Anaplasmosis
by Nolan R. Hartwig, DVM & Douglas T. Ensley, DVM, Department of Veterinary Diagnostic & Production Animal Medicine, Iowa State University
Anaplasmosis is an infectious, vector-borne disease, primarily of adult cattle, caused by a protozoan parasite, Anaplasma marginale. The organism invades and destroys red blood cells, causing anemia, weakness and sometimes death. Infected cattle that recover become carriers and serve as a reservoir of the disease. Anaplasmosis is a major problem in many southern states, but is frequently diagnosed in the Midwest.
Symptoms
Anaplasmosis is commonly diagnosed when a seriously ill or dead adult animal is discovered, usually from midsummer to killing frost. Affected individuals are anemic, weak, gaunt, off-feed and lose weight rapidly. Jaundice, or yellowing of body tissues, is common, but only observed when the eyes, mouth and other membranes are examined. Difficult respiration, fever of 105° F or higher, dehydration and constipation are common. Milk production of lactating cows declines dramatically. Affected animals are often excitable or belligerent. Death may occur within 24 to 48 hours of the onset of symptoms and may be the first indication that anything is wrong in a herd. Abortion after recovery from clinical disease is common.
In an infected herd, recovery from undiagnosed infection and development of a carrier state is more common than death. Many carriers were never known to have been sick. Infected carriers may be more susceptible to other conditions such as gastro-intestinal parasitism, severe weather stress or malnutrition.
Although they may become infected and serve as a reservoir for infection of herd mates, animals younger than 1 1/2 to 2 years of age seldom show symptoms. They can regenerate red blood cells rapidly; replacing those destroyed by the infection. Younger animals may be off feed and moderately anemic, but usually recover quickly.
Anaplasmosis is not a major disease of feedlot cattle. It occasionally affects confined dairy cows, but is much more common in pastured beef cattle.
Cause and Transmission
Anaplasmosis is caused by a small organism, Anaplasma marginale. After exposure, it invades red blood cells and eventually causes them to break up. Many animals develop immunity and transmission of the organism between red blood cells stops before disease is evident. In others, the number of red blood cells destroyed is so large that anemia and death result.
Anaplasmosis is spread from infected to susceptible cattle by ticks and biting insects or through careless use of instruments such as dehorning, castrating, tattooing, or ear-tagging equipment. Biting insects spread the disease by transmitting infected blood from infected to susceptible animals. Insects such as biting (tabanid) flies must feed on an infected animal and then a susceptible animal within a few minutes in order to transmit the organism.
Several species of ticks are important vectors of anaplasmosis. After a tick feeds on an infected animals, the infectious organism localizes and multiplies in the cells lining the gut of the tick. The organism can then be transmitted to succeeding generations of ticks. Thus, the infectious organism can survive outside of infected cattle for long periods of time. This makes eradication of the disease difficult. When future generations of infected ticks feed on susceptible cattle, the disease is transmitted. Tick control is an important aspect of anaplasmosis control. Fortunately, the ticks that transmit anaplasmosis do not survive well in northern states. Total eradication of anaplasmosis from cattle in a herd or area can be accomplished without fear of re-infection in tick-free areas.
Large biting flies, horn flies, and other insects can be mechanical vectors of the disease. Face flies and other non-biting insects do not transmit anaplasmosis. Surgical instruments such as those used for castration, can easily transfer infection through a herd. Disinfection of equipment between animals will prevent transmission.
Deer can become infected, but surveys show that prevalence is low. Researchers do not believe infected deer are an important reservoir of infection for cattle. In most cases, outbreaks of disease are due to purchase of infected, but inapparent carrier cattle. The disease may not manifest itself for several years, depending on vector transmission by biting flies, other insects, or accidental transmission by man. Transmission from nearby infected herds to susceptible cattle by biting flies or other vectors is common.
Economic Effects on Cattle Herds
Losses in breeding herds can be severe, although it is more common for only a few individuals to show symptoms. Mortality may be high, however, especially if there is massive exposure in a herd that has not been exposed to the disease. It is not unusual for blood tests to show that individuals in a herd are infected even though there has never been evidence of disease. Losses depend on degree of exposure, how the infection was introduced into the herd, and possible immunity from previous exposure. The cost of blood testing and treating all exposed cattle can be significant.
Diagnosis
Post mortem examination of cattle that die of anaplasmosis reveals icterus (jaundice) anemia, pale tissues, an enlarged spleen, swollen liver and an enlarged gall bladder. Hemoglobin in the urine is not found in cattle that die of anaplasmosis. Hemoglobinurina is more likely to reflect acute leptospirosis (uncommon), bacillary hemoglobinuria, and several other infectious and toxic conditions of cattle.
Blood tests are used to confirm a diagnosis of anaplasmosis and to detect carrier animals. These tests are normally run at a state veterinary diagnostic laboratory. The tests are based on detecting antibodies against the Anaplasma organism. Antibody levels decline after infection is eliminated, so the success of herd treatment and eradication programs can be monitored.
Prevention and Control
Prevention and control methods should be based on geographic location, exposure to possibly infected cattle, purebred versus commercial herds, and management goals. Complete eradication from herds in northern climates where cattle ticks are not prevalent should be the goal of cattle producers. If this is not done, carriers will remain in the herd and serve as sources of infection that can cause losses at any time in the future. Cooperation of neighbors with infected herds may be necessary, as transmission between herds on nearby pastures readily occurs. A specific plan for control and eradication of anaplasmosis should be worked out with the herd veterinarian.
Anaplasmosis vaccines are available, but have some disadvantages. In southern states where exposure to infected ticks cannot be avoided, vaccination of exposed, susceptible, or infected herds is often advisable. In northern states, this is usually not recommended since aggressive testing and treatment with antibiotics can eliminate the disease. It is important to remember that anaplasmosis vaccines do not eliminate the infection from a herd or from individual animals. They only prevent the development of clinical disease. Vaccinated cattle that are carriers of anaplasmosis will remain carriers, even though they will not develop the disease themselves. Thus, they may be a source of infection for herd mates that are not vaccinated.
The first anaplasmosis vaccines caused a problem with nursing calves called neonatal isoerythrolysis. The vaccine contained red blood cell components that caused cows to develop immunity to some red blood cell components. Antibodies in the cow’s colostrum attack red blood cells of newborn calves that inherited these components. The red cells are quickly broken up or lysed, resulting in an acute hemolytic episode that often resulted in death. Herds commonly reported a one or two percent death loss. Refined manufacturing techniques and carefully timed vaccination have largely eliminated this problem.
Infected herds should be isolated if possible. This prevents spread to other herds and subsequent re-infection of the original herd once disease is eliminated. All replacement cows and bulls should be tested before addition to the herd.
In northern states, infected, but unidentified carriers can be identified by a blood test of all cattle in the herd. It is best to do this two or three months after the first killing frost. Animals that react to the anaplasmosis test are probably infected carriers and should be isolated and treated with tetracyclines. Testing and treating in late fall or early winter precludes possible further spread by insect vectors. In some situations, it may be easier to treat all cattle in infected herds after a killing frost, rather than just those identified as carriers.
The carrier state can be eliminated by one of two methods.
1. Put all cattle on a dosage of 5 mg oxytetracycline or chlortetracycline per pound of body weight daily for 30 days. At this dosage rate, a 1,200 pound cow needs 6 grams daily of the drug. This will eliminate carriers, but it is important that bunk space is adequate, so that all cattle get the required daily dosage. This dose must be recommended by a veterinarian.
2. Inject all cattle with 9 mg per pound body weight of long-acting oxytetracycline. This treatment must be repeated every third day for four treatments. Some veterinarians feel that three treatments are adequate, but published research trials suggest that four treatments should be used. This procedure is often used in smaller herds.
In infected herds, anaplasmosis can be controlled in exposed cattle during the vector season by feeding tetracyclines. Add the drug to a small amount of grain or incorporate into a salt/mineral mixture. Intake should be approximately 0.1 mg. per pound body weight per day. Intake must be carefully monitored.
It is not necessary to provide tetracycline in the salt/mineral mixture of all cattle in northern states. The relatively infrequent occurrence of anaplasmosis and the ability to eliminate the disease by treatment during the winter months precludes routine feeding during the grazing season for most producers. However, if the disease has occurred in the herd or if neighboring cattle are known to be infected preventative treatment may be advisable.
Several states require that cattle be tested for anaplasmosis before importation is allowed. Infected herds are not eligible to export cattle to those states. A strict interpretation of disease control regulations for both interstate and intrastate movement of cattle precludes sale of cattle from herds that are infected with anaplasmosis.