The following diseases are some of the more common diseases known to be transmitted by domestic rodents in San Diego County.
Plague - The Plague is caused by the pathogenic bacterium, Yersinia pestis. Plague usually takes three forms which are bubonic, septicemic, and pneumonic. Bubonic plague is the most common form of plague and is usually caused by the bite of an infected flea. It is characterized by swollen buboes of the lymphatic system. Bacteria can then spread throughout the body which is called septicemic plague. Pneumonic plague involves the lungs, and can be transmitted directly from person-to-person. Fatality is about 50% if left untreated. Roof rats have been implicated in transmitting bubonic plague to humans in many urban areas of the world. They are more susceptible and are more frequent hosts for plague than Norway rats. The Oriental rat flea, Xenopsylla cheopis, is normally found on Norway rats and is one of the most efficient flea vectors for this disease (Weber 1982). However, when roof rats, common in households during the Middle Ages, began dying from plague, Oriental rat fleas sought alternate hosts such as humans and their pets resulting in what was called the black death, killing over 25 million people in Europe alone. In the United States, human cases of urban plague involving domestic rats have not been reported since 1925. After 1925, all cases have been caused by sylvatic rodents in the western states (Barnes 1982). In California, approximately two human cases of plague are reported each year. During a 1991-1992 flea survey of the county, X. cheopis was not found on domestic rodents (Lang 1993). Another rat flea, the northern rat flea, Nosopsyllus fasciatus, occurs on Rattus spp. and house mice, and may cause some human plague cases when flea numbers are high, although it was shown to unlikely initiate epizootics or maintain plague in rat populations for any length of time (Pollitzer 1954).
Salmonellosis - Salmonellosis, caused by a number of different species of Salmonella bacteria, ranks as one of the most widespread animal-borne diseases. Human cases of food poisoning caused by Salmonella can occur when rodents, particularly domestic rats or mice, defecate or urinate on food, dishes, kitchen counters, floors, and other objects associated with humans thus contaminating food or drink. The most common clinical syndrome is acute gastroenteritis accompanied by abdominal cramps, anorexia, diarrhea, head-ache, high fever, and vomiting.
Leptospirosis - Leptospirosis or Weil's disease is caused by the bacterial spirochete, Leptospira icterohaemorrhagicae, and is transmitted among domestic rats and mice and to humans via contact with infected rat urine. It is one of the most important diseases transmitted by rats to humans (Weber 1982). Dogs, cats, and other domestic animals may also become infected and act as host reservoirs. The bacteria appear to enter the skin through small scratches, or via the nose or pharynx, or the conjunctivae when rubbing the eyes. Humans may become infected by swimming or bathing in contaminated water, or from contaminated moist soil or animals, handling infected items, by eating contaminated food, or from rat or mouse bites (Weber 1982).
Seoul Virus - Seoul virus is a strain of hantavirus belonging to the family Bunyaviridae. The primary host reservoirs for this virus are various species of Rattus. The virus is transmitted among rodents and to humans primarily by contaminated feces and urine. Seoul virus causes hemorrhagic fever with renal syndrome, with the mortality rate being 1 % or less. The virus was first identified in Asia but infected rats have also been found in many other parts of the world (Lee et al. 1990). In the United States, R. norvegicus was found to be the major urban reservoir for Seoul-like hantaviruses (Childs et al. 1987). In California, three of 97 R. norvegicus were positive for a Seoul-like virus in San Francisco, and two of 108 of these rats were positive (Tsai et al. 1985). In Orange County, one of 500 R. rattus were positive for Seoul virus (Bennett, S., pers. commun.). However, in San Diego County, during 1992 - 2000, six of 15 R. rattus tested were positive (40 %) from three localities (Felicta County Park, Penasquitos Preserve, and Mariner's Point), with Penasquitos yielding four of the six positives (Lang, unpub. data).
Murine Typhus - Murine (endemic) or flea-borne typhus is caused by Rickettsia typhi. Murine typhus is a disease of rodents, particularly domestic rats. It can be transmitted to rodents or other hosts such as humans when infected fecal material from a flea is scratched into the skin or rubbed into the flea bite or an open sore or wound. In California, X. cheopis and the Norway rat were shown to be the usual respective vector and host reservoir during the first half of the 20th century (Adams et al. 1970). During this period human cases were confined to urban areas of southern California, to include 91 cases in San Diego County (Estey, J., unpub. data). Since then, cases have occurred in more suburban/foothill areas of other counties in southern California where Didelphis marsupialis and their fleas, such as cat fleas, Ctenocephalides felis, may be responsible (Adams et al. 1970). Domestic rats, may also be hosts since they were shown to be positive for the rickettsia (Ramirez, J. pers. comm.). Fleas from domestic rats may also be implicated as vectors. The house mouse flea, Leptopyslla segnis, was found naturally infected with R. typhi and may vector it among rats, while N. fasciatus may not be an important vector although further investigations need to be conducted (Schwan et al. 1985). One human case of murine typhus has been confirmed in San Diego County during 1981 - 1999 (J. Estey, unpub. data.). The patient lived in a rural area and had some evidence of infestation by R. rattus. The source for this infection was never proven. None of the 15 roof rats trapped either on or near her premises were positive for R. typhi.
Rat Bite Fever - Rat bite or Haverhill fever is caused by the bacterium, Streptobacillus moniliformis. Domestic rats and mice can also transmit the bacterium from rat to rat and to humans through secretions of the mouth, nose, or eyes, although usually involvement is by a rat bite (Weber 1982). House mice are not consider-ed as important reservoirs for the bacterium. The fatality rate for untreated cases can be up to about 10%. Secondary infection and tetanus can also occur if the wound is not properly disinfected and treated. Human cases of rat bite fever are not common in California.
Trichinosis - Trichinosis is caused by the nematode, Trichina spiralis, which lives in the muscles of domestic rodents. Domestic rats are the primary reservoir hosts and become infected usually by eating other infected rats (Weber 1982). Pigs, dog and cats, and other animals acquire T. spiralis by eating infected rats or by eating food containing infected rat feces. Humans usually become infected by eating raw or improperly cooked pork. Occasional infections also occur from bear meat or wild boar.
Tropical Rat Mites - The tropical rat mite, Ornithonyssus bacoti, occurs on domestic rats and in their nests. If a rat has a nest in an attic or other site inside a home, and it dies or vacates the nest, these mites will leave the nest and body in search of a host and can attack humans. The bites of these ectoparasites cause irritation and sometimes a painful dermatitis leaving red spots on the infested skin (Ebeling 1975). Scratching can cause secondary infection. O. bacoti has not been found to be a vector for disease, although this mite can harbor various causative agents of disease for long periods following ingestion (Weber 1982). These agents cause tularemia, Q fever, and murine typhus.
References:
- Adams, W. H., R. W. Emmons, and J. E. Brooks. 1970. The changing ecology of murine (endemic) typhus in southern California. Amer. J. Trop.Med. Hyg. 19: 311-318.
- Barnes, A. 1982. Surveillance and control of bubonic plague in the United States. Symp. Zool. Soc. London. 50: 237-270.
- Childs, J. E., G. W. Koreth, G. E. Glass, et al. 1987. Epizootiology of Hantavirus infections in Baltimore: isolation of a virus from Norway rats, and characteristics of infested rat populations. Amer. J. Epidemiol. 126: 55 - 68.
- Ebeling, W. 1975. Urban Entomology. University of California, Los Angeles. Div. Agric. Sci. 695 pp.
- Lang, J. D. 1993. Fleas found on Mammals in San Diego County, California. Bull. Soc. Vector Ecol. 18: 114-124.
- Lee, H. W., P. W. Lee, L. J. Baek, and Y. K. Chu. 1990. Geographical distribution of hemorrhagic fever with renal syndrome and hantavirues. Arch. Virol. (Suppl. 1): 5 - 18.
- Pollitzer, R. 1954. Plague. W. H. O. Monogram Ser. No. 22, Geneva, Switzerland. 698 pp.
- Schwan, T. G., D. Thompson, and B. C. Nelson. 1985. Fleas on roof rats in six areas of Los Angeles County, California: their potential role in the transmission of plague and murine typhus to humans. Amer. J. Trop. Med. Hyg. 34: 372-379.
- Tsai, T. F., S. P. Bauer, D. R. Sassa, et al. 1985. Serological and virological evidence of a Hantaan virus-related enzootic in the Unites States. J. Infect. Dis. 152: 126 - 136.
- Weber, W. J. 1982. Diseases Transmitted by Rats and Mice. Thomson Publications, Fresno, California. 182 pp.
