Transmission And Persistence Of Johne's Disease: Is The Iceberg Phenomenon Consistent With Disease Transmission Dynamics

Johnes disease in cattle is caused by Mycobacterium avium subspecies paratuberculosis (MAP). MAP is associated with rapid weight loss and diarrhea of the infected cattle. This leads to reduced milk production in dairy farming and loses compounded by culling infected animals in order to prevent the disease from spreading within a herd. Disease progression follows four distinct stages, silent, subclinical, clinical and the advanced stage. Methods for early infection diagnosis are yet to be developed. The disease is hardly noticeable in the silent and subclinical stages. Therefore, the infection is noticed when animals are already in the clinical stage. This study investigates the “Iceberg phenomenon”, which attempts to estimate the severity of the disease in a herd once an infected animal is identified. The phenomenon estimates that for one animal in the advanced stage, there are one-to-two in the clinical stage, four-to-eight in the subclinical stage and ten- to-fourteen in the silent stage. Using a system of differential equations our study shows that it is not possible to observe the Iceberg phenomenon using incubation periods associated with the natural course of disease progression. However, the phenomenon can be observed if the incubation period of the silent stage is assumed to be longer than that of the subclinical stage, and incubation period of the clinical stage is less than the subclinical incubation time. This gives rise to the question, is the biology of disease progression accurate or does the Icerberg phenomenon need to be revised?