Study: When two parasites are better than one
by Linda Weiford, WSU News
An international team of scientists has quantified, for the first time, how co-infection significantly reduces the severity of the African cattle-killing disease East Coast fever. Because infections by multiple pathogens are common in nature, the findings can be applied to curbing other parasitic infectious diseases such as human malaria, the researchers conclude.
When calves are infected by two parasite species at the same time, one parasite renders the other far less deadly, said infectious disease epidemiologist Thumbi Mwangi of WSU’s Paul G. Allen School for Global Animal Health. Mwangi co-authored the study published in the journal Science Advances. He is based in Kenya, where the research was conducted.
“We now know that certain parasite co-infections can have strong protective effects—as strong as those offered by vaccines--against certain deadly diseases,” he said.
In east and central Africa, East Coast fever is spread when a tick bites a cow and injects the parasite Theileria parva (T. parva) through its saliva. The resulting disease attacks white blood cells, similar to a fast-moving lymphoma in humans, killing more than a million cattle each year.
In the study, researchers monitored 548 shorthorn zebu calves belonging to consenting smallholder farmers, from birth to one year. Before the end of the year, 17 percent of the calves died. Blood tests revealed that of the 454 that survived, 86 percent were infected by T. parva. And here’s the surprise -- only 18 percent showed symptoms of East Coast fever, while “the rest showed no clinical signs of ever being exposed to the disease,” said Mwangi.
Scientists discovered that the healthy calves were also infected by a related, although less harmful, parasite species: Theileria mutans or Theileria velifera. Calves protected against East Coast fever had contracted one or both of these parasites before being infected by T. parva, the authors say.
The well-known immunomodulatory effects of T. parva are moderated by the presence of T. mutans and T. velifera, thereby reducing the severity of infection, they state.
A vaccine against East Coast fever is made with irradiated T. parva packed into a syringe, but it is expensive and must be administered along with antibiotics. The new study suggests it might be possible to vaccinate calves with the benign T. mutans or T. velifera parasites instead, which would be safer, less costly and easier to administer because no antibiotics would be necessary.
To see the study, go to: http://advances.sciencemag.org/content/1/2/e1400026