Rumen pH increases and true indigestion develops. This pattern of the disease typifies what is referred to as Type 1 Ostertagiasis. Failure to gain or weight loss can occur in calves that do not develop severe Ostertagiasis as a result of less serious infestations. This slight weight loss or failure to gain is undoubtedly more common than severe clinical parasitism and is a potential problem for all calves the first time they graze on contaminated pasture. Figures aren't available, but it seems likely that in this disease, as in many others,the odd calf with clinical signs is the tip of the iceberg, with unnoticed losses occurring in most other apparently parasitized calves.
Clinical signs occur at varying times.
Two clinical situations can occur. These two different phenomena are termed Type 1 and Type 11 Ostertagiasis. Type 1 Ostertagiasis Type 1 is the form of parasitism usually seen. Heifers that have been raised in confinement will be free of Ostertagia. If the pasture they graze was occupied by parasitized cattle the previous year, it will have become contaminated with infective larvae.