Doctors and veterinarians are very familiar with the chain of infection and why it’s important, but it might not be a term producers use regularly. In an article on study.com, professor Jennifer Jurling writes, “Infectious disease results from the interaction of an agent, a host, and an environment. Most of these diseases follow a typical chain of infection starting with a reservoir that contains a pathogen and ending with an infected host. Understanding the chain of infection can help with both the prevention and treatment of infectious diseases.”
Chain of Infection
The chain of infection is made up of six different links: pathogen, reservoir, portal of exit, means of transmission, portal of entry, and the new host. Each link has a unique role in the chain and each can be interrupted, or 'broken', through various means.
The first link is the pathogen or disease-causing organism, which is usually a virus or bacterium. In order to break this link, various methods can be used, including the pasteurization of milk, the chlorination of drinking water, or the use of disinfectants.
The second link is the reservoir, or natural environment that the pathogen requires for survival. Reservoirs can be a person, an animal, or an environmental component, such as soil or water. This link can be broken through medical treatment and testing, insect and rodent eradication, or quarantine.
The third link is the portal of exit and is necessary for the pathogen to leave the reservoir. If the reservoir is a pig, then the portal of exit may be saliva, mucous membranes, feces, blood, or nose or throat discharges. It’s easy to see why it’s difficult to control the portal of exit in a pig operation.
The fourth link is the means of transmission. The pathogen can be transmitted either directly or indirectly. Direct transmission requires close association with the infected host, but not necessarily physical contact. Indirect transmission requires a vector, such as an animal or insect.
The portal of entry is the fifth link. The entry of a pathogen to a swine herd takes place primarily through inhalation, or ingestion. Again, it is difficult to break this link.
The sixth link is the new host. Once a pathogen is in the new host, various factors influence the severity of infection, including the strength of the immune system and the reproductive rate of the pathogen. Immunization, health promotion, and medical treatment can be used to break this link in the chain.
The objective of Dr. Baumert’s study was to determine if the sow herd supplying a nursery flow demonstrating PCVAD was stable for PCV2. Other parties involved in the research were T. Fangman, J. Rustvold, Cargill Pork and Boehringer Ingelheim Vetmedica, Inc. Blood collected from placental umbilical cords, pre-suckle piglets, and sows were evaluated to determine the prevalence of neonatal PCV2 viremia at birth and dam PCV2 viremia. Additionally, environmental samples were collected to assess presence of PCV2 in the environmental.
The 1,200 sow farrow-to-wean operation was selected based on nursery pigs experiencing PCVAD, with an average parity of 2.82. Serum and environmental test results illustrated that 15 of the 16 positive litters were associated with at least one PCV2 PCR positive piglet.
In the Baumert et al. study, the sow herd was shedding PCV2 to its intrauterine piglets at a prevalence higher than expected. Previous diagnostic work in this system would suggest that less than 9 percent of sows were PCV2 positive via PCR. “This diagnostic investigation suggests that serum from placental umbilical cords could be a good indicator of the PCV2 status of the pre-suckle piglets at birth,” write the researchers. “The comparable findings of this diagnostic investigation suggest that additional studies are indicated to determine the future application of the placenta and environmental testing for predicting PCV2 stability status of a sow herd.”