PHNOM PENH (Cambodia Herald) - Researchers at the Institut Pasteur in Phnom Penh have found that soil can transmit highly pathogenic avian influenza (H5N1) which has so far killed more than 350 people worldwide.
In a letter to the September issue of Emerging Infectious Diseases, Ramona Gutiérrez and Philippe Buchy note that direct contact between hosts is the main mode of transmission for avian influenza viruses.
Since the role of the environment as a source of infection has been "rarely studied," the researchers used experimental and simulated field conditions to assess possible transmission in chickens.
All experiments were conducted with a local strain of the virus at the Institut Pasteur using specific pathogen–free chickens from the National Veterinary Research Institute.
The three types of soil used were sandy topsoil collected from around rice fields, sand from a local building company and soil-based compost from a local tree nursery.
The soil samples were seeded with infectious feces. The contaminated soil was then sprinkled on the bottom of an isolator in which the chickens were housed, with swab samples and feathers collected daily.
For the sandy topsoil, the researchers found no clinical symptoms, deaths or seroconversion for H5N1 regardless of the dose protocol.
But for building sand and soil-based compost, the high-dose protocol resulted in 100 percent mortality rate by Day 4.
"Soil-based compost and building sand, although existing in natural settings, are not the most common substrates found in places where free-ranging poultry are raised in Cambodia.
"Therefore, despite the high mortality rate observed in our study after exposure to highly contaminated soils, the role of these soil types in transmission ... infection to poultry or other species, including humans, appears limited."
Sandy topsoil, on the other hand, did not allow any transmission, possibly because soil’s low pH may inactivate enveloped viral particles, as well as bacteria.
"Our results provide evidence that, even when abundantly contaminated, some soil types are unlikely to allow transmission of the virus to poultry and, consequently, probably not to other animals or to humans.
"These results suggest that limited resources could be better concentrated in high-risk areas, where the nature of the soils would be more likely to lead to poultry infection after natural contamination."