Rickettsioses
Rickettsiae are rod-like organisms that are intermediate between viruses and bacteria. They inhabit arthropod hosts, such as fleas, lice and ticks. Rickettsiae parasitize the white blood cells of vertebrate hosts, including humans.
Rickettsiosis [ri-ket-se-o-sis] can be acquired either through the bite of an arthropod, or through skin abrasions, e.g. when crushing a tick between fingers. It can also be acquired from the inhalation of dried faeces. Rickettsiae can be passed transstadially (through each phase of the tick's life cycle) and transovarially (from a female tick to the next generation). Tick-borne types of rickettsiae that can occur in the UK include:
Q - fever
Background
Q-fever (coxiella burnetii) is carried by farm animals, especially sheep, cattle and goats, but it may also be found in cats, and in wild animals (such as birds and rodents). The organism is found in the afterbirth and birth fluids, as well as in the milk, urine, and faeces of infected animals. Ixodid ticks are vectors of Q-fever and may also carry other infections concurrently.
Coxiella burnetii.
Infection of humans generally occurs by inhalation of coxiella burnetii from air that contains barnyard dust contaminated by dried placental material, birth fluids, and excreta from infected herds. Inoculation by an infected tick can also result in clincial disease. Britain is considered to be an endemic zone for Q-fever (Rehacek and Tarasevich 1988).
Signs & Symptoms
Humans are often very susceptible to the disease and only a few organisms are required to cause infection. The incubation period varies depending on the number of organisms that initially infect the patient. A greater number of organisms will result in shorter incubation periods. Most patients become ill within 2-3 weeks of exposure.
Clinical signs are a flu-like onset with fever and fatigue, progressing to pneumonia, anaemia, hepatitis, and cardiac problems.
Testing
An indirect immunofluorescence assay (IFA) is the most widely used method. Coxiella burnetii can also be identified in infected tissues by using immunohistochemical staining and by Polymerase Chain Reaction (PCR) used to detect DNA.
Treatment
Tetracycline antibiotics are usually the treatment of choice, combined with quinolones in chronic and acute cases.
Anaplasmosis / Ehrlichiosis
Background
Anaplasmosis [an-a-plas-mo-sis] and Ehrlichiosis [ehr-lich-i-o-sis] are caused by Anaplasma and Ehrlichia species, which are members of the rickettsiae and are obligate intracellular coccobacilli. The spectrum of disease caused by these species ranges from asymptomatic to fatal. Wild and domestic stock are some of the reservoir-competent hosts.
Human Granulocytotropic Anaplasmosis (HGA), formally known as Human Granulocytic Ehrlichiosis (HGE), is caused by Anaplasma phagocytophilum. This organism is also responsible for Tick-borne Fever in both cattle and sheep, and can cause clinical disease in other animals. It parasitizes the white blood cells of its host, weakening the immune system.
Signs & Symptoms
The onset of clinical disease usually begins within 1-3 weeks of the tick bite. Early symptoms can be non-specific, including flu-like malaise, fever, chills and myalgias. More acute symptoms can include vomiting, acute weight loss, abdominal pain, cough, diarrhoea, haemorrages and renal failure.
Another ehrlichiosis that affects humans is Human Monocytic Ehrlichiosis (HME), which is caused by Ehrlichia chaffeensis. HME is clinically similar to HGA.
Testing
Generally an indirect immunofluorescence assay (IFA) is the principle test used, but Anaplasma / Ehrlichia species can also be identified in infected tissues by using immunohistochemical staining. Polymerase Chain Reaction (PCR) is increasingly used to detect Anaplasma / Ehrlichia DNA.
Treatment
Anaplasma and Ehrlichia infections generally respond to tetracycline antibiotics.
Anaplasma phagocytophilum
Above image courtesy of Mourad Gabriel
