According to the U.S. Centers for Disease Control and Prevention (CDC), 75% of emerging infectious diseases of people are zoonotic, or originated with animals. Besides their effects on animals, those diseases can pose a health risk to farm workers through animal contact, and potentially to the public overall, either through contact with animals or disease vectors such as mosquitos or ticks, or through contaminated food. And anyone who remembers the BSE scare in 2003 knows the damage even the slightest health risk can inflict on our export markets.
The beef industry, so far, has avoided much of the fallout from zoonotic diseases, since the most common, such as zoonotic influenza viruses, have emerged from poultry and pigs. Foot and mouth disease (FMD), probably our top outbreak concern, is not zoonotic, although it is often confused with the human viral disease known as “hand, foot and mouth disease.”
In late 2017, the USDA, CDC and the U.S. Department of the Interior (DOI) organized a One Health Zoonotic Disease Prioritization (OHZDP) workshop involving experts from the human-, animal- and environmental-health sectors. Workshop participants identified these zoonotic diseases as top priorities for the United States:
- Zoonotic influenza – Influenza A viruses primarily affect poultry and pigs
- Salmonellosis -- This is one of the most important foodborne diseases in the United States, potentially infecting ruminants, pigs and poultry.
- West Nile virus – Wild birds serve as the primary reservoir for this mosquito-borne virus. Among livestock species, this virus primarily affects equines.
- Plague – The Yersinia pestis bacteria rarely affects humans in the United States, but has epidemic potential. Wildlife such as ground squirrels and prairie dogs carry the bacteria, with fleas serving as vectors for human infection.
- Emerging coronaviruses – These include severe acute respiratory syndrome-associated coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV). Animals such as camels and cats are known carriers, but these do not appear to affect U.S. livestock.
- Rabies – According to the report, rabies affects about 40,000 to 50,000 people in the United States annually, at a cost of about $510 million. The disease affects cattle, horses, pets and wildlife species.
- Brucellosis -- Brucella abortus, the most important Brucella species in the United States, mainly infects ruminants. Wildlife species, primarily elk and bison, serve as reservoirs for potential transmission to cattle. While national herd prevalence in cattle is less than 0.0001%, the number of affected herds has been increasing since 2005.
- Lyme disease – This tick-borne bacteria (Borrelia burgdorferi) is the most common vector-borne pathogen in the United States. Cattle appear to resist the disease, with wild animals serving as the primary hosts.
According to Virginia Tech veterinarians Kevin Pelzer and Nancy Currin, other zoonotic diseases affecting cattle and occurring in the United States include :
- Anthrax (Bacillus anthracis).
- Brucellosis, caused by the bacterium Brucella, which can affect a wide variety of animals including cattle, pigs, sheep, goats, horses, and dogs.
- Cyrptosporidium is a protozoal parasite that causes diarrhea in humans, often from exposure to young calves.
- Dermatophilosis is a bacterial skin disease caused by Dermatophilus congolensis, affecting cattle, sheep, goats, and horses.
- Escherichia coli (E. coli) are bacteria normally found in the intestines of people and animals. However, some strains cause a severe, often bloody, diarrhea in humans.
- Giardia lamblia is an intestinal protozoal parasite that may or may not cause disease in cattle. Humans become infected by ingestion of contaminated food or water.
- Leptospirosis, a bacterial disease of cattle caused by Leptospira interrogans can spread through the urine of infected animals and can survive in water and soil for months.
- Listeriosis is caused by the bacterium Listeria monocytogenes, commonly infecting cattle, sheep, and goats. Most humans are resistant to infection, but individuals who are immunosuppressed, pregnant, or taking antacids are at increased risk of acquiring infection.
- Pseudocowpox is a virus that causes small raised sores and scabs on the teats and udders of cattle. Humans acquire pseudocowpox by direct contact with infected cows, and can develop painful scabby sores on the hands and arms.
- Q fever is caused by the bacterium Coxiella burneti and causes abortions in cattle, sheep, and goats. Humans are usually infected when they are assisting the birthing process.
- Ringworm is a skin infection caused by fungi, which can infect humans through direct contact with infected animals.
- Bovine tuberculosis is caused by the bacterium Mycobacterium bovis which is shed in respiratory secretions, feces, and milk of infected animals. Humans can acquire tuberculosis from unpasteurized dairy products.
- Vesicular stomatitis is a viral disease transmitted by flies or direct contact. People acquire the virus by direct contact with infected animals.
Risk of Antibiotic Resistance
Compounding the issue, zoonotic diseases such as campylobacteriosis and salmonellosis, continue to build resistance against common antimicrobial drugs, according to a new report from the European Centre for Disease Prevention and Control (ECDC) and European Food Safety Authority (EFSA).
Noting a need for adopting a One Health approach, the authors say the report confirms the rise in antibiotic resistance already identified in previous years.
According to the report, resistance to fluoroquinolones (such as ciprofloxacin) is so high in campylobacter bacteria in some countries that these antimicrobials no longer work for the treatment of severe campylobacteriosis cases.
Most countries reported that salmonella in humans is increasingly resistant to fluoroquinolones. Multidrug resistance (resistance to three or more antimicrobials) is high in salmonella found in humans (28.3%) and animals, particularly in S. Typhimurium.
For campylobacter, the report documents high to extremely high proportions of bacteria found to be resistant to ciprofloxacin and tetracyclines. However, combined resistance to critically important antimicrobials was very low to low in salmonella and campylobacter from humans and animals and in indicator E. coli from animals.
Vytenis Andriukaitis, EU Commissioner for Health and Food Safety, says the report should ring alarm bells, but adds that ambitious national policies in some countries have led to a decrease of antimicrobial resistance. “So before the alarm bells become a deafening siren, let’s make sure that we increasingly act all together, in every country and across the public health, animal health and environment sectors under the One-Health approach umbrella.”
For more on zoonotic diseases, see these articles on Drovers.com.
Could Climate Change Favor Zoonotic Diseases?
Some zoonotic pathogens feature high levels of host specificity, meaning a small number of animal species are capable of harboring the pathogen and passing it to humans. Others, such as rabies, have lower host specificity and can infect a wide variety of animals, generally increasing the risk of human exposure.
Recently, researchers at the University of Queensland and Swansea University demonstrated that environmental factors can play a role in host specificity and the risk of outbreaks in human populations.
In a report published in the journal Trends in Parasitology, the researchers provide an overview of challenges and directions in modelling host specificity under variable environmental conditions. In a previous study, they found climate change could constrain or facilitate the spread of diseases such as avian malaria. For many parasites, they note, host specificity is not fixed and can vary in response to environmental conditions.
"In the past, we've primarily looked at how many different types of animal species a pathogen infects -- widely considered an indicator of its risk to shift between host species,” says Dr Nicholas Clark, from UQ's School of Veterinary Science. "This is just one factor, and we've found that how infected animals are related is also important. Our research also shows that different environments provide new opportunities for pathogens to interact with and infect new host species.”