Summary of the comparison of Isotonic versus Hypertonic Solutions for rehydrating cryptosporidial scours.

The basis of this report and review of scientific evidence is to challenge that hypertonic solutions are advantageous.
I have placed animal welfare considerations in a pre-eminent position. By reviewing the literature, the data is in my opinion overwhelmingly in favour of using isotonic solutions in situations of infectious scours to provide fast re-hydration, kinder rehydration with less discomfort, and reduced exposure of recovering intestinal cells to insult (hypertonic solutions) so that discomfort signs do not hinder early recovery of milk appetite.

Update: (2017)
New Data
1. Abohassani et al (2008) report on the comparison of 3 different molar strength solutions and measured the degree to which pro-inflammatory responses occurred. While completed in rats, the basis of this study identifies that the osmotic stress from hyperosmolarity solutions have heightened levels of the indicators associated with inflammatory signals (which include pain and discomfort). These cytokines include IL-8 and activation of NF -kappaB

2. Published trials in veterinary literature, justifying hypertonic are either based on artificially induced scours (Naylor 1999) or studies reporting on healthy calves treated with hypertonic solutions (Levy et al 1990) i.e. in calves without villus damage. Naylor however produces no data relevant to the claims he makes supporting the claim that there are no deleterious effects of hypertonic solutions.
To support his claims justifying a hypertonic solution Naylor quotes the studies by Levy et al (1990). But this is based on studies comparing solutions of hyperosmolar and isotonic oral electrolyte solutions in healthy calves. In that study the two solutions were shown to have similar beneficial effects on hydration, and this in his opinion alleviates worries about the effect of hypertonicity on gut absorption
However gut absorption is not the same as comparing the return of appetite and the ease and convenience of returning calves to milk feeds. For a review of factors affecting stomach emptying refer to Bell & Mostaghni 1975).

3. The specific relevance of artificially induced diarrhoeas failing to represent naturally-acquired diarrhoeas has been identified by Walker et al (1998). Her studies state that there are morphologic and functional differences to be accounted for. The absence of clinical depression with induced diarrhoeas was considered to be significant and important. Their conclusion is that all effective treatments should be challenged with naturally acquired diarrhoeas.

4. Laboratory Studies: Studies on immature cells are used to study the effects of various strength solutions. As cells are damaged and lost from villi they are replaced by immature cells which take 2-3 days to mature before becoming resistant to the higher strength fluids around them.

gut_villi

Healthy gut villi (left) and damaged gut caused by C. parvum infection (right) in a calf (from Thomson 2016).

These studies indicate that immature intestinal cells at the tip of the villi are probably easily injured with hypertonic fluids as referenced below.

Cell cultures (containing immature cells) mixed with various strength solutions and nutrients for support

Hypotonic (+ nutrients)     Multiply and survive
Isotonic (+ nutrients)         Survive though do not multiply
Hypertonic (+ nutrients)    Death within 1 – 2 hours
Milk                                   Death within 24 hours

5. WHO use hypotonic solutions for infants (with infections) with diarrhoea because they have less complications requiring intravenous therapies, i.e. children suffer less from vomiting with hypotonic solutions. This information highlights the gut comfort of children can be supported with the weaker strength electrolyte solution while at the same time achieving rehydration. Gut comfort for calves is important if calves are to maintain their appetite.

6. I have failed to find trials which show that calves with infectious scours having been used to support hypertonic over isotonic solutions where similar formulations other than tonicity have been used. (PVD trials apart).

7. PVD trials demonstrated that calves had delayed recovery when BCD was added to a
hypertonic commercial electrolyte. This change of electrolyte delayed appetite recovery by 24 hours. It is proposed that this is due to exposure of the immature cells on recovering villi being injured by the high tonicity of high energy solutions, and initiating further inflammatory changes. This observation supports the laboratory animal research by Abolhassani et al (2008)

References:

Abolhassani M, X Wertz, M Pooya, P Chaumet-Riffaud, A Guais, L Schwartz (2008) Hyperosmolarity causes inflammation through the methylation of protein phosphatase 2A. Inflamm. Res 57, 419-429
Bell FR, K. Mostaghni (1975) Duodenal control of gastric emptying in the milk-fed calf.  J.Physiol.245,.387-407
Naylor J M; Oral Electrolyte Therapy Vet Clinics Nth America 1999. 15 (3) p 487 – 504.
(He also notes his conflict of interest stating he has a financial interest in a high energy electrolyte).
Levy M, A M Merritt, L C Levy. Comparison of effects of an iso-osmolar and hyperosmolar oral  rehydrating solution on the hydration status, glycaemia and ileal content composition of healthy neonatal calves. Cornell Vet 80, 143.
Walker PG, PD Constable, DE Morin, JK Drackley, JH Foreman, JC Thurmon. (1998) A reliable, Practical, and  Economical Protocol for Inducing Diarrhea, and Severe Dehydration in the Neonatal Calf. Can J Vet Res 62,  205-213