Understanding how Clostridium perfringens can harm calves
In the animal diseases course I teach at SDSU, I encourage students to take a big-picture view of animal diseases. We typically greatly emphasize the role of germs as the cause of illness. That focus is not wrong – after all, germs are necessary components of infectious diseases. What we often fail to consider, however, are the roles of the animals and their resistance to disease, as well as the environments in which our animals (and the germs) exist. This consideration of all three factors – the agent (germ), host (animal) and the environment – constitutes what we call the “epidemiologic triad.”
Calf scours is a great example of an illness that involves all three parts of the epidemiologic triad. This spring’s weather has resulted in a lot of calls about scours and other calf issues. A frequent subject of those calls has been Clostridium perfringens. While many different germs are involved in calf scours, Clostridium perfringens is one bug we are struggling with as much as we ever have. What can the epidemiologic triad tell us about how this particular germ affects our baby calves?
Let’s start with the germ itself on the first corner of the triad. Clostridium perfringens, like others in its family, is very hardy. When exposed to the outside environment, it turns itself into a spore that can survive a very long time. Once it’s swallowed by a calf, that spore form activates, and the bacteria multiply in the gut.
The trouble caused by Clostridium perfringens in the animal isn’t really from the bacteria themselves, it’s from the toxic chemicals (toxins) those bacteria produce. Different strains of Clostridium perfringens can produce different toxins, which all do different things to the gut and the animal. Based on which toxins they produce, Clostridium perfringens strains are categorized into “types” denoted by letters: Types A, B, C, D, and E.
Historically, Type C infections in baby calves got a lot of attention. This strain produces “beta toxin” that damages the gut, producing bloody scours and sometimes sudden death in very young calves. Nowadays, we’re much more likely to identify Type A Clostridium perfringens in sick calves. This strain produces “alpha toxin,” which affects a calf differently than beta toxin. This strain is associated with abomasal (stomach) problems in somewhat older (2-6 weeks old) calves, making them prone to bloating and scours. Stomach ulcers in young calves are another possible result of Type A infections.
The next corner of the triad concerns the animal itself, and its ability to fight off the effects of Clostridium perfringens toxins. Age-related resistance plays a large role here, but the calf’s immunity is also critical. Calves with antibodies against the different toxins (not just the bacteria itself) are protected against the damage the toxin can inflict on their digestive system. These antibodies come from colostrum, or are created by the calf’s own immune system.
The final corner of the triad is the environment. The “success” of any animal disease germ depends a lot on how well it does in the animal’s environment. As mentioned, Clostridium perfringens has adapted very well to survive in pens, lots, and dirty bedding.
There’s another environment to consider for Clostridium perfringens, too – the environment inside the calf’s gut. Type A strains have adapted to survive well in the calf’s gut, too – to the point where it’s considered a normal finding in a calf’s inventory of gut bacteria (other types are considered to be true pathogens and are encountered from the outside).
There are, however, instances where the growth rate of Type A strains can explode. This happens when there is a sudden increase in the influx of nutrients into the gut: calves with good-milking mothers, or calves temporarily separated from their mother, only to “tank up” once reunited, for example. These nutrients feed the Type A bacteria, which means more bacteria and more toxin production, and a greater chance of illness due to Type A in the calf.
Applying the big-picture concept of the epidemiologic triad to a disease like Clostridium perfringens can really help us understand this germ and what it does to our calves. Now, how can we apply it to help us figure out what to do about Clostridium perfringens? That will be the focus of the next column to come!
Russ Daly, DVM, is the Extension veterinarian at South Dakota State University. He can be reached via e-mail at email@example.com or at 605-688-5171.