Climate change: The heating of the inner atmosphere, oceans, and land surfaces of the earth. The warming is associated with more intense extreme weather events and the altered timing, intensity, and distribution of precipitation.
?Paul R. Epstein, "Climate Change and Public Health: Emerging Infectious Diseases," Harvard Medical School Center for Health and the Global Environment
You've been hearing about weird weather for a while: A historic drought in Texas... followed by regional flooding. Killer tornadoes in Alabama and elsewhere. A rare hurricane on the upper East Coast. Balmy weather in New Hampshire...in January. What gives?
Theories abound. But regardless of cause, changing weather patterns seem to be the new norm. Such weather-pattern changes may cause more than the typical weather-related problems, which can range from inconvenience to destruction. (Last year alone, the U.S. suffered a record-smashing 14 separate billion-dollar weather disasters.) They also may be affecting the spread of infectious disease (and more).
I've witnessed that firsthand in Texas, where I live. (See, "Outbreak!," This Horse Life, page 10.) That led me to contact three experts in equine infectious disease: Dr. Craig Carter, of University of Kentucky, and Dr. Noah Cohen and Dr. Tracy Norman, of Texas A&M University. (See, "The Experts," page 2.) I asked them how climate change might affect our horses' health. Here's what they had to say. Then read on to see how you can help protect your horses.
Dr. Craig Carter: 'A scary thing to watch'
Climate-change scenarios project a shift in the spread of infectious disease, due to warming and associated weather extremes, such as flooding and droughts. "It's a scary thing to watch," says Dr. Carter. "My wife is a master gardener, so she keeps me up to date on plant zones. In the '80s, Kentucky was a Zone 6. Today, the state is mostly a Zone 7, which indicates it's warming. (Such zones, with 1 being the coldest, and 13 the warmest, are the U.S. Department of Agriculture's indicator of whether a plant or tree will survive the winters in a given region.)
"Insect vectors (carriers of disease) are in concert with that trend," he continues. "For instance, West Nile virus, which is transmitted by mosquitoes, appeared in Canada for the first time in 2002. Mosquitoes, ticks, flies, and other insects are moving northward as it warms. It's not just horses (and people) at risk; crops are being affected, as are trees, due to beetle infestations. Climate change affects all forms of life."
It doesn't appear that the warming trend will end anytime soon.
"Ice caps are melting. I read one study done down on the Equator estimating that 2,000 plant and animal species are moving north at a rate of a mile per year," Dr. Carter says. "We need to be ready, such as with vaccine development, better drugs, preventive measures?that all takes increased research dollars."
Pigeon fever, which typically causes deep-muscle abscesses, is an example of an equine infectious disease on the move.
"Drought increased the biting-fly population in areas like Texas, Louisiana, and Colorado," says Dr. Carter. "Flies can mechanically transmit Corynebacterium pseudotuberculosis, the cause of pigeon fever." (More about that later.)
Leptospirosis is also on the move.
"We're currently doing a study on equine leptospirosis at the University of Kentucky," Dr. Carter says. "Interestingly, the horse is the only domestic animal that gets infected by lepto that doesn't have a vaccine. Somehow it's considered a ?Kentucky' problem. But our research team is seeing high blood titers (indicating exposure to the bacteria) all over the country."
Leptospires are one cause of equine recurrent uveitis, an eye inflammation that eventually can cause blindness; lepto also can cause abortion in broodmares. "It's a multi-species disease," explains Dr. Carter. "Horses, cattle, dogs, and other animals pick it up from rodent urine in grass, contaminated hay, and other things in the environment. The Leptospira bacteria bore through the mucous membranes and infect the animal." Outbreaks seem to follow wet years.
"Last year, Kentucky had the highest rainfall on record. We had 67" of rain; normal is about 40"," says Dr. Carter. "That's resulting in a high prevalence of lepto this year. In 2006, another very wet year, we had 41 abortions confirmed in our lab alone, but that was only the tip of the iceberg. Yes, we predict 2012 will be another big lepto/abortion year."
The economic impact on Kentucky farms alone is huge.
"In 2006, we attempted to trace back all the lepto abortions confirmed in our lab. We were only able to get good data from 20 of the affected farms. The value of the foals lost on those farms was $3.5 million. We can't estimate the economic damage and suffering related to the uveitis syndrome. Again, that's just the tip of the iceberg.
"We need more funding for research to better understand the epidemiology of this disease," he states. "But we're not going to give up until we have a vaccine for the horse."
Dr. Carter says a recent graduate-student study at the University of Kentucky demonstrated that infected horses may be a risk factor for humans.
"Leptospirosis affects the kidneys in humans and can be fatal," he says. "It's a worldwide issue?it's one of the most prevalent zoonotic (spreads to humans) diseases seen around the world."
Unusual weather makes disease a moving target for veterinarians.
"Take equine herpes virus, a neurological disease that can be fatal, and one that's highly contagious," Dr. Carter continues. "It tends to follow cold-weather stress, which causes a horse's immune system to be suppressed. EPM (equine protozoal myeloencephalitis), another neurological disease, has also been associated with cold stress. When a cold snap hits areas where horses aren't used to it, they get stressed until they acclimate. They can come down with just about anything."
Dr. Tracy Norman: 'We've seen trends'
"We haven't really hammered out exactly what climate change will mean to the horse world," says Dr. Tracy Norman. "But we have seen trends. Pigeon fever is a really good example.
"Here at Texas A&M, we used to see a case of it once in a blue moon prior to 2009. It was very rare here. In fact, from 2005 to 2008, we had zero cases. In 2009, we had one case. In 2010, we had nine cases. Then came 2011?the year of a historic drought in Texas?when we had 40 cases prior to mid-October. We probably ended up with between 45 and 50, and that was just at our clinic. That's a huge increase.
"In California (where it's endemic)," she continues, "you tend to see outbreaks in late fall and early winter, during the rainy season there. Corynebacterium seems to show up after rain that follows periods of drought."
The bacteria live in the soil; organic matter, especially manure, provides an ample food source for the organism. The mode of transmission has not been fully worked out, but the theory is that it passes primarily via the pus from a draining abscess. If the drainage lands in manure?a likely scenario in a paddock or pasture setting?the manure provides a good growth medium. Biting flies can then pick up the bacteria when feeding on manure, and pass them to other horses. It's also possible that the bacteria can infect abraded skin as well.
"In the past few years, we've seen big spikes of pigeon fever in Colorado, particularly in 2009, in the Fort Collins area, and in Idaho, especially in 2006," says Dr. Norman. "Research is ongoing regarding how outbreaks relate to weather patterns.
Why does Corynebacterium persist in California's high desert, but not in damp areas? Does that relate to the flies in those areas? These factors have not been worked out, but they will be important to understand how weather patterns affect the spread of this disease. That may make increases easier to predict, and perhaps eventually, prevent."
Pigeon fever isn't the only disease on the radar.
"In fall of 2011, we at A&M admitted more cases of salmonella in adult horses than is usual for that time of year," says Dr. Norman. "We'd have horses present with diarrhea, small colon impaction, and/or fever that tested positive for salmonella. In November, at one point, I had nine or 10 horses at the clinic that came in testing positive for the disease. It followed the first significant rain after nine months or more of drought, then seemed to die off soon after.
"Did it release in the soil or manure due to the rain? Or did it have something to do with all the hay being brought in from out of state due to Texas's drought-induced hay shortage? I wondered if perhaps the change in hay caused a change in the population of normal bacteria in the horses' guts that allowed for salmonella to take hold. People were feeding all kinds of new hay sources due to the shortage?imports from other states, as well as processed and pelleted hay. Was that the reason? We don't know."
The 2011 drought had at least one bright side. "We saw fewer strangles cases in July and August of 2011 than we normally do," recalls Dr. Norman. "Was it the weather? Again, we don't know."
She says she also wonders how equine allergies will be affected by climate change.
"It stands to reason that as you change or stress plant populations that live in a region, due to changing weather patterns, that can produce more or different allergens," she says. And that brings up another possible downside to the trucked-in hay necessitated by unusual drought.
"For example, people have had to bring in hay to Texas from New York, Pennsylvania, North Dakota, and Nebraska," Dr. Norman explains. "Horses eat the hay and pass seeds through their manure. That's introducing foreign plants to our pastures. What does that mean down the road? We're not sure."
There's a more sinister threat from the plant world as well.
"As more areas in the country are hit harder by drought, followed by periods of heavy rain, that creates an environment conducive to toxic plants, which are generally very drought-tolerant. Livestock starved by drought conditions will eat plants they'd normally avoid. There's a great potential there for a dramatic increase in liver and other diseases caused by horses eating toxic plants."
Drought also seems to cause an increase in rabies, a fatal neurological disease. "I talked to veterinarians over the last year in Texas who saw rabies cases in multiple species of livestock. We saw an increase of equine rabies at our clinic?none of those horses were vaccinated. It's a $10 vaccination, so skipping that vaccination doesn't make sense. And rabies can be passed to human handlers," she states.
What's the drought connection?
"Drought limits water sources. Stock tanks and ponds provide water. Any wild animal can act as a carrier. Limited water sources bring them in closer contact with horses. We saw a shocking number of equine rabies last year."
What You Can Do
Our experts agreed on the following measures to help protect your horses from becoming victims of climate-change-related conditions.
- Take advantage of vaccines. Work with your veterinarian to be sure your horse is kept up to date on vaccinations against such diseases as rabies, West Nile virus, strangles, Eastern and Western encephalomyelitis (sleeping sickness), tetanus, and flu/rhino, plus any other diseases or organisms endemic in your area for which an equine vaccine exists.
- Keep biting insects to a minimum. Consider a feed-through fly-control product, to limit fly hatches in manure; use chemical barriers (such as sprays) and mechanical barriers (such as fly masks and sheets) to protect your horses; drain standing water to limit mosquito hatches; pick up manure in your stalls and pastures, and dispose of it properly. (Manure also can be a breeding ground for disease-related organisms, such as Corynebacterium and the bacterium responsible for salmonella.)
- Reduce stress. "Horses are like kids," says Dr. Norman. "They crave routine." Maintain a feed, turnout, and exercise schedule, mirroring it as closely as possible when traveling to shows, clinics, trail rides, or other events; bring your own hay and water when possible, to minimize dietary changes when traveling; use sheets, blankets, and shelter to help reduce cold-related stress when unusual winter weather blows in; use fans and shade to keep your horses as cool as possible in case of summer heat waves and/or drought. Make sure every horse has high-quality, dust-free feed and hay (using as consistent a hay source as you can, to help prevent dietary changes), and provide free access to clean water.
- Keep horses' immune systems as healthy as possible. See above, re: reducing stress. Plus, keep this in mind: "The best thing you can do for your horses' (and your) immune system is to maintain a good diet and exercise routine," says Dr. Norman. "It's not sexy, but it works." She also suggest talking to your vet about giving your horses an injectable, systemic immune booster be- fore a high-stress event, such as a long trailer ride or show circuit.
- Reduce weeds. Keep your pastures as improved as possible, and mowed, to reduce broadleaf plants and weeds that could be toxic or cause allergies.
Dr. Noah Cohen: 'We need to be cautious'
I posed a few final questions to Dr. Noah Cohen, an epidemiologist at Texas A&M University. (Epidemiology is the branch of medicine that studies the incidence, distribution, and control of diseases; see "The Experts," page 58.)
Q: Are there emerging or more extreme equine skin-disease risks, due to climate?
A: I'm not aware of evidence linking climate change and equine skin diseases. It makes sense that changes in UV exposure, warmer temperatures that might better support bacterial growth, etc., could lead to increased incidence of skin disease. But to my knowledge, this is purely speculative.
We need to be cautious not only about speculations, but also about observations. For example, just because we see a change in incidence of disease with a change in climate or climatologic conditions (such as rainfall) does not mean the association is causal. There might be other explanations that are unknown and less readily apparent or plausible to us.
Q: I've read that there's concern about new equine diseases migrating to North?
A: I'm not aware of any. It's plausible that climate change will influence the distribution and frequency of diseases. But establishing scientific evidence that associates change in climate and change in disease distribution will be difficult to do, despite the appearance of association. There's a famous example often cited, about the positive association between increase in stork population and human birth rates: The fact that the two parameters were significantly correlated doesn't prove storks are the source of babies.
Another problem is that of trying to forecast disease based on past data: We don't do any better with disease than we can do with the weather or the economy.
Q: Is there any positive news about climate change, such as that it might limit?
A: If there are going to be negatives, there are likely going to be positives. That seems to be nature's way: It never seems to be as simple as "all good" or "all bad." Undoubtedly, if climate change enables some diseases to emerge, it will result in others that are diminished. But I think we are a long way from having solid evidence on the impact of climate on equine diseases.
Craig Carter, DVM, MS, phD. Dr. Carter is a Diplomate of the American College of Veterinary Preventive Medicine, and is Director of the Veterinary Diagnostic Laboratory and Professor, Veterinary Epidemiologist, at the University of Kentucky in Lexington. During his 30-year-career, he's been a leader in the development of epidemiological surveillance and outbreak-cluster detection.
Noah Cohen, VMD, MpH, phD. Dr. Cohen is a Diplomate, American College of Veterinary Internal Medicine. He's Director of the Equine Infectious Disease Laboratory at Texas A&M University, in College Station, Texas. In 2008, he received the Schering-Plough Animal Health Applied Equine Research Award from the World Equine Veterinary Association. His areas of interest are equine genetic epidemiology and the epidemiology of equine infectious disease and equine immunology.
Tracy Norman, VMD, MA. Tracy is a Diplomate, American College of Veterinary Internal Medicine, Clinical Assistant Professor at Texas A&M University, and is a Clinical Assistant Professor there. Her areas of interest are equine internal medicine, diagnostic ultrasound, and respiratory disease.