Anaplasmosis is minute parasite infectious disease that causes destruction of red blood cells. It can be transmitted to healthy animals from infected animals, by biting insects, or surgical instruments. Anaplasmosis (Anaplasma marginale) can be divided into incubation, developmental, convalescent, and carrier stages. These stages are described below.
Incubation stage– Once entering an animal, the anaplasma parasite slowly reproduces in the animal’s blood during this stage. The incubation stage can range from 3 to 8 weeks with wide variations documented because the variation is directly related to the number of parasites introduced into the animal. During incubation stage, the animal looks healthy and shows no sign of being infected. However, after the parasite has reproduced many times and is established in the red blood cells of the animal, the animal’s body will attempt to destroy the parasite.
Developmental stage– Clinical signs usually begin to appear about half-way through this stage. The developmental stage normally lasts about 4 to 9 days. The infected animal’s body destroys the parasite, as well as the red blood cells. The animal will show signs of anemia when substantial loss of red blood cells has occurred. Also, body temperature will commonly rise to 104°—107° F, plus, lactating cows will have a rapid decrease in milk production. The anemic anaplasmosis infected animal is first noticed when it becomes weak and lags behind the rest of the herd. It may refuse to eat, skin becomes pale around eyes and on muzzle, may show constipation, weight loss, excitement, and yellow tinged skin. Once the animal is down, it may be unable to rise. The affected cattle will either die or a recovery will start after 1 to 4 days of the signs of the disease. Unless the infected cattle are detected in the early developmental stage, as a general rule, they should not be treated. The reason for this is antibiotic treatment does little or nothing to affect the outcome when given in the late development or convalescent stage. Second, if the animal is forced to move, or becomes excited, it may die of anoxia (lack of oxygen).
Convalescent stage– Death loss normally occur during the late developmental stage and the convalescent stage. Cattle that do survive lose weight, abort calves, and recover slowly over a 2 to 3-month period. The difference from the developmental stage to the convalescent stage is an increase in red blood cell production in the peripheral blood, an increase in hemoglobin levels and high total white blood cell counts. This stage usually lasts until normal blood values return. All ages of cattle can become infected with anaplasmosis. However, the severity of illness increases with age. Young calves under 6 months of age seldom show signs that indicate they are infected. Cattle up to 3 years of age will become increasingly ill, and more deaths will occur with advancing age. Up to 50% of older cattle past 3 years of age with clinical anaplasmosis can die if not treated.
Carrier stage– Cattle that do recover from the disease remain carriers of anaplasmosis for the rest of their lives, if not adequately treated. Carriers will rarely become ill with anaplasmosis the second time. The animal will not show any clinical signs that are associated with the persistent low-level infection. However, the blood from the carriers will cause anaplasmosis in susceptible cattle if introduced into them. Carriers that are not identified are the most likely source of future infection outbreaks in the herd.
Anaplasmosis is spread by the transfer of blood from an infected animal to a susceptible animal. This mechanism is usually done by biting insects transmitting the A. marginale infected blood with their mouth parts, or by contaminated instruments used in working cattle. Horse flies, stable flies, and mosquitoes are known to transmit the disease by carrying the infected red blood cells from diseased cattle to susceptible cattle. Anaplasmosis is usually not transmitted if more than 5 minutes has lapsed between the time an insect bites a diseased animal and the time it bites a susceptible animal. The disease is more likely to be transmitted by insects when the cattle are gathered close together and making it easier for insects to bite several animals in a short period of time.
Anaplasmosis can be transmitted from one animal to another by the use of equipment that becomes contaminated while working the cattle. The way the disease progresses through the herd can indicate whether it is insects or instruments that are responsible for the disease outbreak. When infected instruments transmit the disease, a large number of cattle in the herd will show signs of anaplasmosis in 4 to 6 weeks after processing the cattle. This kind of outbreak appears suddenly, without any earlier clinical cases being observed. Instrument transmission can be easily prevented by careful handling of equipment between each animal. A quick rinse of instruments in disinfectant and changing needles will usually prevent transmission.
However, when insects are the vectors of the disease, a few cases usually occur first and then are followed 4 to 6 weeks later by another wave of the clinical disease. The first cases are usually caused by biting insects transmitting the disease from carriers in the herd to susceptible animals. The second wave of the disease is from the earlier sick animals. The sick animal is a prime target for eating by the blood-sucking insects. The clinically ill animal is usually down, very weak, and makes no attempt to fight off the biting insects. The blood from the clinically ill animal is 20 times more infectious than the blood from an anaplasmosis carrier.
Researchers have demonstrated that the Pacific Coast tick and the Rocky Mountain wood tick can transmit the disease. The A. marginale parasite can be passed through several developmental stages of the ticks and then transmitted to the susceptible animal. These insect vectors are known as biological vectors. Biological vectors may transmit the disease months or perhaps years after biting an infected animal.
How to interpret an anaplasmosis outbreak can yield information on necessary changes in management. Outbreaks that occur during the vector season indicate infections of susceptible cattle and acute outbreaks of anaplasmosis. Preventative measures such as vaccinations or antibiotic therapy should be implemented. If outbreaks occur during the winter months, it would not be due to recent infection of susceptible cattle, but to stress which can lead to the expression of the disease in infected cattle. Vaccinations work to prevent acute expression of the disease upon infection during initial exposure. However, it does not prevent the development or infection of a carrier animal. Antibiotic therapy may be used to control this type of outbreak. Nutrition and environmental stress are two areas that need to be managed in suspect herds.
Producers are usually not concerned with control until anaplasmosis becomes a problem. It is important the producer knows that there are methods available for controlling an anaplasmosis outbreak. There are federal regulations pertaining to the interstate movement of known anaplasmosis carrier animals. Cattle that are anaplasmosis carriers may be cured of the infection by treatment with certain tetracycline antibiotics. However, programs to eliminate the carrier state should be conducted after the vector season has ended.
Do to upcoming changes, the Food & Drug Administration has changed the regulation of medically important antibiotics used in food animals. The FDA intends to move all medical important antibiotics out of over-the-counter status to veterinary feed directive status. This gives a veterinary oversight to a broad range of products used in animal feed. What does this mean? A veterinary feed directive is a mechanism requiring a producer to get approval from a veterinarian for antibiotics used in animal feeds. A feed type antibiotic control program for control of anaplasmosis in the herd will need to be a veterinary oversight.