Anaplasmosis in Dairy Cattle
Keeping the infection out of your herd is vital to your cows' long-term health, and the most cost effective way to lower your farm's risk
Anaplamosis, the disease caused by cattle infection with Anaplasma marginale, is still a rare occurrence on Ontario dairy farms. However, the risk of infection is likely climbing. Keeping the infection out of your herd is vital to your cows' long-term health, and the most cost-effective way to lower your farm's risk.
A. marginale lives in a cow's red blood cells (RBCs). When a cow is first infected, it may not show any illness signs. Typically, it takes three to 60 days after infection before the organism is seen in a cow's RBCs under a microscope. The organism is very efficient at evading a cow's immune system and can continually infect new RBCs. Cows with A. marginale are infected for life. The organism may no longer be visible in a blood smear and infection will need to be detected by other tests. Although infected cows may not show illness signs, the blood from these animals can be transferred to other cows and cause new infections.
Why is this a concern now?
As of April 1, 2014, Anaplasmosis in cattle was removed from the
list of federally reportable diseases. The federal government is
no longer involved in controlling the infection. Import controls
have been removed. Cows purchased from infected areas of North America
are no longer tested before entering Canada. Testing and preventing
the importation of test positive-cows greatly reduced the risk of
introducing infected cows to Ontario dairy herds.
In Canada, A. marginale has been identified in cattle by either a Canadian Food Inspection Agency serological survey or by clinical case development and follow-up in Manitoba (1968, 2009, 2010, 2013), Quebec (1979), Saskatchewan (1983, 2000, 2008) and Ontario (1996, 2013). Most situations have involved beef cattle; the 2013 case in Ontario was from a dairy herd.
Some experts speculate climate change may help spread vector-borne diseases, which could affect Anaplasmosis by allowing a greater range for tick vectors. However, this likely has a minor impact in Ontario compared with the risk of spreading the infection through cattle movement.
How to recognize Anaplasmosis
Infection severity and how sick a cow becomes depend on her age
at the time of infection and dose of A. marginale initially received.
Cows can become infected at any age. Generally, cows infected at
less than six months of age will not show illness, while those between
six months and one year may have mild disease. Older cows, especially
those over two years of age, tend to have the most severe illness,
and between 25 and 50 per cent of them will die.
A. marginale can spread by the direct transfer of infected RBCs from an infected animal to uninfected herd mates or by the bite from an infected tick. In North America, ticks in the Dermacentor and Boophilus families can become infected by feeding on infected cattle. The organism multiplies within them and spreads to their salivary glands. The tick moves from animal to animal taking blood and spreading A. marginale as it goes.
Tick exposure is minimal on most Ontario dairy farms, although in many parts of the U.S. this is how A. marginale is spread. Following certain practices, such as using the same hypodermic needle to inject more than one animal, can result in new infections in your herd. Injecting oxytocin at milking time directly into cows' bloodstream using the same needle for more than one injection is also concerning. On one New York farm, this practice quickly led to a high infection rate after a carrier cow was introduced to the herd.
Biting flies can also transfer infected RBCs from one animal to another. The organism does not multiply in flies like it does in male ticks; however, biting flies could be an important mode of transfer on Ontario dairy farms, especially for young stock.
Are wildlife or other domestic ruminants a risk to cattle?
The reservoir or source of A. marginale is infected cattle. Other ruminants and wildlife, such as bison, varieties of deer or elk, may become infected and be important in the epidemiology of A. marginale in other areas. However, they are currently not important in Ontario. Experimentally infected White Tailed Deer (WTD) have been shown to become infected but do not develop a persistent blood-borne infection and; therefore, are not important as a reservoir for infection. However, WTD can be hosts for the ticks that spread A. marginale, and where support for an abundance of ticks is important for A. marginale spread, WTD may contribute. Sheep have their own species of Anaplasma (A. ovis), which does not infect cattle. Also, sheep do not become infected with A. marginale.
How to know if your cows have had it
Infected cows with no visible sickness need to be identified using blood tests. In Ontario, cattle can be tested by submitting blood samples collected by your veterinarian to the Animal Health Laboratory in Guelph. The lab tests for antibodies to A.marginale using a cELISA test. The test rarely has false positive results. However, it may not detect all infected animals.
Antibodies can take time to develop, from 16 to 27 days, based on findings from an experimental infection of calves. As well, antibody levels may wax and wane cyclically in chronically infected cows. One recommendation is to do two tests at least eight weeks apart on newly introduced or suspect cows to ensure recently infected cows aren't missed. Additionally, a polymerase chain reaction (PCR) that tests for the organism itself can be used in some circumstances. Blood smears where visible organisms in RBCs are seen, followed by cELISA testing, are also useful for identifying Anaplasmosis in sick cows.
Should I treat or vaccinate?
There is no effective treatment for Anaplasmosis. In the U.S. where Anaplasmosis is endemic and infection risk is high, in some circumstances such as feedlots, antibiotics may be used to reduce the impact of clinical illness. However, they are well documented to not prevent infection. Studies have yet to prove treating Anaplasmosis with antibiotics eliminates the infection. There are currently no vaccines licensed for use for Anaplasmosis in Canada. Like antibiotics, vaccination may help reduce clinical illness but will not prevent new infections or prevent persistent infection from developing. In Ontario, where A. marginale continues to be rare, neither approach is suitable since both would result in a reservoir of persistently infected cows.
The cost of Anaplasmosis in a herd can be profound. Following introduction and spread, milk production by clinically and subclinically infected cows will be reduced, while some cows may show visible sickness and die, and some may abort. Chronically infected cows can serve as a persistent infection source. Once the disease is identified in a herd, extensive testing and culling is necessary to control and eradicate it.
The estimated cost of Anaplasmosis to U.S. dairy producers is $100 million annually. This figure may be underestimated due to chronic but undiagnosed infections.
What should I do now?
Currently, Anaplasmosis is a rare occurrence in Ontario but the risk of introduction is likely increasing. As for all infectious diseases, the most cost effective way to lower the risk of infectious disease is to keep the infection out of your herd. Knowing the health status of the herds you buy from, and that of the cows you are buying, or are bringing into your herd or commingling with your cows at shows or collection centres, is critical. You need to inquire before putting your herd at risk. Where disease status is uncertain or unknown, you should conduct follow-up testing on entry in a timely manner and as recommended by your herd veterinarian. Follow quarantine procedures to keep introduced or returning cows separate from other cows and protected from insects. This may only be feasible for younger cows until testing is complete.
Anaplasma can be present for some time without your knowledge. Before a clinical case occurs, extensive spread can occur if prevention strategies are not in place at all times. It makes sense to have controls in place to prevent the spread of all blood-borne cattle diseases, a list which should now include Anaplasma.
Follow these measures:
Reporting and keeping track
A.marginale in cattle is still a provincially reportable disease. Suspected cases and positive tests must be reported to the provincial government. Ontario vets and laboratories are required to report new cases as stated in the Ontario Animal Health Act.
Regularly review and update your biosecurity plan to ensure the practices you are following provide adequate risk reduction and protection from Anaplasmosis, as well as for other important infectious diseases.
This article originally appeared in the June 2014 Milk Producer Magazine.
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