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A 3-year-old castrated male cat became sluggish and after 2 days stopped eating and developed a swelling in the head region.

The attending veterinarian discovered that the cat and its owners had just moved from a small town in west Texas near the New Mexico border 1 week previously. Two cats and a small dog also were in the household, but only the 1 cat was sick. The cat's diet consisted of dry commercial cat food, and the cat slept in the house at night. However, during the day, the cat was allowed outdoors and, in the previous neighborhood, was especially fond of hunting in a nearby overgrown area. Occasionally, the cat would kill a bird or small rodent and bring it into the backyard to eat. The cat had been seen with a ground squirrel just a few days before moving.

The veterinarian found that the cat was well nourished, but had a rectal temperature of 41 C, swollen, sensitive, bilateral cervical and submandibular lymph nodes, moderate facial edema, and a slight serous nasal exudate. Auscultation indicated that the cat had mild pulmonary congestion. A slightly reddened small bite wound was found over the area of the scapula (the owner indicated that the cats occasionally bit each other during play). Thoracic and abdominal radiographic findings were normal.

The owners reported that rabies was common in the wildlife in the area from which they had moved, and all their animals had been immunized annually against rabies, but had not been immunized against other diseases.

The cat had a WBC count of 18,000/µl, PCV of 44%, plasma protein concentration of 8.4 g/dl, and moderately icteric serum.

Bacterial infection attributable to trauma was diagnosed presumptively, with the infection presumed to be the result of the bite of another cat. The cat was given amoxicillin (5 mg/kg of body weight, q 12 h, IM, for 5 days) and was hospitalized.¹ The following morning, the cat's cervical and submandibular lymph nodes had become more swollen, and one of the submandibular lymph nodes had abscessed. The abscessed node was drained and flushed with sterile normal saline (0.9% NaCl) solution. Concerned about the continued swelling, the veterinarian obtained a specimen of the purulent material for bacteriologic culture. After 48 hours, the report indicated that a gram-negative, bipolar staining, indole-negative organism was isolated. Because results of other biochemical tests indicated that the organism was not Pasteurella multocida and because 48 hours had been required before colonies developed, Yersinia pestis (the causative organism of bubonic plague) was suspected. Because plague was not common to the area, further steps to positively identify the organism were initiated, and the assistance of the Centers for Disease Control Plague Laboratory was requested. The final report confirming that the organism was Y pestis was received 7 days later. However, during those 7 days, the veterinarian was cautioned to consider the cat presumptively infected with Y pestis and was instructed to take necessary precautions.

Although the veterinarian was aware that plague was an infectious disease with serious human consequences, the veterinarian was not sure of the public health implications and how the cat in the hospital should be managed. Consequently, the veterinarian telephoned the state public health veterinarian for answers to the following questions.

Questions from attending veterinarian

Q: What is the public health importance of plague (ie, Y pestis infection) in the cat?

A: The domestic cat can be a source of human infection with Y pestis.^2-4 Veterinarians and owners have been infected while attempting to care for Y pestis infected pets⁵; therefore, cats infected with Y pestis must be considered potential sources of infection and should be handled accordingly. Human infection can result from handling tissues of infected animals, such as blood or purulent material.⁶ Because of the presence of airborne droplets, cats infected with plague pneumonia are a greater risk to human beings handling them than cats infected with the bubonic form. In cats, plague pneumonia may develop secondary to primary bubo infections in the head and neck region.

Because the cat that you treated did not have radiographic evidence of pulmonary involvement the risk of Y pestis infection to those in contact with the cat is reduced somewhat. However, you, your assistants, and the family members that handled the cat after it became ill must be considered to have been exposed; therefore, you should consult a physician immediately.

Q: How can a cat acquire plague, and what is the nature of the disease in the cat?

A: Experimentally, cats have been infected with Y pestis via parenteral and oral inoculation.^7,8 Naturally acquired infection probably results from the bite of infected fleas or the consumption of infected small animals, usually rodents.⁵ The incubation period in cats may be as short as 1 to 2 days. Usually, feline plague is associated with marked lymphadenopathy and fever (up to 41.7 C).³ Primary buboes that result from eating rodents infected with Y pestis generally will be found on the head and neck.³ Although uncommon, primary septicemic plague can develop without buboes.³ Complications secondary to the bacteremia include lymphadenopathy at sites remote from the primary bubo or buboes, subcutaneous abscesses, splenitis, multifocal hepatic necrosis, pneumonia, and pleuritis.³ Initially, the affected lymph nodes are hemorrhagic and necrotic and are surrounded by edema. When an infected cat survives long enough, abscesses containing thick, creamy pus can rupture and drain through fistulous tracts in the skin. Some cats may drool and sneeze, possibly indicating active infection in the oral cavity or in the upper respiratory tract.³

The clinical course may be acute, with death or recovery within 7 days after onset of illness, or may be chronic, with emaciation or death within 2 to 4 weeks after onset.³

The known enzootic area for plague in the United States includes 15 western states and extends from the 101st to about the 97th meridian.⁹ Surveillance data indicates that the plague organism remains widespread and strongly entrenched among wild rodent populations in the enzootic area.^9,10 Quantitative changes in antibody titers against Y pestis in wild carnivores in this enzootic area provides evidence that the disease occurs commonly in these animals. Plague demonstrates a pattern of geographic amplification among susceptible species in the enzootic area resulting in brief epizootics which then regress to enzootic levels in focal areas.⁹ If the cat in question intruded into an area where an epizootic was occurring in the rodent population the probability of the cat acquiring the infection would be high.

Q: What diagnostic steps should be taken if you suspect that an animal has plague?

A: Radiography, microscopic and bacteriologic examinations of blood and lymph node aspirates, and a WBC count should be performed. Radiographic examination of the thorax will help determine whether the animal has pneumonic plague and, thus, whether aerosol transmission of Y pestis via the animal is a danger. Radiography should be repeated if the animal begins to have clinical signs of respiratory tract involvement, such as coughing. A lymph node aspirate can be submitted to a diagnostic laboratory in a syringe, in a blood collection tube, or in a transport medium, such as Cary Blair.^a Serum should be submitted to a laboratory for serologic evaluation. If a lymph node aspirate cannot be obtained, a blood specimen should be obtained immediately, added to a suitable blood transport medium, and submitted for bacteriologic culture. Specimens from animals suspected of having plague may be sent to the Centers for Disease Control, Plague Laboratory, Fort Collins, Colo, provided that authorization of the state public health agency is obtained before shipment. Specimens should be placed in double-plastic bags to preclude leakage and packed and shipped in accordance with US government regulations.^b Sufficient coolant should be packed with the specimens to hold them at 4 C to 8 C during transit.

Q: How should a cat with plague be managed in the hospital?

A: A cat with pneumonic clinical signs should be kept under strict isolation. Hospital staff should wear single-use, high-efficiency filtration surgical masks, gowns, and surgical gloves when entering the isolation room and handling the cat, until 2 days after all clinical signs of respiratory tract illness have ceased. If respiratory signs are not evident, personnel should continue to wear protective clothing until 4 days of specific antimicrobial treatment have been completed and clinical improvement is evident; however, personnel should continue wearing surgical gloves when handling the cat or its excretions.³

Masks, gowns, and gloves should be worn while collecting and examining clinical specimens or performing necropsies. Because of the risks involved, private veterinarians should not perform necropsies of cats suspected of having plague. The intact carcass should be double bagged in water-impervious plastic bags and then submitted to a public health laboratory. The laboratory should be contacted before shipment for special marking and shipping instructions.

A cat with suspected or confirmed infection with Y pestis should be isolated as quickly as possible, and the hospital should be cleaned and disinfected thoroughly. Yersinia pestis is susceptible to drying and does not survive beyond 2 to 3 hours unless protected in organic material such as pus or sputum.¹¹ Therefore, all surfaces contaminated by the cat should be cleaned thoroughly to remove organic matter and an appropriate disinfectant, such as phenolic or iodophor disinfectants, should be applied.^3,11 Animal wastes and litter should be double bagged in water-impervious plastic bags and incinerated. Gowns and other contaminated clothing should be autoclaved or should be incinerated with the animal wastes.

Q: Can cats with plague be treated successfully or should they be euthanatized?

A: If cats with plague can be isolated and treated during the initial phase of the disease, euthanasia is not justified. However, precautions should be taken when treating infected cats, especially if medications are given orally, because Y pestis has been isolated from the pharyngeal fluids of cats up to 10 days after they have been fed infected rodents.⁸ Because of the danger of aerosol transmission of Y pestis from cats with pneumonic plague, cats should be euthanatized when the cat cannot be isolated completely or when precautions to prevent infection during treatment cannot be taken.

In man, streptomycin, tetracycline, and chloramphenicol are highly effective in the treatment of bubonic plague, with streptomycin being considered the drug of choice.^3,6,7 The lack of experience in treating feline plague makes drawing conclusions regarding the preferred drug for treating cats difficult; however, successful treatment of infected cats with tetracycline and amoxicillin has been reported.^12,13 One adult cat infected with Y pestis in the west Texas area was treated successfully with tetracycline oral suspension (20 mg/kg, q 8 h, PO, for 10 days).^c

Q: Fleas were seen on the cat at the time of hospitalization. Are the fleas capable of transmitting the disease?

A: Possibly. Bacteremia can be severe in infected cats and blood from such cats may be infective for fleas feeding on the cat⁸; therefore, fleas must be considered capable of transmitting the disease. The cat, its home environment, and animals and persons that have come in contact with the cat should be treated for fleas with a product that has a residual effect such as 5% carbaryl dust.

Actions taken

After discussing the case with the public health veterinarian, the veterinarian treated the cat for fleas and placed the cat in isolation. The veterinarian's hospital was cleaned and disinfected, and the hospital staff was instructed on the management of the infected cat. The veterinarian called the owner of the cat and reported that the cat was presumed to have plague and that the cat was being given tetracycline (25 mg/kg, q 8 h, PO, for 10 days). The veterinarian advised the owner that members of the family that cared for the cat may have been exposed to the causative agent and suggested that they contact a physician immediately. The owner then asked the veterinarian the following questions.

Questions from owner

Q: What about the other cat? Should it be hospitalized even though it appears to be well?

A: No. The cat does not need to be hospitalized, but it should be treated prophylactically with tetracycline for 7 days. During the 7-day treatment the cat should be observed carefully, and if clinical signs of sickness such as large lymph nodes or fever develop, the cat should be examined immediately by a veterinarian. Because the hospitalized cat had fleas the other cat and its surroundings should be treated with 5% carbaryl.

Q: Do we need to be concerned about the dog?

A: Probably not. However, if the dog appears sick or becomes ill, the dog should be brought in for examination. Generally, the canine family is believed to be more resistant to plague than the feline family and if they develop the disease at all, the disease usually is subclinical, ie, not easily recognizable. Wild carnivores in the dog family, such as coyotes and foxes, can develop blood titers against Y pestis.^7,8 Experimentally, dogs inoculated with Y pestis develop bacteremia and have had Y pestis isolated for 10 days from the oropharynx.⁷ Subclinically and clinically affected dogs can develop antibodies against Y pestis,⁷ and people have contracted plague after skinning infected coyotes and foxes.^7,9

Discussion

Yersinia pestis is well established in the rodent population of the western United States⁹ and devastating epizootics develop periodically among susceptible rodent and flea populations resulting in high morbidity and mortality. Susceptible domestic animals, such as cats, that inadvertently intrude into these areas during an epizootic run a high risk of becoming infected with Y pestis, either as the result of eating an infected rodent or being bitten by an infected flea. Because cats can be transported easily from an enzootic area where they contract the disease to an area where the disease is unknown veterinarians should consider a diagnosis of plague in any cat with lymphadenopathy and fever.

Footnotes

(a) Quan TJ, Centers for Disease Control, Plague Laboratory, Fort Collins, Colo: Personal communication, 1986.

(b) Interstate shipment of etiologic agents, 42CFR 72, 1986.

(c) Gardner BD, Odessa, Tex: Personal communication, 1982.

References

1. Fraser CM (ed). The Merck veterinary manual. 6th ed. Rahway, NJ: Merck & Co Inc, 1986.

2. Rollag OJ, Skeels MR, Nims LJ, et al. Feline plague in New Mexico: report of five cases. J Am Vet Med Assoc 1981.

3. Kaufman AF, Mann JM, Gardiner TM, et al. Public health implications of plague in domestic cats. J Am Vet Med Assoc 1981.

4. Weniger BG, Warren AJ, Forseth V, et al. Human bubonic plague transmitted by a domestic cat scratch. JAMA 1984.

5. Human plague associated with domestic cats--California, Colorado. MMWR 1981.

6. Benenson AS (ed). Control of Communicable diseases in man, US Public Health Association Plague (Pests) ICD-9 020, Washington, DC, 1985.

7. Poland J, Barnes A. Plague. In: Steele JH, ed. CRC handbook series in zoonoses Boca Raton, Fla: CRC Press, 1979.

8. Rust JH Jr, Cavanaugh DC, O'Shita R, et al The role of domestic animals in the epidemiology of plague. J Infect Dis 1971.

9. Barnes AM. Surveillance and control of bubonic plague in the United States. Symp Zool Soc London 1982.

10. Acha PN, Szyfres B. Zoonoses and communicable diseases common to man and animals. Pan American Health Organization, Scientific Publication No. 354, Washington, DC, 1980.

11. Bryan AH, Bryan CA, Bryan CG. Bacteriology. Principals and practice. New York: Barnes and Noble, 1962.

12. Fitch R, Smith L, Christenson SL. Feline plague in plumas county. Calif Morbidity Weekly Rep 1977.

13. Plague in California's review and recommendations for prevention. Calif Morbidity Weekly Rep 1986.

Addendum (1994)

Plague continues to be an endemic, emerging disease in western United States; cases were reported in 13 states between 1984 and 1993. More than 80% of these cases were found in 3 southwestern states: New Mexico, Arizona, and Colorado; another 10 were in California. During early 1993 and 1994, 11 confirmed human plague cases were reported; 7 of these patients were exposed at their homesite. One, a Colorado veterinarian, was exposed while examining a cat with oral lesions and a swollen tongue. Domestic cats were not reported as a source of human infection prior to 1977, but since then, these animals have been the source of infection in 15 human cases. Four cases involved veterinarians or their assistants. Persons working in veterinary practice should be especially aware of the risks involved in handling Yersinia pestis-infected cats.¹

Surveys of plague in wild animal populations during the 1990s have indicated that plague has spread to counties of the western Great Plains region, where it was not known to exist during more than 50 years of plague surveillance. The potential for human cases to develop in eastern Texas was documented in May 1993, when the Texas Department of Health and Centers for Disease Control (Atlanta, Ga) reported that a Y pestis-infected roof rat (Rattus rattus) and a fox squirrel (Sciurus niger) were found in Dallas County. This was followed by discovery of a second infected fox squirrel in June. Active surveillance of area hospitals and clinics failed to identify any human plague cases. A surveillance program was established to monitor plague in the rodent population in the Dallas metropolitan area. In March 1993, the Texas Department of Health reported a plague epizootic in fox squirrels in Taylor County. This 'die-off' was localized on a ranch south of Merkal, Texas. Results of serologic tests performed on carnivores in surrounding counties were positive for Y pestis.

Reference

1. Human Plague-United States, 1993 and early 1994. US Health and Human Services, MMWR Morb Mortal Wkly Rep 1994; 43(13):242-246.