Anthrax in Animals
Anthrax in Animals
Almost all warm-blooded animals are vulnerable to anthrax. Most anthrax victims, however, are herbivores, especially grazing animals such as cattle and sheep. Other domesticated creatures—such as horses, mules, goats, camels, oxen, and llamas—are also very susceptible to the disease. So are many wild animals. For example, anthrax is common in antelope, deer, elk, reindeer, guinea pigs, mice, and rabbits. Anthrax outbreaks caused by contaminated forage have even been reported among elephants and hippopotamuses.
Cats, dogs, pigs, and birds rarely catch anthrax, and cold-blooded creatures never do. Scientists believe that basic differences in anatomy and physiology make some animals more susceptible to certain microbes than others. Thus, many carnivores—including dogs and cats—appear to have some natural resistance to anthrax, as do some omnivores such as pigs.
For other creatures, high or low body temperatures contribute to their immunity to anthrax. Bacillus anthracis bacteria grow best at temperatures ranging from about 77°F to 104°F. Thus, most birds are immune to anthrax because their body temperatures average about 107.6°F. This is several degrees higher than the average body temperatures of cows, goats, and other herbivores (102.2°F) that usually contract anthrax. Conversely, the low body temperature of cold-blooded animals, which varies with the environment, helps protect them from anthrax.
In the past anthrax was common all over the world. Now, it occurs mainly in developing regions that lack the means to control the disease, such as sections of the Middle East, Africa, Australia, southern and eastern Europe, South America, Central America, the Caribbean, and Asia. In the mid-1900s, for instance, a devastating anthrax epidemic killed about 1 million sheep in Iran.
In 2000 at least forty-eight countries suffered from outbreaks of animal anthrax. Of those, forty-three were developing nations, including Nicaragua, Romania, Mongolia, Tajikistan, Kazakhstan, Afghanistan, India, Ethiopia, and Zimbabwe. In January 2000 a terrible anthrax epidemic was reported on an ostrich farm in West Java, Indonesia. Unlike most birds, the average body temperature of ostriches is about 102.6°F, making them susceptible to anthrax. To eliminate the disease from the ostrich farm, over twenty-six hundred ostriches were killed with injections of strychnine (a type of poison). The carcasses were then placed in a deep pit and burned.
A number of industrialized countries, such as Canada, France, Germany, and the United States, also experienced outbreaks of animal anthrax in 2000. Anthrax is not common in the United States, but outbreaks periodically occur, especially in parts of the Southwest, Great Plains, Midwest, and Southeast. In summer 2000, for example, about sixteen hundred animals—including horses, cattle, elk, water buffalo, a llama, and twelve hundred wild, white-tailed deer—died from anthrax in Texas. In the fall of that same year, more than fifty cows died of anthrax at a ranch in Washoe County, Nevada. According to David Thain, Nevada Department of Agriculture state veterinarian, the Nevada outbreak was "believed to be due to … ditch cleaning that released soil borne [anthrax] spores onto pasture grasses."3
In 2001 approximately 100 animals in northwestern Minnesota died from anthrax, as did 21 cattle on a ranch in Santa Clara County, California. In addition, 160 animals in North Dakota perished from the disease. The outbreak in North Dakota was partly triggered by the weather, according to Larry Schuler, the North Dakota state veterinarian. "The [anthrax] spores are always in the ground," observes Schuler. "What appears to bring [anthrax] on is when we have a very wet spring with some flooding followed by a lot of hot dry weather. The spores float up to the surface in the flooding, then land on plants that the cattle eat later."4 A similar weather pattern, hurricane rains followed by hot weather, caused anthrax to break out near Del Rio, Texas, in August 2003.
Life Cycle of Bacillus anthracis
Bacillus anthracis, the organism that causes anthrax, is a large, rod-shaped bacterium. Under normal conditions Bacillus anthracis organisms are surrounded by a gel-like covering called a capsule. This coating protects Bacillus anthracis organisms from the immune system of a host animal.
Anthrax organisms range in size from about 1 to 1.5 microns in width and about 3 to 10 microns in length (1 micron = 1 millionth of a meter). When exposed to adverse conditions, such as the death and decomposition of a host animal, anthrax bacteria form spores.Bacillus anthracis organisms must be exposed to oxygen to make spores, so spore production does not occur inside intact corpses.
Anthrax spores are hardy, thick-walled, oval bodies with an average diameter of about 1 to 3 microns. The spores are highly resistant to drought, heat, cold, disinfectants, and other unfavorable surroundings. The spores can lie dormant in natural environments such as vegetation, soil, or water for hundreds of years. For example, the soil of the Ohio Valley of the eastern United States—which was contaminated by the livestock of homesteaders in the early 1800s—still contains anthrax spores. The same is true of old cattle trails that ran from Texas to Canada, and routes used by pioneers in the Old West. According to Susan Jones, a veterinarian and medical historian at the University of Colorado: "The practice [in the Old West] was to let the animals die and leave them behind. The animal becomes an incubator, and the bacteria survive in spore form for decades and decades in soil."5 Wool, hides, bones, and other parts of deceased anthrax victims can also harbor the spores for years. Lingering spores make it almost impossible to eliminate anthrax from areas where animals have been infected throughout history.
Once anthrax spores enter the body of a living animal, they germinate, or transform back into rod-shaped bacteria. The bacteria begin to multiply near the site of invasion, then spread through the creature's body and continue to reproduce. A huge population of germs develops and produces toxins, resulting in the host animal's death. As the creature's carcass breaks down, the anthrax bacteria form spores, which disperse into the surrounding environment. When the spores are consumed by an animal, the cycle begins again.
Methods of Infection
Though anthrax can strike at any time, grazing animals usually get sick in the dry summer months, when available forage decreases. The animals will eat grass to the ground and may pull plants up and eat the roots as well, taking in anthrax spores in or on the soil. The coarse vegetation can cause small cuts and abrasions in the mouth, throat, and intestine of a grazing animal, allowing the spores to enter the body. Anthrax can also be contracted by livestock through wounds caused by dehorning or castration.
Meat-eating creatures, like predators and scavengers, may contract anthrax from consuming sick prey or infected carcasses. Anthrax spores can also be transmitted by insect bites, polluted water, commercial feed made from diseased animal carcasses, and dust blown off anthrax-contaminated soil.
Once infection occurs, the incubation period, or length of time until symptoms appear, ranges from one to fourteen days but is usually from three to seven days. The course of the illness varies with the form of the disease and the animal infected.
Forms of Animal Anthrax
Four forms of anthrax are seen in animals—peracute anthrax, acute anthrax, subacute anthrax, and chronic anthrax. They differ chiefly in the span of time between the appearance of symptoms and death. Peracute anthrax generally lasts from one to two hours, though some animals go from apparent health to death in moments. Acute anthrax persists for one to two days. Subacute anthrax lasts for three to five days. Chronic anthrax persists longer than five days. Some animals recover from the chronic form of the disease.
Death from anthrax is generally due to septicemia, or blood poisoning, caused by high levels of Bacillus anthracis organisms and the toxins they secrete. At the time of death, most susceptible species—if left untreated—contain about 10 million to 100 million Bacillus anthracis organisms per milliliter (.03 fluid ounces) of blood. Large amounts of anthrax toxins result in kidney failure, tissue damage, massive edema (swelling due to accumulation of fluids in tissue spaces), and shock (decline of body functions due to reduced blood circulation). Animals that survive anthrax become immune to the disease and cannot be reinfected.
Peracute and Acute Anthrax
Ruminants, or grazing animals that chew their cud—such as cattle, sheep, goats, oxen, and llamas—usually exhibit peracute or acute anthrax, the most severe forms of the disease. Horses, which also forage in pastures, generally demonstrate peracute or acute anthrax as well.
Animals with peracute anthrax may die suddenly, without exhibiting any signs of illness. "Even if you're watching your animals like a hawk, all you might notice is a little lethargy," observes Konrad Eugster, executive director of the Texas Veterinary Medical Diagnostic Laboratory in College Station. "For all practical purposes, it's a sudden, unexplained death."6
Usually, though, victims of peracute anthrax exhibit a variety of symptoms such as fever, muscle tremors, difficulty breathing, and convulsions for one to two hours before death. Creatures with acute anthrax demonstrate similar symptoms for one to two days. Acute anthrax sufferers may also display chills, loss of appetite, staggering, diarrhea, convulsions, and aggressiveness (a tendency to charge) followed by listlessness. Affected animals also develop swellings, called tumors, on the body. Pregnant animals may lose the fetus, milk production may be reduced, and milk may be discolored—blood-stained or yellow. Infected horses often experience severe colic (intestinal distress).
After death from peracute or acute anthrax, there may be bloody discharges from the victim's nose, mouth, and anus; rapid bloating and decomposition of the carcass; dark, unclotted blood in the body; reduced rigor mortis (stiffening of the corpse); and an enlarged, pulpy spleen the color of blackberries. Tumors, if cut open, appear black and are filled with a bloody mass of decayed tissue.
Subacute and Chronic Anthrax
Pigs, cats, and dogs generally demonstrate subacute or chronic forms of anthrax. On rare occasions, cattle and horses also exhibit these varieties of the disease. Symptoms of subacute and chronic anthrax might include blood-stained, foamy discharges from the mouth, enteritis (inflammation of the intestine), labored breathing, difficulty swallowing, and swelling of the tongue and throat. In some cases the victim's shoulders, sides, and genital region swell also.
If the swollen throat inhibits breathing, a victim of subacute anthrax can die of suffocation. As in other types of anthrax, subacute and chronic forms of the disease often result in death from septicemia. Sometimes, though, pigs, cats, and dogs—which are somewhat resistant to anthrax—recover from the chronic form of the disease.
On occasion, animals become infected with anthrax cutaneously (through the skin) because of insect bites or injury. In these cases the disease remains restricted to the site of injury in the early stages. The affected area initially becomes hot and swollen, then grows cool and numb.
Without treatment the illness may become systemic seven to ten days after infection, resulting in septicemia. Death then follows within twenty-four to thirty-six hours.
Diagnosing Anthrax in Animals
If an animal perishes after exhibiting some of the symptoms described above, or dies very suddenly, anthrax is usually suspected. In such a case, veterinarians are warned not to perform a necropsy, or animal autopsy, to learn the cause of death. Bacillus anthracisquickly sporulates (forms spores) when exposed to air. Thus, opening the body would induce sporulation and allow anthrax to spread, endangering other animals and humans.
Instead of examining the interior of a suspected anthrax corpse, veterinarians are advised to withdraw blood from an outer vein, such as the jugular vein in the neck. Laboratory workers then prepare a slide of the blood and dye it with a bacterial Gram's stain. To perform a Gram's stain, a laboratory technician immerses the slide in the following series of solutions for about ten seconds each: a purple dye called gentian violet, iodine, alcohol, and a pink dye called safranin. When the stained slide is examined with a microscope, the presence of Bacillus anthracis organisms—which appear as violet blue rods containing colorless, oval spores—would demonstrate infection with anthrax.
How Anthrax Spreads
Anthrax spores, which are easily dispersed, can spread the disease over an extensive area. Animal disseminators of anthrax spores include scavengers such as ravens, vultures, and hyenas that eat the carcasses of anthrax victims. Afterward, the scavengers roam around their ranges dispersing the spores in their feces. Anthrax victims may also contaminate the ground when they die, or pollute water holes if they perish there. Anthrax spores can also be spread by mosquitoes, biting flies, and other blood-sucking insects, which transmit bacteria from one animal to another.
Floodwaters can sweep anthrax spores great distances from their point of origin. Similarly, spores can be dispersed by effluents from factories that use animal parts, such as tanneries, rendering works, carpet mills, and brush factories. The waste products discharged into streams can be carried many miles. The most widespread dispersion of anthrax spores may be via commercial products made with animal parts, which are exported around the globe. These include feedstuffs, protein concentrates, raw bone meal, blood meal, and animal hides.
Economic Impact of Animal Anthrax
Outbreaks of animal anthrax can have severe economic consequences. This is especially true in developing nations, which contain large numbers of poor shepherds and farmers. At the end of the twentieth century, for example, livestock helped support at least 70 percent of the world's rural poor, estimated to be between 800 million and 1 billion people. Thus, anthrax—which can wipe out livestock very quickly—is ranked as one of the twenty conditions that have the greatest impact on poor people around the world.
Anthrax continues to be a major problem in disadvantaged communities because many indigent people lack the means to control the disease. In 2001, for example, a joint mission of the World Food Program and the United Nations Food and Agriculture Organization (FAO) found that shepherds in Afghanistan could not afford anthrax vaccines that cost less than one U.S. penny per dose. Moreover, poor farmers have small herds, so each animal is used for several purposes, such as transportation, plowing, pulling, carrying loads, and producing fertilizer (manure). In addition, livestock supply clothing (hides) and food. In fact, more than 75 percent of the food in shepherding communities comes from milk and livestock products. Thus, to a poor shepherd or farmer, the death of an animal from anthrax is a huge loss.
In some communities, moreover, livestock has enormous cultural and religious significance. The Dinka of Sudan, for instance, use cattle for marriage dowries and religious sacrifices, as well as other customs. The Dinka drink cow's milk and make it into butter and ghee, an oil for cooking. They use cattle urine for washing, dying their hair, and tanning animal hides. The Dinka employ cow dung as fuel for fires and use the resulting ashes to keep their cattle clean and to protect the animals from ticks. The Dinka also use the ashes to decorate themselves (body art) and to make a paste for cleaning their teeth. If cattle die from natural causes or are sacrificed, the animals are butchered. The Dinka then eat the meat and tan the skins. The hides are used to make mats, drums, belts, ropes, and halters. The Dinka also use the horns and bones of the cattle to make tools and decorative items.
To the Dinka, in fact, cattle are the highest form of wealth. According to a 2002 FAO report about the socioeconomic impact of livestock diseases such as anthrax: "[In Dinka society] cattle play an essential role … providing not only milk and dowry but performing important social functions and determining a man's position and influence in the community.… Cattle provide the means by which kinship ties are made and maintained, a process for ensuring the long term viability of the household and a means of receiving support … in the event of disaster."7 Thus, like other livestock keepers, the Dinka are greatly concerned about controlling anthrax. The Dinka have several names for this disease, including "jong nyal," which means a mysterious illness that comes from the sky or from God; "jong de tak," which means spleen disease, and "anguin," which means sudden death.
A February 2003 report from the Climate Information Project of the National Oceanic and Atmospheric Administration (NOAA), which monitors the impact of climate around the world, noted that anthrax had broken out in parts of Sudan during the dry season. According to the NOAA report, there were concerns that the disease would spread quickly, adversely affecting regions that were already experiencing food shortages. To deal with such outbreaks, the Dinka use not only conventional control measures such as vaccine and medicine, but also the services of a "spearmaster," who uses magic to try and ward off the disease.
A study, carried out by the International Livestock Research Institute under the sponsorship of the Department for International Development and published in 2003, notes that "finding [solutions] to the hazards that livestock are exposed to in the developing nations of the world is an excellent approach to rapidly emancipating the resource poor from starvation and poverty."8 Hence, the Animal Production and Health Division of the FAO is fostering and encouraging programs to reduce the incidence of anthrax and other animal diseases around the world. The South African government, for example, disperses about one hundred thousand brochures annually—illustrated with drawings of sad cows—directing farmers to vaccinate their cattle. The goal of these international programs is to reduce poverty and bolster the livelihoods of disadvantaged shepherds and farmers. In addition, controlling zoonotic diseases like anthrax will improve the health of humans.