Chemotherapy of ovine fascioliasis: Use of an analytical model to assess the impact of a series of discrete doses of anthelmintic on the prevalence and intensity of infection

Number of Cryptosporidium parvum oocysts or Giardia spp cysts shed by dairy calves after natural infection

OBJECTIVE

To determine the total number of Cryptosporidium parvum oocysts and Giardia spp cysts shed by dairy calves during the period when they are most at risk after natural infection.

ANIMALS

478 calves naturally infected with C. parvum and 1,016 calves naturally infected with Giardia spp.

PROCEDURE

Oocysts or cysts were enumerated from fecal specimens. Distribution of number of oocysts or cysts versus age was used to determine the best fitting mathematic function. Number of oocysts or cysts per gram of feces for a given duration of shedding was computed by determining the area under the curve. Total number of oocysts or cysts was calculated by taking the product of the resultant and the expected mass of feces.

RESULTS

Intensity of Cparvum oocyst shedding was best described by a second-order polynomial function. Shedding increased from 4 days of age, peaked at day 12, and then decreased. An infected 6-day-old calf would produce 3.89 x 10(10) oocysts until 12 days old. Pattern of shedding of Giardia spp cysts was best described by exponential functions. Intensity of shedding increased from 4 days of age, peaked at day 14, and then decreased. An infected calf would produce 3.8 x 10(7) cysts from day 50 until day 56.

CONCLUSIONS AND CLINICAL RELEVANCE

The large number of oocysts and cysts shed indicates that shedding by dairy cattle poses a risk for susceptible calves and people. Estimates reported here may be useful to aid in designing cost-effective strategies to manage this risk.

The economics of parasite control: obstacles to creating reliable models

Models of the population biology and transmission of helminth parasites of domestic ruminants do not generally include any explicit reference to production because that would require the identification of a systematic relationship between parasitism and production loss. This is difficult because (1) the relationship between parasitism and production loss (in so far as we can identify one) is non-linear, (2) it is by no means clear which index of parasitism we should use in defining the nature of the relationship and (3) time delays in the system and the multicausal nature of production losses make it difficult to tease out those components of the loss that are attributable to parasitism.

New methods of drug application for control of helminths

Several new ways have been developed of delivering anthelmintics to ruminants aimed particularly at reducing labour. For single doses, these include a semi-automatic rumen injector for giving insoluble drugs to cattle more conveniently and efficiently than by oral dosing, and the dermal application of levamisole which is, however, subject to seasonal variation in absorption. Sustained administration offers potentially a high level of preventive control but carries a greater risk of developing drug resistance. Even with the best available methods of group administration in feed supplements or drinking water, there remains some uncontrollable variation in individual intake. Intraruminal sustained release devices largely overcome this problem and constitute the most important new technology. They are represented at present by the commercially successful morantel sustained release bolus, and the more versatile Laby capsule which is under development for anthelmintic delivery. Other new applications include the possible development of synergists, potentiators and drug combinations, the special features of drugs which bind strongly to plasma proteins, and the new possibilities offered by a drug highly effective against all stages of Fasciola hepatica. In the general approach to anthelmintic application in helminth control, there have been advances in knowledge of helminth population biology which can lead to better timing of strategic dosing programs, which show that single treatments can have persistent benefits without a simultaneous reduction in infection rate, and which question the conventional view that control schemes require the treatment of all animals in the group.