It is a fact of life--some people are allergic to rats. this should not seem too surprising since they are furry animals, just like cats and dogs. I must confess to having some symptoms that reflect an allergy to rats. Luckily, the symptoms are not serious. Among the allergic responses that have been reported, are rashes after contact with the rat's toenails or tail and bronchial asthma. Severe conjunctivitis after contact with rat urine has been reported. In extreme cases, allergies have led to anaphylactic shock. Allergic reactions to laboratory animals have caused technicians to change professions and researchers to choose a new research direction. My former major professor developed such an acute allergy to rat dander that he ceased all research with rodents. If you are allergic, don't despair. Face masks, allergy treatments, and antihistamines are sometimes quite helpful.
If you work long enough with rats, you will receive a bite. Laboratory rats are not vicious but will bite if handled improperly and sometimes when they are food deprived. Certain lesions also increase aggression. One thing you must remember. Bite wounds, regardless of species (including humans) must be cleaned thoroughly and an antibiotic ointment applied to the area of the wound. You should also receive a tetanus booster shot before working with rats. If your tetanus shot is not current, get one now!
For a good discussion of disease concerns, consult Percy and Barthold (1993) or the many resources available on the "GC&SU IACUC Animal-Related Links of Interest page."
1. Rodents are involved in the transmission of a variety of diseases found around the world. Although the roles that rodents play in the maintenance and spread of these diseases will vary depending on the disease and the geographic region, certain patterns of involvement emerge.
a. Rodents may serve as intermediate hosts for parasites that ultimately infect man. Although the rodents themselves pose no direct threat to humans, their presence in a geographic region may play a key role in perpetuating infectious organisms that pose a health threat to people in the area. For example, capillariasis, a human liver disease, caused by the adult worm Capillaria hepatica, is primarily an infection in rats.
b. Rodents may serve as reservoirs of disease agents (e.g., Lyme disease spirochete, Borrelia burgdorferi) that are picked up by arthropod vectors and transmitted to humans through bites. In this case, direct contact with the rodent or its excreta pose no health risk; however, the rodent maintains the disease in the geographic area.
c. Rodents may not only serve as reservoirs but may also play a key role in the transmission of the disease. Hantavirus is a good example of this scenario. There is no vector. The disease agent, a virus, is found in the saliva, urine and feces of the infected rodent. Spread to humans is via inhalation of aerosolized excreta, ingestion of excreta, or by direct contact with the rodent itself.
d. Rodents may directly transmit a pathogen to man through bites, as is the case in rat bite fever.
During the first half of this century, laboratory bred rats were frequently subject to outbreaks of clinical diseases of bacterial and parasitic origin. Through selection, improved husbandry, and effective health assessment programs, the nature of the disease problem in laboratory rats has changed markedly over the past 30 years. Overt bacterial disease and major colony health problems from parasites are rare in today's rat colony. The threat now is from sub-clinical infections mostly involving viruses and mycoplasma. The challenges are in their detection and elimination.
Not the least important aspect of health care of laboratory rodents is that of the assessment of the impact of latent infections on the increasingly precise and demanding measurements required of biochemical data. Presumably shifts in the disease spectrum of laboratory rats will continue to occur in response to altering genetic and environmental requirements in biomedical and behavioral research. The role of laboratory animal medicine would, therefore, seem increasingly to involve the development of a better understanding not only of the role of microorganisms in such shifts, but also that of environmental and genetic factors.
At present, the state-of-the-art of health care for laboratory rats relies heavily on the use and establishment of various levels of barrier maintenance for "clean" rats that are the SPF progeny of isolator derived parents. The problems are ones of the effectiveness of various barriers and of weighing need against cost in light of the research objectives. Clearly, the effectiveness of the barrier will be directly reflected in the efficacy of disease control, even though health care in toto obviously involves many other controllable factors (e.g., nutrition, husbandry, etc.) which will be effective under conventional as well as barrier conditions.
Several comprehensive overview chapters on various aspects of health care and diseases in rats may be found in The Laboratory Rat, Volume I, Biology and Diseases (Lindsay, 1979). Other recent monographs on diseases of laboratory rodents (Harkness and Wagner, 1983; Russell, Johnson and Stunkard, 1981) should also be referred to, to supplement the brief annotated listing of rat diseases given here. Treatments have not been included for infectious diseases as they are rarely justifiable in a colony. Either elimination followed by prevention or the use of barrier conditions are the only feasible approaches to the control of contagion.
a.Mycoplasmosis: Murine respiratory mycoplasmosis (MRM) has been advocated as a more appropriate and specific name for CRD (Cassell, Lindsey, Baker et al. 1979). Mycoplasma pulmonis is the organism that plays the major role in chronic respiratory infections in the rat although other bacterial and viral agents may sometimes-be involved. The syndrome is expressed by various signs which may develop either separately or in combinations. Some of these signs have been thought of and described as separate disease entities as in the cases of:
i.Otitis media and/or interna which induces a characteristic circling when the rat is lifted by its tail.
ii.Rhinitis with sneezing and a blood flecked discharge around the nostrils.
iii.Pneumonia with laboured breathing and progressive debility.
M. pulmonis may also infect the genital tract, particularly of females. When present in this form in a breeding colony it may prove a major cause of low fertility through reduced litter sizes or even complete infertility (Cassell and Hill, 1979).
Fortunately, MRM has, over the past few years, been largely eradicated, at least from most major breeding colonies. However, this should not lead to complacency as M. pulmonis is still around in a number of rat colonies and regular monitoring for this and for several viral agents is an essential precaution for all would-be "clean" rat breeding colonies.
b.Bacterial Infections: Severe clinical infections from bacteria are rarely a problem any more in rats. However, mild infections showing clinical signs occasionally occur and latent infections under stress may develop into clinical diseases.
Streptobacillus moniliformis may be present in the nasopharynx of apparently healthy rats and may infect bite wounds (usually to the hands) of persons working with them. The resultant Rat-Bite Fever is a severe systemic infection from blood-borne S. moniliformis (Anderson, Leary and Manning, 1983).
Corynebacterium kutscheri and Streptococcus pneumoniae are the causal organisms of pseudotuberculosis and pneumonococcal pneumonia respectively. Both are usually present only as latent infections but may flare up following stress or when complicated by other pathogens such as M. pulmonis in MRM.
Pseudomonas aeruginosa is ubiquitous in its distribution in conventional colonies but is not normally pathogenic. It is seemingly kept in balance in the gastrointestinal tract of rats by the other microflora normally present, and for this reason infections in barrier colonies may be particularly serious (Weisbroth, 1979).
Leptospera spp. that infect rats will induce only a transitory bacteremia at most. However, this organism settles in the kidney and is shed in the urine. Thirteen serotypes of leptospira have been identified in the rat and at least some of these may cause leptospirosis in man (Geller, 1979). This fact underscores the importance of washing one's hands after handling rats and not smoking or eating in the animal facility.
Salmonella spp. have not generally been considered a problem in rats in recent years and particularly not in barrier colonies. However, these organisms continue to be prevalent in wild rats and other rodents. Infections both in wild and domestic populations will most often be latent, with clinical signs becoming manifest under conditions of stress. The importance of surveillance is indicated by a recent report on a latent S. enteritis outbreak in a commercial barrier colony, which eventually led to the destruction of the whole colony of 35,000 animals (Steffen and Wagner, 1983).
c.Viral Infections: A majority of the viruses known to be naturally infectious for rats cause latent or "silent" infections. Their presence in overly healthy colonies can only be detected by serological monitoring (see discussion on latent virus infections in the chapter on Mice). Agents of the three virus families that are especially widespread among rat colonies will be briefly referred to:
i.Parvoviruses-These DNA viruses are usually latent but may give rise to hepatic, vascular, and neurological lesions under conditions of immunosuppression (Jacoby, Bhatt and Jonas, 1979). Rat parvovirus has frequently been associated with tumours and recently a naturally-occurring rat parvoviral hemorrhagic syndrome has been reported (Coleman, Jacoby, Bhatt et al. 1983).
ii.Coronaviruses-Two antigenically related RNA coated coronaviruses have been isolated from rats as the causal agents of distinct diseases, one of which, sialodacryoadenibs (SDA), is widespread and highly contagious, although also both mild and transitory. Infection with SDA virus leads to an inflammation of the salivary and lacrimal glands. Photophobia, ocular lesions and bulging eyeballs with an overflow of porphyrin (red) tinged tears from the infected harderian glands occur, but usually subside after a week or two (Weisbroth and Peress, 1977). If the inflammatory reaction involves the salivary glands, it may lead to edema in the cervical region. A sub-clinical epizootic of SDA has been reported (Eisenbrandt, Hubbard and Schmidt, 1982). Rat coronavirus (RCV) infection is a related but distinct entity which is primarily pneumotropic, with little or no sialoadenitis being exhibited. RCV infections may prove lethal to neonatal rats but will almost be subclinical in animals over a week old (Jacoby, Bhatt and Jonas, 1979).
iii.Sendax virus (parainfluenza virus) causes a pneumonia in rats which is often associated with intercurrent infections with pneumonia virus of mice (PVM) and/or Mycoplasma pulmonis in MRM.
Spontaneous and experimental infections with Sendai virus alone have caused minimal clinical signs and are of low severity (Jacoby, Bhatt and Jonas, 1979; Castleman, 1983).
a.Dermatomycosis: Ringworm is seen less frequently in rats than in other rodents (mice, guinea pigs). The causal fungal species is probably always one or other form of the polymorphic Tichphyron mentagrophytes. The asymptomatic form of the disease is probably present more frequently than realized (this is particularly true of mouse colonies), and its presence may quite often pass unnoticed until a susceptible person contacts the infection. Treatment with griseofulvin in the feed or drinking water is sometimes successful but the recommended approach in most situations is to destroy the immediately affected group and to thoroughly disinfect everything with which they may have came in contact, before introducing new, clean animals (Weisbroth, 1979).
b.Parasites: Although the rat may harbour very many different ecto- and endoparasites, it is rare for any of these to pose a clinical problem in the properly run laboratory animal rat colony. In theory, parasites of any kind should be completely absent from barrier sustained colonies. In practice, however, occasionally one or other of several species of parasites may be introduced from conventional source animals or through contaminated feed and/or bedding; the more common of these are noted below:
i.Syphacia spp. are the commonly encountered pinworms of mice and rats. These small oxyurid nematodes are ubiquitous. Usually commensal inhabitants of the intestinal tract of clinically normal animals. Syphacia may be transmitted between different species of rodents, have a short, direct life cycle and if pinworm infestations are sufficiently massive they may jeopardize the validity of blood values and distort certain other data in critical behavioral and nutritional research (Kellogg Wagner, 1982). The presence of this parasite may be readily diagnosed by microscopic observation of their ova on a clear cellophane tape anal impression. Once they become established, the control of oxyurid infestations is very difficult. Control measures should include sterilization of bedding, a rigorous campaign against outside rodents, use of filter caps and disinfection of equipment and all ducts (Harkness and Wagner, 1983).
ii.Hymenolepis spp. are dwarf tapeworms and are found in all rodents. The common species in rats are H. diminuta and H. nana, both of which are cestodes that may infect man and other primates. H. nana is the more serious zoonotic hazard as it is capable of a direct life cycle. These parasites are transmitted to and between rodents on contaminated bedding and by insects carrying eggs from one host to another (Harkness and Wagner, 1983; Hsu, 1979). Cysticercus fasciolaris is the larval stage of another adult tapeworm, Taenia taeniaeformis, that may also occasionally be encountered in laboratory rats, gaining entrance through bedding that has been contaminated by cat droppings (Harkness and Wagner, 1983; Hsu, 1979).
b.Alopecia: The behavioral trait of "barbering" was described in the chapter on Mice. This dominant behavioral characteristic is also occasionally encountered in group housed rats and should be differentiated from alopecia due to other, usually more serious, causes (Bresnahan, Kitchell and Wildman, 1983).
c.Allergy: A survey on problems in personnel resulting from association with laboratory rats in the USA and Canada revealed that 23 of 42 responding institutions had personnel who had encountered various allergic reactions to rats. Most sensitive individuals had a personal or family history of allergy (Geller, 1979). In employment situations involving sensitization to laboratory animal danders, the most frequent allergen source appears to be the rat (Lutsky and Neumann, 1975).
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