Manure applicators face a variety of potential health issues from both direct and indirect exposure to livestock manure.

There are risks all along the manure pathway, starting with animals in the barn and storage to manure transfer and field application. Livestock manure contains zoonotic, pathogenic microorganisms that can be transmitted directly to humans during manure handling or processing, or indirectly through contaminated food products and water.

Zoonotic diseases are those that humans can transmit to animals and animals can transmit to humans. Humans can acquire zoonotic disease via vectors, animal bites, bug bites, ingestion (food or water), fecal contamination and aerosol dust.

“The eyes, nose and mouth are open doors for germs to get into your body,” said Dr. Dee Ellis, veterinarian at Texas A&M. “Any way you can mitigate that potential goes a long way toward protecting yourself.”

Ellis pointed out the role of fomites in disease transmission. “Fomites are inanimate objects where disease agents hang out,” he said, citing the “dirty doorknob” as a widely recognized fomite. He also listed dirty boots, clothing, shoes, hair, bedding, water, soil, livestock feeding and handling equipment. “Organic material in general, such as manure, straw or bedding, hides viruses, bacteria and other organisms very well,” he said, adding that half the battle in managing pathogen loads is cleaning to remove organic matter, then disinfecting to kill pathogens.

Disease patterns are based on the relationship between the disease agent, the host and the environment. Breaking the cycle at any point can prevent disease introduction. “The host might be a human,” Ellis said. “The agent could be infectious material (virus or bacteria). The environment is where disease can be mitigated. This isn’t an accident – all three have to work together to transmit disease.”

The environment can be managed through cleaning, disinfection, minimizing dust, using appropriate PPE and avoiding contact with runoff from lagoons. If the infectious agent is present along with a susceptible host, proper mitigation of the environment through best practices can stop disease.

Diseases are spread through either indirect or direct transmission. Direct transmission involves direct contact between an infected and susceptible animal or person, or contact with handling equipment, housing, bedding or manure. Indirect transmission comes from fomites such as dirty boots or a vector like mosquitoes or ticks.

There are number of disease agents, including viruses, bacteria, fungi and protozoa. “Rabies is a virus, E. coli is a bacterium, ringworm is a fungus and giardiasis is a protozoan,” said Ellis. “A transmission cycle that’s going on right now is avian influenza – it works its way to domestic birds through wild birds and can potentially infect humans with type A influenza.”

Avian influenza virus is constantly circulating in wild birds, and manure plays a role in its spread. “Migratory wild birds come down from flyways and bring viruses with them,” said Ellis. “A lot of waterfowl are not that sick – they’re just transporting disease. They come in contact with domestic birds (turkeys, broilers and layers), and when the virus jumps species, it often changes in pathogenicity and its ability to make animals sick. Humans are another type of animals in this case – we can catch it from poultry litter or from handling sick birds, eggs or their crates. This [influenza A] is a classic example of zoonotic disease transmission from animal to animal to humans.”

Manure and disease transmission

Animals in livestock exhibits or petting zoos can harbor zoonotic pathogens, so it’s important to clean up manure and provide hand washing facilities to limit the risk of disease transmission to the public. Photo by Sally Colby

Some of the environments with potential for zoonotic disease transmission include feedlots, dairy farms, poultry houses and swine facilities. However, these facilities have strict biosecurity protocols to keep both animals and humans as safe as possible. There’s higher risk of disease transmission among those who raise chickens in their backyard as a hobby who may not be aware of the risk and don’t take adequate precautions to prevent disease transmission.

Events that involve a mix of livestock and the public such as fairs, livestock shows and petting zoos can spread zoonotic diseases, as can livestock that have contact with wildlife. Ellis said zoonotic diseases can be spread via live birthing exhibitions. “There’s an entire protocol on the public health side to help folks who want to do a live petting zoo or birthing exhibit,” he said. “There are things they can do to mitigate the possibility of disease transmission such as hand washing stations and educational information provided outside the pen.”

Farmers familiar with zoonotic pathogens will have an easier time managing the farm environment to limit such pathogens. One of the most common zoonotic pathogens is rabies, which is transmissible through most mammals. Brucellosis is a bacterium harbored by feral swine and other wild animals which can be transmitted to humans and domestic animals.

Acariasis (mange) can be transmitted from cattle, sheep, goats, poultry and pigs to humans via direct contact with infected animals or their bedding. Rickettsial diseases such as Rocky Mountain spotted fever and Lyme disease are transmitted to humans by ticks. E. coli bacteria live in contaminated soil, water, feces and on equipment. Avian chlamydiosis bacteria are spread by poultry through aerosolized feces, contaminated water, feed or carcasses. Salmonellosis bacteria are harbored by livestock and transmissible to humans. Cysts of the protozoan giardia live in contaminated soil and water.

Ellis stressed the importance of those on the farm notifying their physician that they work with livestock and manure. In the case of illness, knowing a person’s exposure to pathogens helps the physician provide an accurate diagnosis.

The terms “biosafety” and “biosecurity” are important in the context of manure handling. “Biosafety measures are taken by an individual to prevent the introduction and/or spread of harmful organisms to themselves – hand washing, protective equipment and not touching your eyes or mouth,” said Ellis. “Biosecurity is what you and coworkers do to prevent the transmission of disease from your ag setting to another ag setting.”

Understanding the “One Health” concept is key to good biosecurity. Disease organisms and vectors transmit between humans and animals and can persist in the environment. Employees should understand working with healthy (clean) animals prior to working with infected (dirty) animals. Lagoons and manure handling equipment should always be considered dirty, and anyone working around manure should change clothes and footwear prior to entering animal housing areas. Every farm should have policies in place for working with manure as well as clean and dirty animals, and employees should understand and follow protocols.

Employee education is key to biosafety. “Make sure employees understand how to mitigate the risks,” said Ellis. “Make sure they have the right equipment to protect themselves, and that they’re using the equipment.”

by Sally Colby