Molecular tools aid global anaplasmosis vaccine development

As ticks gain new ecological niches, diseases they carry are spreading.

June 5, 2020

3 Min Read
Washington State beef calf.jpg
Anaplasma marginale is transmitted through a tick’s salivary glands and into a cow when it feeds.Washington State University

More than a century ago, veterinarian Sir Arnold Theiler discovered the pathogen Anaplasma marginale — which causes anaplasmosis in cattle — in the gut of ticks.

Now, researchers at Washington State University’s College of Veterinary Medicine and the University of Pretoria in South Africa are using molecular tools to build on Theiler’s work and develop a vaccine against A. marginale.

“It took more than 100 years, but we are inching closer toward what has always been the goal: a safe, global vaccine that is effective against all strains of the pathogen,” said professor Kelly Brayton, a researcher in Washington State University’s veterinary microbiology and pathology department.

A. marginale is transmitted through the tick’s salivary glands and into the host when ticks feed.

The bacteria make their way into the bloodstream and infect red blood cells, causing anemia, fever, breathlessness, weight loss, lethargy, abortion and death. Bovine anaplasmosis, if left untreated, can lead to a 36% mortality rate in infected cattle, the university said in an announcement.

“This is one of the top three widespread livestock diseases in the world,” Brayton said. “In addition to sickness and death, this disease can spread quickly and result in major economic losses for beef producers.”

The goal for Brayton and the team is to produce a vaccine that can be used against the diverse strains of A. marginale in the U.S. and worldwide.

Currently, a blood-based vaccine developed by Theiler is still in use in South Africa, but it does not provide complete protection against A. marginale. It can’t be used in the U.S. because the vaccine can also transmit other blood-borne pathogens.

According to research in Ticks & Tick-borne Diseases, five of A. marginale’s outer proteins have been identified as potential targets for a global vaccine, the announcement said.

For several years the five proteins — Am779, Am854, omp7, omp8 and omp9 — have been identified as potential vaccine targets in the U.S. due to their location and function. What remained unknown is if the proteins’ vaccine candidacy would remain valid across the diverse strains of A. marginale in endemic regions like South Africa.

“If you’re trying to develop a vaccine, you want it to work everywhere. Based on our research, a vaccine targeting these five proteins should be just as effective in the U.S. as it is in South Africa,” Brayton said.

To conduct the research, Brayton and her colleagues at the University of Pretoria collected a wide variety of A. marginale strains from throughout South Africa. Once collected, researchers cloned the candidate genes from the South African strains and compared the potential target proteins with the versions for the U.S.

“The proteins weren’t particularly diverse, which is one reason why they are on our list to serve as good vaccine candidates,” Brayton said.

The research is just the latest publication coming out of the Washington State/University of Pretoria partnership Brayton and her colleagues formed in 2012.

A $625,000 grant recently awarded by the National Institutes of Health will now allow Washington State and the University of Pretoria to collect epidemiological data on tick-borne diseases in South Africa for the next four years. Brayton has partnered with Pretoria professors Nicola Collins and Marinda Oosthuizen on the project.

The team works with the Mnisi people, a community of subsistence farmers, to test ticks and blood from people, captured rodents, community dogs and livestock.

“Malaria in Africa has declined over the past 10-15 years, but the incidence of acute febrile illness has not. Our premise is that some of that acute febrile illness is due to tick-borne disease,” Brayton said. “There’s also this question: Are new disease agents emerging? This grant will also look for those pathogens.

“Ticks are spreading, and they are gaining new ecological niches in areas so the diseases that go with them are spreading and becoming more of a problem,” Brayton said. “This partnership and program will help us keep a pulse on tick-borne disease and detect new diseases early.”

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