Production Animal Clinical Toxicology

Plants | Epidemiology | Pathogenesis | Clinical Signs | Clinical Pathology | Necropsy | Diagnosis | Treatment | Control

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Plants

oats, lucerne, sorghum, maize, kikuyu, rape

Silybum marianum - variegated thistle

Arctotheca calendula - capeweed

Verbisina enceliodes - crown beard

Portulaca spp. - pigweed

Salvia reflexa - mintweed

Epidemiology

animal factors

• most common in ruminants
• cattle more susceptible
• pigs very susceptible but only if nitrites are ingested, e.g. cooked mangels (beets) or nitrite containing wheys
• starved animals given access to potentially toxic plants: related to both quantity ingested and to reduced microbe flora that reduce nitrate
• stressed animals (mustering, droving, other handling)
• lack of acquaintance/adaptation

plant/environmental factors

relate to factors that:

• (1) facilitate uptake of nitrate by plant
• use of high nitrogenous fertilisers
• stockyards
• low soil S and Mo, and,
• (2) reduce the ability of plant reductases to reduce nitrate to ammonia
• drought, dry periods where sufficient moisture at night for nitrate uptake
• during day wilting reduces reductase activity
• eventually nitrate accumulates
• cloudiness, certain herbicides also reduce plant reductases

Pathogenesis

• nitrite absorbed to give metHb
• if greater than 20% metHbaemia - clinical signs
• if greater than 80% metHbaemia - death
• induces an anaemic anoxia
• nitrite may also have some direct effect on tissues
• vasodilation exacerbating peripheral circulatory failure
• most effects are attributable to anoxia

Clinical Signs

• sudden death
• dark mucosae; blood can be chocolate in colour
• respiratory distress from anoxia
• twitching muscle tremor, ataxia
• some animals may have gastroenteritis
• death up to 24 hr post-ingestion

Clinical Pathology

• diphenylamine test for nitrite
• direct on blood smears
• on urine within 48 hr
• metHb - on whole blood submitted within several hours; only some laboratories can do this test
.

Necropsy

• blood may be dark, chocolate in colour; clots poorly
• generally nonspecific

Diagnosis

• history
• clinical pathology

Treatment

• methylene blue i.v. in low doses

Control

• grazing management