Potomac Horse Fever (etiologic agent: Neorickettsia risticii, formerly known as Ehrlichia risticii)

Test code: B0028 - Ultrasensitive qualitative detection of Neorickettsia risticii by real time polymerase chain reaction

Ehrlichiosis is a disease caused by obligatory intracellular bacteria of the genus Ehrlichia that affects a variety of animals including humans. Several species are assigned to this genus; those for which the vectors are known are transmitted by ixodid ticks. Most Ehrlichia infect predominantly white blood cells of their vertebrate hosts; the one exception is Ehrlichia platys which infects blood platelets of dogs. The organisms appear in clusters known as morulae in the cytoplasm of infected cells.

Traditionally, these organisms were classified as agranulocytic or granulocytic based on the cells they infect. With the advent of molecular detection methods and RNA sequencing techniques, these organisms have been reclassified with different names. For example, Ehrlichia risticii is now classified as Neorickettsia risticii and Ehrlichia platys is now Anaplasma platys. Ehrlichia equi, Ehrlichia phagocytophila and human granulocytic Ehrlichial agent have now been confirmed to be the same species and have been reclassified as Anaplasma phagocytophila.

Neorickettsia risticii causes Potomac horse fever, also known as equine monocytic ehrlichiosis. Natural infections have also been reported in dogs, with symptoms resembling E. canis infection. N. risticii is very closely related to Ehrlichia sennetsu.

Potomac horse fever is prevalent in the Midwestern and Northeastern United States and has also been reported in Europe and India. Mode of transmission is unknown, but recent reports suggest oral transmission. Transplacental transmission of this agent has also been reported. Infections are more prevalent during summer months and have a 5 to 30% mortality rate. Clinical signs of infection are pyrexia, anorexia, enteritis, watery diarrhea, colic and laminitis. Hematological findings include thrombocytopenia and leucopenia.

Serological detection of E. risticii is not only insensitive but also not very specific because of cross-reactivity with other Ehrlichia organisms. Detection by culture is slow and also lacks sensitivity. Detection of the pathogen by PCR is sensitive and highly specific (Mott et al., 1997).

Utilities:

• Confirm the disease causing agent
• Shorten the time required to confirm a clinical diagnosis of N. risticii infection.
• Ensure that horse populations are free of N. risticii
• Early prevention of spread of this bacterium
• Minimize personnel exposure to this bacterium
• Safety monitoring of biological products that derive from horses

References:
Mott, J., Rikihisa, Y., Zhang, Y., Reed, S.M. and Yu, C.Y. (1997) Comparison of PCR and culture to the indirect fluorescent-antibody test for diagnosis of Potomac horse fever. J Clin Microbiol. 35:2215-9.

Specimen requirements: 1 ml whole blood in EDTA (purple top) or ACD (yellow top) tube, fecal swab or 1 ml feces, shipped overnight at room temperature.

For specimen types other than those listed here, please call to confirm specimen acceptability and shipping instructions.

For all specimen types, if there will be a delay in shipping, or during very warm weather, refrigerate specimens until shipped and ship with a cold pack unless more stringent shipping requirements are specified. Frozen specimens should be shipped so as to remain frozen in transit. See shipping instructions for more information.

Turnaround time: 2 business days

Methodology: Qualitative real time PCR

Normal range: Nondetected