Potomac Horse Fever Flashcards
What is potomac horse fever caused by?
Neorickettsia risticii
Is it gram (-) or (+)
gram (-)
What type of bacteria is it?
intracellular bacteria that invades macrophages and epithelial cells of the intestine
Neorickettsia risticii
an environmental bacterium that lives inside flukes that are parasitic to aquatic snails, insects, and bats
How is the N. risticii bacterium transmitted to horses?
when a horse accidentally ingests insects harboring flukes infected with N. risticii (causes acute and possibly fatal disease)
When and where was the disease first recognized?
in 1979 along the Potomac river in Maryland
Where in the world does potomac horse fever occur?
-43 states in US
-3 provinces of Canada
-South America
-Europe
-India
Where does potomac horse fever occur?
near fresh water streams, rivers, and irrigated pastures
When is the disease most prevalent?
May-November
Where is the organism present?
in feces
What is most likely the intermediate hosts of the fluke?
snails
What % of snails are infected with the fluke (depending on the time of the year)
3-93%
What are some second intermediate hosts?
-aquatic insects
-mayfly
-dragonfly
-caddisfly
-damselfly
What % of the aquatic insects (second intermediate hosts) test positive for N. risticii?
up to 80%
What is a possible host?
-no definitive host has been found but possibly insect eating bats
How has experimental PHF been transmitted?
-IV
-IM
-SQ
-ID
-PO
How may horses be infected?
-through skin penetration
-ingesting contaminated water or insect
When can accidental ingestion of insects occur?
-grazing near streams
-drinking stream water
-insects are attracted to barn lights and are present in feed and water (turn of barn lights when possible)
What does N. risticii infect after infection?
blood monocytes
What cells does it infect?
epithelial cells of the intestine
Where are lesions confined to?
the GI tract, especially the large colon
What does PHF reduce?
luminal absorption of electrolytes (Na and Cl) and increased water losses
Diarrhea causes…
a loss of epithelial cells
How long are recovered horses immune for?
at least 20 months
How long is the incubation period?
1-3 weeks
What are some risk factors?
-proximity to ponds
-time spent outside
-no use of insecticides
Clinical signs of PHF:
-depression
-anorexia
-Biphasic fever: 102-107 degrees
-decreased intestinal sounds
-severe toxemia, dehydration, cardiac compromise, congested mucosal membranes
-laminitis
Clinical signs within 24-48 hours:
moderate to severe diarrhea, mild abdominal discomfort
Laminitis caused by PHF:
more common than in other causes of enterocolitis
-if it develops it is usually more severe and often refractory to treatment
-very common in PHF
Mortality
5-30% (associated with laminitis and toxemia)
What does mortality depend on:
-living environment
-vet resources
-how severe the horses symptoms get
-how much money the owner can provide
How is PHF diagnosed?
-clinical signs
-time of year
-isolation of N. risticii from blood or feces (difficult and time consuming)
-PCR to detect the DNA in blood and feces (more reliable than culturing the organism)
-serology to detect antibodies may not be useful
Once treatment has started, how quickly is the response?
very fast (within 3 days if therapy is started early)
Treatment antibiotics given:
-IV oxytetracycline
-BID
-5 days
-if given early, a response to treatment is seen within 12 hrs
Other treatments:
-fluids to prevent hypovolemia and shock
-banamine for fever and abdominal discomfort and prevent endotoxemia
-watch for signs of laminitis
What % of horses is the vaccine experimentally reported to protect?
78%
Vaccine failure
proposed explanations for this include
-lack of seroconversion (killed, adjuvant)
-multiple field strains whereas only one strain is present in available vaccines
-attributed to antigenic and genomic heterogeneity among the >14 strains of N. risticii isolated from natural occurring cases
Prevention of PHF:
-vaccination (questionable usefulness)
-limit access of horses to streams, ponds
-turn off the lights of barns in the evening
