When calves are infected by two parasite species at the same time, one parasite renders the other far less deadly, according to a new study published in the current journal of Science Advances.
The 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.
"We now know that certain parasite co-infections can have strong protective effects - as strong as those offered by vaccines - against certain deadly diseases," said infectious disease epidemiologist Thumbi Mwangi of Washington State University's Paul G. Allen School for Global Animal Health. Mwangi is a co-author based in Kenya, where the study was conducted.
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.
"A working hypothesis is that 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 write.
The vaccine that exists against East Coast fever is effective but has drawbacks. Made with irradiated T. parva packed into a shot, it is expensive and must be administered along with a syringe-full of antibiotics. Using knowledge from this new study, it might be possible to vaccinate calves with the benign T. mutans or T. velifera parasites instead, the scientists report, which would be safer, less costly and easier to administer because no antibiotics would be necessary.
The findings that a mild parasitic infection can protect calves from a more potent parasitic foe "may be an important determinant of the burden and distribution of many parasitic diseases in many host populations, including humans," they state.
Story source: Washington State University news release via EurekAlert, 23 Mar 2015
Journal Reference: M. E. J. Woolhouse et al. Co-infections determine patterns of mortality in a population exposed to parasite infection. Science Advances, 2015; 1 (2): e1400026 DOI: 10.1126/sciadv.1400026