Potomac Horse Fever

Question 1

What is potomac horse fever caused by?

Answer 1

Neorickettsia risticii

Question 2

Is it gram (-) or (+)

Answer 2

gram (-)

Question 3

What type of bacteria is it?

Answer 3

intracellular bacteria that invades macrophages and epithelial cells of the intestine

Question 4

Neorickettsia risticii

Answer 4

an environmental bacterium that lives inside flukes that are parasitic to aquatic snails, insects, and bats

Question 5

How is the N. risticii bacterium transmitted to horses?

Answer 5

when a horse accidentally ingests insects harboring flukes infected with N. risticii (causes acute and possibly fatal disease)

Question 6

When and where was the disease first recognized?

Answer 6

in 1979 along the Potomac river in Maryland

Question 7

Where in the world does potomac horse fever occur?

Answer 7

-43 states in US
-3 provinces of Canada
-South America
-Europe
-India

Question 8

Where does potomac horse fever occur?

Answer 8

near fresh water streams, rivers, and irrigated pastures

Question 9

When is the disease most prevalent?

Answer 9

May-November

Question 10

Where is the organism present?

Answer 10

in feces

Question 11

What is most likely the intermediate hosts of the fluke?

Answer 11

snails

Question 12

What % of snails are infected with the fluke (depending on the time of the year)

Answer 12

3-93%

Question 13

What are some second intermediate hosts?

Answer 13

-aquatic insects
-mayfly
-dragonfly
-caddisfly
-damselfly

Question 14

What % of the aquatic insects (second intermediate hosts) test positive for N. risticii?

Answer 14

up to 80%

Question 15

What is a possible host?

Answer 15

-no definitive host has been found but possibly insect eating bats

Question 16

How has experimental PHF been transmitted?

Answer 16

-IV
-IM
-SQ
-ID
-PO

Question 17

How may horses be infected?

Answer 17

-through skin penetration
-ingesting contaminated water or insect

Question 18

When can accidental ingestion of insects occur?

Answer 18

-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)

Question 19

What does N. risticii infect after infection?

Answer 19

blood monocytes

Question 20

What cells does it infect?

Answer 20

epithelial cells of the intestine

Question 21

Where are lesions confined to?

Answer 21

the GI tract, especially the large colon

Question 22

What does PHF reduce?

Answer 22

luminal absorption of electrolytes (Na and Cl) and increased water losses

Question 23

Diarrhea causes…

Answer 23

a loss of epithelial cells

Question 24

How long are recovered horses immune for?

Answer 24

at least 20 months

Question 25

How long is the incubation period?

Answer 25

1-3 weeks

Question 26

What are some risk factors?

Answer 26

-proximity to ponds
-time spent outside
-no use of insecticides

Question 27

Clinical signs of PHF:

Answer 27

-depression
-anorexia
-Biphasic fever: 102-107 degrees
-decreased intestinal sounds
-severe toxemia, dehydration, cardiac compromise, congested mucosal membranes
-laminitis

Question 28

Clinical signs within 24-48 hours:

Answer 28

moderate to severe diarrhea, mild abdominal discomfort

Question 29

Laminitis caused by PHF:

Answer 29

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

Question 30

Mortality

Answer 30

5-30% (associated with laminitis and toxemia)

Question 31

What does mortality depend on:

Answer 31

-living environment
-vet resources
-how severe the horses symptoms get
-how much money the owner can provide

Question 32

How is PHF diagnosed?

Answer 32

-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

Question 33

Once treatment has started, how quickly is the response?

Answer 33

very fast (within 3 days if therapy is started early)

Question 34

Treatment antibiotics given:

Answer 34

-IV oxytetracycline
-BID
-5 days
-if given early, a response to treatment is seen within 12 hrs

Question 35

Other treatments:

Answer 35

-fluids to prevent hypovolemia and shock
-banamine for fever and abdominal discomfort and prevent endotoxemia
-watch for signs of laminitis

Question 36

What % of horses is the vaccine experimentally reported to protect?

Answer 36

78%

Question 37

Vaccine failure

Answer 37

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

Question 38

Prevention of PHF:

Answer 38

-vaccination (questionable usefulness)
-limit access of horses to streams, ponds
-turn off the lights of barns in the evening