The information presented in this Case Study is from Jerome Nietfeld, DVM, PhD, and Brad DeBey, DVM, PhD, Kansas State University Veterinary Diagnostic Laboratory, and appeared in the Kansas Veterinary Quarterly, Summer 2005.
Lead poisioning is one of the most important causes of toxicity in cattle. Although lead poisioning
occurs throughout the year, the number of cases increases in the spring and summer. Two primary reasons are:
The acute toxic dose of lead is much lower for calves than for adult cattle.
Calves are curious and can explore places that adult cattle cannot. If their environment contains an old battery or something else with lead, calves will find it and eat it.
Lead batteries or old bags of insecticide inadvertently left where cattle can find them can cause sudden death. PHOTO:GAVIN MEERDINK
Approximately 20g (0.07 oz.) of lead is an acute lethal dose for a neonatal calf.
Textbooks list the clinical signs of acute lead poisoning as being primarily referable to the central nervous system (CNS).1.2 Signs that appear within 24 to 48 hours of exposure include ataxis, blindness, hyperesthesia, muscle tremors, seizures, head pressing, aggression and excessive salivation. Diarrhea, bloat, tenesmus and constipation can also occur but are often overshadowed by the CNS symptoms. Differential diagnoses are rabies, thromboembolic meningoencephalitis, polioencephalomalacia, listeriosis, nervous coccidiosis, ammoniated forage toxicosis and hepatic encephalopathy.1
Blindness is an important finding and can occur with few other noticeable signs of lead toxicity. The two most common causes of acute blindness in cattle are lead toxicity and polioencephalomalacia. In any outbreak of CNS disease in calves where the clinical signs resemble polio-encephalomalacia, if affected calves do not respond to treatment for polio, test them for lead.
Something that is rarely, if ever, mentioned in textbooks as a clinical sign of lead poisoning is sudden death. Each year we receive several cases where someone has several calves die unexpectedly. The calves are simply found dead in the pasture, but sometimes they are found recumbent and minimally responsive. Usually, there is nothing in the history to suggest CNS disease. If there are signs to suggest CNS disease, they are very subtle, like the case where an owner pulled up to the veterinarian’s clinic with seven dead calves and one live calf in a trailer. The live calf was in lateral recumbency, hypothermic, nonresponsive and died within minutes. The veterinarian thought the calf had nystagmus but noticed nothing else to assist in determining the cause of death. The calves were getting through a small hole in the fence and chewing on a cracked car battery.
Typically, the calves that die suddenly are less than 2-3 months old. Older calves typically display clinical signs of CNS disease before dying. The younger calves may display signs of CNS disease, but the clinical course is probably very short and easy to miss if the calves are not being observed closely. 1
Lead should be suspected in calves that are found dead or dying, do not have clinical evidence of diarrhea or dehydration and do not have significant necropsy findings. Lead poisoning should be considered in cases where several calves have died peracutely for no apparent reason.
For live animals, whole blood is the specimen of choice. More than 90% of circulating lead is absorbed to erythrocytes, so serum is not suitable. Most laboratories can determine lead levels on EDTA, citrated or heparinized blood. If only clotted blood is available, it can be analyzed -- it just takes a little longer. Do not send serum without the clot. EDTA can interfere with some procedures, so if you are unsure of the sample needed, call the diagnostic laboratory to determine the appropriate sample.
In most species, blood lead levels greater than 0.3 ppm indicate significant exposure to lead and are usually considered diagnostic if accompanied by appropriate clinical signs. Levels greater than 0.6 ppm are diagnostic of toxicity.
Kidney and liver are the tissues of choice for diagnosis of toxicity in dead animals. Levels of greater than 10 ppm lead in either organ are usually considered diagnostic of toxicosis. In cases of acute toxicosis, the kidney is more likely to contain elevated lead. In cases of chronic toxicity, lead is more likely to be elevated in the liver. Virtually all cases seen by the Kansas State University Veterinary Diagnostic Laboratory are due to acute exposure, and the lead levels are higher in the kidney than the liver. Occasionally the liver levels are normal while the kidney levels are considerably above 10 ppm.
Sources of lead
Textbooks give long lists of possible sources of lead exposure,1,2 which shows how widespread lead is in the environment. By far, the most common source is discarded or forgotten automotive-type batteries. Lead-based paint accounts for a small percentage of cases and improperly discarded oil and other lubricants that contain lead account for a few cases.
If lead toxicity is suspected or diagnosed and you do not find one of these sources, pull out a toxicology, medical or pathology text and continue looking for one of the lead-containing compounds listed.
1 Gwaltney-Brant, S. (2004). Lead. Clinical Veterinary Toxicology. 204-210.
2 Kahn, C.M., editor (2005). Lead poisoning. The Merck Veterinary Manual (9th ed.). 2404-2405.