Major Vector-Borne and Zoonotic Diseases
Authors
The information in this chapter was prepared by the following individuals:
o David Engelthaler
o Tim Flood, M.D., Medical Director, Chronic Disease Epidemiology, Arizona Department of Health
Introduction
The Vector-borne And Zoonotic Disease section of the Arizona Department of Health Services (ADHS) deals with many animal and bug-borne diseases on a regular basis. It also deals with several more obscure disease on an irregular basis.
Five of the more important diseases under surveillance by the ADHS staff are:
o Plague
o Hantavirus Pulmonary Syndrome
o Rabies
o Mosquito-Borne Encephalitis
o Brucellosis
Outbreaks of disease occur due to complex interactions of weather, uncontrolled growth of animal populations, removal of natural predators, and animal encounters with humans.
The importance of these diseases is not only the individual occurrences, but also the constant threat of an epidemic. Therefore, state law requires the reporting of human cases involving these diseases to the state health department.
The following is a brief synopsis of each of these six diseases.
Plague
Plague is an acute infection of man, rodents, rabbits, and certain carnivores. It is caused by the bacterium Yersinia pestis.
Plague can be transmitted to humans either by flea bites or by direct contact of broken skin with infected animal tissues. In rare cases, it can be spread from person to person, or animal to person, by airborne transmission.
Plague is a treatable disease and responds readily to appropriate antibiotic therapy, particularly if diagnosed early. If untreated, however, it can be fatal.
Plague commonly affects the lymph nodes and on rare occasions can spread to the lungs and the bloodstream. There are three major forms of the disease based on the area of the body it affects:
o Bubonic (lymph system)
o Septicemic (circulatory system)
o Pneumonic (pulmonary system)
Plague is endemic to Arizona in areas above 4,500 feet.
From 1950-1993, there were 51 cases of human plague reported in Arizona.
In the last half of 1994, there were five cases of human plague cases in Arizona. Four of these pneumonic and one was bubonic.
ADHS routinely conducts surveillance of plague by monitoring rodent populations, collecting and testing fleas, and testing blood samples from coyotes.
Hantavirus Pulmonary Syndrome
Hantavirus is the viral cause of hantavirus pulmonary syndrome (HPS), a zoonotic disease that was identified in May of 1993.
Hantavirus is carried by wild rodents, particularly deer mice and other closely related mice. Infected rodents are found throughout the western U.S.
HPS is contracted after hantavirus, shed in the feces, urine and saliva of infected rodents, becomes dispersed in the air and is subsequently inhaled by humans. The most common example of this occurs when people sweep up droppings in enclosed areas.
HPS causes a person's lungs to become filled with fluid, making breathing very difficult. Fast diagnoses and immediate intensive respiratory care is the best treatment.
The present fatality rate for HPS is about 52%.
During 1993 and 1994, there were 95 humans cases of HPS confirmed in 20 states. In Arizona, a total of 18 cases were confirmed, seven of which were from 1994.
ADHS performs trapping at case sites to determine the infection rates of the rodents present. Trapping is also being done in at least 12 different habitats to determine the prevalence of hantavirus throughout the state.
Rabies
Rabies is a viral disease which attacks the central nervous system of its victims, and is almost always fatal once symptoms appear.
The virus can be transmitted through bites from infected animals or through saliva exposure to open wounds or mucous membranes.
Human rabies is rare in the U.S. Only four human cases of rabies have been confirmed in Arizona. The last in-state exposure occurred in 1970. The last human case was from a Mexico dog exposure in 1981. All four were fatal.
In Arizona, rabies most commonly occurs in skunks, bats and foxes, but can show up in any mammal, including dogs and cats.
The last reported rabid dog was in 1986, and the last reported rabid cat was in 1990.
In 1994 there were 51 wild rabid animals reported in Arizona.
ADHS monitors the rate of rabies through reports submitted by counties, and by trapping skunks, coyotes, and other animals.
Encephalitis
Mosquito-borne encephalitis in Arizona is caused by two viruses: St. Louis Encephalitis virus and Western Equine Encephalitis virus.
These viruses cause an acute inflammatory disease of the brain and the central nervous system in humans and horses. Severe infections can lead to seizures, paralysis, coma and/or death. There is about a 3-4% fatality rate.
Both viruses are transmitted by Culex tarsalis and other Culex mosquitoes, which are found in every county in the state. Birds act as the reservoir for the viruses.
In 1994, there was one unconfirmed case of human St. Louis Encephalitis in Maricopa, and the virus was identified in mosquitoes collected in Mojave Valley. Before 1994, the last human case of mosquito-borne encephalitis in Arizona was in 1991.
ADHS maintains surveillance of the viruses mainly through monthly state-wide mosquito collections, with the help of various county health departments. Occasionally the blood of birds is sampled when there is an increased likelihood of an outbreak.
Brucellosis
Brucellosis is a bacterial disease that can last for several days, months, or even for a year or more.
The main carriers of Brucella, the bacteria causing brucellosis, are cattle, swine, goats, sheep and less commonly, dogs.
Most human cases of brucellosis occur from ingesting unpasteurized dairy products from infected domestic animals. It can also be transmitted by coming in contact with tissues and bodily fluids of infected animals.
Airborne infection from pens, stables or laboratories or by self-inoculation of the vaccine are also possible modes of transmission, but are much less common.
In humans, brucellosis causes irregular fever, headache, weakness, muscle and joint pain and weight loss. It is easily diagnosed and treated.
When untreated, brucellosis has about a 2% percent fatality rate.
Arizona was declared brucellosis free in 1988. In 1994, however, there were 15 human cases of brucellosis.
Surveillance of brucellosis in Arizona is a cooperative effort involving the State Agriculture Department conducting livestock surveillance and ADHS conducting disease investigation of human brucellosis cases.
Conclusion
There is a constant threat of disease being passed from animals to humans. Many of these diseases can occur suddenly and spread rapidly.
It is difficult to average the number of humans who contract the diseases, but we estimate the number of deaths to be about three per year.