September 11, 2023
By undertaking a research project based on the One Health approach, which recognizes the interdepency between the health of humans, domesticated and wild animals, and the environment, scientists are working together to develop new knowledge in the Canadian context.
Human granulocytic anaplasmosis (HGA) is the second most common tick-borne disease in North America after Lyme disease. The vector for this disease is once again the black-legged tick, Ixodes scapularis, which spreads the bacteria from infected animals to humans.
HGA is caused by a bacteria called Anaplasma phagocytophilum, which infects blood granulocytes—hence its name. Granulocytes are white blood cells that play a key role in fighting infection. The disease’s zoonotic variant (Ap-ha) is primarily responsible for causing disease in humans and domesticated animals, particularly horses, dogs and cats. In nature, the Ap-ha variant mainly originates in wild rodents.
The rapid rise in reported anaplasmosis cases in Canada can be explained by the expanding geographical distribution of the Ixodes scapularis tick, increased environmental spread (between ticks and rodents), and reinforced vigilance among the public and health care professionals.
Timeline of the largest anaplasmosis outbreak in Canada
In April 2021, the Faculty of Veterinary Medicine at the University of Montreal began detecting anaplasmosis in horses and dogs. What most of the cases had in common was a history of travel to or residence in Bromont, Quebec.
In May and June 2021, the National Microbiology Laboratory (NML) tested blood samples from the animals and confirmed they had been infected with the Ap-ha variant. To confirm the theory that these animals had been exposed to the bacteria in their environment, the NML then began testing ticks collected in Bromont from 2019 to 2021 as part of a Lyme disease research project. The findings showed a rise in the proportion of ticks infected with the bacteria that causes anaplasmosis: 1.1% in 2019, 6.8% in 2020 and 9.5% in 2021. Over that three-year period, 69% of those ticks carried the Ap-ha variant.
In July 2021, human cases of HGA began to be reported. In total, 47 human cases were identified in Quebec in 2021, of which 35 occurred in the Estrie region. In approximately 88% of the cases, tick exposure occurred around the home. This is the largest number of anaplasmosis cases ever reported in a single region in Canada. In 2022, 29 cases of anaplasmosis were reported in Quebec, of which 20 were acquired in the province—12 in Estrie, 5 in Montérégie, 1 in Bas-Saint-Laurent, 1 in Montreal and 1 whose place of acquisition is unknown.
Capturing rodents to better understand the phenomenon
To study the emergence of anaplasmosis, a multidisciplinary team specializing in ecology, veterinary medicine, epidemiology and public health was mobilized in summer 2022 to conduct field research.
The group set out to capture wild rodents in order to identify species that could be considered competent reservoir hosts, in other words, those most likely to carry the Ap-ha variant and spread it to ticks.
Rodents captured in baited traps were sedated in accordance with the ethical standards set out in the study protocol. Once ticks were collected (small animals can sometimes carry dozens) and a blood sample taken, the animals were released back into nature.
In 2022, 341 rodents were captured, 106 blood samples were tested and 1,095 ticks were analyzed by the NML. Twenty-two rodents from three different species tested positive for the Ap-ha variant.
The study, which is continuing in summer 2023, is being carried out in eight peridomestic woodland sites in the Bromont area.
Full findings will not be available for several more months, but greater knowledge of this emerging disease will undoubtedly inform targeted responses to reduce the risk of spread (e.g., deploying bait stations with acaricides to treat rodents for ticks). Stay tuned, there’s more to come!