Anaplasmosis in cattle is an infectious disease caused by a bacteria known as Anaplasma marginale. The disease is common in Missouri. Clinical cases can occur at any time of the year, but the majority of cases are seen in late summer and fall. Older estimates of the economic impact of this disease on the U.S. cattle industry exceeded $300 million a year; however, a current estimate is not available. Losses are due to reduced production, treatment costs, death loss and abortion.
Anaplasma marginale causes disease by infecting red blood cells. The immune system recognizes the infected cells as abnormal and removes them, thereby creating anemia. The severity of disease is related to the number of infected red blood cells and subsequent red blood cell loss. In mild cases, animals may exhibit symptoms such as elevated temperature, depression and pale mucous membranes. In severe cases, animals may be jaundiced, the most obvious symptom of which is a yellow tinge to the mucous membranes around the eyes; exhibit severe depression or nervousness; and eventually die. In herds that are not closely monitored, the first sign of Anaplasmosis may be dead animals.
Adult animals are more significantly affected by Anaplasmosis than younger animals. Younger animals are more tolerant of infection, and their symptoms are often subtle or may go unnoticed altogether. It is believed that young animals are more tolerant because they regenerate red blood cells faster than adults.
Transmission
Transmission of Anaplasma marginale from infected to uninfected animals occurs in one of three ways: mechanically, biologically or transplacentally.
Mechanical transmission
In mechanical transmission, the organism is transferred by blood-contaminated mouthparts of biting flies or by blood-contaminated equipment. Horse flies are capable of transmitting the organism and may remain mechanically infective for up to two hours after feeding on an infected animal. Blood-contaminated equipment, such as needles, tagging or tattoo pliers, and dehorning equipment can also transfer A. marginale from an infected animal to uninfected animals.
Biological transmission
In the U.S., biological transmission occurs primarily through Dermacentor tick species, with the most common species in Missouri being the American dog tick. Ticks are considered the most efficient vector for transmission of A. marginale, although not all strains can infect ticks. Once a tick acquires the organism through a blood meal, A. marginale infects the tick's gut cells and completes part of its life cycle. Over time, other tissues within the tick, including the salivary glands, become infected. Unlike mechanical transmission, in which only infected red blood cells are transferred from one animal to another, A. marginale multiplies within the tick. Therefore, infected ticks transmit large numbers of organisms in their saliva each time they feed. Because ticks remain infected and some species feed intermittently on multiple animals, they are highly efficient at transmitting A. marginale.
Transplacental transmission
Transplacental transmission occurs when the organism is transmitted from dam to fetus. This transmission appears to occur during the second or third trimester of pregnancy. In one study, 10 percent of calves born to infected cows were infected at birth; in another, 16 percent were born infected.
Disease phases
Incubation
The incubation period of a disease is the time from pathogen exposure to the appearance of symptoms. The incubation period for Anaplasmosis ranges from seven to 60 days with an average of 28 days. The length of this period is determined in part by the initial infectious dose, with higher doses leading to shorter incubation periods.
Clinical
The clinical phase begins when about 1 percent of an animal's red blood cells have been infected. The percentage of cells infected doubles every day once the 1 percent threshold has been reached. Clinical signs begin to appear when 15 percent of the red blood cells are infected. The disease progresses rapidly until either the immune system responds or treatment occurs, or both. Clinical signs are related to the number of infected red blood cells, and animals may exhibit a combination of the following symptoms: fever, depression, poor appetite, constipation, jaundice, nervousness, abortion in pregnant animals, and death.
Recovery
The recovery phase begins with the appearance of immature red blood cells. This phase can last for a few weeks or several months. Some animals do not fully recover and are culled because of poor productivity.
Carrier
In the carrier phase, animals that have recovered from Anaplasmosis become persistent carriers of the organism. The benefit of the carrier state is that the animal has life-long immunity and rarely shows clinical disease again, but it does serve as a reservoir of the organism within the herd. Because of the latter, elimination of carrier animals through either culling or chemosterilization has been attempted.
At one time, it was believed chemosterilization could reliably be achieved with repeated administration of injectable oxytetracycline, but newer studies show this method is inconsistent. Long-term feeding of chlortetracycline has also been evaluated, but consistent elimination of the carrier state has only been demonstrated when fed at unapproved concentrations.
Whether through culling or chemosterilization, creating an Anaplasmosis-free herd in a region where the disease is common could have negative consequences. At some point, the organism would likely be reintroduced in the herd and cause a more severe disease event because all the animals are susceptible. Therefore, the decision to eliminate carriers should be made with input from the herd veterinarian.
Treatment
Long-acting injectable oxytetracycline is the antimicrobial most often used for treating Anaplasmosis. Oxytetracycline is bacteriostatic, meaning it stops development of the organism but does not necessarily kill it. Therefore, the animal's immune system must be capable of responding to a disease challenge, and its body must be able to produce enough red blood cells to recover. Administering the antibiotic early in the disease process increases the odds that an animal will recover.
Control
The strategies commonly employed to control Anaplasmosis are minimizing transmission, use of feed antimicrobials, and vaccination.
Minimizing transmission
To minimize transmission, control of arthropod vectors such as ticks and biting flies is recommended when feasible. Several sprays, pour-ons and fly tag products are approved for fly and tick control, but control of horse flies can be more difficult and may require the use of alternative methods.
Procedures to prevent transmission through equipment must also be in place. Procedures to consider include changing vaccination needles and cleaning or disinfecting blood-contaminated equipment between animals.
Finally, eliminating carrier animals from the herd and testing new animals may be warranted in some cases. The benefits versus risk of an Anaplasmosis-free herd in a region where the disease is common, along with the costs associated with testing and eliminating carrier animals, would need to be considered.
Feeding antimicrobials
Chlortetracycline (CTC) is the only antimicrobial approved for control of Anaplasmosis. Typically, it is fed during the arthropod vector season, which in Missouri can range from March to November, depending on spring and fall temperatures. Some veterinarians have determined that feeding year-round is necessary because of risk factors associated with particular herds or locations.
The Food and Drug Administration (FDA) has approved CTC for the control of Anaplasmosis in cattle as follows:
- Beef cattle over 700 pounds
0.5 mg per pound of body weight daily - Beef cattle under 700 pounds
350 mg daily - Beef cattle and nonlactating dairy cattle
0.5–2 mg per pound of body weight daily
The first and second approvals are for medicated feeds that are to be hand-fed daily. Most of the commercial trace mineral supplements sold for control of Anaplasmosis use the first approval. The third approval is reserved for products approved by the FDA to be fed free-choice for control of Anaplasmosis. Only a few products have this approval. It is unlawful to use medicated feeds in an unapproved manner, so be sure to read and follow the feeding directions on the label.
Even when CTC is fed to control Anaplasmosis, clinical cases may still occur. Possible reasons for this include:
- Intake of medicated feedstuffs can vary due to individual animal differences, feedstuff quality and/or environmental influences. Therefore, some animals may not receive sufficient amounts of CTC to control the disease.
- Some strains of A. marginale are less susceptible to tetracycline than other strains. The presence of a less-susceptible strain combined with low intake of medicated feed would increase the risk of a disease event.
- Continuous feeding of CTC could inadvertently chemosterilize a percentage of the herd, thus increasing the number of susceptible animals. How often this occurs is unknown.
Vaccination
The only anaplasmosis vaccine currently available in the United States is produced by University Products LLC. It is classified as an experimental vaccine and potency, and efficacy have not been established. In Missouri, the vaccine is only available through veterinarians.