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When two parasites are better than one: (Unusual) insights into ways to combat human parasitic diseases

Mann2014 Improved Boran87

Portrait of one of Kenya’s  Improved Boran breed of cattle (photo credit: ILRI/Stevie Mann).

‘Parasites found in African cattle could offer a new insight into ways of combatting serious parasitic diseases in humans, including malaria.

A team funded by the Wellcome Trust has found that cows can be protected from parasites that cause deadly diseases if they have been infected with a closely related, but milder species of the parasite earlier in life.

‘The health of 500 Kenyan calves was tracked from birth, including whether they had been infected by any viruses, bacteria or parasites. Deaths from East Coast Fever, the biggest killer of African cattle, dropped by 89% in calves that were already infected by another species of parasite that did not cause disease.

‘As well as the clear economic benefits to African farmers that vaccinating calves with benign parasites could bring, this research could lead to future approaches to human diseases. The study, published in Science Advances, suggests that people infected with a parasite that causes severe malaria may be more likely to survive if they are also infected by a less aggressive species at the same time.

‘Professor Mark Woolhouse, of the University of Edinburgh’s Centre for Immunity, Infection and Evolution, said: “This discovery suggests a completely new way to control a devastating disease in cattle, while reducing the use of antibiotics and environmentally damaging pesticides at the same time. It may also provide clues to new ways of combatting human diseases such as malaria.”’

Scientists and staff of the International Livestock Research Institute (ILRI), in Nairobi and Busia, in Kenya, participated in this 5-year study.

Read this news on the Wellcome Trust Blog: Bovine bugs hold clue to controlling infectious disease, 30 Mar 2015.

Read more reports about this research:
Financial Times Magazine: Parasitology: Parasites as protectors, 27 Mar 2015
Reuters: Cattle parasite study points to possible way to fight malaria,  20 Mar 2015
VOA: ILRI’s Philip Toye VOA interview on East Coast fever, and the benefits of co-parasitic infections, 21 Mar 2015
ILRI News Blog: Fighting fire with fire: New study shows co-parasitic infections of cattle protect the animals from lethal disease, 21 Mar 2015

Read the paper in Science Advances: African parasite that spreads poverty by killing cattle tamed by its less lethal cousins, 20 Mar 2015.

Many individual hosts are infected with multiple parasite species, and this may increase or decrease the pathogenicity of the infections. This phenomenon is termed heterologous reactivity and is potentially an important determinant of both patterns of morbidity and mortality and of the impact of disease control measures at the population level. Using infections with Theileria parva (a tick-borne protozoan, related to Plasmodium) in indigenous African cattle [where it causes East Coast fever (ECF)] as a model system,we obtain the first quantitative estimate of the effects of heterologous reactivity for any parasitic disease. In individual calves, concurrent co-infection with less pathogenic species of Theileria resulted in an 89% reduction in mortality associated with T. parva infection. Across our study population, this corresponds to a net reduction in mortality due to ECF of greater than 40%. Using a mathematical model, we demonstrate that this degree of heterologous protection provides a unifying explanation for apparently disparate epidemiological patterns: variable disease-induced mortality rates, age-mortality profiles, weak correlations between the incidence of infection and disease (known as endemic stability), and poor efficacy of interventions that reduce exposure to multiple parasite species. These findings can be generalized to many other infectious diseases, including human malaria, and illustrate how co-infections can play a key role in determining population-level patterns of morbidity and mortality due to parasite infections.

Researchers from the Universities of Edinburgh, Glasgow, Pretoria and Nottingham, the Roslin Institute and ILRI contributed to the study, which was funded by the Wellcome Trust.

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