19 May 2010

Pathogens, Water, and Animal Agriculture

This week, I am participating in a USDA funded workshop at beautiful Cornell University to discuss waterborne disease occurrence, management and control in agricultural regions. The workshop, put together by Cornell University with funding from the USDA, brings together regulators and industry experts with pathogen researchers across various disciplines that have been funded over the past five years under the National Institute of Food and Agriculture research program. The workshop is peppered with a number of very interesting overview presentations related to waterborne disease. The talks are very accessible to the public and I highly recommend them if you are interested:

Jeffrey Griffiths - Outcome of Waterborne Disease Infections
Jane Hill - Moving Bacteria
Dwight Bowman - Disease from Recreational Water
Rob Atwill - E.coli O157:H7 Outbreaks in California's Leafy Green Industry
Jeanine Plummer - Drinking Water Quality and Disease Outbreak
Jim Smith - Managing Infectious Organisms from Animal Farming Operations
Chuck Gerba - Viruses: Sources, Occurrence, Fate and Risk Assessment
Ron Fayer - Cryptosporidium
(or go here to find links to the above talks)

Also check out this EPA Review of Detecting and Mitigating Fecal Contamination from Animal Agriculture.

Should We Bother?

In my small workgroup, we are discussing various aspects of the risk of waterborne illness exposure in drinking water, recreational water, or on (fresh) foods due to contamination of waters (groundwater or surface water) from animal farming (beef, dairy, poultry, swine). A lot of brainstorming and learning across disciplines (sitting at the table with a couple doctors that are experts on gastrointestinal diseseases, epidemiologists, an ag industry representative, veterinarian, animal scientist,...).

The specific framework we are looking at are animal farming operations (confined or not) on one hand, and what I would call "beneficial uses" and what a colleague here has referred to as "exposure points" on the other hand: drinking water sources (public/private water supplies, that is, surface water intakes, groundwater wells), recreational water (pools, ponds, lakes, rivers, beaches), and fresh food crops (e.g., salad, spinach, tomatoes) that are irrigated with water or treated with manure. Pathogens may travel between the two (farm/ranch to exposure points) via air, soil, surface water, or groundwater or any combination thereof. [Note: our workshop focuses on farm/ranch sources; there are many others related to human activities and to wildlife.]

The big three words that came up in our discussion today are risk assessment, risk management and policy (e.g., the four-barrier approach to mitigating pathogen transport in manure management), and risk communication (including education). Together, these constitute a risk analysis. Risk management and communication/education happens at both ends of the framework: on the farm/ranch side where manure is produced (manure and animal management, farm education and extension) and on the exposure/water use side, where the infectious transmission to humans potentially occurs (water treatment, public education). It can also occur as part of the treatment and education of infected people (for a good laugh, see this No Crypto commercial).

It is well known (and at UC Davis we have down our own share of work on dairies) that pathogens such as E. coli O157:H7, Salmonella, Campylobacter, Cryptosporidium parvum, can be present in animal manure. I have always assumed that controlling pathogen release from animal manure into waterways would intrinsically have a public health benefit. But of course there are many sources other than farms and ranches for waterborne, gastrointestinal diseases, and there are many pathways for transmission other than water for farm fecal pathogens to infect people (think: petting farm, farm camp,....) - see my cartoon above.

As a result, we have an endemic level of these diseases in the population. An interesting question to ask is this: at a national level, if we were to perfectly "sterilize" all manures produced on animal farms before any of it enters a surface or groundwater, would we indeed see a difference in the occurrence of gastrointestinal diseases (the main form of waterborne disease)? Jeff Griffiths from Tufts University pointed us to two interesting publications that show the correlation between animal farming and waterborne disease prevalence (frequency) at a national or large regional level:

Jyotsa Jagai and co-workers recently published a national public health study that shows the relationship between cattle density (cattle are a source of the pathogen Cryptosporidium parvum) and the prevalence of Cryptosporidium infections (so-called cryptosporidiosis). The study convincingly shows that the average occurrence of reported protozoan infections is approximately one-third higher in U.S. counties with high cattle density than in areas with low cattle density. Importantly, the study does not assign risk to specific pathways (water, air, direct contact), so it would be wrong to conclude that waterborne disease occurrence is higher in cattle farming regions than elsewhere. It is indeed a possibility.

Another finding of this study is - in counties with high cattle density - that the occurrence and outbreak intensity of protozoan infections in areas with low population densities (i.e., the country-side) is different from areas with high population densities (i.e., urban areas near/around cattle farms): in rural areas, the outbreaks are more pronounced and spike in the fall, while urban areas experience a more steady infection rate that spikes in the early summer. One hypothesis that I would pose is that this may be the result of the different water sources in these two groups (if drinking water indeed is the main transmission pathway): in rural areas, most water comes from domestic wells (groundwater), whereas most urban areas are on public water supplies, many of which come from surface water sources. How would that affect the timing of disease outbreak? Beef cattle calves (the main source of Cryptosporidium) are typically born in the spring, spring is also a time of much runoff. Hence, surface water sources probably carry the highest pathogen load in late spring and early summer. That same peak would be attenuated in groundwater recharge and not show up in domestic wells until later in the year. Hence rural households wouldn't see a peak in protozoan disease incidence until the fall.

Note that many domestic wells pump water that is much, much older than six months, but that water is unlikely to carry infectious pathogens, since few pathogens survive longer than half a year.

A more conclusive answer on the important role of the waterborne pathogen transmission specifically from animal farming was described in an English study: Sopwith and co-workers showed for northwest England that animal manure management practices (and - at a later time - additional treatment of the drinking water supply) suppressed a well-known re-occurring annual late spring/early summer outbreak of animal-related (zoonotic) Cryptosporidium parvum infections. Annual late summer incidences of human-caused Cryptosporidium homini infections (e.g., via public pools, lakes) remained high during the same time period tested.

These kind of regional/national public health studies on factors controlling the prevalence of waterborne disease are still rare, yet very important in understanding and communicating the national and international importance of animal manure management to reducing the risk of pathogen transport into waterways and ultimately to substantially reduce waterborne infection rates in human populations (see Drs. Griffiths' or Bowman's talks above for all the gory details on waterborne diseases - only bug people can have these kind of dinner conversations... :-).


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