Chronic Wasting Disease (CWD) is a prion disease that affects all members of the Cervidae family. The disease, CWD, affects the nervous system and is a fatal neurodegenerative disease. During the course of infection, CWD prions accumulate in the central nervous system. This accumulation of infected prions in the body causes tissue damage which leads to death of the disease’s host (Almberg et al., 2011). As the disease progresses the animal can show symptoms of loss of body mass, behavioral changes, head tremors, excessive salivation, and aspiration pneumonia. Although CWD is common, the transmission of the disease varies and there have been many studies trying to discover what the main source of transmission is. CWD has been shown to be directly and indirectly transmitted. While there are many sources of transmission of CWD, some being more effective than others, there is not a universal way the disease is transmitted. The three routes of transmission this paper will be focusing on are environmental exposure, oral inoculation, and indirect transmission.
One of the many possible transmissions of CWD is the transmission through environmental exposure. Environmental exposure is what the animals would be exposed to out in the wild. Environmental exposure is proven to be a plausible passage of transmission in many cases. One being the study of two deer being exposed to feed buckets, water, and bedding daily. All of these things were taken from the other pens that were housing deer that were CWD positive. The deer were in the clinical phases of the disease. After 19 months of the deer being exposed to the contaminated feed buckets, water, and bedding, both animals were CWD positive. Proving that deer who have environmental exposure have a high chance of contracting CWD. However, due to the presence of infectious CWD prions in the environment, the study also suggests that the natural prion infection occurs by many routes additional to direct animal-to animal exposure. In this same study, 3 deer received saliva from a CWD positive deer; only 1 out of 3 deer was found to be CWD positive (Mathiason et al.,2009). Based on this information, it is believed that saliva can possible harbor a high concentration of CWD prions for environmental contamination (Mathiason et al., 2009). The transmission of CWD through environmental exposure is a plausible way of transmission supported by the many studies conducted. Although, there are many other possible routes of the transmission of CWD.
One of the other possible transmissions of CWD is the transmission through oral inoculation. Oral inoculation means that the Cervids can get infected leading to acquiring lesions in their oral mucous membranes, which leads to direct prion entry from contact with any possible exposed nerves or blood vessels. With this information, the oral route is considered to be one of the most plausible routes of infection in nature (Denkers, Telling, Hoover, 2011). This transmission being the most plausible due to the Cervids acquiring minor oral lesions in nature as part of foraging. In a recent study, 4 deer were exposed after oral inoculations to CWD positive brain homogenate who were CWD positive. After 12 months of being exposed, all 4 deer were found to be CWD positive (Denkers et al., 2011). In this study, 2 out of 3 animals received infected CWD blood transfusions, and 1 of 3 deer were orally inoculated with infected saliva. After 19 months of being exposed to the contaminates, all animals were found to be CWD positive (Mathiason et al.,2009). This helps prove that the transmission of CWD through oral inoculation is another plausible means of transmission of the disease.
Another possible route for transmission is that of indirect transmission. Soil is one of the likely environmental reservoirs for the infectivity of CWD. Recent laboratory work suggests the possibility that the prions found in soil can be extremely stable. This increases the possibility of the prions having a longer lifespan; they last longer in the soil and their infectivity has a longer lifetime. This makes wildlife more susceptible for a longer period of time due to the prions lasting longer in soil from the decomposing of an infected carcass. The prions in the soil bind to particles, increasing the prion’s infectivity. Prion shedding in CWD is a key factor in the spread of the disease by direct and indirect environmental contact (Henderson et al.,2015). Indirect transmission is yet another plausible source of transmission of CWD. Although, it is possible that the chance of an animal getting infected by indirect transmission is lesser than environment exposure and oral inoculation.
In conclusion, CWD is an emergent and highly contagious disease. CWD is increasingly common although the transmission of the disease varies and there have been many studies trying to discover what the main source of transmission is. Some of the possible transmissions include: environmental exposure, oral inoculation, and indirect transmission. The transmission of CWD through environmental exposure by contaminated environmental exposure is a plausible way of transmission supported by a plethora of studies conducted. Oral inoculation is also plausible due to the Cervids acquiring minor oral lesions in nature as part of foraging, making them more susceptible to this form of transmission out in the wild, versus in clinical studies. As stated previously all of the different types of transmissions can be transmitted in the wild. All in all, there are many routes of possible transmission with some forms being more likely to occur in nature than others.