Cytauxzoonosis in the United States
Cytauxzoon felis is a protozoan parasite primarily found in the central and southeastern United States. The cytauxzoonosis-causing hemoparasite can also be found in areas of Africa and along the Iberian peninsula. Cytauxzoonosis was originally recognized in the United States in the 1970’s, but the affected areas have since then spread. Although not a zoonotic disease, cytauxzoonosis can infect a number of feline species including the Florida panther, bobcats, Iberian lynx, domestic cats, cheetahs, and tigers.4
Cytauxzoonosis is a tick-borne disease, so it is believed that a bite from an infected tick is the main cause of infection. The only species of tick found to have been a carrier is Dermacenter variabilis, although it is not certain whether or not other tick species can carry the disease as well.9 It is also possible that the disease can be passed from mother to progeny. Cytauxzoon felis is a hemoparasite, so the cardiovascular system is the main passageway through the body for the parasite.1
The observed clinical signs are vast and vary greatly, making diagnosis difficult in some cases. Most clinical signs are usually observed during the early stages of infection. What all known cases have in common is a severe and sudden fever spike, with a body temperature of 106 degrees Fahrenheit not uncommon. In most cases death occurs within one to two days after the temperature spike. Infected felines will also often demonstrate pale mucous membranes and a delayed capillary refill time.2 Dypsnea is not uncommon in more advanced cases, which leads to tissue hypoxia. Mucoid nasal discharge and third eyelid protrusion accompanied by scleral injection have been observed in a number of cases.
Increased bilirubin in the blood causes icterus, and the urine itself can take on a dark brown to red tone, presumably due to internal hemorrhaging. Hyperglycemia, neutrophilia, low PCV values (anemia) and elevated liver enzymes are observable on blood taken from infected cats. Even if cats are exhibiting clinical signs of parasitemia the presence of piroplasms is not always evident in the blood, even at later stages of infection. Piroplasms are evident in only about 50% of all known cases, although during the later stages of disease is when merozoites inside of RBCs are more likely to be observable on a blood smear. Splenomegaly and hepatomegaly are sometimes observed on an abdominal palpation.
Secondary disorders have been observed, such as thrombic disease, and in some cases Dissimilar Intravascular Coagulation (DIC), which is attributed as being secondary to dehydration. Towards the later stages of infection, cats often vocalize, presumably due to discomfort and pain. Shock-like symptoms can also be observed, and terminal hypothermia can occur. Some felines will go into a coma before death, while others will experience rapid organ failure. In all cases most cats were dead within one week of the onset of clinical signs.
On most of the infected cats examined tick bites were evident. In order for infection to occur a feline must be inoculated with infected blood by a tick.3 Due to the hemoparasite’s lifecycle, the tick is considered the main vector for cytauxzoonosis. It is speculated that in-utero transmission may occur, but more evidence is necessary before stating this as fact.3 The American Bobcat is the natural reservoir for Cytauxzoon felis. Despite being heavily parasitemic, most bobcats remain asymptomatic, or exhibit very mild signs of infection.9 Some bobcats have been known to die from infection, but most were either very young or already experiencing other health problems.2 Domestic cats are considered a terminal host for the disease and it is very rare that any infected domestic cat survive.9 However, recently in Arkansas and Missouri a less aggressive strain of Cytauxzoon felis has been identified. Several cats infected with this strain have survived with veterinary treatment. When blood samples were taken from healthy domestic cats in households with sick cats, four different felines were found to be infected with the parasite, but remained asymptomatic.9
Most cases of cytauxzoonosis occur in spring and summer months. Virulence varies depending on geographical location. Although the exact disease mechanism is unknown, it is certain that the tick plays a key roll in the hemoparasite’s lifecycle. Cytauxzoonosis occurs in two distinct clinical stages: the tissue phase and the erythrocytic phase. It is imperative that the tissue phase occur fully in order for the disease to be fatal.4 During this phase of the disease intracellular replication of the parasites occurs inside the lining of various blood vessels. As replication progresses, cells enlarge, which impedes blood flow to major organs. The tissue phase also coincides with the schizont stage of the Cytauxzoon felis lifecycle. It is during this stage that merozoites are formed via asexual division. It is the increase in merozoites number inside of the cells that cause them to enlarge. The next phase of disease, the erythrocytic phase, occurs when the macrophages rupture, releasing merozoites into circulation. These merozoites invade new cells, and enter into the parasite’s next life stage of sexual division. This is stage in which piroplasms form and are visible on blood smears taken from infected felines. The erythrocytic phase usually occurs late in the disease cycle. The onset of the disease is so rapid, that most cats do not have a chance to form any type of immunoresponse and thus morbidity is eminent. If a cat does recover, piroplasms can be visible in its bloodstream for the rest of its life.
Cytauxzoonosis is an example of a disease where prevention is the best method of controlling the pathogen. Strict tick control and keeping all domestic cats inside are both strongly advised. There are different species of the parasite in different geographical locations, which makes the severity of disease differ from place to place.4 Certain cat populations seem to have built up an innate resistance to infection; besides the American Bobcat, which is the disease’s natural reservoir, the Florida Panther seems to have built up the most resistance.4 Both United States panther and cougar populations have reasonable prevalence rates with Texas cougars at 39% and Florida panthers at 35%.1 There are currently no proven therapies, and the development of an antibody assay would be ineffective due to the acute nature of the disease. Empirical treatment, such as antibiotic trials are common in infected animals. Treating with fluids, blood transfusion, and antimicrobial agents have also been strategies taken to cure ill cats. Currently, the treatment with the most promise is using imidocarb dipropionate drugs. Cats treated with imidocarb dipropionate tend to survive infection, but there is no conclusive evidence that the drug is the sole or primary reason for recovery. Infected cats are much more likely to respond to treatment if the disease is caught early on.
Necropsy findings on post-mortems of infected cats revealed internal damages not evident during initial treatment. Schizont are evident in the bone marrow, spleen, and lymph nodes of infected cats before parasitemia is clinically evident. Pericardial effusion, as well as liquid in the lungs is also observable post-mortem.2 Because of this, radiographs and bone marrow aspirates are beginning to be used to diagnose cytauxzoonosis. Heavily parasitized macrophages can be found throughout the body, especially within the connective tissue.2 Mineral deposits on the heart and lungs are also observable on necropsy.
There are certain species in which the spread of cytauxzoonosis poses a serious risk to survival. The Iberian Lynx is the most endangered feline in the world. Cytauxzoonosis has been the cause of death in several lynx, and a major outbreak could be enough to wipe out the entire population. Most of the remaining lynx are currently segregated into two major populations. One of those populations has a 33% prevalence rate for the disease, causing serious concerns with those trying to rehabilitate the species.4
The Iberian Lynx is not the only wild feline species threatened by the hemoparasite parasite. Florida cougars are also an endangered species that suffers from high prevalence of the parasite in the remaining population. It is estimated that 1/3 of the remaining Florida cougars have a parasite from the same genus present in their bloodstream.9
Cytauxzoonosis can be difficult to diagnose because the clinical signs can be confused with other parasitic diseases. Also, clinical signs vary extremely from patient to patient.1 Members of the Cytauxzoon genus tend to be indistinguishable from one another, so this can also make diagnosis difficult.9
Thanks for Reading! Literature Consulted:
1Rotstein, David, Sharon Taylor, John Harvey, and Judy Bean. “Hematologic Effects of Cytauxzoonosis in Florida Panthers and Texas Cougars in Florida.” Journal of Wildlife Diseases. 35 (1999): 613-617.
2Nietfeld, Jerome and Christal Pollock. “Fatal Cytauxzoonosis in a Free-Ranging Bobcat (Lynx rufus).” Journal of Wildlife Diseases. 38 (2002): 607-610.
3Gambardella, Paul. “Health Watch.” Animals. 130.4 (1997): 36.
4Millan, J., V. Naranjo, A. Rodriguez, J.M. Perez de la Lastra, A.J. Mangold and J. de la Fuente. “Prevalence of Infection and 18S rRNA Gene Sequences of Cytauxzoon Species in Iberian Lynx (Lynx pardinus) in Spain.” Parasitology. 134 (2006): 995-1001.
9Harvey, John, Mike Dunbar, Terry Norton, and Michael Yabsley. “Laboratory Findings in acute Cytauxzoon felis Infection in Cougars (Puma concolor cougar) in Florida.” Journal of Zoo and Wildlife Medicine. 38.2 (2007): 285-291.