Cattle Country: University of Manitoba entomologists are investigating bovine anaplasmosis transmission in cattle

The following article was written by Mabel Currie and Kateryn Rochon from the Department of Entomology for the National Centre for Livestock and the Environment at the University of Manitoba (NCLE). It was originally published in Cattle Country in March 2025.

Horse flies and deer flies, commonly known as bulldogs, belong to a family of biting flies named Tabanidae. There are thousands of species worldwide, with about 150 in Canada, and approximately 25 different species found in Manitoba. Many Manitobans have fallen victim to a bite from these persistent flies, especially after a swim during the summer months. Typically, horse flies emerge during early June, with the level of activity for different species changing over the summer. Numbers decline into mid to late August. Horse flies have large compound eyes, and paired with their strong flight muscles, they are able to spot and pursue potential host animals from great distances. Their fine-tuned skills as hunters, however, are to the chagrin of many a-farmer when their cattle become the targets. Their painful bites cause distress to the animals, resulting in negative effects including reduced weight gain and milk production in addition to blood loss. Horse fly bites can also transmit some blood-borne pathogens such as bovine anaplasmosis. In Manitoba, there have been outbreaks of bovine anaplasmosis since 2013, especially in the southeastern areas of the province.

Effects of anaplasmosis on cow health

Bovine anaplasmosis is a production-limiting disease caused by Anaplasma marginale, a bacteria that attacks red blood cells. Symptoms in infected cows vary from showing no signs of disease to anemia, fatigue, and reduced feed intake, leading to poor overall cow health and significant losses to farmers. Depending on the herd, up to 25% of infected animals can die. Currently, no cure is available for the infected animals and, perhaps more importantly, animals that do recover remain infected for life, serving as a source of bacteria that can be transmitted to healthy animals in the herd. Therefore, the presence of this pathogen in Manitoba is concerning.

Transmission of anaplasmosis between cattle

Bovine anaplasmosis is a blood-borne disease spread by contaminated tools, ticks, and biting flies. Ticks are ‘biological’ vectors, meaning that when they feed on an infected cow, the bacteria will multiply and remain within the tick for a long time. The tick can then transmit the bacteria to any animal it feeds on. Horse flies, on the other hand, are referred to as ‘mechanical’ vectors. The anaplasmosis bacteria does not grow and persist within their body, but instead lingers on their mouthparts after a blood meal, much like it would on a contaminated needle or sharp tool. When a horse fly feeds on an infected animal, contaminated blood remains on its mouthparts for a short time. Because the bite of horse flies is painful, cows react strongly to the bites and those defensive behaviours can dislodge the flies that have started to feed. But the flies are persistent, so they will go land on another animal to continue to feed. If the fly then bites an uninfected animal, the residual blood remaining on its mouth can transfer the disease.

Horse flies: the feeding mechanism.

There are common misconceptions about how horse flies feed. Despite popular belief, they do not ‘take a chunk’ of flesh, but rather cut into the skin like we cut into a steak. Unlike mosquitoes, which feed directly from blood vessels in the skin, horse flies slash the skin and blood vessels with serrated mouthparts, and then feed from the blood pooling to the surface using a large sponge-like structure similar to that of a house fly. Only the female horse flies feed on blood as it serves as a source of protein to make eggs. Horse flies get their energy from sugars from flower nectar, which is the only thing males feed on.

UM research explores horse flies and bovine anaplasmosis transmission

During the summer of 2024, Mabel Currie and a team led by Kateryn Rochon from the Department of Entomology at the University of Manitoba set up fly traps at two locations in southeastern Manitoba where they collected over 1300 flies! The traps were inverted vinyl cones with collection containers on top, and a black yoga ball suspended in the middle. The ball acted as a bait for female horse flies, as the colour and gleam tricks them into thinking they are animals. After the flies were collected, they were taken back to the lab to be frozen, until they were ready to be identified. Using a microscope to look at each individual’s features, the species were were identified, which allowed the researchers to determine their presence at different farms. After identification, each fly was dissected so their gut could be tested for the bacteria that causes bovine anaplasmosis. By examining the flies’ ovaries, the researchers also determined how many batches of eggs each female fly has laid. As each batch of eggs requires a blood meal, the researchers can see which species bite more and are more likely to spread diseases among cattle, and at what point in the season.

Understanding the role of ticks and horse flies play in the emergence of bovine anaplasmosis is part of a larger project funded by the Beef Cattle Research Council involving researchers in Manitoba, Saskatchewan, and Alberta. One objective is to determine the prevalence of bovine anaplasmosis in cattle operations by testing the blood of cattle in Manitoba, Alberta, and British Columbia. This is where the Manitoba team comes in, collecting ticks and flies on the farms and looking for relationships between tick and fly abundance, potentially infected vectors, as well as detecting anaplasmosis. The other exciting objective of this research is to develop a new rapid test for bovine anaplasmosis, and validate it with the blood collected from cows in the study.

We are looking for more farms to join the program, especially in Manitoba’s central and western regions. For more information about how you can get involved, please contact Dr. Kateryn Rochon (project researcher, kateryn.rochon@umanitoba.ca) or Dr. Shaun Dergousoff (project lead, shaun.dergousoff@agr.gc.ca).