Two zoonotic infections have made headlines in the United States this year, plague and tularemia, due to increased reports of each infection in humans, which has some people scratching their heads asking “why?”.
The Centers for Disease Control and Prevention (CDC) defines a zoonotic disease as a “disease that can be passed between animals and humans. Zoonotic diseases can be caused by viruses, bacteria, parasites, and fungi. These diseases are very common. Scientists estimate that more than 6 out of every 10 infectious diseases in humans are spread from animals.”
Last week, New Mexico reported the latest human plague case in the US this year in a Santa Fe County woman. This was the 15th human plague case of the year. The increase in plague in 2015 has created headlines in the media like “Plague Besieges California, Colorado, and New Mexico”, but what is the reality?
There has been 15 cases so far [Arizona (two), California (one), Colorado (four), Georgia (one), New Mexico (four), Oregon (one) and Utah (one)], definitely an increase compared to the average of seven human plague cases reported annually in the US, but still not as high as the 17 cases reported in 2006, the highest number since 2000.
However, it is double the average number, so what’s going on?
Dr. Paul Ettestad is the Public Health Veterinarian in New Mexico and an expert on zoonotic diseases. His state sees cases of plague, tularemia and hantavirus most years and in 2015, four human plague cases have been reported, including one fatality.
When I posed the question to Dr. Ettestad, “Why do you believe we are seeing so many more cases in 2015 than in recent years?”, he replied as follows:
In New Mexico, the frequency of plague cases can fluctuate from year to year and broadly over time; anywhere from a high of 27 cases in the state in 1983 to several years where there were no cases at all. The exact reasons for this are unknown but most likely relate to the complex ecology of the disease, which is influenced by environmental factors such as increased rainfall leading to an increase in rodent populations and increased flea survival; and even predator population dynamics, along with variable opportunities for spillover into humans.
Then there is tularemia. This zoonotic disease is not nearly as famous or feared; however, it can be deadly and the US is seeing quite an uptick in 2015, particularly in Colorado and surrounding states.
In an Morbidity and Mortality Weekly Report (MMWR) published in late 2013, CDC officials reported 1,208 human tularemia cases for the 10 year period of 2001 through 2010, an average of 120 cases annually.
Through the first 8 1/2 months of 2015, the CDC reports a provisional cumulative total of 175 tularemia cases to date. Certain states are seeing much higher numbers than usual.
In Nebraska, health department officials have reported 18 human cases, which represents a 25-year high.
In Wyoming, 16 cases have been reported. This in a state that just sees one or two cases annually. In fact, Wyoming Department of Health state epidemiologist, Dr. Tracy Murphy said, “To see this many cases reported in Wyoming in a single year is striking. Over the last 25 years the highest number of cases reported in Wyoming was six in 2001.”
According to the latest published data from the Colorado Department of Public Health and Environment, 40 human cases have been reported to date in 2015. From 2005 through 2014, Colorado saw 42 human cases.
I posed the question of “Why do you believe we are seeing so many more cases (plague and tularemia) in 2015 than in recent years? to CDC Division of Vector-Borne Diseases Medical Epidemiologist and coauthor of the above referenced MMWR, Dr. Christina Nelson and she emailed me the following statement:
While the number of plague and tularemia cases are above average this year, the exact reasons are hard to pinpoint. For both diseases, the number of cases tends to fluctuate from year-to-year, most likely due to the complicated relationship between arthropods (fleas in the case of plague and ticks or deer flies in the case of tularemia), rodents or rabbits, and human activity.
Despite agreement on the difficulty in determining the cause of the increases in both diseases, the answers from both experts were incredibly similar–factors like increased vector populations, rodent populations and animal to human interactions tended to overlap.
Educating the public about prevention is key, particularly during months when people are more likely to be involved in outdoor activities.
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