Outside environmental temperatures exceeding 100°F can cause significant stress on cattle. This heat stress is often enhanced by excessive humidity. When the heat index (a combination of temperature and humidity are added together to give one how hot it feels outside) exceeds 100°F, cattle may become significantly stressed resulting in decreased milk production, poor reproductive performance, increased incidences of mastitis, uterine infections, other infections, and death. An excellent example of extreme heat and humidity problems is the heat period of July 2006 which resulted in the death of approximately 20,000 animals.
Animals with underlying chronic disease are the first to die during extreme heat periods. The chronic illness decreases their ability to properly regulate their core body temperature. The most common chronic diseases noted are unresolved pneumonias, chronic liver abscesses, hardware disease, and lymphosarcoma. Animals with unresolved pneumonia may fail to demonstrate significant respiratory distress prior to death. Animals with hardware disease often have extensive peritoneal abscesses associated with the rumenal wall. In some cases, the wire has penetrated the diaphragm and caused necrosis and secondary pneumonia or inflammation of the pericardial sac of the heart. Chronic liver abscesses from a previous case of rumen acidosis are also a common finding in animals submitted for necropsy during these times. In lymphosarcoma (caused by bovine leukemia virus), peripheral lymph node enlargement can be seen, but often the lesions of lymphosarcoma are only internal and involve the heart, spleen, uterus and/or abomasum.
Fresh cows are severely stressed by the heat. Recently fresh cows brought to the diagnostic laboratory for examination had severe uterine infections (endometritis). These animals usually had freshened 10 and 20 days earlier. A mixed bacterial infection consisting of E. coli, Arcanobacterium pyogenes, Proteus and numerous anaerobic bacteria were often identified in the uterine discharge. No common cause for these uterine infections is noted; however dystocia leading to damage of the uterine wall, retained placentas, and retrograde reflux of bacteria into the uterus during delivery can cause such problems. Often these infections are associated with unclean or poorly managed calving areas. These animals die due to high fevers and poor thermoregulation caused by endotoxin release from bacteria in the uterus. Good fresh cow management and uterine discharge monitoring for the development of uterine infections are critical in protecting these animals from heat related deaths.
Moderate to severe diarrhea during periods of high heat makes animals more susceptible to death secondary to dehydration. The identification of the agent causing the diarrhea (this can assist in identifying the source of infection) and keeping the animal hydrated are critical in keeping the animals alive and preventing further infections in other cows.
Vaccination with gram negative organisms during intense heat periods can stress cattle and in some cases, may lead to death. In many instances, these animals are vaccinated in the morning, followed by deaths in the late afternoon or early evening. The cause of the fever (often over 104°F) was due to the bacterial products in the vaccine (endotoxins that develop the antibody response in the animal) which, under normal temperature conditions, would lead to only mild fever production. During extreme heat periods, these bacterial products can have a serious impact on the affected animals. These fevers, along with the intense environmental temperatures, cause ineffective heat dissipation in the animal. Herdsmen who have vaccinated in the evening in hopes of preventing heat associated vaccine deaths by allowing the animals to have a fever during the cooler period of the night can still have heat associated deaths. This is due to prolonged fevers lasting longer than 12 hours which result in stressed animals exposed to another day of intense heat. Consequently, it is wise to refrain from vaccinating animals with fever producing vaccines during these extreme heat periods.
Over-conditioned (fat) dry cows are significantly affected by the intense heat. Animals most commonly affected are overly fat cows that are placed in the dry pen. Often, these animals had been conditioned to sprinklers and misters while in the milking pens. However, when animals are dried off, if they are placed in pens with marginal shade and inadequate misting, the animals cannot adequately stay comfortable, which makes them more susceptible to heat related injuries and possibly death. Adding additional shade and misters to the dry pens often has a beneficial effect, not only to these over-conditioned cows but also to the other dry cows. Since these animals have more difficulty managing excessive heat, over-conditioned animals should be critically evaluated during intense heat periods to see if they should remain in the herd or be replaced. Also, cattle in the dry pen are commonly the animals that receive fever producing vaccines. This may predispose these over-conditioned animals to additional temperatureassociated stresses due to fever and possible death.
The number of abortions commonly increases during excessive periods of heat. In fetuses presented for examination during these periods, we often fail to identify a cause of these abortions. This finding would suggest that the increased abortions may be due to heat associated stress. In addition, vaccination of animals with gram negative vaccines may lead to excessive heat induced abortions. Although the dairy producer may assume that the abortions are due to heat related problems, one needs to submit these fetuses to their veterinarian or regional diagnostic laboratory to insure that infectious agents (leptospirosis, Campylobacter, IBR, BVDV, Listeria, or Neospora) are not the cause of the abortions.
The feed ration management is important during heat. If the TMR ration is left out in the extreme heat and sun for too long of a time, the ration can spoil resulting in intestinal upset, decreased feed intake at later feedings, diarrhea, and decreased milk production.
Outbreaks of bacterial septicemias in calves are often an overlooked problem associated with an extensive heat period that can last for 3 to 4 weeks after the end of the heat period. Most of the septicemias noted are caused by E.coli in 3 to 8 day old calves. However, Salmonella infections (Salmonella Typhimurium, S. Newport, and S. Dublin) were also common causes of septicemia. These septicemias are often associated with poor colostrum and milk management at the dairy due to colostrum/milk degradation and contamination from improper handling and storage resulting in bacterial overgrowth prior to ingestion by the calf. Older calves with septicemias can also have similar problems due to improper storage of milk resulting in bacterial contamination after pasteurization. Once milk is prepared for calves, it should be maintained such that spoilage will not occur resulting in excessive numbers of pathogenic bacteria growing in the milk product.
Source: Robert B. Moeller Jr. DVM, DACVP, California Animal Health and Food Safety Laboratory, Tulare, CA