Following is an article I put together for distribution by the KY Cattlemen's Assn. back in 2013; to my knowledge, it's all still relevant:
It's mid-August and bovine anaplasmosis season is upon us once again. Over the past week or so, diagnosticians at Murray State University-Breathitt Veterinary Center have fielded numerous telephone calls from veterinary practitioners in our service area regarding suspected cases of bovine anaplasmosis. Some producers have experienced mortality rates among mature animals as high as 20%, with deaths occurring over a 3-4 day period, and potentially more to come. MSU-BVC personnel have confirmed bovine anaplasmosis in sample submissions by cELISA testing and examination of blood and splenic impression smears on a number of submissions.
The causative agent of bovine anaplasmosis, Anaplasma marginale, is a rickettsial (bacterial) organism which is an obligate intracellular parasite, infecting the erythrocytes (red blood cells) of cattle. As the infected animal's immune system recognizes parasitized erythrocytes, they are removed from circulation by the spleen and are destroyed - often more rapidly than they can be replaced - causing the animals to rapidly become anemic and icteric (jaundiced).
Affected animals will typically be greater than 2 years of age, will be febrile (103-105F), may lag behind or isolate themselves from the herd, exhibit anorexia, lethargy, increased respiratory rate and decreased milk production. Animals will be anemic, and in latter stages of the disease, mucous membranes may be noticeably icteric (jaundiced). Constipation is common, and normally docile animals may exhibit aggression, due to cerebral hypoxia. Affected cows may abort, and bulls may be rendered infertile for 6 months or longer. Producers frequently find animals dead, without having noticed significant behavioral abnormalities beforehand. At necropsy, connective tissues are diffusely yellow, blood is thin and watery, and the spleen is typically enlarged and pulpy.
Onset of clinical signs typically ensues 3-6 weeks following infection. Once 1% of erythrocytes are infected, the animals enter the acute phase of the disease, with the level of parasitemia doubling daily. Clinical signs generally ensue once 50% of erythrocytes are infected. Affected animals rapidly become anemic due to destruction of parasitized red blood cells and may die very quickly following the first indications that they are sick. In-herd mortality rates may occasionally approach 60-80%.
Clinical disease is rare in cattle less than 6 months of age. Affected animals less than 2 years of age may be 'misdiagnosed' as having bovine respiratory disease, as they may present with fever and elevated respiratory rate. If these animals are anemic and/or icteric, one should consider the possibility of anaplasmosis.
Anaplasma marginale strains present in our area of the country are primarily tick-vectored; biting flies such as horseflies and horn flies may play a minor role in transmission, but livestock producers should also be aware that they can effectively transmit the organism from animal to animal, if they re-use needles/equipment between animals when doing routine herd work such as vaccination, dehorning, etc. Trials performed at Kansas State University demonstrated animal-to-animal transmission in as many as 60% of cattle injected with a needle which had been previously inserted into a known A.marginale –infected animal.
We frequently hear claims from producers and veterinarians that whitetail deer are the source of infection for cattle herds, but long-term monitoring of the Southeastern deer herd, and specific research trials conducted by the Southeastern Cooperative Wildlife Disease Study at University of Georgia have shown that whitetail deer are not an appropriate host for maintenance of A.maginale, and thus are unlikely to be involved in dissemination or transmission of this disease, other than possibly acting as a vector for carrying infected ticks from one premise to another.
The Anaplasma organism changes two of its major surface proteins every six weeks, so infected animals' immune systems are constantly 're-exposed' to a 'new' strain of the organism on a continuing basis – but are unable to effectively 'clear' the infection. Adult animals which have become infected with A.marginale and survive or recover from clinical disease, as well as calves which may have been infected in utero or during their first year, are unlikely to develop subsequent clinical disease, but must be considered to be persistently-infected with the organism and may continually serve as a potential source of infection for naïve animals in the herd.
Laboratory diagnostic techniques have improved in recent years; the complement fixation(CF) card test used in the past had very low sensitivity(20%), with high incidence of false negatives – which likely fostered dissemination of infected animals. The competitive ELISA (cELISA) serologic test for anaplasmosis is highly specific and sensitive, but may not detect animals during the incubation phase 3-6 weeks following infection. Real-time PCR tests may be positive as early as 3 weeks post-infection. Microscopic examination of blood smears is generally unfruitful until parasitemia exceeds the 1% infection rate- which is approximately the time that clinical disease ensues. Recovered, persistently-infected animals will remain seropositive, but organisms may be present in numbers too few to demonstrate on microscopic examination of stained blood smears, but may be detected with PCR.
As most veterinarians are aware, treatment of clinically-affected animals must be undertaken with care, as some of these animals may be extremely anemic and may die during the process and stress of handling and treatment.
Oxytetracycline (OTC) has historically been used as a treatment for anaplasmosis, although no oxytetracycline products carry a USDA label claim for that use, which places it in an Extra-Label Drug Use category. OTC – even multiple doses - will not clear A.marginale infection. As OTC is a bacteriostatic antimicrobial agent, it merely slows replication of the organism, allowing the infected animal to (hopefully) mount an immune response and accelerate production of new red blood cells. Whole-herd treatment with OTC may result in varied success, as animals treated early in the incubation period may merely have the onset of clinical disease delayed as a result of treatment. Deaths may continue to occur several weeks following OTC treatment in animals which were treated during the incubation phase of the disease.
Feeding Chlortetracycline (CTC), at a level of 0.5mg/lb/day throughout the vector season, has been shown to control active disease due to Anaplasma marginale; it will not, however, prevent infection. Unfortunately, most CTC-medicated mineral mixes will not reliably supply this level of drug as fed, and, as animals may not consume mineral on a daily basis, CTC-medicated mineral cannot generally be relied upon to provide effective control of clinical anaplasmosis. Feeding CTC at levels of 2mg/lb/day for 60 days may clear most infections – but possibly not all. Animals cleared by feeding high-level CTC will test seronegative within 4-6 months – but 'cleared' animals will once again be susceptible to infection and development of clinical disease.
A proven, commercially-produced, inactivated Anaplasma marginale vaccine was approved in 2013 for sale and use in Kentucky cattle by Dr. Bob Stout, KY State Veterinarian. The vaccine does not prevent A.marginale infection, but does appear to prevent clinical disease in properly vaccinated animals. Reports from producers who are currently using this vaccine have been favorable.
Please contact your veterinarian for further guidance and assistance in planning and instituting an anaplasmosis control and treatment program. You can be certain that diagnosticians at MSU-BVC and UK-VDL are ready and willing to assist you and your veterinarian in diagnosis of this common cattle disease.
