By: Ron Gill, Tom Hairgrove and Tom Craig
Texas cattle producers have reported an increasing number of cases of bovine anaplasmosis, a disease that causes anemia, abortion, decreased production, and death. The disease appears to be most prevalent in the Edwards Plateau and Rolling Plains of West Texas.
Bovine anaplasmosis is caused by a bacterium, Anaplasma marginale, which infects red blood cells. The infection is spread from animal to animal via any mechanism that transmits these cells, such as contaminated needles, dehorners, ear taggers, and biting flies.
During mechanical transmission, the bacteria are transferred without increasing in numbers. For example, only the blood contained on the mouthparts of biting flies is infective. Mechanical transmission must occur within a short period before the bacteria die.
Biological transmission occurs via ticks. Once a tick consumes a blood meal infected with A.marginale, the bacteria continue to develop and multiply within it. The tick can remain infective for long periods.
Importance to Texas producers
According to a national survey, bovine anaplasmosis was the second most important disease threat to cattle in the 1970s. Today, some consider it a disease of bygone times that no longer threatens Texas producers.
However, more cases of clinical disease have been reported recently, possibly because of increased cattle movement. When infected and uninfected cattle are coming led, the unprotected cattle can become acutely ill.
Field research conducted by the Texas A&M AgriLife Extension Service indicates that the highest percentage of persistently infected animals in Texas is in the Edwards Plateau and Rolling Plains. More research is warranted, but the geographical area of highest persistent A. marginale infection coincides with the location of the winter tick (Dermacentor albipictus).
After infection, the animals may become very ill. Symptoms include anemia, jaundice, abortion, sudden weight loss, and often death. Anemic cattle are weak, have difficulty breathing, and behave more aggressively. In jaundiced animals, the eyes and gums are yellowish.
If the animals recover, they will be infected with the bacteria for life (persistently infected). However, the persistent infection will protect the infected animals from showing active disease.
The period from infection until the appearance of symptoms varies from 2 to 5 weeks, depending on the animal’s age:
- Under 1 year: Animals seldom show signs of disease but can become persistently infected.
- 1 to 2 years: Animals may show signs of disease, but they are usually mild.
- Older than 2 years: Animals often exhibit classical anemia, jaundice, abortion, and sudden weight loss. Often they die.
Disease prevention and management
No USDA-licensed vaccine is available to control bovine anaplasmosis. The source of infection is persistently infected animals. To reduce the spread of this disease, producers must man- age persistently infected cattle properly.
Test cattle for disease before commingling
Take precautions to reduce the risk of disease when introducing replacement cattle into an existing herd:
- If you live in an area where the disease is prevalent, be careful when introducing non-infected cattle.
- If the disease is not prevalent in your area, take precautions when introducing cattle from regions where the disease is more common.
Diagnostic tests can determine whether cattle are infected. To have cattle tested, send blood samples to the Texas A&M Veterinary Medicine Diagnostic Laboratory (http://tvmdl.tamu.edu/).
Use the test results when analyzing the risks of commingling infected and uninfected cattle. It can be just as disastrous to bring uninfected animals into a herd of diseased cattle as it is to introduce diseased animals into an uninfected herd.
Take precautions when handling cattle
Cattle are at greatest risk when A. marginale is spread to susceptible cattle during husbandry practices such as dehorning, ear tagging, and vaccinating. Many animals can be infected at once during these procedures. About 40 days later, many may die.
To limit disease transmission during routine husbandry practices:
- Remove the blood from dehorners, ear taggers, and surgical instruments and disinfect them between animals.
- Change needles between animals.
Control horse and deer flies in spring and summer
The disease is also spread mechanically by horse flies and deer flies, which are difficult to control. Because symptoms take 2 to 7 weeks to appear and biting flies usually transfer A. marginale in the spring and summer, disease appears most often in late summer and fall.
In East Texas, manage grazing
In East Texas, the disease is more likely to be spread by biting flies. If possible, use pastures with low fly pressure in late spring and summer, and rotate into the higher-pressure pastures in the winter.
Producers in East Texas should watch for symptoms of anaplasmosis in the summer.
In West Texas, control winter ticks from late fall to early spring
The disease is spread biologically in Texas by winter ticks. In the Rolling Plains and Edwards Plateau, these ticks are active from October through March. Tick control practices in October will reduce but not eliminate the spread of disease.
Producers in West Texas should be vigilant for symptoms of anaplasmosis from January through May.
Add tetracycline to supplements when flies are biting
Continuously feeding tetracycline incorporated in mineral or feed supplements will not prevent infections. However, this practice does help control disease by decreasing the number of bacteria in infected animals and reducing bacterial reproduction in newly infected animals.
Time the treatment to coincide with the period that the vectors (organisms that transmit disease-causing agents from infected to uninfected animals) are active:
- In East Texas, where the vector is biting flies, feed from May through July.
- In West Texas, where the vector is the winter tick, feed from October through March.
It is very difficult to clear persistently infected animals of A. marginale organisms using tetracycline. Although long-acting injectable oxytetracycline was previously thought to be effective, current studies show that it is unlikely to eliminate a persistent infection.
Clearing a persistent animal can be detrimental because it will be susceptible to becoming reinfected and suffering clinical disease.
Although this publication provides a general description of bovine anaplasmosis in Texas, you should consult with your large animal veterinarian to evaluate your risks and develop a strategy for health management in your herd.
For more information
The Tick App for Texas and the Southern Region. By Pete Teel and collaborators. http://tickapp.tamu.edu/.
“Bovine Anaplasmosis.” Manual of Diagnostic Tests and Vaccines for Terrestrial Animals 2008, 6th edition, pp 599–10. World Organization for Animal Health, Paris.
“The Natural History of Anaplasma marginale.” By K. M. Kocan, J. de la Fuente, E. F. Blouin, F. Coetzee, and S.A. Ewing, 2010. Veterinary Parasitology. 167. 95–107.
“Review of Bovine Anaplasmosis.” By P. Aubry and D. W. Geale. Transboundary and Emerging Diseases. 2011. 58: 1–30.
Veterinary Medicine: A Text of the Diseases of Cattle, Horses, Sheep, Pigs and Goats, 10th ed. By O. M. Radostits, C. C. Gay, K. W. Hinchcliff, and P.D. Constable. 2007. Elsevier Saunders, New York.
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