The Centers for Disease Control (CDC) defines zoonotic diseases—or “zoonoses”—as infectious diseases spread between animals and people. Scientists estimate that over 60 percent of known infectious diseases in people worldwide are spread from animals.
Although those stats don’t necessarily apply to the western U.S. countryside, forested areas and mountains, several zoonotic diseases can be significant to livestock owners, hunters and outdoor enthusiasts. These diseases can be transmitted by vectors, direct contact, inhalation or ingestion of organisms. Season, environment and weather factor into exposure.
The short list for noteworthy zoonoses in the West include the bacterial diseases plague, Rocky Mountain spotted fever (RMSF), Lyme disease, Q fever, tularemia, anthrax, leptospirosis, and brucellosis. Viral zoonotic diseases include West Nile Virus, hantavirus, and rabies. Parasitic diseases trichinella and cysticercosis can be spread to humans. These are serious zoonotic diseases that need to be on ranchers’ radar.
Some of these zoonotic diseases may show no clinical signs in animals and have varying incubation periods. Many of the above mentioned might be considered “wolves in sheep’s clothing” or “great imitators” of other disease processes in humans. Simply put, many zoonoses mimic other diseases.
Whether the onset of symptoms is slow or sudden, clinical signs are often called “flu like;” they include fever, dehydration, depression, anorexia, weakness, and lethargy. Weight loss can be manifested by some of these diseases if they become chronic. Certain diseases can develop lesions specific to that zoonotic organism. Consequently, knowledge of habitat, potential exposure, and clinical signs of these diseases is pertinent information when venturing into animal and/or disease habitats.
Unique lesions, exposure history, and information associated with the following zoonoses include:
- Plague—Flea transmission; rodent disease spills into human populations; lymph node abscesses also source of bacteria; geographic “hot spots” include NM, CO, CA, AZ, TX; prairie dogs are the hosts/sentinel species for plague in the U.S.
- RMSF—Tick transmission; canine association; rash begins on the palms of the hands and soles of the feet in humans; red to purple spots/hemorrhages predispose to anemia in dogs.
- Lyme disease—Deer tick transmission; due to small tick size and minimal pain at attachment, bites may go unnoticed; infection in humans; if left untreated, infection can spread to the heart and the nervous system leading to chronic symptoms; infection can occur in dogs though a canine vaccine exists.
- Tularemia—Also called “rabbit fever” for its high visibility in rabbits that are susceptible to large die-offs; human exposure via tick or fly bites, skin contact, aerosol inhalation, and ingestion of contaminated water; rabbit hunters in infected areas are especially at risk; considered a potential bioterrorism agent.
- Leptospirosis—Spread through urine of infected wildlife, livestock, and (increasingly) dogs; possible continual shedding after clinical signs decrease; infection via contact with tissues/urine/contaminated water; organism can concentrate at periphery of tanks during drought due to concentration of animals watering; many varieties of the disease and a wide range of symptoms.
- Brucellosis—Brucellosis abortis; aka ‘Bangs;’ abortion disease in cattle and wildlife; causes “undulant fever,” an inconsistent rising and falling fever; in humans largely eradicated in the U.S.; some presence currently in states bordering Yellowstone; B. suis reported in feral hogs; fetal fluids transmission; different varieties of brucellosis in other animals, i.e., goats and dogs.
- Q fever—Tick transmission; flu-like symptoms; livestock disease usually subclinical; rare small ruminant abortion outbreaks; milk transmission plus direct contact/aspiration of reproductive fluids; “at risk” occupations include veterinary personnel, farmers, and sheep shearers.
- Anthrax—Fatal and reportable contagious disease; anthrax is a soil-borne organism; humans and animals can inhale bacteria from contaminated soil or carcasses; also considered a bioterrorism agent; discontinue handling a carcass if characteristic “grape jelly spleen” is observed during necropsy or processing; carcasses must be incinerated to minimize spread; primarily confined to persistent pockets in ND, SD, MT, NE, NV, TX.
- Hantavirus—Deer mouse host; transmission by inhalation of mice urine and droppings; 36 percent fatality rate; acute respiratory failure; first recognized in Four Corners 1993; has been diagnosed in all western states; highest number of cases in NM, CO, AZ, CA; recent outbreaks include Yosemite National Park in 2012, six cases in New Mexico (four fatal) in 2016, and Klamath County, OR, in October 2017.
- West Nile Virus—Bird hosts; mosquito vectors; horses and humans can acquire but are dead-end hosts that cannot transmit to each other; typically seasonal; equine typically neurologic syndrome; vaccine available; the death rate among U.S. horses ranges from 30-40 percent for West Nile disease; of horses that recover from the disease, up to 40 percent may exhibit neurological signs for six months or more after the initial diagnosis; about one in five people infected develop a fever and other symptoms and about one out of 150 infected people develop a serious, sometimes fatal, illness.
- Rabies—Fatal and reportable neurologic disease; can infect all warm-blooded animals; vaccines available; regional hosts include skunks, bats, raccoons, coyotes, foxes, feral cats; prophylactic treatment is available for humans with known and potential exposure; death is almost certain for symptomatic cases; brain tissue needed for testing animals.
- Cysticercosis—Also known as “swine brain worm;” caused by larval form of swine tapeworms; larval cysts form in the brain, muscle, organs or other tissues; source of potential brain cysts in humans; identified in feral hogs and recently, though rarely, in canines including wolves, coyotes, foxes and dogs; transmission can occur via ingestion or inhalation of infected animal tissues or feces.
- Trichinosis—Roundworm larvae and cysts in gastrointestinal tract and muscle of wildlife (feral hogs, bear, etc.); gastrointestinal symptoms in humans; exposure via consumption of raw or undercooked meat; smoking, freezing or curing game meat does not kill all Trichinella species.
That’s a lengthy list of zoonotic diseases with potential exposure to any variety of outdoorsmen. The potentially good news is that antibiotics can be used to treat the bacterial diseases plus parasitic diseases have therapeutic medications. Nonetheless, many of these diseases have lengthy recovery or complications, so preventative measures are imperative to prevent contact if possible. These include:
- Minimize exposure to vectors by utilizing flea and other insect spray and remove ticks as soon as possible. Proper tick removal involves using fine-tipped tweezers to grasp the tick as close to the skin’s surface as possible. Pull upward with steady, even pressure. Don’t twist or jerk the tick; this can cause the mouth-parts to break off and remain in the skin. Do not follow old wives’ tales that suggest coating the tick in petroleum jelly or touching a hot match to it to make it withdraw. According to the CDC, these methods are not effective at removing the tick and will increase the likelihood of infection if the tick is carrying a zoonotic disease.
- Stuff dryer sheets in boot tops, shirt sleeves, collars, caps, hats to repel certain insects (i.e., mosquitoes).
- Wear masks and open windows when cleaning closed vacant ranch camps, hunting cabins, wood sheds, saddle houses, and feed rooms. Spray areas with dilute chlorine bleach before and after sweeping in areas where there is evidence of mice.
- Don’t handle/process animals that show any signs of illness or small mammals known to carry tick and flea-borne diseases.
- Wear protective gloves, eyewear and masks if possible to decrease exposure to disease via inhalation or direct contact with zoonoses. Utilize good hygiene and sanitation.
- Be observant for lesions or damage to internal organs while processing game. Cook game meat thoroughly.
History of exposure to vectors, wildlife and livestock is important when visiting your health care provider if you are experiencing some nonresponsive and nondescript illnesses.
A time-honored veterinary diagnostic statement goes like this: “When you hear hoof beats, think horses, not zebras.” But unanticipated zoonotic diseases may give a “head’s up” for potential zebra surveillance. — Ginger Elliott, DVM
