Although not protozoan parasites, we will discuss several important vector transmitted rickettsial organisms.
Anaplasma marginale - Agent of gall disease throughout the world.
This important disease of cattle is a tick and /or fly transmitted disease with a complicated life cycle. Many cases may also be the result of mechanical or accidental transmission by humans. Anaplasmosis is a disease of adult cattle as severe clinical disease does not occur until the animal is about 18 months of age. Younger animals are susceptible to infection but do not exhibit clinical disease. In mature cows, the incubation period is 15-36 days averaging 26. There is an increase in body temperature during this period and mature animals may die during it. Organisms (initial bodies) (above) appear in the blood several days before the fever. Anerexia develops along with an anemia. In severe cases, the severe anemia may make the animal susceptible to other problems. Pathogenicity in the disease is related to RBC loss and the resultant anemia. Treatment is with tetracycline although it is not known if the animals totally eliminate the organism, once infected. Carriers, either treated animals or animals infected while young, are common and serve as a source of infection for either arthropods or mechanical transmission A vaccine is available but it is not of value in the face of an outbreak and may produce isohaemolytic problems in some calves. Tick and other arthropod control is important in prevention as well as prophylactic antibiotic therapy.
Diagnosis is dependent on microscopic detection of initial bodies in stained thin blood films, serology, and case history.
Tetracycline theropy canbe used prophylactically if proper dosage is insured and continuous application can be accomplished. Vaccination with presently available products requires yearly administration. Potentially useful new vaccines may become available in the near future if newly developed tick cell lines are capable of sustaining organism growth.
Other species of Anaplasma occur including A. centrale and A. ovis.
Ehrlichia spp.- Agent of ehrlichiosis.
This is a tick transmitted rickettsial organisms that can effect a variety of mammals. Ehrlichia canis, E. platys, E. ewingii, E. equi and E. risticii are obligate intracellular organisms that parasitize circulating leukocytes (above) or thrombocytes of the host animal. Several species in this genus cause both clinical and subclinical diseases in canids.. Although disease caused by these organisms was once thought to be rare, improved diagnostic capabilities, an awareness of the presence of these organisms, and probably an increase in incidence have resulted in more frequent reportings. Presently, canine ehrlichiosis is one of the most commonly reported tick-transmitted diseases of canines in the U.S. .
E. canis, the causative agent of canine monocytic ehrlichiosis occurs most commonly during the summer months but can occur throughout the year. Three phases of the disease are recognized; acute, subclinical, and chronic. The chronic phase is often divided into mild chronic and severe chronic for a better clinical presentation. Pathogenicity often varies from species to species. As a result, making a clear differentiation between the acute and chronic phases of infection is often difficult. The use of hematologic and serum chemistry changes should be considered in each case along with the case history.
The incubation period lasts 8 to 20 days. The progression of infection consists of an acute phase of 2 to 4 weeks during which the organism multiplies. Nonspecific signs such as fever, oculonasal discharge, anorexia, depression, weight loss and mild anemia may occur during this phase. The acute phase usually resolves spontaneously and is followed by the subclinical phase. During this phase, the body weight normalizes and the dog appears clinically normal. Laboratory abnormalities such as mild thrombocytopenia and hyperglobulinemia often occur. Serum antibody titers usually begin to rise 7 to 28 days after infection and continue to rise during the subclinical phase. The subclinical phase may last 40 to 129 days but may also persist for years.
In the mild chronic phase of infection, clinical signs reappear as vague signs of illness. The severe chronic phase may occur with or without the occurrence of a mild chronic phase.
Clinical abnormalities usually seen as presenting complaints include depression/lethargy, weight loss, anorexia, hemorrhagic tendencies, pyrexia, lymphadenomegaly, splenomegaly, heart murmur, vomition, and ataxia.
E. ewingii is the causative agent of canine granulocytic ehrlichiosis. The disease caused by this organism is generally milder than that caused by E. canis. Signs include mild fever (102-103F), thrombocytopenia, leukopenia and a mild normocytic, normochromic anemia. Chronic cases may present with arthritis with numerous organisms in neutrophils and eosinophils in synobial fluid. The only known vector is Amblyomma americanum, the Lone-Star tick.
Infections with E. platys (infectious cyclic thrombocytopenia) result after an incubation period of 8 to 15 days. Infected dogs usually are not clinically ill and rarely show signs of hemorrhage even with severe thrombocytopenias. Dual infections with E. canis are common. Infections with E. equi, usually seen as infections of circulating neutrophils of horses, are reported from a number of animals, including dogs. Although this organism can cause severe illness in horses, it usually only results in a mild or inapparent infection in dogs. E. risticii, the agent of Potomac horse fever, can also occur in dogs, at least under experimental conditions. Again, infections in dogs with this organism appear to be sub-clinical.
Diagnosis in all cases requires microscopic identification of the morulae in circulating cells of Giemsa-stained blood films. Examining buffy-coat preparations, tissue aspirates and impression smears from lung, lymph node or spleen increases the chances detecting infected cells. IFA serology appears to be relatively specific and can also be used. A single titer below 1:200 is questionable unless followed by a rising titer.
Treatment of ehrlichial infections is usually accomplished successfully with tetracycline, oxytetracycline, doxycycline, minocycline and chloramphenicol. Good supportive care is also necessary, especially in chronically infected animals.
A recently identified organism, E. chaffeensis, has been shown to be the causative agent of human monocytic ehrlichiosis (HME) A second, yet unnamed species which is found in human neutrophils has been shown to be responsible for human granulocytic ehrlichiosis (HGE). Like all ehrlichieae, the agent for HME is a small gram-negative bacterium that is an obligate intracellular parasite. In the case of E. chaffeensis, infections usually occur in monocytic phagocytes. Amblyomma americanum, the lone-star tick, and Dermacentor variabilus, the American dog tick, are the only arthropods that have been implicated in transmission. A high proportion of reported cases occur within the range of A. americanum. White-tailed deer appear to be important in the epidemiology of E. chaffeensis. HGE which was first reported in 1994 has now been identified in over 170 human cases from Minnesota, Wisconsin, Massachusetts, Connecticut, New York, Rhode Island, Pennsylvania, Maryland, Florida, Arkansas and California. Molecular analysis suggests that the Ehrlichia spp that causes HGE is the same agent that is responsible for granulocytotrophic ehrlichiosis of horses in California and elsewhere and in dogs from the Upper Midwest. The present understanding of HGE epidemiology indicates that Ixodes scapularis is the major vector in the Upper Midwest and New England states and that I. pacificus is probably involved in northern California.
An additional unnamed species of Ehrlichia is reported to infect white-tailed deer. Although a distinct species, it does cross-react serologically with both E. canis and E. chaffeensis.
Link to OSU Ehrlichia Research
Eperythrozoon spp- This parasite is found on RBCs and free in plasma. There are several species that effect cattle, rodents, sheep and swine. Although not known, it is thought to be arthropod transmitted. Latent non-clinical infections are common. In severe cases, anemia, anorexia and jaundice occur. Pathological changes consist of anemia, jaundice, enlarged spleen and hyperplasia of the bone marrow. In E. suis infections, the highest mortality and morbidity are in suckling pigs which may die of acute infection in less than 5 days. Significant losses are usually not noted although the organism appears to be wide spread. Tetracycline treatment appears to help although the infection may never be totally eliminated. Recent findings indicate that both Eperythrozoon and Haemobartonella may be more closely related to memebrs of the family Mycoplasmataceae than rickettsia.
Haemobartonella spp. are parasite of canines, felines and rodents. H. canis is an epierythrocytic parasite of erythrocytes The organism occurs on RBCs only, with latent infections being common. The organism occurs as chains across the surface of the infected cell or as small dots, rods, or rings. Transmission is mainly be the brown dog tick with both transstadial and transovarial transmission occurring. Most naturally infected dogs do not develop clinical disease but when they do, it is usually associated with a developing anemia. Diagnosis is based on microscopic identification of peripheral blood smears. Treatment appear to be similar to that used for canine ehrlichiosis involving tetracycline, oxytetracycline, etc. Antibiotic therapy probably does not eliminate the organism completely from infected dogs. As noted above, this genus may be more closely related to the Mycoplasmataceae than rickettsia.
Rickettsia rickettsii- Agent of Rocky Mountain spotted fever.
Rocky Mountain spotted fever is a tickborne disease with a worldwide occurrence. A variety of hosts, including man, are susceptible to infection. Infection may result in a spectrum of disease ranging from subclinical to severe or fatal multiorgan collapse. The disease is maintained in nature in small mammals and ticks, especially Dermacentor andersoni and D. variabilis. Ticks can transmit the disease both transstadially and transovarially.
Canines are infected following bites from infected ticks. Dogs do not develop a sufficient enough rickettsemia to act as a reservoir and as such are not involved in the direct transmission of the organism. They do serve as an ideal sentinel indicating infection rates in specific areas.
Early clinical signs in the dog are often nonspecific. Laboratory testing generally indicate a thrombocytopenia. Serologic testing is necessary to confirm infection. Treatment with tetracycline and chloramphenicol for 14 to 21 days is effective.
Differentiation of RMSF cases from canine ehrlichiosis is often difficult. Both diseases are characterized by fever, depression, lymphadenopathy, and signs of neurologic disfunction. Petechial hemorrhages and other signs of hemorrhagic diathesia are not common in either disease. RMSF tends to be more seasonal, effects younger dogs, and is a more rapid and severe clinical disease. Anemia, leukopenia and thrombocytopenia are more common with Ehrlichia while leucytosis and thrombocytopenia are more common with RMSF. Perhaps most importantly, arising serum titer (paired samples) is needed to confirm cases of RMSF while a single high Ehrlichia titer is generally sufficient to make a diagnosis.
Cowdria ruminatium- Agent of heartwater disease.
This tick-transmitted disease effects a variety of animals in Africa south of the Sahara and in some Caribbean Islands. It is an intracytoplasmic parasite that causes a wide range of clinical signs The most pathoneumonic indication of infection is fluid in the pericardial sac of the heart. At present, there is a great deal of concern over possible accidential introduction of this organism into the southern U.S.