ANNUAL PUBLIC HEALTH: Zoonoses: Nothing You Get At The Zoo

Zoonoses are diseases of animals that are transmissible to humans. Those of us in the United States are fortunate to only have a few zoonotic diseases that significantly impact humans, most notably West Nile virus and Lyme disease. While the incidence of other zoonoses is low and, thus, less notable, in some cases the three discussed in this article can be fatal.

Most zoonotic diseases have the following characteristics: disease organism, arthropod vector, animal reservoir and human host.

The typical sequence of events in zoonotic disease transmission is that the vector picks up the organism while biting an infected reservoir animal. The reservoir animal may or may not be directly affected by the pathogenic organism. If the arthropod subsequently feeds upon a human, the organism typically is transferred through the bite. In zoonoses, humans usually are considered dead-end hosts, i.e., the organism is not transferred from person-to-person through subsequent feeding of another arthropod.

ROCKY MOUNTAIN SPOTTED FEVER. Until the discovery of Lyme disease, Rocky Mountain spotted fever (RMSF) was the most significant zoonotic disease within the U.S. In terms of incidence, it ranks second to Lyme disease. The causative organism for this disease is Rickettsia rickettsii and is transmitted by the bite of an infected Ixodid tick (hard tick).

According to the Centers for Disease Control and Prevention (CDC), RMSF is the most severe and frequently reported rickettsial disease in the United States. In the past 50 years, 250 to 1,200 cases have been reported to CDC annually. Most infections are reported during the summer months when ticks are most active. The initial flu-like symptoms include headache, fever and muscle pain followed by a rash. If misdiagnosed and/or untreated, RMSF can be fatal; even when treated, 3 to 5 percent of affected individuals die. Despite its name, the disease is widespread across the continental United States and from southern Canada to parts of South America.

The reservoir animal for the disease organism are infected ticks, which are capable of passing the organism to successive generations in several ways:
• Males can pass the organism in their sperm to female ticks.
• Transovarial transmission: the female passes the organism to the eggs.
• Transstadial transmission: an infected larva or nymph passes the organism to the next stage, i.e., nymph and adult, respectively, when it molts.

The two primary vectors of the disease in the United States are the American dog tick (Dermacentor variabilis), found east of the Rocky Mountains and on the Pacific coast, and the Rocky Mountain wood tick (Dermacentor andersoni), found in the Rocky Mountain states and Canada. The incidence of infected ticks is low, 1 to 3 percent.

Both ticks have similar life cycles. In summer, the engorged American dog tick female drops off the host animal and lays from 4,000 to 6,500 yellow-brown eggs in a sheltered location. The eggs hatch in 36 to 57 days and the larvae seek rodents or other small animals for their blood meal. After molting to the nymphal stage, the ticks once again seek hosts. The engorged nymphs molt to the adult stage, which usually feeds on dogs or other large mammals.

Development (egg to egg) can be completed within three months, but each stage is remarkably resistant to starvation and the life cycle may be prolonged for up to two years. Unfed adults can live for two to three years.

Larval and nymphal activity begins in March and continues until mid-July. Nymphs are more abundant during the summer. Adults usually are active in the spring when they are found in “waiting positions” on vegetation along paths and trails. They are picked up by passing animals, attach, begin to feed and mate. It is during this questing activity that the likelihood of human exposure increases.

EHRLICHIOSIS. The second of the tick-borne zoonotic diseases discussed in this article was first diagnosed within the United States during the mid-1980s. During an 11-year period from 1986 to 1997, more than 1,200 cases of human ehrlich-iosis were reported to CDC. Ehrlichiosis is caused by at least three types of Ehrlichia bacterial organisms that occur in the United States. The disease was first diagnosed by veterinarians in animals, e.g., dogs, cattle, sheep and goats. Human cases are typically reported in individuals older than 40 years old. However, the occasional fatalities from the disease span a wide range of ages.

Transovarial transmission of the disease organism does not appear to occur. The most significant means of disease transmission is transstadial, i.e., the larvae are uninfected when they first feed. Those that become infected as they feed on an infected animal transfer the organism to the nymphal stage which, subsequently can transmit it to another animal, e.g., a human. Currently, it appears that only tick nymphs and adults are involved in transmission.

About 80 to 90 percent of human cases are reported in April through September. This corresponds with the period of greatest tick activity. Most cases are reported during the summer months.

The first human cases of Ehrlichiosis (Ehrlichia chaffeensis) were reported in 1987 from the Southeast and South Central states. The primary vector of the disease in these geographic regions is the lone star tick (Amblyomma americanum) and the primary reservoir animal appears to be white-tailed deer.
Human granulocytic ehrlichiosis (HGE) was the second form of the disease discovered in 1994. The causative organism has yet to be named. The primary vector of HGE is the black-legged tick, Ixodes scapularis, in the eastern half of the United States and the western black-legged tick, Ixodes pacificus, along the Pacific coast. Both tick species are better known as vectors of Lyme disease. Deer, elk and wild rodents are believed to be reservoirs for the organism.

The third and most recently discovered human ehrlichia pathogen is Ehrlichia ewingii. The isolation of this pathogen is limited to a few patients in Tennessee, Missouri and Oklahoma. The vector, distribution and disease etiology currently are under investigation. However, it appears that the lone star tick, Amblyomma ameri-canum, may be a primary vector and dogs may be one of the principal reservoir animals based on findings that this tick is responsible for the transmission of the organism between dogs in the Southeast United States.

HANTAVIRUS. Hantavirus is a type of RNA virus related to the Bunyaviridae. The organisms are capable of causing either Hantavirus pulmonary syndrome (HPS) in the western hemisphere or hemorrhagic fever with renal syndrome (HFRA) occurring only in east Asia.

There are many types of Hantavirus that are commonly found in host animals, but only a few cause disease in humans. The most prevalent type of Hantavirus causing HPS in the U.S. is Sin Nombre virus or “Four Corners virus.” Three other types, New York-1, Black Creek Canal and the Bayou, have been identified in the U.S.

The CDC reported 289 HPS cases through January 2002; 38 percent of people with reported cases died. Males account for 60 percent of the cases reported. The mean age for all cases is 37 years (range: 10 to 75 years). Caucasians represent 78 percent of the cases followed by Native Americans with 19 percent of the cases.

Cases have been reported from 31 states, most west of the Mississippi River and nearly 75 percent of the cases are reported from rural residents. The disease has also been reported from several countries in South and Central America.

The hosts of the Hantavirus are rodents and, in the United Status, the primary species is the deer mouse, Peromyscus maniculatus. To lesser extents, the cotton rat, Sigmodon hispidus, the marsh rat Oryzomys palustris and the white-footed deer mouse, Peromyscus leucopus, serve as reservoirs.

Recent studies have found that infected rodents are present in all habitats, but the prevalence of infection is higher in Peromyscus and specific geographic areas. The fact that Peromyscus is the primary genera involved in the transmission of the disease is of concern because of its propensity to live in close association with humans when compared to other rodent genera. Two additional factors that enhance this genera’s ability to transmit the disease are larger population densities and a wider range of habitats. 

Typically HPS infections are acquired by inhaling virus-laden dust contaminated with urine and/or feces. This usually occurs when people work in areas that have been heavily infested with one of the above rodent species and where there are significant accumulations of urine and droppings. In addition, the disease can be contracted by exposure to contaminated fresh or dry materials, which are introduced into broken skin and eyes, or by ingestion. Persons bitten by rodents have also been infected. Transmission does not occur through contact with infected humans or arthropod bites.

Care should be taken when working in environments where there have been large Peromyscus infestations. Areas of greatest concern are vacant buildings, sheds, garages, crawlspaces and attics. Areas should only be entered when wearing proper personal protective equipment, i.e., respirator with HEPA filter, rubber gloves, coveralls and goggles.

As a pest management professional, you are probably at greater risk for exposure to these diseases than many of your customers. Be sure to use proper personal protective measures and be careful out there.

Editor’s note: The table on pages 64 and 65 presents a selected list of the diseases most often associated or perceived to be common with rodent populations in urban areas. Diseases such as lymphocytic choriomeningitis, tularemia, dysentery, and other rodent-borne diseases are important in certain parts of the world.

The table is courtesy of Robert Corrigan, author of Rodent Control: A Practical Guide For Pest Management Professionals. Your can order the book by calling 800/456-0707. 

The author is technical director of American Pest Management, Takoma Park, Md. He can be reached at 301/891-2600 or rkramer@pctonline.com.