Anaplasma marginale is a bacterial pathogen responsible for Bovine Anaplasmosis, a significant disease primarily affecting cattle. This condition impacts livestock health globally and presents a substantial economic burden on agricultural economies. The disease is prevalent in tropical and subtropical regions worldwide, and in the United States, it is endemic in southern, midwestern, and western states.
The Pathogen and Its Host
Anaplasma marginale is a rickettsial bacterium. This microorganism is an obligate intracellular parasite. It infects and multiplies within cattle’s red blood cells.
Other domestic and wild ruminants can also become infected. The infection leads to the destruction of red blood cells by the animal’s immune system, causing progressive anemia. While calves under one year old typically show no or mild clinical signs, they can become lifelong carriers, serving as a reservoir for the bacteria.
Transmission Pathways
Anaplasma marginale spreads between animals through various mechanisms, primarily via biological vectors such as ticks. Several tick species transmit the bacteria. Ticks acquire the bacteria when feeding on infected animals and transmit it to new hosts.
Mechanical transmission also plays a significant role. Biting flies can transfer infected blood from one animal to another on their mouthparts. Additionally, human-mediated activities contribute to mechanical transmission, particularly through the reuse of contaminated equipment like needles, dehorning instruments, and castration tools. Strict biosecurity, including cleaning veterinary instruments between uses, limits this spread.
Recognizing the Disease
Bovine Anaplasmosis presents with a range of clinical signs, with severity often increasing with the age of the infected animal. Calves under one year old generally exhibit no or mild symptoms, while older cattle, especially those over two years, can experience severe and often fatal disease. A defining symptom is progressive anemia, visible as pale mucous membranes in the eyes and gums, due to the destruction of red blood cells.
Other common signs include fever, lethargy, and weight loss. Infected cows may also show a decrease in milk production and, if pregnant, can experience abortions. In severe cases, sudden death can occur. The disease can manifest in acute, chronic, or carrier states.
Managing and Preventing Anaplasmosis
Diagnosing Anaplasmosis involves several methods to detect Anaplasma marginale. Microscopic examination of Giemsa-stained blood smears can identify the organism within red blood cells, particularly during the acute phase of infection. For detecting carrier animals or in later stages, serological tests like the competitive Enzyme-Linked Immunosorbent Assay (cELISA) are highly sensitive, identifying antibodies against the pathogen. Polymerase Chain Reaction (PCR) tests, which detect the bacterium’s DNA, are also used and can identify low-level infections.
Treatment primarily relies on tetracycline antibiotics, such as oxytetracycline and chlortetracycline, which are effective against the bacterial infection. Early intervention with these antimicrobials is important for improving outcomes. While treatment can manage acute disease, it may not eliminate the carrier state, meaning animals can remain infected and serve as a source for transmission.
Prevention strategies focus on reducing exposure and managing the pathogen within herds. Vector control is important, involving measures to reduce tick and biting fly populations through pasture management and insecticide use. Vaccination, where available and appropriate, can help reduce the severity of clinical disease, though current vaccines may not prevent persistent infection. Strategic use of antibiotics, such as feeding chlortetracycline during high-risk seasons, can help control anaplasmosis, often requiring veterinary approval. Herd management practices, including testing for infected animals and implementing strict biosecurity protocols like using clean needles and sterilizing veterinary instruments, are essential for limiting the spread of Anaplasma marginale.