Anaplasma marginale is a bacterial pathogen that impacts livestock health worldwide. This obligate intracellular bacterium belongs to the order Rickettsiales and causes bovine anaplasmosis, a disease characterized by severe anemia and other clinical signs in cattle. Understanding where this organism exists is important for managing its spread and mitigating its economic effects on the livestock industry.
Global Presence
Anaplasma marginale is found in tropical and subtropical regions across the globe, primarily determined by the distribution of its tick vectors. Bovine anaplasmosis occurs in areas approximately between 40°N and 32°S latitude, including extensive parts of North and South America, southern Europe, Africa, Asia, and Australia.
Warmer climates in these areas support the life cycles of various tick species responsible for transmitting the pathogen. In the United States, detections in beef cattle are most common from late summer through late fall. Climate change, leading to rising temperatures and altered rainfall patterns, could potentially expand the geographic range of these tick populations, increasing the risk of Anaplasma marginale spreading into previously unaffected areas.
Within Host Animals
Anaplasma marginale primarily infects cattle, causing bovine anaplasmosis, but also resides in other domestic and wild ruminants. These animals serve as natural reservoirs, allowing the bacterium to persist in various environments. Susceptible hosts include water buffalo, bison, African antelopes, and certain deer species like white-tailed deer. The bacterium resides within the red blood cells (erythrocytes) of these infected animals.
Once inside red blood cells, Anaplasma marginale multiplies, leading to their destruction by the animal’s immune system. This destruction results in progressive anemia, a hallmark of anaplasmosis. The percentage of infected red blood cells can vary, sometimes exceeding 50% during severe stages of the disease. Animals that survive acute infection often become persistently infected carriers, showing no overt symptoms but serving as a continuous source of infection for new hosts.
Transmission Routes
The primary method by which Anaplasma marginale moves between host animals is through biological vectors, mainly various species of ticks. Ticks acquire the bacterium by feeding on an infected animal, and the pathogen undergoes a developmental cycle within the tick. When the tick subsequently feeds on a susceptible animal, it transmits the bacterium through its saliva. Different tick genera, including Dermacentor, Rhipicephalus, and Amblyomma, are known to transmit Anaplasma marginale.
While ticks are the main biological vectors, mechanical transmission also plays a role. This occurs when blood containing the bacterium is transferred from an infected animal to a susceptible one through contaminated instruments, such as needles, dehorners, or castration tools. Biting flies, including stable flies (Stomoxys calcitrans) and horse flies (Tabanus species), can also mechanically transmit the bacterium by transferring infected blood between animals during interrupted feeding. These mechanical routes contribute to the circulation of Anaplasma marginale in livestock populations, especially in areas with low tick activity or during routine veterinary procedures.