Chapter 9 Pansystemic Diseases
Pansystemic diseases include those that involve multiple body systems in addition to the primary target organ. The causes of these diseases may be viral, bacterial, or parasitic, and secondary infections are common. Box 9-1 lists some of the most commonly seen pansystemic diseases of the dog and cat.
Box 9-1 Common Pansystemic Diseases
Feline panleukopenia is caused by a DNA virus of the family Parvoviridae, which is closely related antigenically to the canine parvovirus (CPV), type 2. The disease is primarily one of young, unvaccinated cats and feral animals. Transmission is by direct contact or from a contaminated environment. The virus shed into the environment may remain infectious for years.
Feline parvovirus multiplies within mitotic cells of the neonatal brain, bone marrow, lymphoid tissue, and in the intestinal lymphoid tissue, causing destruction of the cells with release of a large number of virions. The incubation period is usually 4 to 5 days. Signs may be peracute, acute, subacute, or subclinical.
Feline infectious peritonitis (FIP) is primarily a disease of catteries and multicat households. FIP does not occur without exposure to feline coronavirus. Eighty percent to 90% of cats in catteries have antibodies to feline coronaviruses (mostly feline enteric coronavirus [FECV]), and these cats shed virus intermittently. FECV is highly contagious through the feces, as well as urine and saliva. Current thinking is that this virus may mutate to feline infectious peritonitis (FIPV) within some infected cats. FIPV then enters the macrophages, spreading throughout the body.
FIPV and FECV are difficult to differentiate with current testing procedures. Enzyme-linked immunosorbent assay (ELISA) and immunofluorescence assays are nonspecific for FIPV. Even the polymerase chain reaction cannot differentiate the two viruses.
FIP occurs in two forms: the effusive, or “wet,” form (75%) and the noneffusive, or “dry,” form. About 45% of cats that have the dry form will have ocular or neurologic lesions. In the effusive form, perivasculitis results in the accumulation of a protein-rich fluid in the thoracic and/or abdominal cavity, the scrotum, the pericardial cavity, and the renal subcapsular space. The inflammatory process may also involve the liver and the pancreas. The clinical progression is more rapid than with the dry form.
Signs of noneffusive FIP are vaguer. The pyogranulomatous lesions may be found anywhere in the body, especially the eyes and the neurologic system. Clinical signs may include ataxia, seizures, behavioral changes, paresis, and/or hyperesthesia. Ocular signs include iritis, retinitis, uveitis, hyphema, corneal edema, retinal hemorrhage, and retinal detachment.
Feline leukemia is caused by a retrovirus that is associated with both neoplastic and nonneoplastic (immunosuppressive) disease. Both vertical and horizontal transmission occurs. The virus is unstable in the environment; therefore, close contact between cats is required for infection to occur. The virus can be isolated from saliva, urine, tears, and milk, and it can be spread through fighting, grooming, or exposure to contaminated food bowls, food, water, or litter pans. Transplacental and transmammary transmission does occur. The outcome of exposure to the virus is variable and depends on several host factors: age, immunocompetence, concurrent disease, viral strain, dose, and the duration of exposure.
Exposed cats may experience development of the following conditions: (1) a regressive infection (transient), (2) a progressive infection (persistent viremia) with no clinical signs, or (3) active infection with clinical signs. Clinical signs associated with feline leukemia virus (FeLV) include anemia, anorexia, depression, weight loss, nervous system disease, and secondary infections. Vomiting and diarrhea may be seen if the gastrointestinal tract is involved.
Treatment is primarily supportive, and prevention is through vaccination and limited contact with infected cats. All cats should be tested for FeLV using the standard peripheral-blood ELISA test before vaccination. If positive, cats should undergo an immunofluorescent antibody (IFA) test or be retested by ELISA in 3 to 4 months. Cats with recurring positive results will usually be positive for life and should be isolated from all other nonvaccinated cats. Many may remain in good health for prolonged periods if not stressed.
Feline immunodeficiency virus (FIV, or feline AIDS) is a lentivirus associated with an immunodeficiency disease in domestic cats, which is morphologically and biochemically similar to the HIV virus but is antigenically distinct. FIV is highly species specific, growing only in feline-derived cells. Most infections are acquired by horizontal transmission among adult cats. Male, sexually intact cats living outdoors are at greatest risk for acquiring FIV infection. Fighting and bite wounds appear to be the major route of transmission. There is little or no sexual transmission in cats. Neonatal kittens may become infected by contact with infected queens, although plasma antibodies against FIV may be passed to kittens in colostrum when nursing. Because the ELISA test for FIV detects antibodies, kittens should not be diagnosed using these tests until after 6 months of age.
Clinical signs of FIV involve chronic, unresponsive infections (gingivitis, stomatitis, and skin, ear, and/or respiratory tract infections), anemia, ocular and/or neurologic signs, and weight loss. Chronic fever and cachexia are common findings. Cats may remain asymptomatic for long periods after infection or may suffer from recurring bouts of illness interspersed with periods of relatively good health. Cats infected with FIV are at increased risk for development of chronic renal insufficiency.
Prevention of infection is by limiting exposure to outdoor cats. Spaying and neutering outdoor cats can limit exposure by decreasing aggressive behaviors. A vaccine for FIV currently is available. Cats receiving this vaccine may test positive for FIV at a later date.
Toxoplasmosis is caused by Toxoplasma gondii, an intracellular coccidian parasite with worldwide distribution. The feline is the only definitive host, but other warm-blooded animals, including humans, can serve as intermediate hosts. Exposure to Toxoplasma is common; an estimated 30% to 60% of adult humans are seropositive for exposure.
Transmission can occur by three routes: (1) eating contaminated meat from an intermediate host, (2) fecal-oral route, and (3) transplacental route. In carnivores, ingestion of infected intermediate hosts is responsible for most infections.
Once sporulated oocysts are ingested, tachyzoites form and invade any tissue in the body. Clinical signs of disease are related to the tissue involved. The disease may be especially severe in immunocompromised animals or in very young animals. In the cat, the two tissues most commonly involved are the lung and the eyes, whereas in the dog, the gastrointestinal, neurologic, and the respiratory systems are commonly infected. However, Toxoplasma infections are rare in the dog.
After infection, the cat sheds oocysts in the feces for 1 to 2 weeks. Because of this limited shedding of organisms, exposure to these infective oocysts is probably not an important source of infection for humans and other cats. Ingestion of uncooked or undercooked meat is most likely the main route of infection in both cats and humans. Therefore, prevention of infection involves eliminating hunting and feeding of raw meat to the cat, cooking all meat properly before feeding, and following good hygiene practices when handling cat feces.
Humans who are immunosuppressed should avoid contact with infected cats. Congenital infection in the first or second trimester can result in serious birth defects. Although infected cats are unlikely to pose a major threat to most pregnant women, the following steps can be taken to prevent infection: