potomac horse fever

Question 1

etiology

Answer 1

caused by neorickettsia risticii, gram negative coccus, intracellular bacteria-invades macrophages and epithelial cells of the intestines

Question 2

epidemiology

Answer 2

recognized in may 1979 along potomac river in MD, occurs in 43 states of US, 3 canada provinces, south america, europe and india, occurs fresh water streams, rivers and irrigated pastures, May-November, organism present in feces.

Question 3

role of snails

Answer 3

strong evidence that snails infected with the fluke are involved in the life cycle as intermediate hosts., many species of snails incriminated with the life cycle of N. Ristikii, 3.93% of snails infected with the fluke, second intermediate host=aquatic insects (may flies, dragonflies, caddisfly and damselfly), up to 80% of aquatic flies are positive for N. Risticii, no definitive host has been identified

Question 4

how is PHF transmitted?

Answer 4

PHF has been transmitted IV,IM,SQ,ID and PO, horses have been infected through skin penetration or by ingesting contaminated water or insect, when horses are fed infected insects they contract the disease, under natural conditions: horses can ingest aquatic insects while they graze near streams or drink stream water and swallow the insect, accidental ingestion of insects that are attracted to barn lights and are present in feed and water

Question 5

what type of water do horses need to drink?

Answer 5

treated city water or uncontaminated well water

Question 6

pathogenesis

Answer 6

N. Risticii infects blood monocytes, infects epithelial cells of the intestines, lesions are confined to the GI tract (especially the large colon), diarrhea is caused by loss of epithelial cells and there is reduced luminal absorption of electrolytes (Na and Cl) and increased water losses, recovered horses are immune to reinfection for at least 20 mo., incubation period is 1-3 weeks

Question 7

risk factors for PHF

Answer 7

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

Question 8

clinical signs

Answer 8

depression, anorexia, biphasic fever (102-107), decreased intestinal sounds, moderate to severe diarrhea within 24-48 hrs, mild abdominal discomfort
some horses: severe toxemia, dehydration, cardiac compromise, congested mucosal membranes, laminitis, mortality=5-30% associated with toxemia and laminitis

Question 9

diagnosis

Answer 9

clinical signs, time of year, isolation of N. Risticii from blood or feces, PCR is more sensitive and reliable than culturing the organism, serology to detect antibodies may not be useful because it takes time after infection for antibody titers to rise, differential diagnosis includes any syndrome that produces diarrhea

Question 10

treatment

Answer 10

IV, oxytetracycline 2x per day for 5 days, fluids to prevent hypovolemia and shock, ban amine for fever and abdominal discomfort and to prevent endotoxemia

Question 11

prevention

Answer 11

vaccination, questionable usefulness, limit access to streams and turn off lights of barns in the evenings