Salmonella is a zoonotic organism, which means it can be passed from an
animal to a human.
According to a 1996 U.S. study, salmonella-tainted food costs between
$600 million and $3.5 billion US a year in human medical expenses and
productivity losses.
Salmonella typhimurium DT 104 is of particular concern. This strain is
resistant to at least five antibiotics.
Of all salmonella strains, this one causes the highest mortality rates
in people.
At least five outbreaks of DT104 have been reported in the United
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States. Three were linked to consumption of noncommercial or homemade
cheese that was contaminated with salmonella. One was traced to
exposure to infected dairy cattle. The other occurred in elementary
school children believed to have consumed contaminated milk or
contacted infected animals.
Though the most recognized source of salmonella DT104 is cattle, it has
also been isolated from sheep, goats, pigs, dogs, mice, cats, horses
and turtles.
It is thought to pass between animals via a fecal-oral route, which
means the bacterium is passed in the feces and consumed by the next
host. Overcrowded conditions and stress will promote the spread of
salmonella organisms.
Humans can be infected directly through contact with ill farm animals
or indirectly by consuming contaminated food. Occasionally, infected
pets can pass it to humans. Cats’ coats can become contaminated through
grooming because they shed large numbers of salmonella from their
mouths.
Salmonella is not a benign illness in people. More than 40 percent of
afflicted individuals need to be hospitalized and up to three percent
will die. The infection typically causes intestinal signs such as
vomiting and diarrhea, but in some people the bacteria can reach the
bloodstream, causing the blood infection septicemia.
The exact cost of salmonella to cattle producers is difficult to
determine because it includes treatment costs, deaths, increased cull
rates, reduced efficiency and decreased weight gain.
Infected cattle will suffer from fever, mental dullness, loss of
condition, decreased milk production, loss of appetite, dehydration and
diarrhea that can progress to dysentery. Because many other infections
can cause similar symptoms, the only way to diagnose salmonella is to
culture feces or blood.
Treating salmonella DT104 is difficult because it is resistant to
antibiotics commonly used in cattle.
It is unlikely salmonella will be eradicated from domestic animals in
the near future. The infection can, however, be controlled by a number
of management procedures:
- Clean up fecal material and compost it away from cattle.
- Use footbaths to prevent fecal transfer.
- Work with calves first, and then adult cattle.
- Keep feed and water off the ground.
- Quarantine newly purchased livestock for a month.
- Isolate sick animals. Minimize animal density.
- Prevent access to feed storage areas by animals other than cattle.
Because of DT104’s resistance to antibiotics, concern has been raised
that antibiotic use in cattle could cause more resistance to develop.
In the United Kingdom, DT104 became harder and harder to kill with
fluoroquinolones, which was a new antibiotic at the time. The
resistance was blamed on the use of those antibiotics in cattle, but
the claim was not substantiated because fluoroquinolones weren’t
available for use in cattle until a year after the resistance pattern
emerged.
Jeff Grognet is a veterinarian and writer practising in Qualicum Beach,
B.C.
