Emerging zoonoses and DVMs: Knowledge for everyday
Dr. Radford Davis explains why veterinarians need to take the lead in talking to their clients about the potential risks of disease transmission from pets to clients and why prevention is so important.
During the last 100 years, there has been a dramatic change in the cause of death within the United States and throughout the world. In 1900, Americans were more likely to die from common infectious disease ailments such as pneumonia, diarrhea or tuberculosis. Today our greatest health foes are obesity, heart disease, diabetes and cancer. We are a wealthy country with an advanced medical care infrastructure and a well developed, though admittedly not perfect, public health system.
Consequently, the life expectancy in the United States now stands at an average of 76.9 years, up from 47.3 in 1900. This trend toward a more chronic disease burden is relatively new to many developing countries, where infectious diseases still remain the biggest cause of death.
The burden of infectious diseases across our planet remains staggering, though few can conceptualize to what degree. Of the roughly 56 million deaths that occurred worldwide in 2000, the World Health Organization estimates that more than 14 million, or 25 percent, were due to an infectious cause.
Lower respiratory diseases are the leading infectious cause of death, followed next by AIDS, diarrhea/dysentery, tuberculosis, malaria and lastly measles. This figure does not take into consideration that some diseases previously thought to be unrelated to infectious agents are now known to be intimately connected. Cervical cancer, for example, is associated with the human papillomavirus. In one hour, more than 1,500 people die from an infectious disease across our planet, and more than half of these are children under 5. Hardest hit are developing countries where medical care is meager and public health may be nonexistent. Indeed, developing countries do most of the living and dying of our world. In the last 30 years or so, infectious diseases have emerged to impact our society and touch our lives in ways thought relegated to the history books. Diseases once thought conquered, such as tuberculosis, have risen to new heights due to lack of public health funding and a shift in national priorities.
Role of the veterinarian
So what does this have to do with veterinary medicine? Of human infectious pathogens, 61percent are zoonotic. Add to this the fact that 75 percent of all emerging pathogens of humans are zoonotic and we immediately realize the important role animals play in human disease (Table 1). A recent American Veterinary Medical Association (AVMA) study showed that 58.3 percent of households in the United States today own at least one pet, and that from 1996 to 2001, the number of dogs, cats, and horses in these households increased by 16.5 percent, 16.6 percent, and 27.5 percent respectively. And the risk of reptile-acquired salmonellosis has changed little even with the decrease in lizard numbers since families owning turtles increased by 12.6 percent.
Table 1: examples of zoonotic emerging diseases
Domestic animals play an integral part in our lives and bring us much joy, but we must not forget the risks, which are often small, they pose in the transmission of zoonotic agents. These risks tend to be highest in children, who often forego basic hygiene measures, such as hand washing, and in the various immunocompromised populations of our society.
What is still more interesting is that it is believed that nearly all emerging disease are not newly evolved microbes, but agents that have existed in nature for some time. Diseases, and the animals that transmit them to people, have shaped history and civilizations. The Black Death pandemic that swept Europe in 1345, for example, took tens of millions of lives and led the way for the Protestant reformation, helped finish off an ailing feudal system, and set the stage for the Renaissance.
So why do new infectious diseases emerge? There is no one simple reason, and the answer is most likely tied to multiple factors acting in concert.
- Microbial adaptation: Microbes mutate, change, and adapt, either through natural mechanisms, such as the influenza virus recombination that occurs in water fowl, pigs and humans, or through man-made pressures, such as the result from the inappropriate usage of antibiotics.
- Population movement: Increases in travel and transport can seed microbes in new territories and expose fully susceptible populations. From 1989-1999, the number of international travelers visiting the United States increased 33 percent, while Americans traveling outside the country increased 44 percent. Microbes from the jungles of Africa can be brought to the cities of the United States within 24 hours.
- Intrusion into new areas: Population movement and construct are also factors that promote disease emergence and spread. Today there are more than 6 billion people on our planet, with 45 percent of them residing in cities. By 2025 this will be nearly 65 percent, with the number of mega-cities (those with 5 million or more citizens) growing from 41 to over 64. With the increase in people comes crowding and the higher likelihood of communicable disease transmission. Also come the standard difficulties in meeting the needs for proper solid and liquid waste disposal, safe drinking water, and maintaining quality public health.
Interface building also occurs in the Southwest, where outdoor cats acquire Yersinia pestis infection from hunting infected rodents or by bites from infected fleas. If not caught early, the disease in cats can develop into pneumonic plague and directly infect people by coughing, sneezing or breathing on them. There have been at least 23 human cases in the United States as a result of pneumonic transmission of Yersinia pestis from cats. Some of these have been in veterinarians who treated the cats. Even a few of these cases have been fatal (Photo 1).
n Population changes: The number and categories of immunocompromised people have significantly risen in the last several decades. The number of elderly, those over 65, in the United States makes up 13 percent of our population. This has increased ten fold since 1900 and it is expected that by 2030 one in five Americans will be over 65 (Figure 3).
This trend in not limited just to the United States either. Immunocompromised populations also include infants, chemotherapy patients, transplant patients and others. Prior to the discovery of AIDS in the United States in 1981, and the viral isolation of HIV-1 in 1983, HIV had been insidiously working its way around the world. HIV is speculated to have emerged in people through direct contact with non-human primates infected with a closely related simian immunodeficiency virus (SIV). There are now approximately 40 million people living with HIV and roughly 3 million deaths each year. This essentially makes the SIV-HIV mutation and transmission one of the most influential and devastating zoonotic diseases in history.
Photo 1: Right hand of a plague patient displaying acral gangrene. Gangrene is one of the manifestations of plague, and is the origin of the term "Black Death" given to plague throughout the ages.
Along with HIV has come the recognition of a host of opportunistic infections, including many that are zoonotic. Prior to 1976, cryptospori-diosis was an uncommon disease in people. This changed with the appearance of AIDS and now occurs in 50-80 percent of all AIDS cases. The organism, Cryptosporidium parvum, has made inroads into other highly susceptible populations too. Each year we can expect to see a number of children acquire cryptosporidium from petting zoos, a source that is also responsible for a number of E. coli 0157:h7 outbreaks (Figure 2).
Human ingenuity has led to a paradox: While we have developed miraculous medical procedures and devices to extend life, we have simultaneously paved the way for disease agents to take advantage of these inventions. Medical treatments may impair the immune system and result in a greater chance of infection. Bone marrow or solid organ transplants, chemotherapy, renal dialysis, chronic corticosteroid therapy, implanted medical devices, radiation treatment and other therapies put patients at higher risk for infectious diseases.
The probability of zoonotic disease emergence and transmission is greatest when public health infrastructure, and/or medical and veterinary care deteriorates. Of the many routes by which zoonoses can be transmitted, indirect contact and vector transmission are the most likely routes when compared to all infectious agents that infect humans.
Ectoparasites, primarily fleas and ticks, serve as vectors for many zoonotic diseases. The Centers for Disease Control and Prevention (CDC) estimates that there are 22,000 cases of Cat Scratch Disease every year. There is some evidence that incriminates fleas in the transmission of Bartonella henselae, the primary causative agent, in cats. This seems fitting since many other Bartonella agents are known to be transmitted by other ectoparasites: Bartonella quintana, the cause of trench fever, is transmitted by lice and was a common disease in WWI where it affected nearly 1 million troops. Today, Bartonella henselae and quintana are both a cause of bacillary angiomatosis in AIDS patients, a vasoproliferative condition that can affect skin and sometimes the liver. In the 1990s, Bartonella quintana emerged among the homeless of large cities in the U.S. and Europe. Lice have not been fully implicated in these settings and it is possible that unidentified vectors or animal reservoirs may be responsible for the spread of this agent.
In a study of zoonotic agents and parents and pediatricians, parents noted that they received their information on toxoplasmosis mainly from newspapers or magazines (40 percent), their obstetrician (35 percent), television (11 percent), their pediatrician (7 percent), and their pet store or veterinarian (5 percent combined).
The veterinarian was last to be consulted. But less than half of the pediatricians knew that undercooked meat and soil served as a source for acquiring toxoplasmosis. Only 5 percent of pediatricians regularly educated patients or their families about pet-associated salmonellosis, even though 95 percent of them knew that reptiles carried salmonella.
In a separate study of 327 veterinarians and 332 physicians in Wisconsin by Grant and Olsen, veterinarians encountered zoonotic diseases or discussed zoonotic diseases more often than physicians, with small animal practitioners encountering zoonotic diseases most frequently.
Overall, veterinarians encountered or discussed zoonotic diseases about once a week, and were almost never contacted by physicians about these diseases. Both veterinarians and physicians listed Salmonella spp. and Toxoplasma gondii as the agents of most concern to immunocompromised people, despite the fact that most infections due to these agents are acquired from food, not directly from animals.
Veterinarians must be proactive when it comes to zoonoses. We should not wait until an owner becomes ill before taking action. We have the educational background, experience and opportunity to serve as experts and should counsel the immunocompromised, the non-immunocompromised, and the human medical community. As veterinarians, we are protectors of animal health, welfare and suffering, but we are also guardians of human health. Indeed, we have an ethical and legal obligation to address public health and have taken a sworn oath to do so. But the job need not be daunting. Barriers between veterinarians and physicians must be overcome and communication enhanced; we cannot let the client and issues of zoonoses fall through the cracks thinking their health care provider will carry the load. The veterinarian clearly has an excellent opportunity to disseminate advice and education when he/she has the animal and the owner together. We must be the cheerleaders for zoonosis education.
There are many resources on the Internet and within published literature that can help in tackling zoonosis prevention and control. The best prevention efforts, however, begin with education. Basic educational efforts target improved hygiene (e.g., close child supervision in hand washing after animal or soil contact, immediate disposal of animal waste), proper pet selection, and use of insect repellent on people and flea and tick products on animals. There are very specific guidelines for the prevention of zoonoses in immunocompromised people that all veterinarians need to be aware of and incorporate into everyday practice. Each of us must seek out the essential educational materials, attend conferences, read journals, and talk to colleagues.
Zoonoses will continue to emerge; we cannot stop them. Indeed, the route of transmission for more than 200 human pathogens is still unknown, and it is likely that many of these may come from animals. As veterinarians, we can make the difference in the health of all species, including humans, through a bolder effort to address known and emerging zoonoses.