Diagnostic tests used in cattle:
- Fecal flotation: Fecal flotation gives a broad overview of the parasites present (nematodes, cestodes, protozoa). Results are assigned semi-quantitative results (Rare, few, moderate, high, very high). It should not be used to assess treatment efficacy for strongyles.
- Modified Wisconsin Egg Count: This technique is the preferred technique for quantifying strongyle nematodes in cattle because it has a lower limit of detection and cattle typically have low egg counts. If egg counts conducted before and 14 days after deworming show less than a 90% reduction in egg counts, the strongyles are considered drug On a herd level, it is recommended to test at least 10 to 15 animals to assess drug resistance. This strategy is best used in cattle with pre-treatment egg counts of >200 eggs per gram. Lower egg counts make calculation of treatment efficacy unreliable.
- Fecal/Baermann sedimentation: Sedimentation methods are used to detect trematode eggs and lungworm larvae.
- McMaster Egg Count: The McMaster method is a poor method for quantitating strongyles in cattle because most cattle egg counts fall below the limit of detection of this Occasionally practitioners will request a McMaster count as a measure of coccidia (Eimeria) oocysts. While we occasionally perform the test for this purpose, submitting DVMs should be aware that it has relatively low levels of precision due to variability in oocyst shedding, the nature of the assay, and the vast number of organisms present in feces. In general, this test has poor utility for cattle samples.
Considerations for interpretation of results:
At a population level, it is expected that grazing ruminants have some level of parasitism. Accordingly, nematodes and coccidia will almost always be detected, especially on fecal flotation. Therefore, diagnostic reports of parasitism should be critically interpreted before implementing herd-level recommendations. Since parasites are normally present in herbivores, it is critical to rule out other differential diagnoses for the clinical sign of concern. In general, fecal flotation results of “rare” or “few” parasites are not considered to be clinically significant. Animals that have concurrent disease processes (bacterial, viral, nutritional, etc) are likely to have increased numbers of parasites as a result of a compromised immune system.
At a population level, under normal conditions, parasites are over dispersed. That means that a few individual animals have many parasites while most animals in the herd have a moderate to low level of parasites. In general, the fecal exam from a single animal or few animals from a large population are unlikely to represent the status of the entire group. It is recommended to run at least 10 samples from animals in the same age class when evaluating a herd.
Interpretation of specific parasitology test findings:
Nematodes: Strongyles are the most important group of nematodes infecting cattle. Ostertagia, Cooperia, and other strongyles can cause reduced weight gain and heavy infections can cause diarrhea. Strongyle egg counts should be determined by the modified Wisconsin method because it has a lower limit of detection and cattle have relatively low egg counts due to the large volume of feces they produce. There are many species of strongyles that shed similar eggs in feces. Accordingly, fecal flotation and modified Wisconsin egg counts are reported as “strongyles” without further differentiation of individual species.
Cows tend to have lower egg counts than calves. There is not a standard for egg count that is considered to be significant or trigger treatment decisions. We suggest that calves in Iowa with >200 eggs per gram or cows with >100 eggs per gram may require some intervention. Again, it should be emphasized that it is important to assess the population of animals and not single animals when making treatment decisions. On a herd level it is common for many animals to have 0-50 eggs per gram while a few have several hundred eggs per gram.
Cattle also become infected with other nematodes such as Trichuris, Strongyloides, and lungworms. These nematodes are less commonly a cause of disease in ruminants in Iowa. Fecal flotation results of “high” or “very high” levels of these parasites may indicate clinical significance.
Cestodes: Ruminants become infected with Moniezia spp. tapeworms. In general, these parasites do not typically cause clinical signs but they may result in decreased feed efficiency. Fecal flotation results of “high” or “very high” levels of these parasites may indicate significance.
Protozoa: The most common protozoan infecting small ruminants is Eimeria, commonly referred to as coccidia. Fecal flotation results of “moderate”, “high”, or “very high” may indicate that the finding is clinically significant. Counting Eimeria organisms (McMaster) for assessing treatment efficiency has poor repeatability due to the nature of the assay and the non-homogenous distribution of oocysts in feces.