borne disease caused by Borrelia burgdorferi is
strongly associated with deer, and the infection risk for humans and animals is
highest in woodland and nearby pasture.
It is 'one to watch' as our wild and farmed deer population grows. It
causes a fever and rash and can progress to cause central nervous system
disease, arthritis and blood vessel damage in humans.
Again, little is known about the disease in Ireland, as it is rarely
diagnosed, although blood testing shows that exposure to infection is common.
This is a serious and potentially fatal disease in humans, so it is very
important to seek medical attention if any relevant symptoms are seen after
a tick bite.
Louping Ill
This virus causes encephalitis (brain inflammation) in sheep and is often
fatal. It tends to occur in ticks in well-defined areas and is best
controlled on affected farms by sourcing replacements from home-bred animals, or
at least from within those areas.
Control
A common feature of many tick-borne diseases is the strong, often
life-long, immunity that results from infection. As a result, strong herd immunity
develops, and very little disease tends to occur in stable, closed herds,
even in heavily infested areas.
The exception here would be tick pyemia in lambs, where certain farms have
a problem year after year unless they control the ticks.
Control of ticks requires the recognition and elimination of ideal tick
habitat. Because ticks require mild, moist conditions, they are usually found
at the base of dense vegetation. Ideal conditions for ticks occur where
grass is growing through one or two years of dead previous growth (areas
ungrazed for several years), and there is a moist decaying mat of old
vegetation at the base of the sward. This is something to watch for when renting
grazing land that may have been fallow for some time.
Control is achieved by minimising this phenomenon and by keeping animals
fenced out of likely areas. Good pasture management and the rotation (where
possible) of forage and grazing areas should minimise the amount of tick
habitat.
The use of acaricides (chemicals that kill ticks) with residual effect
will give protection against ticks for several weeks, and is a common
practice, as a way to protect cattle or lambs during periods of peak risk.
However, preventing tick bites will also prevent the acquisition of
immunity, so these animals will continue to be vulnerable once the protection
offered by the acaricide wears off.
Another useful control measure is to source replacements within the herd,
or at least locally, so that they will have been exposed to the range of
tick-borne diseases that occur in that area.
Great care is needed when introducing animals from tick-free farms to
areas where they will face a significant challenge, as these animals will have
no immunity to tick-borne disease.
_http://www.farmersjournal.ie/site/farming-Tick-borne-disease-on-Irish-farms
-12922.html_
(http://www.farmersjournal.ie/site/farming-Tick-borne-disease-on-Irish-farms-12922.html)
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