August 7, 2012
CIDRAP: Certain soil types contaminated with H5N1 avian flu viruses led to chicken deaths when added to their houses, but sandy topsoil collected from area rice fields seemed to resist that pathogen, a Cambodian study in Emerging Infectious Diseases yesterday revealed. Investigators studied sandy topsoil collected from around rice fields in Phnom Penh province, as well as building sand and soil-based compost from a local tree nursery. They contaminated each soil type with low doses of H5N1 (1 infectious unit on day 0, 2 on day 6, 4 on day 12, and 8 on day 18) and high doses (8, 12, 16, and 20 infectious units on the various days, respectively). The soil was then sprinkled on the bottom of an isolator containing 10 to 20 chickens. High-dose building sand and compost both led to a 100% fatality rate after 2 days, whereas high-dose sandy topsoil led to no deaths. Low-dose compost led to a 50% H5N1 seroconversion rate after 24 days, compared with 33% for building sand and 1% for sandy soil. The authors hypothesize that the sandy soil's acidity inhibits H5N1 spread. They state that about 40% of rice fields in the country have this type of soil and that it is most commonly found in H5N1 outbreak areas. In contrast, "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," they write.