Cytauxzoonosis / Bobcat fever

Cytauxzoonosis, commonly known as bobcat fever, is a severe and often fatal tick-borne disease affecting domestic cats, caused by the protozoal parasite Cytauxzoon felis. This devastating infection is transmitted through the bite of infected ticks, primarily the lone star tick (Amblyomma americanum), and is endemic to the central and southeastern regions of the United States. The disease is named for its natural reservoir host, the bobcat, which typically carries the parasite without developing clinical illness. Domestic cats, unfortunately, are highly susceptible to severe disease and face mortality rates that historically exceeded ninety percent without treatment.

The causative organism, Cytauxzoon felis, undergoes a complex life cycle involving both the tick vector and the mammalian host. When an infected tick feeds on a domestic cat, sporozoites are injected into the bloodstream and rapidly invade macrophages and other mononuclear cells. Within these cells, the parasites undergo extensive replication, causing the cells to enlarge dramatically and occlude blood vessels throughout the body. This vascular occlusion, rather than the red blood cell infection that follows, is primarily responsible for the severe and often fatal course of the disease. The incubation period from tick bite to clinical signs is typically one to three weeks.

The impact of cytauxzoonosis on affected cats is profound and rapidly progressive. Cats typically become severely ill within days of symptom onset, developing high fever, profound depression, and multiorgan dysfunction. The occlusion of blood vessels by parasite-laden cells causes tissue ischemia affecting the liver, lungs, spleen, kidneys, and brain. Without aggressive treatment, most cats succumb within two to five days of showing clinical signs. Even with treatment, the disease carries significant morbidity, and survivors may experience lasting effects. The condition represents one of the most serious feline health threats in endemic areas.

Recent advances in treatment protocols have significantly improved survival rates for cats with cytauxzoonosis when treatment is initiated early and aggressively. The combination of atovaquone and azithromycin has emerged as the most effective treatment option, increasing survival rates to approximately sixty percent in some studies. However, early recognition of the disease remains critical, as delays in treatment dramatically reduce the chance of survival. Prevention through tick control is essential for cats living in or traveling to endemic areas, and awareness among cat owners and veterinarians in these regions is crucial for improving outcomes.

The causative agent of cytauxzoonosis is Cytauxzoon felis, a protozoal parasite belonging to the phylum Apicomplexa, which also includes the organisms responsible for malaria and toxoplasmosis. This parasite requires two hosts to complete its life cycle: a mammalian host, where it reproduces asexually, and a tick vector, where sexual reproduction occurs. The lone star tick (Amblyomma americanum) is the primary vector in the United States, though other tick species may play a role in transmission. Bobcats serve as the natural reservoir host, maintaining the parasite in the environment without typically developing clinical disease.

Cytauxzoonosis is not a genetic or hereditary condition, and there is no known genetic predisposition that makes certain cats more or less susceptible to infection. However, the outcome of infection may be influenced by individual immune factors that are not fully understood. Cats that survive infection develop immunity and may become chronic carriers, potentially serving as a source of infection for ticks even though they no longer show clinical signs. This carrier state does not appear to cause ongoing health problems for the infected cat, though it may contribute to maintenance of the parasite in domestic cat populations.

Environmental factors are the primary determinants of cytauxzoonosis risk. Geographic location is crucial, with the disease endemic to the central and southeastern United States, including Texas, Oklahoma, Arkansas, Missouri, Kansas, Louisiana, Mississippi, Alabama, Georgia, Florida, Tennessee, Kentucky, Virginia, and the Carolinas. The disease has been expanding its range northward, with cases now reported in states previously considered non-endemic. Seasonal factors influence risk, with most cases occurring during spring and summer months when tick activity is highest. Outdoor access, particularly in wooded or brushy areas where ticks and bobcats are present, represents the primary risk factor.

The risk of cytauxzoonosis is determined almost entirely by exposure to infected ticks rather than by cat-specific factors. Outdoor cats, especially those in rural areas with bobcat populations, face the highest risk. Cats that hunt or spend time in wooded areas are more likely to encounter infected ticks. Male cats may face slightly higher risk due to larger territories and more roaming behavior, but sex differences in susceptibility to disease are not established. Age does not appear to significantly influence susceptibility, though very young or immunocompromised cats may progress more rapidly once infected.

Once introduced into a cat through a tick bite, C. felis sporozoites invade mononuclear phagocytes, particularly macrophages. Within these cells, the parasites undergo asexual reproduction called schizogony, producing large numbers of merozoites. The infected cells become dramatically enlarged, forming structures called schizonts that can reach many times the size of normal cells. These enlarged cells accumulate in blood vessels, particularly in the liver, lungs, spleen, and lymph nodes, causing vascular occlusion and tissue ischemia. This schizogenous phase is responsible for the severe clinical disease seen in cats. Merozoites eventually released from schizonts invade red blood cells, but this erythrocytic phase, while diagnostically useful, contributes less to the severe pathology than the tissue phase.

The early warning signs of cytauxzoonosis can be subtle and nonspecific, making early diagnosis challenging even for experienced veterinarians. Initial symptoms often include mild lethargy and decreased appetite, which many owners attribute to minor illness or behavioral changes. Cats may seem slightly less interactive or sleep more than usual in the day or two before obvious illness develops. Some cats develop mild fever during this prodromal phase that may go unnoticed. Because cats naturally hide signs of illness and the early symptoms are nonspecific, precious time for early treatment intervention is often lost before the severity of the condition becomes apparent.

As cytauxzoonosis progresses, the most common and recognizable symptoms become increasingly severe and dramatic. High fever, often reaching 104 to 106 degrees Fahrenheit or higher, develops as the disease enters its acute phase. Profound depression and complete loss of appetite are characteristic, with cats becoming progressively unresponsive to their environment. Rapid breathing or labored respiration may indicate pulmonary involvement as parasite-laden cells occlude lung vessels. Icterus or jaundice, evidenced by yellowing of the whites of the eyes, gums, and skin, frequently develops as the liver becomes affected. These symptoms typically progress rapidly over twenty-four to seventy-two hours.

Behavioral changes in cats with cytauxzoonosis reflect the severe systemic nature of the disease. Affected cats typically show profound depression, becoming unresponsive or only minimally responsive to stimuli. Vocalization may occur, possibly indicating pain or distress, though many cats become silent as they deteriorate. Complete anorexia is the rule, with cats refusing even highly palatable foods. Hiding behavior may initially occur as cats seek solitude when feeling unwell, but as weakness progresses, cats may be unable to move from wherever they have settled. Decreased or absent grooming leads to rapid deterioration of coat quality.

Physical examination findings in cytauxzoonosis reflect multisystem involvement and circulatory compromise. Pale or yellow mucous membranes indicate anemia and liver dysfunction respectively. Dehydration develops rapidly due to fever and lack of fluid intake. Enlarged lymph nodes may be palpable, particularly early in the disease course. The spleen and liver may be enlarged and painful on abdominal palpation. Heart sounds may be muffled, and pulse quality often deteriorates as circulatory shock develops. Body temperature is typically markedly elevated early in the disease but may become subnormal in terminal stages as circulatory collapse occurs.

The progression of cytauxzoonosis symptoms is characteristically rapid and relentless without treatment. From the first obvious signs of illness, cats typically deteriorate over two to five days to a terminal state. Fever may fluctuate but remains elevated throughout the acute phase. Icterus deepens as liver function deteriorates. Respiratory distress may develop as lung vessels become occluded. Neurological signs including disorientation, seizures, or coma may occur as brain circulation is compromised. Disseminated intravascular coagulation may develop, causing abnormal bleeding. The rapid progression leaves a very narrow window for effective treatment intervention.

Emergency symptoms requiring immediate veterinary intervention include any combination of fever with depression and decreased appetite in a cat from an endemic area, particularly during tick season. Jaundice in a previously healthy cat should always prompt consideration of cytauxzoonosis. Respiratory distress, collapse, or unresponsiveness indicates advanced disease requiring emergency care. Pale gums with rapid heart rate suggest significant anemia and circulatory compromise. Any outdoor cat from an endemic region showing acute illness during spring or summer should receive immediate veterinary evaluation with cytauxzoonosis on the differential list. Delays of even hours can mean the difference between survival and death.

Diagnosis of cytauxzoonosis begins with clinical suspicion based on history and presenting signs. The veterinarian will inquire about geographic location, outdoor access, tick exposure, and the timeline of symptom development. Physical examination typically reveals fever, depression, icterus, and signs of circulatory compromise in affected cats. The combination of acute illness with high fever and jaundice in an outdoor cat from an endemic area during tick season should prompt immediate consideration of cytauxzoonosis. Given the rapid progression of the disease, diagnostic testing should proceed urgently while supportive care is initiated.

Blood testing provides essential diagnostic information and supports the clinical diagnosis of cytauxzoonosis. Complete blood count typically reveals anemia, which may range from mild to severe, and often shows abnormalities in white blood cell numbers and platelet counts. Blood chemistry panel frequently demonstrates elevated liver enzymes reflecting hepatic damage, elevated bilirubin causing the visible jaundice, and may show kidney compromise and electrolyte abnormalities. Blood smear examination to identify organisms within red blood cells can provide rapid confirmation when positive, though organisms may be sparse early in the disease. Polymerase chain reaction (PCR) testing offers the most sensitive and specific diagnostic confirmation and can detect infection before organisms are visible on blood smear.

Differentiating cytauxzoonosis from other conditions with similar presentations is important for appropriate treatment selection. Hemotropic mycoplasma infections can cause anemia and fever but typically progress less rapidly. Feline infectious peritonitis may cause fever and icterus but has a different clinical course. Immune-mediated hemolytic anemia can cause similar blood abnormalities but responds to immunosuppressive therapy rather than antiprotozoal treatment. Hepatic lipidosis, pancreatitis, and cholangiohepatitis can cause icterus but typically lack the severe systemic inflammation seen with cytauxzoonosis. Toxin exposure should be considered, particularly rodenticide ingestion. The geographic location and history of tick exposure help distinguish cytauxzoonosis from these alternatives.

Confirmation of cytauxzoonosis diagnosis is most definitively achieved through identification of the organism combined with compatible clinical signs. Blood smear examination revealing characteristic signet ring-shaped organisms within red blood cells provides visual confirmation, though sensitivity is limited early in infection. PCR testing detecting C. felis DNA offers higher sensitivity and can confirm infection before organisms are microscopically visible. Tissue biopsy or cytology demonstrating schizont-laden macrophages provides definitive diagnosis but is rarely performed in living patients. In practice, treatment is often initiated based on high clinical suspicion in endemic areas, with diagnostic confirmation obtained while treatment proceeds. Post-mortem examination in fatal cases typically reveals characteristic pathology including schizont-laden macrophages occluding vessels in multiple organs.

Emergency treatment for cytauxzoonosis must be initiated immediately upon clinical suspicion, as delays significantly reduce survival probability. Intravenous fluid therapy addresses dehydration and supports circulation in cats experiencing vascular compromise. Fever reduction may be attempted, though antipyretic medications have variable effectiveness against the intense fever of cytauxzoonosis. Oxygen supplementation may be necessary for cats with respiratory compromise due to pulmonary vessel occlusion. Intensive monitoring of vital signs, hydration status, and response to treatment guides ongoing supportive care. Many severely affected cats require hospitalization in a veterinary intensive care unit for optimal management.

Antiprotozoal medication forms the cornerstone of cytauxzoonosis treatment, with recent protocols dramatically improving survival rates. The combination of atovaquone and azithromycin has emerged as the most effective treatment regimen, with studies showing survival rates of approximately sixty percent compared to less than twenty-five percent with older protocols. Atovaquone is typically administered at 15 mg/kg orally every eight hours with a fatty meal to enhance absorption, while azithromycin is given at 10 mg/kg orally every twenty-four hours. Treatment duration is typically ten days, though some protocols extend treatment for longer periods. Imidocarb dipropionate, used in older protocols, may still be employed but is considered less effective than the atovaquone-azithromycin combination.

Surgical intervention has no role in treating cytauxzoonosis, as this is entirely a medical condition requiring antiprotozoal therapy and supportive care. However, intensive medical intervention that may feel invasive to cats and owners is often necessary for survival. Blood transfusion may be required for severely anemic cats, providing both oxygen-carrying capacity and clotting factors if disseminated intravascular coagulation is present. Intravenous catheter placement for fluid and medication administration is standard. Feeding tubes may be necessary to provide nutritional support in cats that refuse food. Central venous catheters may be placed in critical patients for intensive monitoring and medication delivery.

Supportive care is essential and often determines outcome in cats receiving appropriate antiprotozoal therapy. Maintaining hydration through aggressive intravenous fluid therapy supports organ perfusion and helps maintain blood pressure. Nutritional support through syringe feeding, appetite stimulants, or feeding tube placement ensures cats receive adequate calories during recovery. Anti-nausea medications may improve appetite and comfort. Pain management addresses discomfort from illness and invasive treatments. Temperature regulation ensures cats remain warm without overheating. Nursing care including soft bedding, gentle handling, and quiet environment minimizes stress during critical illness.

Complementary approaches focus on supporting the cat's ability to recover while antiprotozoal medications eliminate the infection. Hepatoprotective supplements such as S-adenosylmethionine (SAMe) or milk thistle may support liver recovery from the damage caused by the infection. Probiotics may help maintain gastrointestinal function during aggressive medical treatment. Vitamin B12 supplementation supports red blood cell production during recovery from anemia. These complementary measures do not replace antiprotozoal therapy but may enhance overall recovery. Physical therapy and rehabilitation are rarely necessary but may help cats that developed neurological complications regain function.

Treatment decisions in cytauxzoonosis are influenced by disease severity, available resources, and realistic outcome expectations. The cost of intensive care hospitalization can be substantial, reaching several thousand dollars for prolonged stays. Even with optimal treatment, approximately forty percent of cats do not survive, and some survivors experience lasting effects. Owners should be counseled honestly about prognosis while being offered the chance to attempt treatment. Cats presenting with severe circulatory collapse or coma face very poor prognosis regardless of treatment intensity. Geographic access to veterinary facilities capable of intensive care may influence treatment options. When treatment is pursued, early initiation of appropriate antiprotozoal therapy combined with aggressive supportive care offers the best chance of survival.

Recovery from cytauxzoonosis, when it occurs, follows a variable timeline depending on disease severity and treatment response. Cats that respond to treatment typically show improvement in fever and depression within forty-eight to seventy-two hours of starting appropriate antiprotozoal therapy. Appetite often returns within three to five days, though it may take longer for cats that were critically ill. Icterus resolves more slowly, often persisting for one to two weeks after other clinical signs improve. Complete recovery to normal activity and health typically requires two to four weeks, though some cats recover faster while others take longer. Hospitalization duration varies from three to seven days for most survivors.

Post-treatment care continues after hospital discharge and includes completion of the full antiprotozoal medication course at home. Atovaquone and azithromycin are typically continued for a total of ten days, requiring owners to administer medications reliably. Follow-up bloodwork monitoring liver function, red blood cell counts, and other parameters helps track recovery and identify any complications. Activity restriction during initial recovery prevents overexertion while the body heals. Gradual return to normal diet from any prescription or supportive feeding regimen allows the gastrointestinal tract to readjust. Follow-up PCR testing may be performed to confirm clearance of the organism, though many survivors become chronic carriers without clinical significance.

Prognosis for cytauxzoonosis survivors depends on several factors. The severity of disease at presentation strongly influences outcome, with cats that present earlier in the disease course facing better prognosis than those with advanced circulatory collapse. Response to initial treatment within the first forty-eight hours is prognostic, with cats that show improvement more likely to survive than those that continue deteriorating despite treatment. Development of complications such as disseminated intravascular coagulation or acute kidney injury worsens prognosis. With appropriate treatment initiated early, survival rates of approximately sixty percent are achievable, though this still means significant mortality from this devastating disease.

Long-term outlook for cats that survive cytauxzoonosis is generally good, though some lasting effects may occur. Most survivors develop immunity and are protected against future clinical disease, though they may become chronic carriers of the organism. Chronic carrier cats can serve as a source of infection for feeding ticks, potentially contributing to local disease transmission. Some survivors experience mild chronic anemia or liver enzyme elevations that persist after clinical recovery. Neurological deficits from brain ischemia during acute illness may persist in some cases. Regular monitoring through wellness examinations and bloodwork helps identify any ongoing effects. Tick prevention remains essential for survivors to prevent potential reinfection with different strains and to reduce their role as reservoirs for environmental contamination.

Primary prevention of cytauxzoonosis centers on preventing tick bites, as transmission requires a tick vector and direct cat-to-cat transmission does not occur. Year-round tick prevention using veterinary-approved products is essential for cats in endemic areas. Several tick preventive products are available for cats, including topical solutions and collars, though options are more limited than for dogs. Isoxazoline products approved for cats provide effective tick prevention and should be used according to label directions. Products containing fipronil or flumethrin have also demonstrated efficacy against the lone star tick. Regular reapplication according to product directions maintains protection throughout the tick season and year-round in areas with extended tick activity.

Environmental management reduces tick exposure and complements individual prevention strategies. Keeping cats indoors eliminates tick exposure entirely and represents the most effective prevention strategy, particularly during peak tick season from spring through fall. For cats with outdoor access, limiting time outside during peak tick activity periods of dawn and dusk reduces exposure. Maintaining yards by removing brush, leaf litter, and tall grass eliminates tick habitat. Keeping grass mowed short and creating gravel or wood chip barriers between wooded areas and lawns discourages tick migration into yards. Discouraging wildlife, particularly deer and small mammals that serve as tick hosts, from yards reduces local tick populations.

No vaccine is currently available for cytauxzoonosis, and dietary interventions play no direct role in prevention. However, maintaining overall health through good nutrition supports immune function that may help cats fight infection if exposure occurs. Some survivors of cytauxzoonosis develop immunity and may be protected against future clinical disease, though intentional exposure for immunity development is not recommended given the high mortality rate of initial infection. Research into vaccine development is ongoing, but no commercial product is currently available. Natural immunity in bobcats that serve as reservoir hosts is not understood well enough to apply to domestic cat protection.

Regular veterinary care supports prevention efforts through education and early detection. Veterinarians in endemic areas should counsel clients about cytauxzoonosis risk and prevention strategies. Regular wellness examinations provide opportunities to assess tick prevention compliance and adjust strategies as needed. Prompt veterinary evaluation when cats show signs of illness during tick season enables early diagnosis and treatment of cytauxzoonosis before the disease becomes advanced. Parasitologic screening is not useful for prevention since infection develops rapidly after exposure, but awareness of endemic status helps guide diagnostic and treatment decisions when illness occurs.

Early intervention when exposure or symptoms occur dramatically affects outcome. Daily tick checks for cats with outdoor access enable prompt removal of attached ticks before transmission can occur, as ticks typically must feed for several hours before transmitting C. felis. Prompt removal using fine-tipped tweezers and appropriate technique minimizes infection risk. Any cat in an endemic area showing fever and depression during tick season should receive immediate veterinary evaluation. Starting treatment at the first sign of illness, rather than waiting for definitive diagnosis, improves survival rates. Educating household members about symptoms and the need for urgent care ensures prompt response when illness develops.

Daily management of a cat recovering from cytauxzoonosis requires attention to medication administration, monitoring, and supportive care. Antiprotozoal medications must be given exactly as prescribed, with atovaquone requiring administration with fatty food for proper absorption. Monitoring appetite, activity level, and elimination provides important information about recovery progress. Affected cats should be kept indoors in a quiet, comfortable environment during recovery. Temperature monitoring may be recommended during early recovery to ensure fever has resolved. Maintaining hydration through encouraging water intake or providing water fountains supports organ function during healing.

Home environment modifications support recovery and future prevention. Creating a completely indoor environment eliminates future tick exposure and represents the safest option for cats in endemic areas. If outdoor access is permitted, limiting it to enclosed catios or supervised yard time reduces tick exposure. Ensuring the home environment is calm and low-stress supports immune function during recovery. Providing easily accessible food, water, and litter box locations reduces exertion required during early recovery when cats may be weak. Comfortable bedding in warm, quiet areas allows rest without disturbance.

Maintaining quality of life for recovered cats focuses on balancing safety with feline behavioral needs. Indoor cats benefit from environmental enrichment including climbing structures, window perches for bird watching, interactive toys, and regular play sessions with owners. If the decision is made to allow supervised outdoor access, timing outings to avoid peak tick activity periods and conducting thorough tick checks afterward reduces risk. Providing mental stimulation through puzzle feeders, hiding treats, and rotating toys prevents boredom. For cats previously accustomed to outdoor access, the transition to indoor living may require patience and creative enrichment solutions.

Ongoing monitoring ensures any complications are detected early and confirms complete recovery. Follow-up appointments scheduled according to veterinary recommendations assess recovery through physical examination and laboratory testing. Bloodwork monitoring liver function, red blood cell counts, and other parameters should return to normal over several weeks in recovering cats. Any return of fever, depression, or appetite loss should prompt immediate veterinary contact, as relapse or other illness may occur. Long-term wellness examinations help detect any chronic effects of the infection and ensure overall health maintenance.

Caregiver support is important given the emotional and financial stress of managing cytauxzoonosis. The disease's high mortality rate and intensive treatment requirements create significant stress for owners. Support groups and online communities can provide emotional support and practical advice. Financial counseling may be available through veterinary social workers or nonprofit organizations for owners facing difficult decisions about treatment costs. Clear communication with the veterinary team about prognosis, treatment options, and expected costs helps owners make informed decisions. For owners who have lost cats to cytauxzoonosis, grief support resources may be helpful. Prevention education for any future feline additions to the household helps protect against repeat experiences with this devastating disease.

Cytauxzoonosis does not demonstrate breed-specific predisposition, as susceptibility to infection and disease severity are determined by tick exposure and host immune factors rather than genetics. All breeds of domestic cats, from common domestic shorthairs to rare purebred cats, face equal risk when exposed to infected ticks in endemic areas. The disease affects cats regardless of coat length, color, or body type. Breed does not influence outcome once infection occurs, with treatment response and survival depending on disease severity at presentation and treatment timing rather than genetic factors.

The primary risk factors for cytauxzoonosis are environmental and behavioral rather than breed-related. Outdoor cats face the highest risk due to tick exposure, with rural cats in wooded areas of endemic regions at greatest risk. Hunting cats that pursue prey in brushy areas where ticks and bobcats are present encounter the most dangerous exposure scenarios. Male cats may face slightly higher risk due to larger territories and increased roaming, but this reflects behavior rather than biological susceptibility. Age does not significantly influence susceptibility, though outcome may be worse in very young cats or those with concurrent illness.

For breeders and rescues operating in endemic areas, risk mitigation focuses on tick prevention rather than breed selection. All cats should receive appropriate tick prevention regardless of breed. Maintaining strictly indoor facilities eliminates tick exposure entirely. Quarantine protocols for new arrivals from unknown origins should include thorough examination for ticks and monitoring for illness. Breeding decisions need not consider cytauxzoonosis susceptibility, as no genetic factors influence risk. Education of adopters about tick prevention and the risks of outdoor access in endemic areas helps protect cats placed in new homes.

Several conditions may occur alongside cytauxzoonosis or complicate its diagnosis and treatment. Concurrent tick-borne infections such as ehrlichiosis or anaplasmosis may occur in heavily tick-infested cats, though these are rare in cats compared to dogs. Secondary bacterial infections may develop in immunocompromised cats with cytauxzoonosis. Disseminated intravascular coagulation is a common and serious complication of severe cytauxzoonosis, causing abnormal bleeding and further compromising already critical patients. Acute kidney injury and hepatic failure may develop as complications of severe disease, requiring additional supportive care and worsening overall prognosis.

Differentiating cytauxzoonosis from conditions with similar presentations ensures appropriate treatment. Hemotropic mycoplasma infections (feline infectious anemia) cause anemia and fever but typically progress more slowly and lack the severe systemic inflammation of cytauxzoonosis. Feline infectious peritonitis can cause fever, icterus, and depression but has different laboratory findings and disease course. Immune-mediated hemolytic anemia causes anemia and icterus but responds to immunosuppressive rather than antiprotozoal therapy. Hepatic lipidosis causes icterus but typically occurs in obese cats with history of anorexia and lacks fever. Rodenticide toxicosis can cause bleeding and liver damage but has characteristic coagulation abnormalities. Geographic location and tick exposure history help distinguish cytauxzoonosis from these alternatives.

Complications of cytauxzoonosis can lead to secondary conditions requiring additional treatment. Severe anemia may necessitate blood transfusion, with associated risks of transfusion reactions. Disseminated intravascular coagulation requires plasma transfusion and careful monitoring for bleeding complications. Acute kidney injury may require intensive fluid management and monitoring. Hepatic injury may lead to coagulopathy and encephalopathy requiring specific treatment. Pulmonary complications from vessel occlusion may cause acute respiratory distress syndrome. Neurological complications from brain ischemia may result in permanent deficits. Recognition and treatment of these complications is essential for maximizing survival in severely affected cats.