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Epidemiology

Epidemiology is the study of diseases and how they spread through a population. The veterinarian has played a key role in the development of epidemiology. In the midst of our current pandemic, here are a few examples of disease outbreaks, and responses with successful outcomes: Canine parvovirus, New world screwworm, rabies, and Rinderpest.


If you are a dog owner, you are likely familiar with the vaccine against canine parvovirus as it is one of the "core" vaccines most dogs receive. Canine parvovirus is a highly contagious disease targeting white blood cells, the cells in the gastrointestinal tract, and in some cases cardiovascular cells. It has an 80% fatality rate in infected dogs and puppies. Canine parvovirus is a relatively new disease, showing up in the 1970s and being formally recognized in 1978. The disease is believed to have jumped from the feline disease panleukopenia virus, being nearly identical genetically (only differing by two amino acids). When this disease first jumped species, people were unsure of what was going on; why were so many dogs becoming sick and dying? Within two years the disease was worldwide, infecting and killing millions of dogs. In 1979 Drs. Leland Carmichael, DVM and Max Appel, DVM developed the first vaccine for the disease, and by 1981 it was commercially available and widely distributed. Today we only see canine parvovirus sporadically in unvaccinated dogs as the vaccine is nearly 100% effective.


New world screwworms (Cochliomyia hominivorax) are a species of fly whose larvae (maggots) feed on living tissue of animals. The screwworm's main host is cattle. They plagued the early settlers of the United States by feeding on their cattle, causing many cows to become ill or die. The disease continued to be a serious problem into the mid-1900s as the U.S. Department of Agriculture was spending millions of dollars to try to treat and control infections in the nation's cattle population; it seemed like a losing battle. Then, in 1958, scientists and entomologists Edward F. Knipling and Raymond C. Bushland studied the flies' life cycle and proposed a revolutionary idea called the "sterile insect technique". These scientists discovered that the female screwworm only mates once, lays eggs, and dies. They proposed that if they could release large numbers of sterile males, these females would mate, lay sterile eggs, and die, and over time, the populations of screwworms would decline. The U.S. Department of Agriculture was willing to try anything and did the first large-scale test in Florida. By 1959 the fly population had been eradicated from the state. The rest of the United States quickly adopted the technique, and by 1966, the screwworm had been eradicated from the United States.


Rabies has been known to humans for over 4,000 years with the first written records in Mesopotamia in 1930 BCE. The virus infects many mammals such as dogs, cats, foxes, raccoons, skunks and bats. The virus spreads through the saliva of infected animals. When an infected animal bites a host, the virus causes a local infection in the muscle and then moves through the nervous system into the brain, where it starts to cause erratic aggressive behavior. At the same time, it replicates in high numbers in the salivary glands. At this point the animal bites another animal or human and the cycle continues. After the onset of clinical signs, there is a near 100% fatality rate. In the early 1990s, about 50,000 human cases/deaths occurred annually worldwide. Most cases of human rabies in the world occur from the bites of dogs, most often in children under the age of 15 in developing countries in Asia, Africa and South America. In 1885 Louis Pasteur and Émile Roux, developed the first rabies vaccine from the neural tissue of an infected rabbit. Over the next few decades the vaccine was developed. Post-exposure immunoglobulin was also introduced to treat people before the onset of clinical signs and prevent them from becoming sick. Vaccination of pets in the U.S. became widespread in the 1940s and 1950s, which coincided with a marked decrease in human cases nationwide. Due to the development and mandated nature of the rabies vaccine in domestic pets (dogs and cats), and the development of post-exposure immunoglobulin, the United States rarely sees any human cases of rabies. These vaccination strategies are also being applied worldwide, and the incidence of rabies in humans has dropped to about 24,000 as of 2010 with the goal being zero cases of human rabies by 2030!


Rinderpest, also known as "cattle plague", was a viral disease of even-toed ungulates (hoofed animals). The virus is similar to the human measles virus and the canine distemper virus. The main animal of human significance it impacted was cattle; though it also infected a myriad of wild animals such as bison, deer, antelope and warthogs. The disease spread by contact of animals with each other and communal bodies of drinking water. Clinical signs were usually fever, lethargy, diarrhea, ulceration of the mouth, and within 6-12 days, most infected animals were dead. In native populations, the disease could have a 100% fatality rate, effectively wiping out whole populations of cattle and causing widespread famine. The disease plagued human civilization since the time of the ancient Egyptians, and is even cited as a factor in the fall of Rome. It continued to plague Europe throughout the Middle Ages and into the nineteenth and twentieth century. It seemed there was no hope until veterinary pathologist William Hutchins Boynton developed the first vaccine for the disease in 1918. After about 10 years of working on the vaccine, a worldwide vaccination campaign began to eliminate Rinderpest. For the next several decades, most eradication efforts were on an individual-country basis. Then, nations started working together, and the Global Rinderpest Eradication Programme initiated in 1994. The last documented case of Rinderpest was in Kenya in 2001. Since then, all surveillance efforts have not shown a single case and the disease is believed to be completely eradicated.


These diseases are all very different in how they infect and how they have been managed. However, there are a few key points that carry through. First, the focus on science; critically studying the disease, whether it is the pathogens, DNA, or reproductive strategy, and figuring out a way to outsmart them with vaccine development (or sterile male flies). Secondly, working together as one; it was not until the whole world came together in one effort that the eradication of Rinderpest was achieved, or that the number of rabies cases in humans has drastically decreased. Finally, not giving up; for thousands of years, rabies and Rinderpest plagued our animal civilizations, but eventually we came up with a way to fight back!


Should you have any questions, comments or concerns, please don't hesitate to contact us by phone at 845-876-6008, or by e-mail at [email protected].

Thank you for choosing us to be part of your pet's healthcare team!

With warmest regards,
Your friends at Rhinebeck Animal Hospital

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